Datasheet

R
trkb
=
0.8V
V
OUT2
- 0.8V
x R
trkt
VOLTAGE
TIME
RATIOMETRIC STARTUP
V
OUT2
EN
V
OUT1
R
trkb
=
R
trkt
V
OUT1
-1.0V
LMZ10503EXT
www.ti.com SNVS668G –JUNE 2010–REVISED OCTOBER 2013
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 LMZ10503EXT 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 LMZ10503EXT. This offset will cause the LMZ10503EXT 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 LMZ10503EXT 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.
Tracking - Equal Slew Rates
Alternatively, the tracking feature can be used to have similar output voltage ramp rates. This is referred to as
simultaneous startup. In this case, the tracking resistors can be determined based on the following equation
(23)
and to ensure proper overdrive of the SS pin
V
OUT2
< 0.8 x V
OUT1
(24)
For the example case of V
OUT1
= 5V and V
OUT2
= 2.5V, with R
trkt
set to 33 kΩ as before, R
trkb
is calculated from
the above equation to be 15.5 kΩ. A timing diagram for the case of equal slew rates is shown below.
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