ST AN2728 Application note
AN2728
Application note
ST1S12 small synchronous buck converter
Introduction
The ST1S12 family of synchronous step-down DC-DC converters optimized for powering low-voltage digital cores in HDD applications is generally used to replace the high-current linear solution when the power dissipation may cause high heating of the application environment. It provides up to 0.7 A over an input voltage range of 2.5 V to 5.5 V.
A high switching frequency (1.7 MHz) allows the use of tiny surface-mount components. A resistor divider to set the output voltage value, an inductor, and two capacitors are required for the adjustable version. Only an inductor and 2 capacitors are needed for the 1.2 V and 1.8 V fixed version. A low output ripple is guaranteed by the current mode PWM topology and by the use of low ESR surface-mount ceramic capacitors.
The device is thermal protected and current limited to prevent damages due to accidental short-circuit. This family of products is available in the TSOT23-5L package.
Figure 1. ST1S12 - simplified schematic
VIN
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UVLO |
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Rsense |
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I_SEN |
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- |
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Σ |
VSUM |
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Comp |
VRST |
MOSFET |
VDRV_P Driver |
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CONTROL |
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VSET |
LOGIC |
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OSC |
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SW |
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VC |
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DMD |
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DMD |
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- |
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SHUT DOWN |
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VDRV_N Driver |
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SOFT START |
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REF1 |
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FB/VO |
EN |
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Gnd |
April 2008 |
Rev 1 |
1/20 |
www.st.com
Contents |
AN2728 |
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Contents
1 |
ST1S12 description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
4 |
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1.1 |
Enable function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
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1.2 |
Current limit and short-circuit protection . . . . . . . . . . . . . . . . . . . . . . . . . . |
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2 |
Selecting components for your applications . . . . . . . . . . . . . . . . . . . . . |
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2.1 Output voltage selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.2 Input capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 2.3 Output capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 2.4 Inductor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 2.5 Layout considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3 |
Thermal considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
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Demonstration board usage recommendation . . . . . . . . . . . . . . . . . . . |
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4.1 |
External component selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
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4.2 |
Capacitors selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
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4.3 |
Inductor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
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Bill of materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
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Recommended footprint . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
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Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
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2/20
AN2728 |
List of figures |
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List of figures
Figure 1. ST1S12 - simplified schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Figure 2. Inductor current at no load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Figure 3. Inductor current at medium load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Figure 4. Inrush current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Figure 5. Enable voltage vs. temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Figure 6. Short-circuit protection simplified schematic. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Figure 7. Typical application schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Figure 8. Drop vs. temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Figure 9. Feedback voltage vs. temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Figure 10. Layout considerations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Figure 11. ST1S12 demonstration board layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Figure 12. Demonstration board layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Figure 13. Demonstration board schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Figure 14. Efficiency vs. output current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Figure 15. Efficiency vs. output voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Figure 16. Efficiency vs. inductor at VO = 1.8 V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Figure 17. Efficiency vs. inductor at VO = 3.3 V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Figure 18. Efficiency vs. input voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Figure 19. TSOT23-5L footprint dimensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
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ST1S12 description |
AN2728 |
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1 ST1S12 description
The ST1S12 is an adjustable current mode PWM synchronous step-down DC-DC converter with an internal 0.7 A power switch. It is a complete 0.7 A switching regulator with internal compensation which eliminates the need for additional components.
The device operates with typically 1.7 MHz fixed frequency, and in order to guarantee the lowest switching ripple, operates in pulse width modulation (PWM) mode even at low-load condition. (Figure 2 and Figure 3)
Figure 2. Inductor current at no load |
Figure 3. Inductor current at medium load |
VOUT (AC) |
Vout (AC) |
IL |
IL |
SW |
SW |
VEN=VIN=5 V, VOUT=1.8 V, no load, CH1=SW, |
VEN=VIN=5 V, VOUT=1.8 V, RLOAD=4.7 Ω, CH1=SW, |
CH2=VOUT, CH4=IL |
CH2=VOUT, CH4=IL |
To clamp the error amplifier reference voltage, a soft-start control block generating a voltage ramp is implemented. When switching on the power supply, it allows controlling the inrush current value (Figure 4).
Figure 4. Inrush current
VOUT
EN
SW
IIN
VEN=VIN=5 V, VOUT=1.8 V, RLOAD =15 Ω,
Ch1=SW, CH2=VOUT, CH3=EN, CH4=IIN
4/20
AN2728 |
ST1S12 description |
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Other protection circuits in the device are the thermal shutdown block which turns off the regulator when the junction temperature exceeds 150 °C (typ.) and the cycle-by-cycle current limiting that provides protection against shorted outputs.
The few components required for operation of the device are an inductor, two capacitors, and a resistor divider. The inductor chosen must be capable of withstanding peak current level without saturating. The value of the inductor should be selected keeping in mind that a large inductor value increases the efficiency at low output current and reduces output voltage ripple, while a smaller inductor can be chosen when it is important to reduce the package size and the total application cost. The ST1S12 has been designed to work properly with X5R or X7R SMD ceramic capacitors both at the input and at the output. These types of capacitors, thanks to their very low series resistance (ESR), minimize the output voltage ripple. Other low ESR capacitors can be used according to the need of the application without compromising the right functioning of the device.
Finally, if the input voltage falls close to the output voltage, the ST1S12 can run at 100 % duty cycle, in this mode the PMOS switch is continuously maintained ON. In this case the output voltage value is the input voltage minus the voltage drop across the PMOS switch and the resistance of the inductor.
The minimum input voltage to guarantee the right output voltage is:
VIN_MIN = IOUT_MAX x (RDS(on)_P + DCRL) + VOUT
where DCRL is DC resistance of the inductor and RDS(on)_P is the resistance of the PMOS.
Due to the high switching frequency and peak current, it is important to optimize the application environment such as reducing the length of the PCB traces and placing all external components near the device.
1.1Enable function
The ST1S12 features an enable function (pin 1). When the EN voltage is higher than 1.5 V the device is ON and if it is lower than 0.5 V the device is OFF, Figure 5 shows the enable voltage vs temperature. In shutdown mode consumption is lower than 1 µA.
The EN pin does not have an internal pull-up, which means that the enable pin cannot be left floating.
If the enable function is not used, the EN pin must be connected to VIN.
Figure 5. Enable voltage vs. temperature
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1.8 |
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1.6 |
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ON |
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1.4 |
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OFF |
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1.2 |
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[V] |
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1 |
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EN |
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0.8 |
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V |
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0.6 |
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0.4 |
VIN = 5.5 V, IOUT = 10 mA |
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0.2 |
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0 |
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-75 |
-50 |
-25 |
0 |
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75 |
100 |
125 |
150 |
175 |
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T [°C] |
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5/20
ST1S12 description |
AN2728 |
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1.2Current limit and short-circuit protection
In overcurrent protection mode, when the peak current reaches the current limit, the device reduces tON down to its minimum value. In these conditions, the duty cycle is strongly reduced and, in most applications, this is enough to limit the current to Ilim.
In case of heavy short-circuit when the feedback voltage is lower than 0.1 V (typ.), the loop switches to short mode automatically. In this condition the voltage Vsum=Isen * Rsen is compared with 0.4 V (typ.) to clamp the upper limit of the inductor current. In this condition the maximum output limitation current is reduced to 300 mA instead of 1 A. At the same time the DMD circuit clamps the lower boundary of the inductor current. One RS flip-flop is being used to control the PMOS and NMOS switches. When the feedback voltage is higher than 0.1 V (typ.) voltage, the device returns to normal closed-loop switching operation (Figure 6).
Figure 6. Short-circuit protection simplified schematic
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VIN |
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Vsum=Isen*Rsen |
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VREF1 |
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COM |
S |
L1 |
VOUT |
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VC |
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Q |
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R |
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R1 |
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C2 |
COM |
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DMD |
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R2 |
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0.1V |
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VFB |
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GND |
GND |
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GND |
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