Datasheet

DocID023280 Rev 3 13/28
ST1S15 Detailed description
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 to heavy load
currents. At light load currents the converter can automatically enter PFM (pulse frequency
mode) mode.
To make the device work, few components are required: an inductor and two capacitors. It
has been designed to 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 be able to
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 only in PWM mode 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 in order
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 are increased to about 350 ns so as to provide higher current
capability. After startup of the PWM circuitry, the ST1S15 switches to PWM operation.
Figure 22. PFM to PWM transition
I
L
Consecutive 3 PFM pulses
t
t
PFM
Transition
Time
PWM
MODE
t
I
LOAD
Light Load
High Load
500 mA
AM11906v1