Datasheet

VIN
EN
GND
SS
R
trkb
R
trkt
V
IN
C
in1
Master Power
Supply
V
OUT1
V
OUT2
C
O1
V
SS
LMZ10503EXT
VOUT
C
SS
=
t
SS
x I
SS
V
FB
VIN
EN
GND
VOUT
R
enb
R
ent
V
IN
C
in1
Master Power Supply
V
OUT1
V
OUT2
C
O1
LMZ10503EXT
LMZ10503EXT
SNVS668G –JUNE 2010–REVISED OCTOBER 2013 www.ti.com
Soft-Start
The LMZ10503EXT begins to operate when both the VIN and EN, voltages exceed the rising UVLO and enable
thresholds, respectively. A controlled soft-start eliminates inrush currents during startup and allows the user more
control and flexibility when sequencing the LMZ10503EXT with other power supplies.
In the event of either VIN or EN decreasing below the falling UVLO or enable threshold respectively, the voltage
on the soft-start pin is collapsed by discharging the soft-start capacitor by a 14 µA (typ.) current sink to ground.
Soft-Start Capacitor
Determine the soft-start capacitance with the following relationship
(20)
where V
FB
is the internal reference voltage (nominally 0.8V), I
SS
is the soft-start charging current (nominally 2 µA)
and C
SS
is the external soft-start capacitance.
Thus, the required soft-start capacitor per unit output voltage startup time is given by
C
SS
= 2.5 nF / ms (21)
For example, a 4 ms soft-start time will yield a 10 nF capacitance. The minimum soft-start capacitance is 680 pF.
Tracking
The LMZ10503EXT can track the output of a master power supply during soft-start by connecting a resistor
divider to the SS pin. In this way, the output voltage slew rate of the LMZ10503EXT will be controlled by a
master supply for loads that require precise sequencing. When the tracking function is used, a small value soft-
start capacitor should be connected to the SS pin to alleviate output voltage overshoot when recovering from a
current limit fault.
16 Submit Documentation Feedback Copyright © 2010–2013, Texas Instruments Incorporated
Product Folder Links: LMZ10503EXT