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LT1934/LT1934-1 Micropower Step-Down Switching Regulators in ThinSOT
FEATURES
s s s s s s s s s
DESCRIPTIO
Wide Input Voltage Range: 3.2V to 34V Micropower Operation: IQ = 12A 5V at 250mA from 6.5V to 34V Input (LT1934) 5V at 60mA from 6.5V to 34V Input (LT1934-1) 3.3V at 250mA from 4.5V to 34V Input (LT1934) 3.3V at 60mA from 4.5V to 34V Input (LT1934-1) Low Shutdown Current: <1A Low VCESAT Switch: 200mV at 300mA Low Profile (1mm) SOT-23 (ThinSOTTM) Package
APPLICATIO S
s s s s s
The LT (R)1934 is a micropower step-down DC/DC converter with internal 400mA power switch, packaged in a low profile (1mm) ThinSOT. With its wide input range of 3.2V to 34V, the LT1934 can regulate a wide variety of power sources, from 4-cell alkaline batteries and 5V logic rails to unregulated wall transformers and lead-acid batteries. Quiescent current is just 12A and a zero current shutdown mode disconnects the load from the input source, simplifying power management in battery-powered systems. Burst Mode(R) operation and the low drop internal power switch result in high efficiency over a broad range of load current. The LT1934 provides up to 300mA of output current. The LT1934-1 has a lower current limit, allowing optimum choice of external components when the required output current is less than 60mA. Fast current limiting protects the LT1934 and external components against shorted outputs, even at 34V input.
, LTC and LT are registered trademarks of Linear Technology Corporation. Burst Mode is a registered trademark of Linear Technology Corporation. ThinSOT is a trademark of Linear Technology Corporation.
Wall Transformer Regulation Automotive Battery Regulation Standby Power for Portable Products Distributed Supply Regulation Industrial Control Supplies
TYPICAL APPLICATIO
3.3V Step-Down Converter
D2
100 LT1934 VIN = 12V
BOOST VIN 4.5V TO 34V VIN LT1934 SHDN GND FB SW
0.22F
EFFICIENCY (%)
L1 47H
90
C2 2.2F
D1 10pF 1M
VOUT 3.3V 250mA
80 VOUT = 3.3V 70
+
ON OFF
C1 100F
604k
60
C1: SANYO 4TPB100M C2: TAIYO YUDEN GMK325BJ225MN D1: ON SEMICONDUCTOR MBR0540 D2: CENTRAL CMDSH-3 L1: SUMIDA CDRH4D28-470
1934 TA01
50 0.1
U
Efficiency
VOUT = 5V 1 10 100 LOAD CURRENT (mA)
1934 TA02
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1934f
1
LT1934/LT1934-1
ABSOLUTE
(Note 1)
AXI U
RATI GS
PACKAGE/ORDER I FOR ATIO
ORDER PART NUMBER
TOP VIEW BOOST 1 GND 2 FB 3 6 SW 5 VIN 4 SHDN
Input Voltage (VIN) ................................................. 34V BOOST Pin Voltage ................................................. 40V BOOST Pin Above SW Pin ...................................... 20V SHDN Pin ............................................................... 34V FB Voltage ................................................................ 6V SW Voltage ............................................................... VIN Operating Temperature Range (Note 2) .......................... LT1934E/LT1934E-1 ......................... - 40C to 85C LT1934I/LT1934I-1 ......................... - 40C to 125C Maximum Junction Temperature .......................... 125C Storage Temperature Range ................. - 65C to 150C Lead Temperature (Soldering, 10 sec).................. 300C
LT1934ES6 LT1934ES6-1 LT1934IS6 LT1934IS6-1 S6 PART MARKING LTXP LTF8 LTAJB LTAJC
S6 PACKAGE 6-LEAD PLASTIC SOT-23
TJMAX = 125C, JA = 250C/ W, JC = 102C/ W
Consult LTC Marketing for parts specified with wider operating temperature ranges.
The q denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 10V, VBOOST = 15V, unless otherwise noted.
PARAMETER Undervoltage Lockout
-40C TA 85C -40C TA 125C q q q q
ELECTRICAL CHARACTERISTICS
CONDITIONS
MIN
TYP 3 3 3 12 12 12 0.01
MAX 3.2 3.6 3.6 22 26 26 2 1.27 1.27 15 60 2.3
UNITS V V V A A A A V V mV nA nA %/V s s % %
Quiescent Current
VFB = 1.3V
-40C TA 85C -40C TA 125C -40C TA 85C -40C TA 125C -40C TA 85C -40C TA 125C
VSHDN = 0V FB Comparator Trip Voltage FB Comparator Hysteresis FB Pin Bias Current FB Voltage Line Regulation Switch Off Time Maximum Duty Cycle Switch VCESAT Switch Current Limit BOOST Pin Current Minimum Boost Voltage (Note 3) Switch Leakage Current VFB = 1.25V 4V < VIN < 34V VFB > 1V VFB = 0V VFB = 1V
-40C TA 85C -40C TA 125C q q q q
VFB Falling
q q
1.22 1.21
1.25 1.25 10 2 2 0.007
1.4 85 83
1.8 12 88 88 200 65
ISW = 300mA (LT1934) ISW = 75mA (LT1934-1) LT1934 LT1934-1 ISW = 300mA (LT1934) ISW = 75mA (LT1934-1) ISW = 300mA (LT1934) ISW = 75mA (LT1934-1) 350 90
300 120 490 160 12 10 2.5 2.5 2
400 120 8.5 6.0 1.8 1.7
2
U
mV mV mA mA mA mA V V A
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WW
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LT1934/LT1934-1
The q denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 10V, VBOOST = 15V, unless otherwise noted.
PARAMETER SHDN Pin Current SHDN Input Voltage High SHDN Input Voltage Low Note 1: Absolute Maximum Ratings are those values beyond which the life of the device may be impaired. Note 2: The LT1934E and LT1934E-1 are guaranteed to meet performance specifications from 0C to 70C. Specifications over the -40C to 85C operating temperature range are assured by design, characterization and CONDITIONS VSHDN = 2.3V VSHDN = 34V 2.3 0.25 MIN TYP 0.5 1.5 MAX 5 UNITS A A V V
ELECTRICAL CHARACTERISTICS
correlation with statistical process controls. The LT1934I and LT1934I-1 specifications are guaranteed over the -40C to 125C temperature range. Note 3: This is the minimum voltage across the boost capacitor needed to guarantee full saturation of the internal power switch.
TYPICAL PERFOR A CE CHARACTERISTICS
LT1934 Efficiency, VOUT = 5V
100 LT1934 VOUT = 5V L = 47H 90 TA = 25C 80 100
VIN = 12V
EFFICIENCY (%)
EFFICIENCY (%)
EFFICIENCY (%)
VIN = 24V
70
60
50 0.1
1 10 100 LOAD CURRENT (mA)
1934 G01
LT1934-1 Efficiency, VOUT = 3.3V
100
500
SWITCH CURRENT LIMIT (mA)
90
EFFICIENCY (%)
LT1934-1 VOUT = 3.3V L = 100H TA = 25C VIN = 12V VIN = 24V
80
300
OFF TIME (s)
70
60
50 0.1 1 10 100
1934 G04
LOAD CURRENT (mA)
UW
LT1934 Efficiency, VOUT = 3.3V
LT1934 VOUT = 3.3V L = 47H 90 TA = 25C 80 VIN = 24V VIN = 12V 70 100
LT1934-1 Efficiency, VOUT = 5V
LT1934-1 VOUT = 5V L = 150H TA = 25C VIN = 12V 80 VIN = 24V 70
90 VIN = 5V
60
60
50 0.1
50 1 10 100 LOAD CURRENT (mA)
1934 G02
0.1
1
10
100
1934 G03
LOAD CURRENT (mA)
Current Limit vs Temperature
3.0 LT1934 400 2.5 2.0 1.5 1.0 0.5
Off Time vs Temperature
200 LT1934-1 100
0 -50
-25
50 25 0 75 TEMPERATURE (C)
100
125
0 -50 -25
50 25 75 0 TEMPERATURE (C)
100
125
1934 G05
1934 G06
1934f
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LT1934/LT1934-1 TYPICAL PERFOR A CE CHARACTERISTICS
Frequency Foldback
16 14
SHDN PIN CURRENT (A)
TA = 25C
10 8 6 4 2 0 0 0.2 0.8 1.0 0.6 FEEDBACK PIN VOLTAGE (V) 0.4 1.2
1934 G07
FEEDBACK VOLTAGE (V)
SWITCH OFF TIME (s)
12
Quiescent Current vs Temperature
20 4.0
QUIESCENT CURRENT (A)
15
UVLO (V)
10
5
0 -50 -25
Minimum Input Voltage VOUT = 3.3V
6.0 LT1934 VOUT = 3.3V 5.5 TA = 25C BOOST DIODE TIED TO OUTPUT
INPUT VOLTAGE (V)
5.0 4.5 4.0
INPUT VOLTAGE (V)
VIN TO START
VIN TO RUN 3.5 3.0 0.1
4
UW
VFB vs Temperature
1.27
2.0
SHDN Bias Current vs SHDN Voltage
TA = 25C
1.26
1.5
1.25
1.0
1.24
0.5
1.23
1.22 -50 -25
0
50 25 0 75 TEMPERATURE (C)
100
125
0
2
4 6 8 SHDN PIN VOLTAGE (V)
10
12
1934 G09
1934 G08
Undervoltage Lockout vs Temperature
3.5
3.0
2.5
75 0 25 50 TEMPERATURE (C)
100
125
2.0 -50 -25
75 0 25 50 TEMPERATURE (C)
100
125
1934 G10
1934 G11
Minimum Input Voltage VOUT = 5V
8 LT1934 VOUT = 5V TA = 25C 7 BOOST DIODE TIED TO OUTPUT VIN TO START 6 VIN TO RUN 5
1 10 100 LOAD CURRENT (mA)
1934 G12
4 0.1
1 10 100 LOAD CURRENT (mA)
1934 G13
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LT1934/LT1934-1
PI FU CTIO S
BOOST (Pin 1): The BOOST pin is used to provide a drive voltage, higher than the input voltage, to the internal bipolar NPN power switch. GND (Pin 2): Tie the GND pin to a local ground plane below the LT1934 and the circuit components. Return the feedback divider to this pin. FB (Pin 3): The LT1934 regulates its feedback pin to 1.25V. Connect the feedback resistor divider tap to this pin. Set the output voltage according to VOUT = 1.25V (1 + R1/R2) or R1 = R2 (VOUT/1.25 - 1). SHDN (Pin 4): The SHDN pin is used to put the LT1934 in shutdown mode. Tie to ground to shut down the LT1934. Apply 2.3V or more for normal operation. If the shutdown feature is not used, tie this pin to the VIN pin. VIN (Pin 5): The VIN pin supplies current to the LT1934's internal regulator and to the internal power switch. This pin must be locally bypassed. SW (Pin 6): The SW pin is the output of the internal power switch. Connect this pin to the inductor, catch diode and boost capacitor.
BLOCK DIAGRA
VIN
VIN C2
5
+ + -
BOOST ON TIME 12s DELAY OFF TIME 1.8s DELAY R Q S Q SW L1 6 D1 C1 VOUT C3 D2 1
ON OFF
4
SHDN
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VREF 1.25V
+
ENABLE
-
2 GND R2 3 FB R1
FEEDBACK COMPARATOR
1934 BD
1934f
5
LT1934/LT1934-1
OPERATIO
The LT1934 uses Burst Mode control, combining both low quiescent current operation and high switching frequency, which result in high efficiency across a wide range of load currents and a small total circuit size. A comparator monitors the voltage at the FB pin of the LT1934. If this voltage is higher than the internal 1.25V reference, the comparator disables the oscillator and power switch. In this state, only the comparator, reference and undervoltage lockout circuits are active, and the current into the VIN pin is just 12A. As the load current discharges the output capacitor, the voltage at the FB pin falls below 1.25V and the comparator enables the oscillator. The LT1934 begins to switch, delivering current to the output capacitor. The output voltage rises, and when it overcomes the feedback comparator's hysteresis, the oscillator is disabled and the LT1934 returns to its micropower state. The oscillator consists of two one-shots and a flip-flop. A rising edge from the off-time one-shot sets the flipflop, which turns on the internal NPN power switch. The switch remains on until either the on-time one-shot trips or the current limit is reached. A sense resistor and amplifier monitor the current through the switch and resets
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(Refer to Block Diagram)
the flip-flop when this current reaches 400mA (120mA for the LT1934-1). After the 1.8s delay of the off-time one-shot, the cycle repeats. Generally, the LT1934 will reach current limit on every cycle--the off time is fixed and the on time is regulated so that the LT1934 operates at the correct duty cycle. The 1.8s off time is lengthened when the FB pin voltage falls below 0.8V; this foldback behavior helps control the output current during start-up and overload. Figure 1 shows several waveforms of an LT1934 producing 3.3V from a 10V input. When the switch is on, the SW pin voltage is at 10V. When the switch is off, the inductor current pulls the SW pin down until it is clamped near ground by the external catch diode. The switch driver operates from either the input or from the BOOST pin. An external capacitor and diode are used to generate a voltage at the BOOST pin that is higher than the input supply. This allows the driver to fully saturate the bipolar switch for efficient operation. If the SHDN pin is grounded, all internal circuits are turned off and VIN current reduces to the device leakage current, typically a few nA.
VOUT 50mV/DIV VSW 10V/DIV
ISW 0.5A/DIV
ILI 0.5A/DIV
5s/DIV
1934 F01a
Figure 1. Operating Waveforms of the LT1934 Converting 10V to 3.3V at 180mA (Front Page Schematic)
1934f
LT1934/LT1934-1
APPLICATIO S I FOR ATIO
Which One to Use: LT1934 or LT1934-1? The only difference between the LT1934 and LT1934-1 is the peak current through the internal switch and the inductor. If your maximum load current is less than 60mA, use the LT1934-1. If your maximum load is higher, use the LT1934; it can supply up to ~300mA. While the LT1934-1 can't deliver as much output current, it has other advantages. The lower peak switch current allows the use of smaller components (input capacitor, inductor and output capacitor). The ripple current at the input of the LT1934-1 circuit will be smaller and may be an important consideration if the input supply is current limited or has high impedance. The LT1934-1's current draw during faults (output overload or short) and start-up is lower. The maximum load current that the LT1934 or LT1934-1 can deliver depends on the value of the inductor used. Table 1 lists inductor value, minimum output capacitor and maximum load for 3.3V and 5V circuits. Increasing the value of the capacitor will lower the output voltage ripple. Component selection is covered in more detail in the following sections. Minimum Input Voltage The minimum input voltage required to generate a particular output voltage is determined by either the LT1934's undervoltage lockout of ~3V or by its maximum duty
Table 1
PART LT1934 VOUT 3.3V L 100H 47H 33H 150H 68H 47H 150H 100H 68H 220H 150H 100H MINIMUM COUT 100F 47F 33F 47F 33F 22F 15F 10F 10F 10F 4.7F 4.7F MAXIMUM LOAD 300mA 250mA 200mA 300mA 250mA 200mA 60mA 45mA 20mA 60mA 45mA 20mA
5V
LT1934-1
3.3V
5V
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cycle. The duty cycle is the fraction of time that the internal switch is on and is determined by the input and output voltages: DC = (VOUT + VD)/(VIN - VSW + VD) where VD is the forward voltage drop of the catch diode (~0.4V) and VSW is the voltage drop of the internal switch (~0.3V at maximum load for the LT1934, ~0.1V for the LT1934-1). This leads to a minimum input voltage of: VIN(MIN) = (VOUT + VD)/DCMAX - VD + VSW with DCMAX = 0.85. Inductor Selection A good first choice for the inductor value is: L = 2.5 * (VOUT + VD) * 1.8s/ILIM where ILIM is the switch current limit (400mA for the LT1934 and 120mA for the LT1934-1). This choice provides a worst-case maximum load current of 250mA (60mA for the LT1934-1). The inductor's RMS current rating must be greater than the load current and its saturation current should be greater than ILIM. To keep efficiency high, the series resistance (DCR) should be less than 0.3 (1 for the LT1934-1). Table 2 lists several vendors and types that are suitable. This simple rule may not provide the optimum value for your application. If the load current is less, then you can relax the value of the inductor and operate with higher ripple current. This allows you to use a physically smaller inductor, or one with a lower DCR resulting in higher efficiency. The following provides more details to guide inductor selection. First, the value must be chosen so that the LT1934 can supply the maximum load current drawn from the output. Second, the inductor must be rated appropriately so that the LT1934 will function reliably and the inductor itself will not be overly stressed. Detailed Inductor Selection and Maximum Load Current The square wave that the LT1934 produces at its switch pin results in a triangle wave of current in the inductor. The LT1934 limits the peak inductor current to ILIM. Because
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LT1934/LT1934-1
APPLICATIO S I FOR ATIO
Table 2. Inductor Vendors
Vendor Murata Sumida Phone (404) 426-1300 (847) 956-0666 URL www.murata.com www.sumida.com Part Series LQH3C CR43 CDRH4D28 CDRH5D28 DO1607C DO1608C DT1608C WE-PD1, 2, 3, 4 Comments Small, Low Cost, 2mm Height
Coilcraft
(847) 639-6400
www.coilcraft.com
Wurth Electronics
(866) 362-6673
www.we-online.com
the average inductor current equals the load current, the maximum load current is: IOUT(MAX) = IPK - IL /2 where IPK is the peak inductor current and IL is the peakto-peak ripple current in the inductor. The ripple current is determined by the off time, tOFF = 1.8s, and the inductor value: IL = (VOUT + VD) * tOFF /L IPK is nominally equal to ILIM. However, there is a slight delay in the control circuitry that results in a higher peak current and a more accurate value is: IPK = ILIM + 150ns * (VIN - VOUT)/L These expressions are combined to give the maximum load current that the LT1934 will deliver: IOUT(MAX) = 350mA + 150ns * (VIN - VOUT)/L - 1.8s * (VOUT + VD)/2L (LT1934) IOUT(MAX) = 90mA + 150ns * (VIN - VOUT)/L - 1.8s * (VOUT + VD)/2L (LT1934-1) The minimum current limit is used here to be conservative. The third term is generally larger than the second term, so that increasing the inductor value results in a higher output current. This equation can be used to evaluate a chosen inductor or it can be used to choose L for a given maximum load current. The simple, single equation rule given above for choosing L was found by setting IL = ILIM /2.5. This results in IOUT(MAX) ~0.8ILIM (ignoring the delay term). Note that this analysis assumes that the inductor current is continuous, which is true if the ripple current is less than the peak current or IL < IPK.
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The inductor must carry the peak current without saturating excessively. When an inductor carries too much current, its core material can no longer generate additional magnetic flux (it saturates) and the inductance drops, sometimes very rapidly with increasing current. This condition allows the inductor current to increase at a very high rate, leading to high ripple current and decreased overload protection. Inductor vendors provide current ratings for power inductors. These are based on either the saturation current or on the RMS current that the inductor can carry without dissipating too much power. In some cases it is not clear which of these two determine the current rating. Some data sheets are more thorough and show two current ratings, one for saturation and one for dissipation. For LT1934 applications, the RMS current rating should be higher than the load current, while the saturation current should be higher than the peak inductor current calculated above. Input Capacitor Step-down regulators draw current from the input supply in pulses with very fast rise and fall times. The input capacitor is required to reduce the resulting voltage ripple at the LT1934 and to force this switching current into a tight local loop, minimizing EMI. The input capacitor must have low impedance at the switching frequency to do this effectively. A 2.2F ceramic capacitor (1F for the LT1934-1) satisfies these requirements. If the input source impedance is high, a larger value capacitor may be required to keep input ripple low. In this case, an electrolytic of 10F or more in parallel with a 1F ceramic is a good combination. Be aware that the input
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LT1934/LT1934-1
APPLICATIO S I FOR ATIO
capacitor is subject to large surge currents if the LT1934 circuit is connected to a low impedance supply, and that some electrolytic capacitors (in particular tantalum) must be specified for such use. Output Capacitor and Output Ripple The output capacitor filters the inductor's ripple current and stores energy to satisfy the load current when the LT1934 is quiescent. In order to keep output voltage ripple low, the impedance of the capacitor must be low at the LT1934's switching frequency. The capacitor's equivalent series resistance (ESR) determines this impedance. Choose one with low ESR intended for use in switching regulators. The contribution to ripple voltage due to the ESR is approximately ILIM * ESR. ESR should be less than ~150m for the LT1934 and less than ~500m for the LT1934-1. The value of the output capacitor must be large enough to accept the energy stored in the inductor without a large change in output voltage. Setting this voltage step equal to 1% of the output voltage, the output capacitor must be: COUT > 50 * L * (ILIM /VOUT)2 For example, an LT1934 producing 3.3V with L = 47H requires 33F. This value can be relaxed if small circuit size is more important than low output ripple. Sanyo's POSCAP series in B-case and C-case sizes provides very good performance in a small package for the LT1934. Similar performance in traditional tantalum capacitors requires a larger package (C- or D-case). The
Table 3. Capacitor Vendors
Vendor Panasonic Phone (714) 373-7366 URL
Kemet Sanyo
(864) 963-6300 (408) 749-9714
Murata AVX Taiyo Yuden
(404) 436-1300
(864) 963-6300
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LT1934-1, with its lower switch current, can use a B-case tantalum capacitor. With a high quality capacitor filtering the ripple current from the inductor, the output voltage ripple is determined by the hysteresis and delay in the LT1934's feedback comparator. This ripple can be reduced further by adding a small (typically 10pF) phase lead capacitor between the output and the feedback pin. Ceramic Capacitors Ceramic capacitors are small, robust and have very low ESR. However, ceramic capacitors can cause problems when used with the LT1934. Not all ceramic capacitors are suitable. X5R and X7R types are stable over temperature and applied voltage and give dependable service. Other types (Y5V and Z5U) have very large temperature and voltage coefficients of capacitance. In the application circuit they may have only a small fraction of their nominal capacitance and voltage ripple may be much larger than expected. Ceramic capacitors are piezoelectric. The LT1934's switching frequency depends on the load current, and at light loads the LT1934 can excite the ceramic capacitor at audio frequencies, generating audible noise. If this is unacceptable, use a high performance electrolytic capacitor at the output. The input capacitor can be a parallel combination of a 2.2F ceramic capacitor and a low cost electrolytic capacitor. The level of noise produced by the LT1934-1
Part Series Ceramic, Polymer, Tantalum Ceramic, Tantalum Ceramic, Polymer, Tantalum Ceramic Ceramic, Tantalum Ceramic
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Comments EEF Series
www.panasonic.com
www.kemet.com www.sanyovideo.com
T494, T495 POSCAP
www.murata.com www.avxcorp.com www.taiyo-yuden.com
TPS Series
9
LT1934/LT1934-1
APPLICATIO S I FOR ATIO
when used with ceramic capacitors will be lower and may be acceptable.
BOOST C3 SW GND VBOOST - VSW VOUT MAX VBOOST VIN + VOUT
A final precaution regarding ceramic capacitors concerns the maximum input voltage rating of the LT1934. A ceramic input capacitor combined with trace or cable inductance forms a high quality (under damped) tank circuit. If the LT1934 circuit is plugged into a live supply, the input voltage can ring to twice its nominal value, possibly exceeding the LT1934's rating. This situation is easily avoided; see the Hot Plugging Safely section. Catch Diode A 0.5A Schottky diode is recommended for the catch diode, D1. The diode must have a reverse voltage rating equal to or greater than the maximum input voltage. The ON Semiconductor MBR0540 is a good choice; it is rated for 0.5A forward current and a maximum reverse voltage of 40V. Schottky diodes with lower reverse voltage ratings usually have a lower forward drop and may result in higher efficiency with moderate to high load currents. However, these diodes also have higher leakage currents. This leakage current mimics a load current at the output and can raise the quiescent current of the LT1934 circuit, especially at elevated temperatures. BOOST Pin Considerations Capacitor C3 and diode D2 are used to generate a boost voltage that is higher than the input voltage. In most cases a 0.1F capacitor and fast switching diode (such as the 1N4148 or 1N914) will work well. Figure 2 shows two ways to arrange the boost circuit. The BOOST pin must be more than 2.5V above the SW pin for best efficiency. For outputs of 3.3V and above, the standard circuit (Figure 2a) is best. For outputs between 2.8V and 3V, use a 0.22F capacitor and a small Schottky diode (such as the BAT-54). For lower output voltages the boost diode can be tied to the input (Figure 2b). The circuit in Figure 2a is more efficient because the BOOST pin current comes from a lower voltage source. You must also be sure that the maximum voltage rating of the BOOST pin is not exceeded. The minimum operating voltage of an LT1934 application is limited by the undervoltage lockout (~3V) and by the
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D2 LT1934 VIN VIN VOUT
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(2a)
D2
BOOST LT1934 VIN VIN GND SW
C3 VOUT
1934 F02
VBOOST - VSW VIN MAX VBOOST 2VIN
(2b)
Figure 2. Two Circuits for Generating the Boost Voltage
maximum duty cycle as outlined above. For proper startup, the minimum input voltage is also limited by the boost circuit. If the input voltage is ramped slowly, or the LT1934 is turned on with its SHDN pin when the output is already in regulation, then the boost capacitor may not be fully charged. Because the boost capacitor is charged with the energy stored in the inductor, the circuit will rely on some minimum load current to get the boost circuit running properly. This minimum load will depend on input and output voltages, and on the arrangement of the boost circuit. The minimum load generally goes to zero once the circuit has started. Figure 3 shows a plot of minimum load to start and to run as a function of input voltage. In many cases the discharged output capacitor will present a load to the switcher which will allow it to start. The plots show the worst-case situation where VIN is ramping very slowly. Use a Schottky diode (such as the BAT-54) for the lowest start-up voltage. At light loads, the inductor current becomes discontinuous and the effective duty cycle can be very high. This reduces the minimum input voltage to approximately 300mV above VOUT. At higher load currents, the inductor current is continuous and the duty cycle is limited by the
1934f
LT1934/LT1934-1
APPLICATIO S I FOR ATIO
Minimum Input Voltage VOUT = 3.3V
6.0 LT1934 VOUT = 3.3V 5.5 TA = 25C BOOST DIODE TIED TO OUTPUT
INPUT VOLTAGE (V)
5.0 4.5 4.0
VIN TO START
VIN TO RUN 3.5 3.0 0.1
1 10 100 LOAD CURRENT (mA)
1934 G12
Minimum Input Voltage VOUT = 5V
8 LT1934 VOUT = 5V TA = 25C 7 BOOST DIODE TIED TO OUTPUT
INPUT VOLTAGE (V)
VIN TO START 6 VIN TO RUN 5
4 0.1
1 10 100 LOAD CURRENT (mA)
1934 G13
Figure 3. The Minimum Input Voltage Depends on Output Voltage, Load Current and Boost Circuit
maximum duty cycle of the LT1934, requiring a higher input voltage to maintain regulation. Shorted Input Protection If the inductor is chosen so that it won't saturate excessively, an LT1934 buck regulator will tolerate a shorted output. There is another situation to consider in systems where the output will be held high when the input to the LT1934 is absent. This may occur in battery charging applications or in battery backup systems where a battery or some other supply is diode OR-ed with the LT1934's output. If the VIN pin is allowed to float and the SHDN pin is held high (either by a logic signal or because it is tied to
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VIN), then the LT1934's internal circuitry will pull its quiescent current through its SW pin. This is fine if your system can tolerate a few mA in this state. If you ground the SHDN pin, the SW pin current will drop to essentially zero. However, if the VIN pin is grounded while the output is held high, then parasitic diodes inside the LT1934 can pull large currents from the output through the SW pin and the VIN pin. Figure 4 shows a circuit that will run only when the input voltage is present and that protects against a shorted or reversed input.
D4 VIN 5 VIN BOOST LT1934 100k 4 SHDN GND 1M 2 SW FB 3 BACKUP 6 VOUT 1 D4: MBR0530
1934 F07
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Figure 4. Diode D4 Prevents a Shorted Input from Discharging a Backup Battery Tied to the Output; It Also Protects the Circuit from a Reversed Input. The LT1934 Runs Only When the Input is Present
PCB Layout For proper operation and minimum EMI, care must be taken during printed circuit board layout. Figure 5 shows the high current paths in the buck regulator circuit. Note that large, switched currents flow in the power switch, the catch diode (D1) and the input capacitor (C2). The loop formed by these components should be as small as possible. Furthermore, the system ground should be tied to the regulator ground in only one place; this prevents the switched current from injecting noise into the system ground. These components, along with the inductor and output capacitor, should be placed on the same side of the circuit board, and their connections should be made on that layer. Place a local, unbroken ground plane below these components, and tie this ground plane to system ground at one location, ideally at the ground terminal of the output capacitor C1. Additionally, the SW and BOOST nodes should be kept as small as possible. Finally, keep the FB node as small as possible so that the ground pin and
1934f
11
LT1934/LT1934-1
APPLICATIO S I FOR ATIO
VIN SW
GND
(5a)
IC1 VIN SW L1
C2
Figure 5. Subtracting the Current When the Switch is On (a) from the Current When the Switch is Off (b) Reveals the Path of the High Frequency Switching Current (c). Keep This Loop Small. The Voltage on the SW and BOOST Nodes Will Also be Switched; Keep These Nodes as Small as Possible. Finally, Make Sure the Circuit is Shielded with a Local Ground Plane
SHUTDOWN VIN VOUT SYSTEM GROUND
VIAS TO LOCAL GROUND PLANE OUTLINE OF LOCAL GROUND PLANE
Figure 6. A Good PCB Layout Ensures Proper, Low EMI Operation
ground traces will shield it from the SW and BOOST nodes. Figure 6 shows component placement with trace, ground plane and via locations. Include two vias near the GND pin of the LT1934 to help remove heat from the LT1934 to the ground plane. Hot Plugging Safely The small size, robustness and low impedance of ceramic capacitors make them an attractive option for the input bypass capacitor of LT1934 and LT1934-1 circuits. However, these capacitors can cause problems if the LT1934
12
U
VIN SW GND
W
UU
(5b)
VSW
GND
D1
C1
1934 F05
(5c)
1934 F06
is plugged into a live supply (see Linear Technology Application Note 88 for a complete discussion). The low loss ceramic capacitor combined with stray inductance in series with the power source forms an under damped tank circuit, and the voltage at the VIN pin of the LT1934 can ring to twice the nominal input voltage, possibly exceeding the LT1934's rating and damaging the part. If the input supply is poorly controlled or the user will be plugging the LT1934 into an energized supply, the input network should be designed to prevent this overshoot.
1934f
LT1934/LT1934-1
APPLICATIO S I FOR ATIO
Figure 7 shows the waveforms that result when an LT1934 circuit is connected to a 24V supply through six feet of 24-gauge twisted pair. The first plot is the response with a 2.2F ceramic capacitor at the input. The input voltage rings as high as 35V and the input current peaks at 20A. One method of damping the tank circuit is to add another capacitor with a series resistor to the circuit. In Figure 7b
CLOSING SWITCH SIMULATES HOT PLUG IIN VIN LT1934
+
2.2F
LOW IMPEDANCE ENERGIZED 24V SUPPLY
STRAY INDUCTANCE DUE TO 6 FEET (2 METERS) OF TWISTED PAIR
10F 35V AI.EI.
+
2.2F
1 LT1934 0.1F 2.2F
4.7 LT1934-1 0.1F 1F
Figure 7. A Well Chosen Input Network Prevents Input Voltage Overshoot and Ensures Reliable Operation When the LT1934 is Connected to a Live Supply
1934f
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an aluminum electrolytic capacitor has been added. This capacitor's high equivalent series resistance damps the circuit and eliminates the voltage overshoot. The extra capacitor improves low frequency ripple filtering and can slightly improve the efficiency of the circuit, though it is likely to be the largest component in the circuit. An alternative solution is shown in Figure 7c. A 1 resistor is
VIN 10V/DIV IIN 10A/DIV 10s/DIV
W
UU
(7a)
LT1934
(7b)
(7c)
LT1934-1 1F
(7d)
(7e)
1934 F07
13
LT1934/LT1934-1
APPLICATIO S I FOR ATIO
added in series with the input to eliminate the voltage overshoot (it also reduces the peak input current). A 0.1F capacitor improves high frequency filtering. This solution is smaller and less expensive than the electrolytic capacitor. For high input voltages its impact on efficiency is minor, reducing efficiency less than one half percent for a 5V output at full load operating from 24V. Voltage overshoot gets worse with reduced input capacitance. Figure 7d shows the hot plug response with a 1F ceramic input capacitor, with the input ringing above 40V. The LT1934-1 can tolerate a larger input resistance, such as shown in Figure 7e where a 4.7 resistor damps the voltage transient and greatly reduces the input current glitch on the 24V supply. High Temperature Considerations The die temperature of the LT1934 must be lower than the maximum rating of 125C. This is generally not a concern unless the ambient temperature is above 85C. For higher temperatures, care should be taken in the layout of the circuit to ensure good heat sinking of the LT1934. The maximum load current should be derated as the ambient temperature approaches 125C. The die temperature is calculated by multiplying the LT1934 power dissipation by the thermal resistance from junction to ambient. Power dissipation within the LT1934 can be
14
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estimated by calculating the total power loss from an efficiency measurement and subtracting the catch diode loss. The resulting temperature rise at full load is nearly independent of input voltage. Thermal resistance depends on the layout of the circuit board, but a value of 150C/W is typical. The temperature rise for an LT1934 producing 5V at 250mA is approximately 25C, allowing it to deliver full load to 100C ambient. Above this temperature the load current should be reduced. For 3.3V at 250mA the temperature rise is 15C. Finally, be aware that at high ambient temperatures the external Schottky diode, D1, is likely to have significant leakage current, increasing the quiescent current of the LT1934 converter. Outputs Greater Than 6V For outputs greater than 6V, tie a diode (such as a 1N4148) from the SW pin to VIN to prevent the SW pin from ringing above VIN during discontinuous mode operation. The 12V output circuit in Typical Applications shows the location of this diode. Also note that for outputs above 6V, the input voltage range will be limited by the maximum rating of the BOOST pin. The 12V circuit shows how to overcome this limitation using an additional Zener diode.
1934f
W
UU
LT1934/LT1934-1
TYPICAL APPLICATIO S
3.3V Step-Down Converter
D2
VIN 4.5V TO 34V
VIN 6.5V TO 34V
U
BOOST VIN C2 1F SW
0.1F
L1 100H
LT1934-1 SHDN GND FB
D1 10pF 1M
VOUT 3.3V 45mA
+
ON OFF
C1 22F
604k
C1: TAIYO YUDEN JMK316BJ226ML C2: TAIYO YUDEN GMK316BJ105ML D1: ZETEX ZHCS400 OR ON SEMI MBR0540 D2: CENTRAL CMDSH-3 L1: COILCRAFT DO1608C-104 OR WURTH ELECTRONICS WE-PD4 TYPE S
1934 TA04
5V Step-Down Converter
D2
BOOST VIN C2 1F SW
0.1F
L1 150H
LT1934-1 SHDN GND FB
D1 10pF 1M
VOUT 5V 45mA
+
ON OFF
C1 22F
332k
C1: TAIYO YUDEN JMK316BJ226ML C2: TAIYO YUDEN GMK316BJ105ML D1: ZETEX ZHCS400 OR ON SEMI MBR0540 D2: CENTRAL CMPD914 L1: COILCRAFT DO1608C-154 OR WURTH ELECTRONICS WE-PD4 TYPE S
1934 TA05
1934f
15
LT1934/LT1934-1
TYPICAL APPLICATIO S
1.8V Step-Down Converter
D2
VIN 3.6V TO 16V
Loop Powered 3.3V Supply with Additional Isolated Output
D3 ISOLATED OUT 3V 10F 3mA
BOOST VIN 14V TO 32V <3.6mA D4 10V 1F 390k VIN SW
C1 L1A 50H
*
D1 1M 10pF
LT1934-1 SHDN GND FB
D1: ON SEMICONDUCTOR MBR0540 D2, D3: BAT54 D4: CENTRAL CMPZ5240B L1: COILTRONICS CTX50-1 ZENER DIODE D4 PROVIDES AN UNDERVOLTAGE LOCKOUT, REDUCING THE INPUT CURRENT REQUIRED AT START-UP
16
*
U
BOOST VIN LT1934 SHDN GND FB SW
0.1F
L1 33H
C2 2.2F
D1 147k
VOUT 1.8V 250mA
+
ON OFF
C1 100F
332k
C1: SANYO 2R5TPB100M C2: TAIYO YUDEN EMK316BJ225ML D1: ZETEX ZHCS400 OR ON SEMI MBR0540 D2: CENTRAL CMPD914 L1: SUMIDA CR43-330
1934 TA06
L1B 50H
+
D2 VOUT 3V 9mA
+
33F 715k
1934 TA08
1934f
LT1934/LT1934-1
TYPICAL APPLICATIO S
Standalone 350mA Li-Ion Battery Charger
D2 0.1F L1 47H SW CHRG LT1934 C2 1F SHDN GND FB 332k D1 1M
D3 VIN 7V TO 28V VIN
C1: SANYO 6TPB47M C2: TAIYO YUDEN GMK316BJ105ML D1, D3: ON SEMICONDUCTOR MBR0540 D2: CENTRAL CMDSH-3 L1: SUMIDA CR43-470
CHARGE CURRENT (mA)
U
+
1k 1k VIN GATE LTC4052 ACPR SENSE BAT C1 47F CTIMER 0.1F
10k 0.047F 0.022F
BOOST
+
350mA TIMER GND 1-CELL 4.2V Li-Ion BATTERY
1934 TA07a
C5 10F (619) 661-6835 (408) 573-4150 (602) 244-6600 (516) 435-1110 (847) 956-0667
CHARGE STATUS AC PRESENT
500 VIN = 24V
400 VIN = 8V
300
VIN = 12V 200
100
0 2.5
3.5 3 4 BATTERY VOLTAGE (V)
4.5
1934 TA07b
1934f
17
LT1934/LT1934-1
TYPICAL APPLICATIO S
12V Step-Down Converter
D2
VIN 15V TO 32V
ON OFF
18
U
D4
0.1F D3
BOOST VIN LT1934 SHDN GND FB SW D1
L1 100H
C2 2.2F
VOUT 12V 170mA 866k
+
C1 22F
100k
C1: KEMET T495D226K020AS C2: TAIYO YUDEN GMK325BJ225MN D1: ON SEMI MBR0540 D2, D4: CENTRAL CMPD914 D3: CENTRAL CMPZ5234B 6.2V ZENER L1: TDK SLF6028T-101MR42
1934 TA09
1934f
LT1934/LT1934-1
PACKAGE DESCRIPTION
S6 Package 6-Lead Plastic TSOT-23
(Reference LTC DWG # 05-08-1636)
0.62 MAX
0.95 REF
3.85 MAX 2.62 REF
RECOMMENDED SOLDER PAD LAYOUT PER IPC CALCULATOR
0.20 BSC 1.00 MAX DATUM `A'
0.30 - 0.50 REF NOTE: 1. DIMENSIONS ARE IN MILLIMETERS 2. DRAWING NOT TO SCALE 3. DIMENSIONS ARE INCLUSIVE OF PLATING 4. DIMENSIONS ARE EXCLUSIVE OF MOLD FLASH AND METAL BURR 5. MOLD FLASH SHALL NOT EXCEED 0.254mm 6. JEDEC PACKAGE REFERENCE IS MO-193 0.09 - 0.20 (NOTE 3)
Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
U
2.90 BSC (NOTE 4)
1.22 REF
1.4 MIN
2.80 BSC
1.50 - 1.75 (NOTE 4) PIN ONE ID
0.95 BSC
0.30 - 0.45 6 PLCS (NOTE 3)
0.80 - 0.90 0.01 - 0.10
1.90 BSC
S6 TSOT-23 0302
1934f
19
LT1934/LT1934-1
TYPICAL APPLICATIO
VIN 6.5V TO 34V
ON OFF
RELATED PARTS
PART NUMBER LT1616 LT1676 LT1765 LT1766 LT1767 LT1776 LTC(R)1877 LTC1879 LT1956 LTC3405/LTC3405A LTC3406/LTC3406B LTC3411 LTC3412 LTC3430 DESCRIPTION 25V, 500mA (IOUT), 1.4MHz, High Efficiency Step-Down DC/DC Converter 60V, 440mA (IOUT), 100kHz, High Efficiency Step-Down DC/DC Converter 25V, 2.75A (IOUT), 1.25MHz, High Efficiency Step-Down DC/DC Converter 60V, 1.2A (IOUT), 200kHz, High Efficiency Step-Down DC/DC Converter 25V, 1.2A (IOUT), 1.25MHz, High Efficiency Step-Down DC/DC Converter 40V, 550mA (IOUT), 200kHz, High Efficiency Step-Down DC/DC Converter 600mA (IOUT), 550kHz, Synchronous Step-Down DC/DC Converter 1.2A (IOUT), 550kHz, Synchronous Step-Down DC/DC Converter 60V, 1.2A (IOUT), 500kHz, High Efficiency Step-Down DC/DC Converter 300mA (IOUT), 1.5MHz, Synchronous Step-Down DC/DC Converter 600mA (IOUT), 1.5MHz, Synchronous Step-Down DC/DC Converter 1.25A (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter 2.5A (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter 60V, 2.75A (IOUT), 200kHz, High Efficiency Step-Down DC/DC Converter COMMENTS VIN = 3.6V to 25V, VOUT = 1.25V, IQ = 1.9mA, ISD = <1A, ThinSOT Package VIN = 7.4V to 60V, VOUT = 1.24V, IQ = 3.2mA, ISD = 2.5A, S8 Package VIN = 3V to 25V, VOUT = 1.2V, IQ = 1mA, ISD = 15A, S8, TSSOP16E Packages VIN = 5.5V to 60V, VOUT = 1.2V, IQ = 2.5mA, ISD = 25A, TSSOP16/E Package VIN = 3V to 25V; VOUT = 1.2V, IQ = 1mA, ISD = 6A, MS8/E Packages VIN = 7.4V to 40V; VOUT = 1.24V, IQ = 3.2mA, ISD = 30A, N8, S8 Packages VIN = 2.7V to 10V; VOUT = 0.8V, IQ = 10A, ISD = <1A, MS8 Package VIN = 2.7V to 10V; VOUT = 0.8V, IQ = 15A, ISD = <1A, TSSOP16 Package VIN = 5.5V to 60V, VOUT = 1.2V, IQ = 2.5mA, ISD = 25A, TSSOP16/E Package VIN = 2.7V to 6V, VOUT = 0.8V, IQ = 20A, ISD = <1A, ThinSOT Package VIN = 2.5V to 5.5V, VOUT = 0.6V, IQ = 20A, ISD = <1A, ThinSOT Package VIN = 2.5V to 5.5V, VOUT = 0.8V, IQ = 60A, ISD = <1A, MS Package VIN = 2.5V to 5.5V, VOUT = 0.8V, IQ = 60A, ISD = <1A, TSSOP16E Package VIN = 5.5V to 60V, VOUT = 1.2V, IQ = 2.5mA, ISD = 30A, TSSOP16E Package
1934f LT/TP 0703 1K * PRINTED IN USA
20
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 q FAX: (408) 434-0507
q
U
5V Step-Down Converter
D2 BOOST VIN LT1934 SHDN GND FB 332k SW D1 10pF 1M 0.1F L1 68H C2 2.2F VOUT 5V 250mA
+
C1 68F
C1: SANYO 6TPB68M C2: TAIYO YUDEN GMK325BJ225MN D1: ZETEX ZHCS400 OR ON SEMI MBR0540 D2: CENTRAL CMPD914 L1: SUMIDA CDRH5D28-680
1934 TA03
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2002

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