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Linear Fet Controller
POWER MANAGEMENT Description
The SC1548 is a power supply controller designed to provide a simple single regulated power supply with over current protection. It is part of Semtechs SmartLDO family of products. The SC1548 can provide a 1.818V power supply for the I/O plane or 1.515V for GTL+ / AGP from either 3.3V or 2.5V. An adjustable option allows generation and control of any voltage from 1.263V up to 5V. SC1548 features include tight output voltage regulation, an enable control and over current protection. Over current protection is provided by feedback to the sense pin. If the output drops below 50% of the nominal output voltage (typical) for greater than 4ms (typical), the output will be shut down. The SC1548 is available in a tiny 5-pin SOT-23 surface mount package.
SC1548
Features
u 2.5% output accuracy over line, load and u 1.515V, 1.818V and adjustable output voltage u Enable control u Over current protection u 5-pin SOT-23 package
options available temperature
Applications
u u u u
Motherboards Graphics cards Microcontrollers Simple power supplies
Typical Application Circuit
Fixed Output Voltage Versions
1.818V OUT + C1 100uF + C2 100uF Q1 IRL530N 3.3V IN 12V IN
+
C3 22uF 1 2 3
U1 SNS GND DRV IN 4 C4 0.1uF EN 5 ENABLE
SC1548CSK-1.8
Adjustable Output Voltage Version
Q1 IRL530N 3.3V IN 12V IN
2.5V OUT R1 97.6 + C1 100uF + C2 100uF
+
C3 22uF U1 1 2 ADJ GND DRV IN 4 C4 0.1uF EN 5 ENABLE
R2 100
3
SC1548CSK
Revision 1, January 2001
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SC1548
POWER MANAGEMENT Absolute Maximum Ratings
Parameter Input Supply Voltage Input Pi ns Thermal Impedance Juncti on to C ase Thermal Impedance Juncti on to Ambi ent Operati ng Ambi ent Temperature Range Operati ng Juncti on Temperature Range Storage Temperature Range Lead Temperature (Solderi ng) 10 Sec Sy mbol VIN VADJ, VEN, VSNS q JC q JA TA TJ TSTG TLEAD Maximum -0.5 to +15 -0.5 to +7 81 256 0 to +70 0 to +125 -65 to +150 300 U nits V V C /W C /W C C C C
Electrical Characteristics(1)
Unless specified: TA = 25C, VIN = 12V, VPWR = 3.3V, IOUT = 0A. Values in bold apply over full operating temperature range.
Parameter IN Supply Voltage Qui escent C urrent
Sy mbol
Test C onditions
Min
Ty p
Max
U nits
VIN IQ
11.28
12.00 1.0
12.72 1.5 2.0
V mA
U nderv oltage Lockout Start Threshold EN Enable Pi n C urrent Threshold Voltage Hysteresi s Enable D elay Ti me(2)(3) D i sable D elay Ti me(2)(3) IEN VTH(EN) VHYST tD(ON) tD(OFF) VEN = Low to Hi gh, measured from VEN = VTH(EN) to 10% VDRV VEN = Hi gh to Low, measured from VEN = VTH(EN) to 90% VDRV V E N = 0V VEN ri si ng 1.8 200 500 150 100 150 2.3 A V mV ns ns UVLO 7 8 9 V
SN S (Fixed Output Voltage Parts) Sense Pi n C urrent ISNS Si nki ng 75 100 125 A
AD J (Adjustable Output Voltage Parts) Adjust Pi n C urrent Reference Voltage(2) IADJ V AD J Sourci ng 3.0V VPWR(4) 3.6V, 1mA IOUT 1A -1.5% -2.5%
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0.25 1.263 +1.5% +2.5%
A V
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SC1548
POWER MANAGEMENT Electrical Characteristics (Cont.)(1)
Unless specified: TA = 25C, VIN = 12V, VPWR = 3.3V, IOUT = 0A. Values in bold apply over full operating temperature range.
Parameter
Sy mbol
Test C onditions
Min
Ty p
Max
U nits
Output Voltage R egulation (Fixed Output Voltage Parts) Output Voltage(2) VOUT 3.0V VPWR(4) 3.6V, 1mA IOUT 1A -1.5% -2.5% DRV Output C urrent Output Voltage Ri se Ti me(2)(3) Fall Ti me(2)(3) Ov ercurrent Protection Tri p Threshold Power-up Output Short C i rcui t Immuni ty Output Short C i rcui t Gli tch Immuni ty C ontrol Section Bandwi dth VDRV = 9V, THD = 5%, C L = 600pF 5 MHz VTH(OC) 30 1 0.5 50 5 4 70 60 10 %VOUT ms ms IDRV VDRV tr tf VDRV = 4V, VSNS = 1.2V Full On, IDRV = 0mA VEN = Low to Hi gh, measured from VEN = VTH(EN) to 90% VDRV VEN = Hi gh to Low, measured from VEN = VTH(EN) to 10% VDRV 5 9.0 10 10.5 1.0 550 mA V ms s VOUT +1.5% +2.5% V
Notes: (1) This device is ESD sensitive. Use of standard ESD handling precautions is required. (2) See Application Circuit on page 1. (3) See Timing Diagram on page 4. (4) Connected to FET drain.
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SC1548
POWER MANAGEMENT Timing Diagram
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SC1548
POWER MANAGEMENT Pin Configuration
Top View
Ordering Information
Part N umber(1)(2) SC 1548C SK-X.X.TR P ackag e SOT-23-5
SOT-23-5L
Notes: (1) Where -X.X denotes voltage options. Available voltages are: 1.515V (-1.5) and 1.818V (-1.8). Leave blank for adjustable version. (2) Only available in tape and reel packaging. A reel contains 3000 units.
Block Diagram
Pin Descriptions
Pin 1 Pin N ame Pin Function SNS AD J Regulator sense i nput for fi xed output voltage opti ons. Use as a remote sense to the source of the N-channel MOSFET. Regulator sense i nput for adjustable output voltage versi on. S et output voltage as follows (refer to appli cati on ci rcui t on page 1):
R1 VO = 1 . 263 * 1 + R2
2 3 4 5
GND D RV IN EN
Ground. O u t p u t o f r e g u l a t o r. D r i v e s t h e g a t e o f a n N - c h a n n e l M O S F E T t o m a i n t a i n t h e o u t p u t voltage desi red. +12V supply. A c t i ve hi g h e na b l e c o nt r o l w i t h i nt e r na l p ul l up . O ut p ut o f r e g ul a t o r t ur ns o f f w he n E N i s taken low.
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SC1548
POWER MANAGEMENT Typical Characteristics(1)
Quiescent Current vs. Junction Temperature
1000 900 800 700 UVLO (V) IQ (A) 600 500 400 300 200 100 0 0 25 50 TJ (C) 75 100 125 6.0 0 25 50 TJ (C) 75 100 125 VIN = 12V, VEN = 3.3V 8.5 8.0 7.5 7.0 6.5 9.0
Start Threshold vs. Junction Temperature
Enable Threshold Voltage vs. Junction Temperature
2.30 2.25 2.20 2.15 VTH(EN) (V) 2.10 2.05 2.00 1.95 1.90 1.85 1.80 0 25 50 TJ (C) 75 100 125 VHYST (mV) VIN = 12V VEN rising 500 450 400 350 300 250 200 150 100 50 0 0 VIN = 12V VEN falling
Enable Hysteresis vs. Junction Temperature
25
50 TJ (C)
75
100
125
Enable Pin Current vs. Junction Temperature
150 125 100 75 50 25 0 0 25 50 TJ (C) 75 100 125 tD(ON) (ns) IEN (A) VIN = 12V VEN = 0V 1000 900 800 700 600 500 400 300 200 100 0 0 VIN = 12V
Enable Delay Time vs. Junction Temperature
25
50 TJ (C)
75
100
125
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SC1548
POWER MANAGEMENT Typical Characteristics (Cont.)(1)
Disable Delay Time vs. Junction Temperature
200 VIN = 12V 175 150 110 tD(OFF) (ns) ISNS (A) 125 100 75 50 85 25 0 0 25 50 TJ (C) 75 100 125 80 75 0 25 50 TJ (C) 75 100 125 105 100 95 90 120 115 125 VIN = 12V VEN = 3.3V VSNS = VO(NOM)
Sense Pin Current vs. Junction Temperature
Drive Output Voltage vs. Junction Temperature
12.00 11.50 11.00 VDRV (V) VO (V) 10.50 10.00 9.50 9.00 0 25 50 TJ (C) 75 100 125 VIN = 12V VSNS = 0V IDRV = 0mA 1.845 1.840 1.835 1.830 1.825 1.820 1.815 1.810 1.805 1.800 1.795 1.790 0
Output Voltage (SC1548CSK-1.8) vs. Junction Temperature
VIN = 12V VEN = 3.3V 3.0V VPWR 3.6V 1mA IO 1A
25
50 TJ (C)
75
100
125
OCP Trip Threshold (SC1548CSK-1.8) vs. Junction Temperature
1.2 1 0.8 VTH(OC) (V) 0.6 0.4 0.2 0 0 25 50 TJ (C) 75 100 125 Power-up Short Circuit Immunity (ms) VIN = 12V VEN = 3.3V 10 9 8 7 6 5 4 3 2 1 0 0
Power-Up Output Short Circuit Immunity vs. Junction Temperature
VIN = 12V VEN switched from 0V to 3.3V ROUT = 0 Two representative parts shown
25
50 TJ (C)
75
100
125
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SC1548
POWER MANAGEMENT Typical Characteristics (Cont.)(1)
Output Short Circuit Glitch Immunity vs. Junction Temperature
8 Short Circuit Glitch Immunity (ms) 7 6 5
tr (s) 1200 1000 800 600 400 200 VIN = 12V VEN switched from 0V to 3.3V Two representative parts shown 0 25 50 TJ (C) 75 100 125
Drive Pin Rise Time vs. Junction Temperature
VIN = 12V VEN = 3.3V ROUT of 0 applied to output Two representative parts shown
4 3 2 1 0 0 25 50 TJ (C) 75 100 125
0
Drive Pin Fall Time vs. Junction Temperature
1000 900 800 700 600 tf (s) 500 400 300 200 100 0 0 25 50 TJ (C) 75 100 125 -40 -60 f (Hz) 20 0 -20 VIN = 12V VEN switched from 3.3V to 0V Two representative parts shown 80 60 40
SC1548CSK-1.8 Small Signal Gain and Phase Shift vs. Frequency
0 IOUT = 1.8A -45 -90 Gain Phase () Phase () -135 -180 -225 -270 Phase 1.00E+03 1.00E+04 Gain (dB) 1.00E+05 -315
-80 1.00E+02
-360 1.00E+06
SC1548CSK-1.5 Small Signal Gain and Phase Shift vs. Frequency
80 80 IOUT = 1.8A 60 40 Gain Gain (dB) Phase () 20 0 -20 -40 -60 -80 1.00E+02 Phase -135 -180 -225 -270 -315 -45 40 -90 Gain (dB) 20 0 -20 0 60
SC1548CSK Small Signal Gain and Phase Shift vs. Frequency
0 VOUT = 2V IOUT = 1.8A -45 -90 Gain -135 -180 -225 Phase -40 -60 -80 1.00E+02 -270 -315 -360 1.00E+06
1.00E+03
1.00E+04 f (Hz)
1.00E+05
-360 1.00E+06
1.00E+03
1.00E+04 f (Hz)
1.00E+05
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SC1548
POWER MANAGEMENT Typical Characteristics (Cont.)(1)
Load Transient Response Load Transient Response, Expanded
Trace 1: VOUT, AC coupled, 50mV/div. Trace 2: VDRV, 2V/div. Trace M3: load stepping from 0A to 1A to 0A Timebase: 10s/div
Trace 1: VOUT, AC coupled, 50mV/div. Trace 2: VDRV, 2V/div. Trace M3: load stepping from 0A to 1A Timebase: 1s/div
Load Transient Response, Expanded
Disable Delay Time, tD(OFF)
Trace 1: VOUT, AC coupled, 50mV/div. Trace 2: VDRV, 2V/div. Trace M3: load stepping from 1A to 0A Timebase: 1s/div
Trace 1: VDRV, 1V/div. Trace 2: VEN, 2V/div. Timebase: 100ns/div tD(OFF) 36ns
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SC1548
POWER MANAGEMENT Typical Characteristics (Cont.)(1)
Drive Output Fall Time, tf Enable Delay Time, tD(ON)
Trace 1: VDRV, 1V/div. Trace 2: VEN, 2V/div. Timebase: 100ns/div tf 350ns
Trace 1: VDRV, 1V/div. Trace 2: VEN, 2V/div. Timebase: 250ns/div tD(ON) 550ns
Drive Output Rise Time, tr
Power-up Output Short Circuit Immunity
Trace 1: VDRV, 1V/div. Trace 2: VEN, 2V/div. Timebase: 500s/div tr 1ms
Trace 1: VDRV, 5V/div. Trace 2: VEN, 2V/div. Timebase: 2ms/div SC1548 enabled into a short, therefore VOUT < VTH(OC) immediately the device is enabled. This device shuts down after 8ms.
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SC1548
POWER MANAGEMENT Typical Characteristics (Cont.)(1)
Output Short Circuit Glitch Immunity Note: (1) See Applications Circuit on page 1.
Trace 1: VDRV, 5V/div. Trace 2: VOUT, 1V/div. Timebase: 1ms/div SC1548 enabled, then shorted, therefore VOUT < VTH(OC) immediately the short is applied. This device shuts down after 5ms.
Applications Infomation
Theory Of Operation The SC1548 linear FET controller provides a simple way to drive an N-channel MOSFET to produce a tightly regulated output voltage from an available, higher, supply voltage. It takes its power from a 12V supply, drawing typically 2mA while operating. It contains an internal bandgap reference which is compared to the output voltage via a resistor divider. This resistor divider is internal on the fixed output voltage options, and user selectable on the adjustable option. Since the drive pin can pull up to a 9V guaranteed minimum, the device can be used to regulate a large range of output voltages by careful selection of the external MOSFET (see component selection, below). The SC1548 includes an active high enable control with an internal pullup resistor. If this pin is pulled low, the drive pin is pulled low, turning off the N-channel MOSFET. If the pin is left open or pulled up to 2.5V, 3.3V or 5V, then the drive pin will be enabled.
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Also included is an overcurrent protection circuit that monitors the output voltage. If the output voltage drops below 50% of nominal, as would occur during an overcurrent or short condition, the device will pull the drive pin low and latch off. Fixed Output Voltage Options Please refer to the Application Circuit on Page 1. The fixed output voltage parts have an internal resistor divider that draws a nominal 100A from the output. The voltage at the common node of the resistor divider is then compared to the bandgap reference voltage of 1.263V. The drive pin voltage is then adjusted to maintain the output voltage set by the resistor divider. Referring to the block diagram on page 5, the nominal resistor values are:
Output Voltage 1.515V 1.818V R 1 (kW) 2.52 5.55 R 1 (kW) 12.63 12.63
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SC1548
POWER MANAGEMENT Applications Infomation (Cont.)
It is possible to adjust the output voltage of the fixed voltage options, by applying an external resistor divider to the sense pin (please refer to Figure 1 below). Since the sense pin sinks a nominal 100A, the resistor values should be selected to allow 10mA to flow through the divider. This will ensure that variations in this current do not adversely affect output voltage regulation. Thus a target value for R2 (maximum) can be calculated:
R2 V OUT ( FIXED ) 10 mA
The output voltage can only be adjusted upwards from the fixed output voltage, and can be calculated using the following equation:
VOUT ( ADJUSTED
)
R1 = VOUT ( FIXED ) * 1 + + R1 * 100 A R2
Volts
Q1 VOUT R1 + C1 100uF + C2 100uF + C3 22uF
VPWR
12V IN
U1 1 2 R2 3 SNS EN GND DRV IN 4 C4 0.1uF 5 ENABLE
SC1548CSK-X.X
Figure 1: Adjusting The Output Voltage of Fixed Output Voltage Options
Q1 VOUT R1 + C1 100uF + C2 100uF + C3 22uF U1 1 2 R2 3 ADJ GND DRV IN 4 C4 0.1uF EN 5 ENABLE VPWR 12V IN
SC1548CSK
Figure 2: Setting The Output Voltage of the Adjustable Output Voltage Option Adjustable Output Voltage Option The adjustable output voltage option does not have an internal resistor divider. The adjust pin connects directly to the inverting input of the error amplifier, and the output voltage is set using external resistors (please refer to Figure 2 above). In this case, the adjust pin sources a nominal 0.5A, so the resistor values should be selected to allow 50A to flow through the divider.
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Again, a target value for R2 (maximum) can be calculated:
R2 1 . 263 V 50 A
The output voltage can be calculated as follows:
R1 V OUT = 1 . 263 * 1 + - 0 . 5 A * R1 R2
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SC1548
POWER MANAGEMENT Applications Infomation (Cont.)
Please see Table 1 below for recommended resistor values for some standard output voltages. All resistors are 1%, 1/10W.
V OU T (V ) 1.5 1.8 2.5 2.8 3.0 3.3 R 1 (W) 18.7 42.2 97.6 124 140 169 R 2 (W) 100 100 100 102 102 105
To be most effective, the MOSFET RDS(ON) should not be selected artificially low. The MOSFET should be chosen so that at maximum required current, it is almost fully turned on. If, for example, a supply of 1.5V at 4A is required from a 3.3V 5% rail, the maximum allowable RDS(ON) would be:
R DS ( ON )( MAX ) =
(0 . 95 * 3 . 3 - 1 . 5 * 1 . 025 ) 400 m
4
To allow for temperature effects 200mW would be a suitable room temperature maximum, allowing a peak short circuit current of approximately 15A for a short time before shutdown. Capacitor Selection Output Capacitors: low ESR aluminum electrolytic or tantalum capacitors are recommended for bulk capacitance, with ceramic bypass capacitors for decoupling high frequency transients. Input Capacitors: placement of low ESR aluminum electrolytic or tantalum capacitors at the input to the MOSFET (VPWR) will help to hold up the power supply during fast load changes, thus improving overall transient response. The 12V supply should be bypassed with a 0.1F ceramic capacitor. Layout Guidelines One of the advantages of using the SC1548 to drive an external MOSFET is that the bandgap reference and control circuitry do not need to be located right next to the power device, thus a very accurate output voltage can be obtained since heating effects will be minimal. The 0.1F bypass capacitor should be located close to the supply pin, and connected directly to the ground plane. The ground pin of the device should also be connected directly to the ground plane. The sense or adjust pin does not need to be close to the output voltage plane, but should be routed to avoid noisy traces if at all possible. Power dissipation within the device is practically negligible, requiring no special consideration during layout.
Table 1: Recommended Resistor Values For SC1548 The maximum output voltage that can be obtained from the adjustable option is determined by the input supply voltage and the RDS(ON) and gate threshold voltage of the external MOSFET. Assuming that the MOSFET gate threshold voltage is sufficiently low for the output voltage chosen and a worst-case drive voltage of 9V, VOUT(MAX) is given by:
V OUT ( MAX ) = V PWR
( MIN )
- I OUT ( MAX ) * R DS ( ON )( MAX )
Short Circuit Protection The short circuit protection feature of the SC1548 is implemented by using the RDS(ON) of the MOSFET. As the output current increases, the regulation loop maintains the output voltage by turning the FET on more and more. Eventually, as the RDS(ON) limit is reached, the MOSFET will be unable to turn on any further, and the output voltage will start to fall. When the output voltage falls to approximately 50% of nominal, the LDO controller is latched off, setting output voltage to 0V. Power must be cycled to reset the latch. To prevent false latching due to capacitor inrush currents or low supply rails, the current limit latch is initially disabled. It is enabled at a preset time (nominally 5ms) after both IN and EN rise above their lockout points. If EN is left floating (using the internal resistor pullup), then VPWR should come up before VIN, or the device will latch off. If the enable function is not being used, EN should be tied to VPWR.
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SC1548
POWER MANAGEMENT Outline Drawing - SOT-23-5
Land Pattern - SOT-23-5
Contact Information
Semtech Corporation Power Management Products Division 652 Mitchell Rd., Newbury Park, CA 91320 Phone: (805)498-2111 FAX (805)498-3804
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