AN2603

ST1S09 description

Doc ID 13778 Rev 3

7/23

Equation 1

Few components are required for operation of the device: an inductor, two capacitors and

the resistor divider. The chosen inductor must be capable of withstanding peak current level

without saturating. The inductor value can be selected taking into consideration 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 total application cost. Moreover, the ST1S09 family has been designed to function

properly with X5R or X7R SMD ceramic capacitors both at the input and at the output. This

type of capacitor minimizes the output voltage ripple, thanks to its very low series resistance

(ESR). Other low ESR capacitors can be used based on application requirements without

compromising the correct functioning of the device.

Due to the high switching frequency and peak current, it is important to optimize the

application environment by reducing the length of the PCB traces and placing all external

components in close proximity to the device.

1.1

Inhibit function (ST1S09I only)

The ST1S09I features an Inhibit function (pin6). When the INH voltage is higher than 1.3 V

the device is ON, and if it is lower than 0.4 V the device is OFF. In shutdown mode,

consumption is lower than 1 µA.

The INH pin does not have an internal pull-up, which means that the Inhibit pin cannot be

left floating.

1.2

Power Good function (ST1S09 only)

Most ODD applications require a flag showing that the output voltage is in the correct range.

The Power Good threshold depends on the feedback voltage. When the feedback is higher

0.736 V (PG threshold, 0.92* 0.8 V) and the Power Good pin is at high impedance.

Figure 7.

Power Good block diagram

V

OUT

V

FB

1

R1

R2

--------

+

⋅

=

FB

PG

+

-

0.92*Vref