before the master supply. A value of 33 k

Ω 1% is recom-

mended for R

low quiescent current through the divider while minimizing the

effect of the 2 µA soft-start current source.

For example, if the master supply voltage V

the LMZ10503EXT output voltage was 1.8V, then the value

of R

times would be 14.3 k

Ω. A timing diagram for this example,

the equal soft-start time case, is shown below.

30118259

Tracking - Equal Slew Rates

Alternatively, the tracking feature can be used to have similar

output voltage ramp rates. This is referred to as simultaneous

startup. In this case, the tracking resistors can be determined

based on the following equation

and to ensure proper overdrive of the SS pin

V

For the example case of V

R

equation to be 15.5 k

Ω. A timing diagram for the case of equal

slew rates is shown below.

30118261

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 appli-

cations through parasitic conduction paths from one supply

rail to another. Even though the LMZ10503EXT is a syn-

chronous 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 pro-

tects 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

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 dis-

charging 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 re-

moved, 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.

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