1V/Div
20 MHz BW
5 µs/Div
SWITCH NODE
INPUT VOLTAGE
1V/Div
100 µs/Div
1V/Div
1V/Div
0.3A/Div
50 mA/Div
VCON
IL
IIN
VOUT
LMZ10500
www.ti.com
SNVS723C – OCTOBER 2011 – REVISED MARCH 2013
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 V
OUT
ramp up to nominal voltage. However, using excessively
large output capacitance or V
CON
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 650mA 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 LMZ10500 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 V
OUT
, 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 LMZ10500 input voltage is
lowered, such that the off-time of the PFET reduces to less than 35ns, the LMZ10500 doubles the switching
period to extend the off-time for that V
IN
and maintain regulation. If V
IN
is lowered even more, the off-time of the
PFET will reach the 35ns mark again. The LMZ10500 will then reduce the frequency again, achieving less than
100% duty cycle operation and maintaining regulation. As V
IN
is lowered even more, the LMZ10500 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 V
IN
regulation range is illustrated in the
following plot.
Figure 37. High duty cycle operation and switching frequency reduction.
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