Datasheet
0 10 20 30 40 50 60 70 80 90
300
400
500
600
700
800
SOFT OFF TIME (EN TO 10% VOUT) (s)
TEMP (°C)
VIN = 2.8 V
VIN = 3.0 V
VIN = 4.0 V
VIN = 5.0 V
2.5 3.0 3.5 4.0 4.5 5.0 5.5
0
100
200
300
400
500
600
700
800
SOFT START TIME (NO LOAD) (s)
VIN (V)
Vout = -5.0V
Vout = -3.3V
Vout = -2.5V
Vout = -1.5V
Vout = -0.9V
LMR70503
SNVS850A –JUNE 2012–REVISED APRIL 2013
www.ti.com
Soft Start And Soft Off
The LMR70503 begins to operate when EN goes high with the presence of valid V
IN
, or V
IN
swings below UVLO
level and back up with the presence of valid EN voltage. The soft start action is inherent with the maximum peak
current limit and minimum off time. During start up, the inductor current rises to the maximum peak current limit,
then the high-side switch is turned off for T
OFF-MIN
and the output capacitor(s) is charged during this time. Then
the high-side turns on to repeat the cycle. After the output voltage is charged to the regulation level, the
LMR70503 will operate in steady state. The soft start time will be longer with more output capacitance, and / or
lower supply voltage V
IN
, and / or more loading during start up. Figure 36 shows soft start vs V
IN
with L= 6.8 µH
and no load. Soft-start is reset any time the part is shut down or a thermal shutdown event occurs.
Figure 36. Soft Start Time (No Load) vs. V
IN
The LMR70503 will shutdown when EN pin voltage goes below the falling threshold, or V
IN
goes below UVLO
falling threshold. When shutdown, the LMR70503 incorporates an output voltage discharge feature to bring the
output voltage to zero volts, regardless of the load current. When the EN input is taken below its lower threshold,
an internal MOSFET turns on and discharges the output capacitors. Typical soft off times (from EN falling edge
to 10% of Vout ) over V
IN
and temperature are shown in Figure 37. Figure 38 shows the typical off time from 90%
to 10% of Vout.
Figure 37. Soft Off Time (EN Falling Edge To 10% Vout) vs. Temperature, V
OUT
= -5.5 V, No Load
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