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LTC1574 LTC1574-3.3/LTC1574-5 High Efficiency Step-Down DC/DC Converters with Internal Schottky Diode
FEATURES
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DESCRIPTION
The LTC(R)1574 is a family of easy-to-use current mode DC/DC converters ideally suited for 9V to 5V, 5V to 3.3V and inverting operation. With an internal 0.9 switch (at a supply voltage of 12V) and a low forward drop Schottky diode (0.450V typ at 200mA, TA = 25C), the LTC1574 requires only three external components to construct a complete high efficiency DC/DC converter. Under no load condition, the LTC1574 draws only 130A. In shutdown, it draws a mere 2A making this converter ideal for battery-powered applications. In dropout, the internal P-channel MOSFET switch is turned on continuously allowing the user to maximize the life of the battery source. The maximum inductor current of the LTC1574 family is pin selectable to either 340mA or 600mA, optimizing efficiency for a wide range of applications. Operation up to 200kHz permits the use of small surface mount inductors and capacitors. For applications requiring higher output current or ultrahigh efficiency, see the LTC1148 and LTC1265 data sheets. For detailed applications information, see the LTC1174 data sheet.
High Efficiency: Up to 94% Usable in Noise-Sensitive Products Peak Inductor Current Independent of Inductor Value Short-Circuit Protection Internal Low Forward Drop Schottky Diode Only Three External Components Required Wide VIN Range: 4V to 18.5V (Absolute Maximum) Low Dropout Operation Low-Battery Detector Pin Selectable Current Limit Internal 0.9 Power Switch: VIN = 12V Standby Current: 130A Active Low Micropower Shutdown
APPLICATIONS
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Inverting Converters Step-Down Converters Memory Backup Supply Portable Instruments Battery-Powered Equipment Distributed Power Systems
and LTC are registered trademarks and LT is a trademark of Linear Technology Corporation.
TYPICAL APPLICATION
High Efficiency Step-Down Converter
VIN 5.5V to 16V 12 11 6
100
LTC1574-5 Efficiency
L = 100H VOUT = 5V IPGM = 0V
5 VIN LBIN SHDN 7 10 3, 14 100H
+
22F* 35V
95
EFFICIENCY (%)
90 VIN = 9V 85 80 75 70
LBOUT LTC1574-5 VOUT IPGM GND 2, 4, 13, 15 SW
+
5V 175mA 100F* 10V
* AVX TPSD226K035 ** AVX TPSD107K010 COILTRONICS CTX100-4
1574 TA01
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VIN = 6V
10 LOAD CURRENT (mA)
100 200
1574 TA02
1
LTC1574 LTC1574-3.3/LTC1574-5
ABSOLUTE MAXIMUM RATINGS
(Voltage Referred to GND Pin) Input Supply Voltage (Pin 5).................. - 0.3V to 18.5V Switch Current (Pin 3, 14) ........................................ 1A Switch Voltage (Pin 3, 14) .......................... VIN - 18.5V Operating Temperature Range .................... 0C to 70C Junction Temperature (Note 1) ............................ 125C Storage Temperature Range ................. - 65C to 150C Lead Temperature (Soldering, 10 sec).................. 300C
PACKAGE/ORDER INFORMATION
TOP VIEW NC 1 GND 2 SW 3 GND 4 VIN 5 IPGM 6 SHDN 7 NC 8 16 NC 15 GND 14 SW 13 GND 12 LBIN 11 LBOUT 10 VOUT (VFB*) 9 NC
ORDER PART NUMBER LTC1574CS LTC1574CS-3.3 LTC1574CS-5
S PACKAGE 16-LEAD PLASTIC SO *ADJUSTABLE OUTPUT VERSION TJMAX = 125C, JA = 110C/W
Consult factory for Industrial and Military grade parts.
ELECTRICAL CHARACTERISTICS
SYMBOL PARAMETER IFB VFB VOUT VOUT Feedback Current into Pin 10 Feedback Voltage Regulated Output Voltage Output Voltage Line Regulation Output Voltage Load Regulation CONDITIONS LTC1574 LTC1574 LTC1574-3.3 LTC1574-5
TA = 25C, VIN = 9V, VSHUTDOWN = VIN, IPGM = 0V, unless otherwise specified.
MIN TYP MAX 1
q q q
UNITS A V V V mV mV mV mV mV A A A V A mA A mV A A s V
1.20 3.14 4.75
1.25 3.30 5.00 10 -5 - 45 -5 - 50 450 130 2 1.25
1.30 3.46 5.25 70 - 70 - 70 - 70 - 70 600 180 25 1.4 0.5
VIN = 6V to 12V, ILOAD = 100mA, IPGM = VIN (Note 2) LTC1574-3.3 (Note 2) LTC1574-5 (Note 2) 20mA < ILOAD < 175mA, IPGM = 0V 20mA < ILOAD < 400mA, IPGM = VIN 20mA < ILOAD < 175mA, IPGM = 0V 20mA < ILOAD < 400mA, IPGM = VIN
IQ
Input DC Supply Current (Note 3) Active Mode Sleep Mode Shutdown (Note 4) Low-Battery Trip Point Current into Pin 12 Current Sunk by Pin 11 Comparator Hysteresis Current Limit ON Resistance of Switch Switch Off Time Shutdown Pin High Shutdown Pin Low Shutdown Pin Input Current
4V < VIN < 16V, IPGM = 0V 4V < VIN < 16V VSHUTDOWN = 0V, 4V < VIN < 16V
VLBTRIP ILBIN ILBOUT VHYST IPEAK RON tOFF VIH VIL IIH
VLBOUT = 0.4V, VLBIN = 0V VLBOUT = 5V, VLBIN = 10V IPGM = VIN, VOUT = 0V IPGM = 0V, VOUT = 0V VOUT at Regulated Value Minimum Voltage at Pin 7 for Device to Be Active Maximum Voltage at Pin 7 for Device to Be in Shutdown VSHUTDOWN = 16V
q q q
0.5 7.5 0.54 0.27
1.0 15 0.60 0.34 0.9
1.5 1.0 30 0.78 0.50 1.55 5
3 1.2
4
0.75 2
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V A
LTC1574 LTC1574-3.3/LTC1574-5
ELECTRICAL CHARACTERISTICS
SYMBOL PARAMETER IIL VF IR Shutdown Pin Input Current Schottky Diode Forward Voltage Schottky Reverse Current CONDITIONS 0 VSHUTDOWN 0.8V Forward Current = 200mA Reverse Voltage = 5V Reverse Voltage = 18.5V 0.450 10 100
TA = 25C, VIN = 9V, VSHUTDOWN = VIN, IPGM = 0V, unless otherwise specified.
MIN TYP MAX 0.5 0.570 25 250 UNITS A V A A
The q denotes specifications which apply over the full operating temperature range. Note 1: TJ is calculated from the ambient temperature TA and power dissipation PD according to the following formulas: TJ = TA + (PD x 110C/W) Note 2: Guaranteed by Design.
Note 3: Does not include Schottky reverse current. Dynamic supply current is higher due to the gate charge being delivered at the switching frequency. Note 4: Current into Pin 5 only, measured without electrolytic input capacitor.
TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency vs Load Current
100 VIN = 5V 100 95
90
EFFICIENCY (%)
EFFICIENCY (%)
80
VIN = 9V
VIN = 9V 85 80 75 70 L = 50H VOUT = 5V IPGM = VIN COIL = CTX50-4 1 10 100 LOAD CURRENT (mA) 400
1574 * TPC02
EFFICIENCY (%)
70 L = 50H VOUT = 3.3V IPGM = VIN COIL = CTX50-4 1 10 100 LOAD CURRENT (mA) 500
1574 * TPC01
60
50
Efficiency Using Different Types of Inductor Core Material
100 CTX50-4
90
EFFICIENCY (%)
LEAKAGE CURRENT (nA)
RDS(ON) ()
CTX50-4P 80
70
60
VIN = 5V VOUT = 3.3V IPGM = VIN 1 10 100 LOAD CURRENT (mA) 500
1574 * TPC04
50
UW
Efficiency vs Load Current
95 94
Efficiency vs Input Voltage
VOUT = 5V L = 100H COIL = CTX100-4
90
VIN = 6V
93 92 91 90 89 5 6 7 8 9 10 11 12 INPUT VOLTAGE (V) 13 14 ILOAD = 100mA IPGM = 0V
ILOAD = 300mA IPGM = VIN
1574 * TPC03
Switch Leakage Current vs Temperature
180 160 140 120 100 80 60 40 20 0 0 20 60 40 TEMPERATURE (C) 80 100
1574 * TPC05
Switch Resistance vs Input Voltage
1.7 1.6 1.5 1.4 1.3 1.2 1.1 1.0 0.9 0.8 0.7 4 6 8 10 12 14 16 INPUT VOLTAGE (V) 18 20 TA = 25C
VIN = 13.5V
1574 * TPC06
3
LTC1574 LTC1574-3.3/LTC1574-5
PIN FUNCTIONS
NC (Pins 1, 8, 9, 16): No Connection. GND (Pins 2, 4, 13, 15): Ground. SW (Pins 3, 14): Drain of P-Channel MOSFET Switch and Cathode of Schottky Diode. VIN (Pin 5): Input Supply Voltage. It must be decoupled close to ground (Pin 4). IPGM (Pin 6): This pin selects the current limit of the P-channel switch. With IPGM = VIN, the current trip point is 600mA and with IPGM = 0V, the current trip point is reduced to 340mA. SHDN (Pin 7): Pulling this pin to ground keeps the internal switch off and puts the LTC1574 in micropower shutdown. VOUT or VFB (Pin 10): For the LTC1574, this pin connects to the main voltage comparator input. On the LTC1574-5 and LTC1574-3.3, this pin goes to an internal resistive divider which sets the output voltage. LBOUT (Pin 11): Open drain of an N-Channel Pull-Down. This pin will sink current when (Pin 12) LBIN goes below 1.25V. LBIN (Pin 12): The (-) Input of the Low-Battery Voltage Comparator. The (+) input is connected to a reference voltage of 1.25V.
APPLICATIONS INFORMATION
Operating Frequency and Inductor Since the LTC1574 utilizes a constant off-time architecture, its operating frequency is dependent on the value of VIN. The frequency of operation can be expressed as:
f= 1 VIN - VOUT t OFF VIN + VD
(Hz)
IPGM = VIN 100mA/DIV
where tOFF = 4s and VD is the voltage drop across the internal Schottky diode. Note that the operating frequency is a function of the input and output voltage. Although the size of the inductor does not affect the frequency or inductor peak current, it does affect the ripple current. The peak-to-peak ripple current is given by: V + VD IRIPPLE = 4 x 10 -6 OUT L
(AP-P )
By choosing a smaller inductor, a low ESR (Effective Series Resistance) output filter capacitor has to be used. Core loss will increase due to higher ripple current. Short-Circuit Protection The LTC1574 is protected from output short circuits by its internal current limit. Depending on the condition of the
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IPGM pin, the limit is either set to 340mA or 600mA. In addition, the off-time of the switch is increased to allow the inductor current to decay far enough to prevent any current build-up (see Figure 1).
IPGM = 0
GND
L = 100H VIN = 13.5V
20s/DIV
1574 * F01
Figure 1. Inductor Current with Output Shorted
Low-Battery Detector The low-battery indicator senses the input voltage through an external resistive divider. This divided voltage connects to the "-" input of a voltage comparator (Pin 12) which is compared with a 1.25V reference voltage. With the current
LTC1574 LTC1574-3.3/LTC1574-5
APPLICATIONS INFORMATION
going into Pin 12 being negligible, the following expression is used for setting the trip limit: R4 VLBTRIP = 1.25 1 + R3
VIN R4 12 R3 LTC1574
- +
1.25V REFERENCE
1574 * F02
Figure 2. Low-Battery Comparator
LTC1574 Adjustable Applications The LTC1574 develops a 1.25V reference voltage between the feedback terminal (Pin 10) and ground (see Figure 3). By selecting resistor R1, a constant current is caused to flow through R1 and R2 to set the overall output voltage. The regulated output voltage is determined by: R2 VOUT = 1.25 1 + R1 For most applications, a 30k resistor is suggested for R1. To prevent stray pickup, a 100pF capacitor is suggested across R1 located close to the LTC1574.
VOUT R2 LTC1574 VFB 10 100pF R1
1574 * F03
Figure 3. LTC1574 Adjustable Configuration
Inverting Applications The LTC1574 can easily be set up for a negative output voltage. If - 5V is desired, the LTC1574-5 is ideal for this application as it requires the least components. Figure 4 shows the schematic for this application. Note that the output voltage is now taken off the GND pins. Therefore, the maximum input voltage is now determined by the
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difference between the absolute maximum voltage rating and the output voltage. A maximum of 12V is specified in Figure 4, giving the circuit 1.5V of headroom for VIN. Note that the circuit can operate from a minimum of 4V, making it ideal for a four NiCd cell application. For a higher output current circuit, please refer to the Typical Applications section.
INPUT VOLTAGE 4V TO 12V
12 11 6
5 VIN LBIN LBOUT IPGM LTC1574-5 GND 2, 4, 13, 15 SHDN VOUT SW
+
0.1F 7 10 3, 14 50H**
+
2 x 47F* 16V
+
* AVX TPSD476K016 ** COILTRONICS CTX50-4
2 x 47F* 16V VOUT -5V 45mA
1574 * F04
Figure 4. Positive-to-Negative 5V Converter
Low Noise Regulators In some applications it is important not to introduce any switching noise within the audio frequency range. Due to the nature of the LTC1574 during Burst ModeTM operation, there is a possibility that the regulator will introduce audio noise at some load currents. To circumvent this problem, a feed-forward capacitor can be used to shift the noise spectrum up and out of the audio band. Figure 5 shows the low noise connection with C2 being the feed-forward capacitor. The peak-to-peak output ripple is reduced to 30mV over the entire load range. A toroidal surface mount
Burst Mode is a trademark of Linear Technology Corporation
5 VIN 12 11 6 LTC1574 LBIN LBOUT IPGM GND 2, 4, 13, 15 SHDN SW VFB 7 3, 14 10 L1** 100H
+
100F* 10V
VIN 5V
56k
C2 6.8nF
VOUT 3.3V 425mA 100F* 10V
+
33k * AVX TPSD107K010 ** COILTRONICS CTX100-4
1574 * F05
Figure 5. Low Noise 5V to 3.3V Regulator
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LTC1574 LTC1574-3.3/LTC1574-5
APPLICATIONS INFORMATION
inductor L1 is chosen for its excellent self-shielding properties. Open magnetic structures such as drum and rod cores are to be avoided since they inject high flux levels into their surroundings. This can become a major source of noise in any converter circuit. Design Example As a design example, assume VIN = 9V (nominal), VOUT = 5V and IOUT = 350mA maximum. The LTC1574-5 is used for this application with IPGM (Pin 6) connected to VIN. The minimum value of L is determined by assuming the LTC1574-5 is operating in continuous mode.
INDUCTOR CURRENT
IPEAK AVG CURRENT = IOUT +I I = PEAK V IV 2 = 350mA
TIME
Figure 6. Continuous Inductor Current
With IOUT = 350mA and IPEAK = 0.6A (IPGM = VIN), IV = 0.1A. The peak-to-peak ripple inductor current, IRIPPLE, is 0.5A and is also equal to:
TYPICAL APPLICATIONS
Low Noise, High Efficiency 3.3V Regulator
VIN 4V TO 12.5V
VIN 6 12 11 IPGM LBIN LBOUT GND 2, 4, 13, 15 * AVX TPSD226K025 ** AVX TPSD107K010 COILTRONICS CTX50-4 100pF LTC1574 SHDN VFB SW 7 10 3, 14
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V + VD IRIPPLE = 4 x 10 -6 OUT L
(AP-P )
Solving for L in the above equation and with VD = 0.5V, L = 44H. The next higher standard value of L is 50H (example: Coiltronics CTX50-4). The operating frequency, ignoring voltage across diode VD is:
V f 2.5 x 105 1 - OUT VIN = 111kHz
With the value of L determined, the requirements for CIN and COUT are calculated. For CIN, its RMS current rating should be at least: IRMS = IOUT VOUT VIN - VOUT VIN
[
(
)]
1/ 2
(ARMS)
1574 * F06
= 174mA For COUT, the RMS current rating should be at least:
IRMS
IPEAK A RMS 2 = 300mA
(
)
+
22F* 25V x2
0.1F
50H
6.8nF 56k
VOUT 3.3V 450mA
+
100F** 10V x2
33k
1574 TA03
LTC1574 LTC1574-3.3/LTC1574-5
TYPICAL APPLICATIONS
Low Dropout 5V Step-Down Regulator with Low-Battery Detection
VIN 5.5V to 12.5V
* LOW-BATTERY INDICATOR IS SET UP TO TRIP AT V IN = 5.5V ** AVX TPSD476K016
SELECTION MANUFACTURER COILTRONICS SUMIDA GOWANDA
PART NO. CTX100-4 CD75-101 GA10-103K
TYPE SURFACE MOUNT SURFACE MOUNT THROUGH HOLE
VIN 4V TO 12.5V
* AVX TPSD226K025 ** AVX TPSD476K016 COILTRONICS CTX50-4
* LOW-BATTERY INDICATOR IS SET TO TRIP AT V IN = 4.4V ** AVX TPSD106K035 *** AVX TPSD107K010
SELECTION MANUFACTURER COILTRONICS COILCRAFT SUMIDA GOWANDA
PART NO. CTX50-3 DT3316-473 CD54-470 GA10-472K
TYPE SURFACE MOUNT SURFACE MOUNT SURFACE MOUNT THROUGH HOLE
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.
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4.7k *LOWBATTERY INDICATOR 162k 6 11 12 47.5k IPGM
5 VIN SHDN 7 10 3, 14 L1 100H 0.1F
+
47F** 16V x2
LBOUT LTC1574-5 VOUT LBIN GND 2, 4, 13, 15 SW
+
VOUT 5V 47F** 365mA 16V x2
1574 TA04
High Efficiency 3.3V Regulator
5 VIN 6 12 11 IPGM LBIN LBOUT GND 2, 4, 13, 15 SHDN LTC1574-3.3 VOUT SW 7 10 3, 14
+
22F* 25V x2
0.1F
50H
+
47F* 16V x2
VOUT 3.3V 425mA
1574 TA05
Positive to -5V Converter
VIN 4V TO 12.5V
4.7k *LOWBATTERY INDICATOR 280k 6 11 12 43k IPGM LBOUT
5 VIN SHDN VOUT LTC1574-5 LBIN GND 2, 4, 13, 15 SW 7 10 3, 14 L1 50H 0.1F
+
10F** 35V x2
V IN (V) I OUT (mA) 4 110 6 140 8 170 10 200 12.5 235
+
100F*** 10V VOUT -5V
1574 TA06
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LTC1574 LTC1574-3.3/LTC1574-5
PACKAGE DESCRIPTION
0.010 - 0.020 x 45 (0.254 - 0.508) 0.008 - 0.010 (0.203 - 0.254) 0 - 8 TYP
0.016 - 0.050 0.406 - 1.270 *DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE **DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE
RELATED PARTS
PART NUMBER LT (R) 1076 LTC1174 LTC1265 LT1375/LT1376 DESCRIPTION Step-Down Switching Regulator High Efficiency Step-Down/Inverting DC/DC Converter 1.2A, High Efficiency Step-Down DC/DC Converter 1.5A, 500kHz Step-Down Switching Regulator COMMENTS 2A Monolithic Bipolar Switcher for VIN to 60V Same as LTC1574 Without Schottky Diode in SO-8 Package Current Mode with 0.3 Switch for Higher Current High Frequency, Synchronizable in SO-8 Package
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Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7487
(408) 432-1900 q FAX: (408) 434-0507 q TELEX: 499-3977
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Dimension in inches (millimeters) unless otherwise noted. S Package 16-Lead Plastic SOIC
0.386 - 0.394* (9.804 - 10.008) 16 15 14 13 12 11 10 9
0.228 - 0.244 (5.791 - 6.197)
0.150 - 0.157** (3.810 - 3.988)
1 0.053 - 0.069 (1.346 - 1.752)
2
3
4
5
6
7
8
0.004 - 0.010 (0.101 - 0.254)
0.014 - 0.019 (0.355 - 0.483)
0.050 (1.270) TYP
SO16 0695
LT/GP 0795 6K * PRINTED IN USA
(c) LINEAR TECHNOLOGY CORPORATION 1995

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