10 µs/Div
500 mV/Div
500 mV/Div
200 mA/Div
200 mA/Div
VCON
IL
IOUT
VOUT
LMZ10500
SNVS723C – OCTOBER 2011 – REVISED MARCH 2013
www.ti.com
CURRENT LIMIT
The LMZ10500 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 1.2A (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 LMZ10500 features a current limit based soft start circuit in order to prevent large in-rush current and output
overshoot as V
OUT
is ramping up. This is achieved by gradually increasing the PFET current limit threshold to the
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
smooth V
OUT
ramp up.
Figure 35. Startup behavior of current limit based softstart.
The soft start rate is also limited by the V
CON
ramp up rate. The V
CON
pin is discharged internally through a pull
down device before startup occurs. This is done to deplete any residual charge on the V
CON
filter capacitor and
allow the V
CON
voltage to ramp up from 0V when the part is started. The events that cause V
CON
discharge are
thermal shutdown, UVLO, EN low, or output short circuit detection. The minimum recommended capacitance on
V
CON
is 220pF and the maximum is 1nF. The duration of startup current limiting sequence takes approximately
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 LMZ10500 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 LMZ10500 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
.
14
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