Datasheet

t
SSCL
+
ǒ
V
OUT
(
NOM
)
C
OUT
Ǔ
I
CL
(
MIN
)
t
SS
+
ǒ
V
REF
C
SS
Ǔ
I
SS
TPS74401
GND
SS
OUT
FB
EN
IN
BIAS
V
IN
V
OUT
R
2
R
1
C
SS
C
IN
1 Fm
C
V
BIAS
C
BIAS
1 Fm
R
TPS744xx
www.ti.com SBVS066O –DECEMBER 2005–REVISED MARCH 2013
PROGRAMMABLE SOFT-START SEQUENCING REQUIREMENTS
The TPS744xx features a programmable, monotonic, The device can have V
IN
, V
BIAS
, and V
EN
sequenced
voltage-controlled soft-start that is set with an in any order without causing damage to the device.
external capacitor (C
SS
). This feature is important for However, for the soft-start function to work as
many applications because it eliminates power-up intended, certain sequencing rules must be applied.
initialization problems when powering FPGAs, DSPs, Enabling the device after V
IN
and V
BIAS
are present is
or other processors. The controlled voltage ramp of preferred, and can be accomplished using a digital
the output also reduces peak inrush current during output from a processor or supply supervisor. An
start-up, minimizing start-up transients to the input analog signal from an external RC circuit, as shown
power bus. in Figure 34, can also be used as long as the delay
time is long enough for V
IN
and V
BIAS
to be present.
To achieve a linear and monotonic soft-start, the
TPS744xx 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
),
the soft-start capacitance (C
SS
), and the internal
reference voltage (V
REF
), and can be calculated using
Equation 1:
(1)
Figure 34. Soft-Start Delay Using an RC Circuit
If large output capacitors are used, the device current
on Enable
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:
If a signal is not available to enable the device after
IN and BIAS, simply connecting EN to IN is
acceptable for most applications as long as V
IN
is
greater than 1.1V and the ramp rate of V
IN
and V
BIAS
(2)
is faster the set soft-start ramp rate. If the ramp rate
V
OUT(NOM)
is the nominal set output voltage as set by
of the input sources is slower than the set soft-start
the user, C
OUT
is the output capacitance, and I
CL(MIN)
time, the output will track the slower supply minus the
is the minimum current limit for the device. In
dropout voltage until it reaches the set output voltage.
applications where monotonic startup is required, the
If EN is connected to BIAS, the device will soft-start
soft-start time given by Equation 1 should be set to
as programmed provided that V
IN
is present before
be greater than Equation 2.
V
BIAS
. If V
BIAS
and V
EN
are present before V
IN
is
applied and the set soft-start time has expired then
The maximum recommended soft-start capacitor is
V
OUT
will track V
IN
.
0.015μF. Larger soft-start capacitors can be used and
will not damage the device; however, the soft-start
NOTE: When V
BIAS
and V
EN
are present and V
IN
is
capacitor discharge circuit may not be able to fully
not supplied, this device outputs approximately 50μA
discharge the soft-start capacitor when re-enabled.
of current from OUT. Although this condition will not
Soft-start capacitors larger than 0.015μF could be a
cause any damage to the device, the output current
problem in applications where the user needs to
may charge up the OUT node if total resistance
rapidly pulse the enable pin and still requires the
between OUT and GND (including external feedback
device to soft-start from ground. C
SS
must be low-
resistors) is greater than 10kΩ.
leakage; X7R, X5R, or C0G dielectric materials are
preferred. Refer to Table 2 for suggested soft-start
capacitor values.
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