Datasheet

R
T1
=
0.8V
V
OUT2
- 0.8V
R
T2
VOLTAGE
TIME
RATIOMETRIC STARTUP
V
OUT2
EN
V
OUT1
R
T1
=
R
T2
V
OUT1
-1.0V
C
SS
=
t
SS
x 2 PA
0.8V
LM2854
www.ti.com SNVS560C –MARCH 2008–REVISED APRIL 2013
(5)
Thus, the required soft start capacitor per unit output voltage startup time is given by
C
SS
= 2.5 nF / ms (6)
For example, a 10 nF soft-start capacitor will yield a 4 ms soft-start time.
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 LM2854 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
T1
and R
T2
is given by
(7)
The above equation includes an offset voltage to ensure that the final value of the SS pin voltage exceeds the
reference voltage of the LM2854. This offset will cause the LM2854 output voltage to reach regulation slightly
before the master supply. A value of 33 kΩ 1% is recommended for R
T2
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 LM2854 output voltage was 1.8V, then the value
of R
T1
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
(8)
and to ensure proper overdrive of the SS pin
V
OUT2
< 0.8 V
OUT1
(9)
For the example case of V
OUT1
= 5V and V
OUT2
= 2.5V, with RT2 set to 33 kΩ as before, R
T1
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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