Datasheet

VOLTAGE

TIME

SIMULTANEOUS STARTUP

OUT2

OUT1

LMZ10503EXT

SNVS668G –JUNE 2010–REVISED OCTOBER 2013

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Pre-Bias Startup Capability

At startup, the LMZ10503EXT is in a pre-biased state when the output voltage is greater than zero. This often

occurs in many multi-rail applications such as when powering an ASIC, FPGA, or DSP. The output can be pre-

biased in these applications through parasitic conduction paths from one supply rail to another. Even though the

LMZ10503EXT is a synchronous converter, it will not pull the output low when a pre-bias condition exists. The

LMZ10503EXT will not sink current during startup until the soft-start voltage exceeds the voltage on the FB pin.

Since the device does not sink current it protects the load from damage that might otherwise occur if current is

conducted through the parasitic paths of the load.

Current Limit

When a current greater than the output current limit (I

OCL

) is sensed, the on-time is immediately terminated and

the low side MOSFET is activated. The low side MOSFET stays on for the entire next four switching cycles.

During these skipped pulses, the voltage on the soft-start pin is reduced by discharging the soft-start capacitor by

a current sink on the soft-start pin of nominally 14 µA. Subsequent over-current events will drain more and more

charge from the soft-start capacitor, effectively decreasing the reference voltage as the output droops due to the

pulse skipping. Reactivation of the soft-start circuitry ensures that when the over-current situation is removed, the

part will resume normal operation smoothly.

Over-Temperature Protection

When the LMZ10503EXT senses a junction temperature greater than 145°C (typ.), both switching MOSFETs are

turned off and the part enters a standby state. Upon sensing a junction temperature below 135°C (typ.), the part

will re-initiate the soft-start sequence and begin switching once again.

LMZ10503EXT Application Circuit Schematic and BOMs

This section provides several application solutions with an associated bill of materials. The compensation for

each solution was optimized to work over the full input range. Many applications have a fixed input voltage rail. It

is possible to modify the compensation to obtain a faster transient response for a given input voltage operating

point.

Product Folder Links: LMZ10503EXT