Application information

ST1S09

12/18

Doc ID 13632 Rev 4

7

Application information

The ST1S09 is an adjustable current mode PWM step-down DC-DC converter with internal

2 A power switch, packaged in a DFN6 3 x 3 mm.

The device is a complete 2 A switching regulator with its internal compensation eliminating

the need for additional components.

The constant frequency, current mode, PWM architecture and stable operation with ceramic

capacitors results in low, predictable output ripple.

The over-voltage protection circuit acts when the output voltage is over 10 % of the rated

voltage and within 200 ns the low side MOSFET will be turned on to clamp the output

transient. The current limit for clamping is about 400 mA. When the output voltage drops to

about 5 % above the nominal level, the device returns to nominal closed loop switching

operation.

The open drain Power Good (PG) pin is released when the output voltage is higher than

Other circuits fitted to the device protection are the thermal shut-down block, which turns off

the regulator when the junction temperature exceeds 150 °C (typ), and the cycle-by-cycle

current limiting, which provides protection against shorted outputs.

As an adjustable regulator, the ST1S09’s output voltage is determined by an external

resistor divider. The desired value is given by the following equation:

Equation 1

To utilize the device, only a few components are required: an inductor, two capacitors and

the resistor divider. The inductor chosen must be able to reach peak current level without

saturating. Its value can be selected while taking into account 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 package size and the total

cost of the application. Finally, the ST1S09 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,

due 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 correct functioning of the device.