Detailed description

ST1S15

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DocID023280 Rev 4

7

Detailed description

7.1

General description

The ST1S15 is a fixed voltage mode PWM step-down DC-DC converter, which operates

with typically 6 MHz fixed frequency pulse width modulation (PWM) at moderate and heavy

load currents. At light load currents the converter can automatically enter PFM (pulse

frequency mode) mode.

Few components are required: an inductor and two capacitors. It can work properly with

X5R or X7R SMD ceramic capacitors both at the input and at the output. These kinds of

capacitors, thanks to their very low series resistance (ESR), minimize the output voltage

ripple. In addition, the chosen inductor must not saturate at the peak current level.

7.2

Mode transition

The ST1S15 can work in PWM mode or in PFM mode according to the different operating

conditions. If the MODE pin is pulled high, the device works in PWM mode only even at

light or no-load. If the MODE pin is low, the operation changes according to the average

input current handled by the device. At low output current the device works in PFM mode

so to obtain very low power consumption and very good efficiency. When the output current

increases, the device automatically switches to PWM mode in order to deliver the power

needed by the load.

The ST1S15 passes from PFM to PWM when 3 consecutive PFM pulses occur. This

means that the PFM has reached its maximum current capability and the device needs to

go into PWM mode. The whole PWM circuitry starts after a transition time. During this time

the duration of the PFM pulses rises about 350 ns so to provide higher current capability.

After the PWM circuitry startup, the ST1S15 switches to PWM operation.

Figure 20: PFM to PWM transition