Datasheet

Reference
SimplifiedBlock Diagram
BIAS
FB
IN
V
IN
V =3.3V 5%
BIAS
±
V =3.3V 5%
V =1.5V
I =1.5A
Efficiency=45%
IN
OUT
OUT
±
V
OUT
OUT
Reference
SimplifiedBlock Diagram
V
OUT
OUT
BIAS
FB
IN
V =5V 5%
BIAS
±
V =1.8V
V =1.5V
I =1.5A
Efficiency=83%
IN
OUT
OUT
t
SS
+
ǒ
V
REF
C
SS
Ǔ
I
SS
t
SSCL
+
ǒ
V
OUT
(
NOM
)
C
OUT
Ǔ
I
CL
(
MIN
)
TPS74401-EP
www.ti.com SBVS122B –MARCH 2010–REVISED SEPTEMBER 2010
DROPOUT VOLTAGE
The TPS74401 offers industry-leading dropout
performance, making it well-suited for high-current
low V
IN
/low V
OUT
applications. The extremely low
dropout of the TPS74401 allows the device to be
used in place of a DC/DC converter and still achieve
good efficiencies. This efficiency allows users to
rethink the power architecture for their applications to
achieve the smallest, simplest, and lowest cost
solution.
There are two different specifications for dropout
voltage with the TPS74401. The first specification
(see Figure 29) is referred to as V
IN
Dropout and is
for users that wish to apply an external bias voltage
to achieve low dropout. This specification assumes
that V
BIAS
is at least 1.62 V above V
OUT
, which is the
Figure 30. Typical Application of the TPS74401
case for V
BIAS
when powered by a 3.3-V rail with 5% Without an Auxiliary Bias
tolerance and with V
OUT
= 1.5V. If V
BIAS
is higher than
3.3 V × 0.95 or V
OUT
is less than 1.5 V, V
IN
dropout is
PROGRAMMABLE SOFT-START
less than specified.
The TPS74401 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 transients to the input
power bus.
To achieve a linear and monotonic soft-start, the
TPS74401 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
Figure 29. Typical Application of the TPS74401
reference voltage (V
REF
), and can be calculated using
Using an Auxiliary Bias Rail
Equation 1:
The second specification (see Figure 30) is referred
to as V
BIAS
Dropout and is for users that wish to tie IN
(1)
and BIAS together. This option allows the device to
If large output capacitors are used, the device current
be used in applications where an auxiliary bias
limit (I
CL
) and the output capacitor may set the
voltage is not available or low dropout is not required.
start-up time. In this case, the start-up time is given
Dropout is limited by BIAS in these applications
by Equation 2:
because V
BIAS
provides the gate drive to the pass
FET and therefore must be 1.62 V above V
OUT
.
Because of this usage, IN and BIAS tied together
(2)
easily consume huge power. Pay attention not to
exceed the power rating of the IC package.
V
OUT(NOM)
is the nominal set output voltage as set by
the user, 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 to
be greater than Equation 2.
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