7

Operation description

The ST1PS01 is an ultra-low quiescent new generation buck converter. It targets a very small quiescent current

consumption (typical 500 nA) and it guarantees high efficiency operation even down to few microampere loads.

It is based on a hysteretic comparator that senses the coil ripple current that is held constant in all operation

modes. The device has seamless transition between PFM (pulse frequency modulation) and PWM (pulse width

modulation) mode with low ripple and good load transient response.

In order to maintain constant ripple current on the selected coil, the device changes switching frequency, which

also depends on input supply voltage. During PWM mode (heavy load), the device operates in continuous

conduction up to 400 mA and switching frequency can reach 2 MHz maximum.

7.1

Power save mode

At light load the device enters automatically power save mode with total current consumption from the input power

supply of 500 nA typical; during this condition most of the internal blocks are turned off in order to reach ultra-low

power consumption. During this time, the load current is supported by the output capacitor.

7.2

Output voltage

The device allows output voltage selection without external resistor divider. A couple of standard digital inputs are

used to configure the device to supply a fixed output voltage according to Section 5 . The VOUT pin must be

connected directly and as close as possible to the inductor terminal to obtain the best performance and get

the best output voltage regulation. The output voltage can be dynamically changed to implement voltage scaling.

7.3

Output discharge and UVLO

The device embeds a fast output discharge circuitry active when the enable pin is held to ground (EN=gnd) or

when the input supply voltage reaches the minimum voltage level set by the UVLO protection circuit

(undervoltage lock-out protection circuit). The UVLO rising threshold at 1.63 V (typ.) guarantees a proper device

supply voltage operation.

7.4

Soft-start and current limitation

The device embeds a fixed soft-start circuit active during a limited time period (few ms). This feature allows the

inrush current to be minimized from the power supply in case of weak source. During this period internal circuit

reduces to 280 mA the typical switch current limit.

7.5

100% duty cycle operation

The device enters 100% duty cycle operation if the input voltage comes close to the selected output voltage.

During this mode, the regulator is turned off and output pin is directly connected to the input pin through the high-

side MOSFET. The output voltage follows the input level minus the voltage drop across the internal MOSFET and

the inductor. Once the input voltage exceeds the 100% duty cyle leave threshold, the device restarts to switch and

regulates the select output voltage again.

7.6

Power Good flag

The Power Good comparator monitors the selected VOUT voltage. The Power Good open drain output is in high

impedance when the VOUT reaches the correct voltage level while it switches to low level when VOUT falls below

the normal voltage level.

ST1PS01

Operation description

DS12799 - Rev 5

page 9/20