10 µs/Div

500 mV/Div

500 mV/Div

300 mA/Div

300 mA/Div

VCON

IL

IOUT

VOUT

LMZ10501

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SNVS677D – MAY 2011 – REVISED MARCH 2013

CURRENT LIMIT

The LMZ10501 current limit feature protects the module during an overload condition. The circuit employs

positive peak current limit in the PFET and negative peak current limit in the NFET switch. The positive peak

current through the PFET is limited to 1.7A (typ.). When the current reaches this limit threshold the PFET switch

is immediately turned off until the next switching cycle. This behavior continues on a cycle-by-cycle basis until the

overload condition is removed from the output. The typical negative peak current limit through the NFET switch is

-0.6A (typ.).

The ripple of the inductor current depends on the input and output voltages. This means that the DC level of the

output current when the peak current limiting occurs will also vary over the line voltage and the output voltage

level. Refer to the DC Output Current Limit plots in the Typical Performance Characteristics section for more

information.

STARTUP BEHAVIOR AND SOFTSTART

The LMZ10501 features a current limit based soft start circuit in order to prevent large in-rush current and output

final operating value as the output voltage ramps during startup. The maximum allowed current in the inductor is

stepped up in a staircase profile for a fixed number of switching periods in each step. Additionally, the switching

frequency in the first step is set at 450kHz and is then increased for each of the following steps until it reaches

2MHz at the final step of current limiting. This current limiting behavior is illustrated in Figure 35 and allows for a

Figure 35. Startup behavior of current limit based softstart.

thermal shutdown, UVLO, EN low, or output short circuit detection. The minimum recommended capacitance on

75µs. After the sequence is completed, the feedback voltage is monitored for output short circuit events.

OUTPUT SHORT CIRCUIT PROTECTION

In addition to cycle by cycle current limit, the LMZ10501 features a second level of short circuit protection. If the

load pulls the output voltage down and the feedback voltage falls to 0.375V, the output short circuit protection will

engage. In this mode the internal PFET switch is turned OFF after the current limit comparator trips and the

beginning of the next cycle is inhibited for approximately 230µs. This forces the inductor current to ramp down

and limits excessive current draw from the input supply when the output of the regulator is shorted. The

synchronous rectifier is always OFF in this mode. After 230µs of non-switching a new startup sequence is

initiated. During this new startup sequence the current limit is gradually stepped up to the nominal value as

illustrated in the STARTUP BEHAVIOR AND SOFTSTART section. After the startup sequence is completed

again, the feedback voltage is monitored for output short circuit. If the short circuit is still persistent after the new

startup sequence, switching will be stopped again and there will be another 230µs off period. A persistent output

short condition results in a hiccup behavior where the LMZ10501 goes through the normal startup sequence,

then detects the output short at the end of startup, terminates switching for 230µs, and repeats this cycle until the

output short is released. This behavior is illustrated in Figure 36.

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