L7292 datasheet(45/60 Pages) STMICROELECTRONICS

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L7292

Buck regulator description

11.6

PFM mode

To maintain high efficiency in light-load condition the device enters automatically the PFM

mode for a load current of around IDCM/2 or lower. This happens when the inductor current

becomes discontinuous.

During the PFM operation, the converter fixes the output voltage to the nominal DC value

plus ½ Vout ripple that is affected by the output capacitor, the current ripple in the inductor

and the load current.

In this phase the high-side PMOS current is limited at IDCM and the pulses frequency is

proportional to the load current. When the PMOS current reaches IDCM the PMOS is

switched OFF while the NMOS is forced ON. This is valid until a zero current is detected,

then the regulator output is set in high impedance until Vout crosses the nominal value.

11.7

Forced PWM

When enabled, the buck regulator always operates in the PWM mode regardless of the

output current. In the light-load the efficiency will be worse than in the PFM mode due to the

inductor current becoming negative.

This function enables the ability to have a constant frequency in light-load condition and also

reduces the output voltage ripple.

11.8

Current limiting

According to the max. load current, an ILIMIT is embedded in each regulator to protect the

device and any external components from overload conditions.

When the high-side PMOS current reaches the ILIMIT, the regulator output is forced in high

impedance for around 1

s, allowing the inductor current to decrease to a safe level before

allowing another on-time to happen.

11.9

Short-circuit protection

In case the high-side PMOS current reaches the current limit and Vout falls below the short-

circuit voltage threshold, the regulator is switched off permanently, until another full power-

up of the whole device is performed.

11.10

Deep sleep mode

This feature is enabled when the device has to provide the minimum functionality of the

system, to reduce the quiescent current and to be ready to restart the operative conditions

when needed.

The device can be placed in the “Deep Sleep Mode” (DSM) by writing bit 7 REGCTRL = 1 in

this mode.



Regulator 3 is enabled - no load current, in the PFM mode.



Regulators 1 - 2 - 4 - 5 are OFF.

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