LM10502

SNVS884B – AUGUST 2012 – REVISED MAY 2013

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If the load current should increase during PFM mode causing the output voltage to fall below the ‘low2’ PFM

threshold, the part will automatically transition into fixed-frequency PWM mode.

Soft Start

Each of the buck converters has an internal soft-start circuit that limits the in-rush current during startup. This

allows the converters to gradually reach the steady-state operating point, thus reducing startup stresses and

surges. During startup, the switch current limit is increased in steps.

For Buck 1 and 2 the soft start is implemented by increasing the switch current limit in steps that are gradually

set higher. The startup time depends on the output capacitor size, load current and output voltage. Typical

startup time with the recommended output capacitor of 22 µF is 0.2 to 1ms. It is expected that in the final

application the load current condition will be more likely in the lower load current range during the start up.

Current Limiting

A current limit feature protects the device and any external components during overload conditions. In PWM

mode the current limiting is implemented by using an internal comparator that trips at current levels according to

the buck capability. If the output is shorted to ground the device enters a timed current limit mode where the

NFET is turned on for a longer duration until the inductor current falls below a low threshold, ensuring inductor

current has more time to decay, thereby preventing runaway.

Internal Synchronous Rectification

While in PWM mode, the bucks use an internal NFET as a synchronous rectifier to reduce the rectifier forward

voltage drop and the associated power loss. Synchronous rectification provides a significant improvement in

efficiency whenever the output voltage is relatively low compared to the voltage drop across an ordinary rectifier

diode.

Low Dropout Operation

The device can operate at 100% duty cycle (no switching; PMOS switch completely on) for low dropout support.

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

operates near 100% duty cycle, output voltage ripple is approximately 25 mV.

The minimum input voltage needed to support the output voltage:

VIN_MIN=VOUT+ILOAD*(RDSON_PFET+RIND)

Where,

•

ILOAD = Load Current

•

RDSON_PFET = Drain to source resistance of PFET (high side) .

•

RIND = Inductor resistance

(1)

Out Of Regulation

When any of the Buck outputs are taken out of regulation (below 85% of the output level) the device will start a

shutdown sequence and all other ouputs will switch off normally. The device will restart when the forced out-of-

regulation condition is removed.

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