LMZ10501SH datasheet(14/25 Pages) TI

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1V/Div

20 MHz BW

5 µs/Div

SWITCH NODE

INPUT VOLTAGE

1V/Div

100 µs/Div

1V/Div

0.5A/Div

100 mA/Div

VCON

IIN

VOUT

LMZ10501

SNVS677D – MAY 2011 – REVISED MARCH 2013

www.ti.com

Figure 36. Hiccup behavior with persistent output short circuit.

Since the output current is limited during normal startup by the softstart function, the current charging the output

capacitor is also limited. This results in a smooth VOUT ramp up to nominal voltage. However, using excessively

large output capacitance or VCON capacitance under normal conditions can prevent the output voltage from

reaching 0.375V at the end of the startup sequence. In such cases the module will maintain the described above

hiccup mode and the output voltage will not ramp up to final value. To cause this condition, one would have to

use unnecessarily large output capacitance for 1A load applications. See the INPUT AND OUTPUT CAPACITOR

SELECTION section for guidance on maximum capacitances for different output voltage settings.

HIGH DUTY CYCLE OPERATION

The LMZ10501 features a transition mode designed to extend the output regulation range to the minimum

possible input voltage. As the input voltage decreases closer and closer to VOUT, the off-time of the PFET gets

smaller and smaller and the duty cycle eventually needs to reach 100% to support the output voltage. The input

voltage at which the duty cycle reaches 100% is the edge of regulation. When the LMZ10501 input voltage is

lowered, such that the off-time of the PFET reduces to less than 35ns, the LMZ10501 doubles the switching

period to extend the off-time for that VIN and maintain regulation. If VIN is lowered even more, the off-time of the

PFET will reach the 35ns mark again. The LMZ10501 will then reduce the frequency again, achieving less than

100% duty cycle operation and maintaining regulation. As VIN is lowered even more, the LMZ10501 will continue

to scale down the frequency, aiming to maintain at least 35ns off time. Eventually, as the input voltage decreases

further, 100% duty cycle is reached. This behavior of extending the VIN regulation range is illustrated in the

following plot.

Figure 37. High duty cycle operation and switching frequency reduction.

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LMZ10501SH Datasheet(PDF) 14 Page - Texas Instruments