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LM3370

SNVS406M – NOVEMBER 2005 – REVISED AUGUST 2011

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PFM OPERATION

At very light loads, the converter enters PFM mode and operates with reduced switching frequency and supply

current to maintain high efficiency.

The part will automatically transition into PFM mode when either of two conditions are true, for a duration of 32 or

more clock cycles:

1. The NFET current reaches zero.

(3)

Supply current during this PFM mode is less than 20 µA per channel, which allows the part to achieve high

efficiency under extremely light load conditions. When the output drops below the ‘low’ PFM threshold, the cycle

repeats to restore the output voltage to

∼1.2% above the nominal PWM output voltage.

If the load current should increase during PFM mode (see Figure 2) causing the output voltage to fall below the

‘low2’ PFM threshold, the part will automatically transition into fixed-frequency PWM mode.

During PFM operation, the converter positions the output voltage slightly higher than the nominal output voltage

during PWM operation, allowing additional headroom for voltage drop during a load transient from light to heavy

load. The PFM comparators sense the output voltage via the feedback pin and control the switching of the output

FETs such that the output voltage ramps between 0.8% and 1.6% (typical) above the nominal PWM output

voltage. If the output voltage is below the ‘high’ PFM comparator threshold, the PFET power switch is turned on.

set for PFM mode. The typical peak current in PFM mode is:

(4)

Once the PFET power switch is turned off, the NFET power switch is turned on until the inductor current ramps

to zero. When the NFET zero-current condition is detected, the NFET power switch is turned off. If the output

voltage is below the ‘high’ PFM comparator threshold (see Figure 2), the PFET switch is again turned on and the

cycle is repeated until the output reaches the desired level. Once the output reaches the ‘high’ PFM threshold,

the NFET switch is turned on briefly to ramp the inductor current to zero and then both output switches are

turned off and the part enters an extremely low power mode.

FORCED PWM MODE

The LM3370 auto mode can be bypassed by forcing the device to operate in PWM mode, this can be

SOFT-START

The LM3370 has a soft start circuit that limits in-rush current during start up. Soft start is activated only if EN

LDO - LOW DROP OUT OPERATION

The LM3370 can operate at 100% duty cycle (no switching, PFET switch completely on) for low drop out support

of the output voltage. In this way the output voltage will be controlled down to the lowest possible input voltage.

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