6TPE100MPB2 datasheet(14/22 Pages) NSC

14 / 22 page

the master supply. A value of 33 k

Ω 1% is recommended for

trkt as a compromise between high precision and low quies-

cent current through the divider while minimizing the effect of

the 2 µA soft-start current source.

For example, if the master supply voltage V

OUT1 is 3.3V and

the LMZ10503 output voltage was 1.8V, then the value of

trkb needed to give the two supplies identical soft-start times

would be 14.3 k

Ω. A timing diagram for this example, the

equal soft-start time case, is shown below.

30111859

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

OUT2 < 0.8 x V OUT1

For the example case of V

OUT1 = 5V and VOUT2 = 2.5V, with

trkt set to 33 kΩ as before, Rtrkb is calculated from the above

equation to be 15.5 k

Ω. A timing diagram for the case of equal

slew rates is shown below.

30111861

Pre-Bias Startup Capability

At startup, the LMZ10503 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, FP-

GA, or DSP. The output can be pre-biased in these applica-

tions through parasitic conduction paths from one supply rail

to another. Even though the LMZ10503 is a synchronous

converter, it will not pull the output low when a pre-bias con-

dition exists. The LMZ10503 will not sink current during start-

up 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 conduct-

ed 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 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 LMZ10503 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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