Datasheet

t =
SSCL
(V C )´
OUT(NOM) OUT
I
CL(MIN)
t =
SS
(V C )´
REF SS
I
SS
TPS74801-Q1
www.ti.com SLVSAI4B –OCTOBER 2010–REVISED JULY 2013
PROGRAMMABLE SOFT-START
The TPS74801-Q1 features a programmable, monotonic, voltage-controlled soft-start that is set with an external
capacitor (C
SS
). This feature is important for many applications because it eliminates power-up initialization
problems when powering FPGAs, DSPs, or other processors. The controlled voltage ramp of the output also
reduces peak inrush current during start-up, minimizing start-up transient events to the input power bus.
To achieve a linear and monotonic soft-start, the TPS74801-Q1 error amplifier tracks the voltage ramp of the
external soft-start capacitor until the voltage exceeds the internal reference. The soft-start ramp time depends on
the soft-start charging current (I
SS
), soft-start capacitance (C
SS
), and the internal reference voltage (V
REF
), and
can be calculated using Equation 1:
(1)
If large output capacitors are used, the device current limit (I
CL
) and the output capacitor may set the start-up
time. In this case, the start-up time is given by Equation 2:
(2)
where:
V
OUT(NOM)
is the nominal output voltage,
C
OUT
is the output capacitance, and
I
CL(MIN)
is the minimum current limit for the device.
In applications where monotonic startup is required, the soft-start time given by Equation 1 should be set greater
than Equation 2.
The maximum recommended soft-start capacitor is 0.015 μF. Larger soft-start capacitors can be used and do not
damage the device; however, the soft-start capacitor discharge circuit may not be able to fully discharge the soft-
start capacitor when enabled. Soft-start capacitors larger than 0.015 μF could be a problem in applications where
it is necessary to rapidly pulse the enable pin and still require the device to soft-start from ground. C
SS
must be
low-leakage; X7R, X5R, or C0G dielectric materials are preferred. Refer to Table 2 for suggested soft-start
capacitor values.
SEQUENCING REQUIREMENTS
V
IN
, V
BIAS
, and V
EN
can be sequenced in any order without causing damage to the device. However, for the soft-
start function to work as intended, certain sequencing rules must be applied. Connecting EN to IN is acceptable
for most applications, as long as V
IN
is greater than 1.1 V and the ramp rate of V
IN
and V
BIAS
is faster than the
set soft-start ramp rate. If the ramp rate of the input sources is slower than the set soft-start time, the output
tracks the slower supply minus the dropout voltage until it reaches the set output voltage. If EN is connected to
BIAS, the device soft-starts as programmed, provided that V
IN
is present before V
BIAS
. If V
BIAS
and V
EN
are
present before V
IN
is applied and the set soft-start time has expired, then V
OUT
tracks V
IN
. If the soft-start time
has not expired, the output tracks V
IN
until V
OUT
reaches the value set by the charging soft-start capacitor.
Figure 28 shows the use of an RC-delay circuit to hold off V
EN
until V
BIAS
has ramped. This technique can also
be used to drive EN from V
IN
. An external control signal can also be used to enable the device after V
IN
and
V
BIAS
are present.
NOTE
When V
BIAS
and V
EN
are present and V
IN
is not supplied, this device outputs approximately
50 μA of current from OUT. Although this condition does not cause any damage to the
device, the output current may charge up the OUT node if total resistance between OUT
and GND (including external feedback resistors) is greater than 10 kΩ.
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