Datasheet
R
trkb
=
R
trkt
V
OUT1
-1.0V
VIN
EN
GND
SS
R
trkb
R
trkt
V
IN
C
in1
Master Power
Supply
V
OUT1
V
OUT2
C
O1
V
SS
LMZ10503
VOUT
C
SS
=
t
SS
x I
SS
V
FB
LMZ10503
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SNVS641I –JANUARY 2010–REVISED OCTOBER 2013
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 LMZ10503 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 LMZ10503 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.
Tracking - Equal Soft-Start Time
One way to use the tracking feature is to design the tracking resistor divider so that the master supply output
voltage, V
OUT1
, and the LMZ10503 output voltage, V
OUT2
, both rise together and reach their target values at the
same time. This is termed ratiometric startup. For this case, the equation governing the values of tracking divider
resistors R
trkb
and R
trkt
is given by
(22)
The above equation includes an offset voltage, of 200 mV, to ensure that the final value of the SS pin voltage
exceeds the reference voltage of the LMZ10503. This offset will cause the LMZ10503 output voltage to reach
regulation slightly before the master supply. A value of 33 kΩ 1% is recommended for R
trkt
as a compromise
between high precision and low quiescent current through the divider while minimizing the effect of the 2 µA soft-
start current source.
For example, if the master supply voltage V
OUT1
is 3.3V and the LMZ10503 output voltage was 1.8V, then the
value of R
trkb
needed to give the two supplies identical soft-start times would be 14.3 kΩ. A timing diagram for
this example, the equal soft-start time case, is shown below.
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