AN2728 Datasheet, PDF (5/20 Pages) STMicroelectronics – ST1S12 small synchronous buck converter

AN2728
ST1S12 description
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:
V_{IN_MIN} = I_{OUT_MAX} x (R_{DS(on)_P} + DCR_L) + V_OUT
where DCR_L is DC resistance of the inductor and R_{DS(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.
1.1
Enable function
Figure 5.
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 V_IN.
Enable voltage vs. temperature
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
-75
V_IN = 5.5 V, I_OUT = 10 mA
-50 -25 0 25 50 75
T [°C]
ON
OFF
100 125 150 175
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