Datasheet

Reference
SimplifiedBlock Diagram
V
OUT
OUT
BIAS
FB
IN
V
IN
V =3.3V 5%
BIAS
±
V =3.3V 5V
V =1.5V
I =1.5A
Efficiency=45%
IN
OUT
OUT
±
C
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
C
OUT
t =
SS
(V xC )
REF SS
I
SS
t =
SSCL
(V xC )
OUT(NOM) OUT
I
CL(MIN)
TPS749xx
SBVS082G –JUNE 2007–REVISED NOVEMBER 2010 www.ti.com
DROPOUT VOLTAGE
The TPS749xx offers very low dropout performance,
making it well-suited for high-current low V
IN
/low V
OUT
applications. The low dropout of the TPS749xx allows
the device to be used in place of a DC/DC converter
and still achieve good efficiencies. This provides
designers with 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 TPS749xx. The first specification
(see Figure 26) is referred to as V
IN
Dropout and is
used when an external bias voltage is applied to
achieve low dropout. This specification assumes that
V
BIAS
is at least 3.25V
(1)
above V
OUT
, which is the
Figure 27. Typical Application of the TPS749xx
case for V
BIAS
when powered by a 5.0V rail with 5%
Without an Auxiliary Bias
tolerance and with V
OUT
= 1.5V. If V
BIAS
is
higher than V
OUT
+ 3.25V, V
IN
dropout is less than
specified
(1)
.
PROGRAMMABLE SOFT-START
The TPS749xx 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
TPS749xx error amplifier tracks the voltage ramp of
the external soft-start capacitor until the voltage
exceeds the internal reference. The soft-start ramp
time is dependent on the soft-start charging current
Figure 26. Typical Application of the TPS749xx
(I
SS
), soft-start capacitance (C
SS
), and the internal
Using an Auxiliary Bias Rail
reference voltage (V
REF
), and can be calculated using
Equation 1:
The second specification (shown in Figure 27) is
referred to as V
BIAS
Dropout and applied to
applications where IN and BIAS are tied together.
(1)
This option allows the device to be used in
If large output capacitors are used, the device current
applications where an auxiliary bias voltage is not
limit (I
CL
) and the output capacitor may set the
available or low dropout is not required. Dropout is
start-up time. In this case, the start-up time is given
limited by BIAS in these applications because V
BIAS
by Equation 2:
provides the gate drive to the pass FET; therefore,
V
BIAS
must be 1.75V above V
OUT
. Dropout is limited
by BIAS in these applications because V
BIAS
provides
(2)
the gate drive to the pass FET; therefore, V
BIAS
must
be 1.75V above V
OUT
. Because of this usage, IN and
where:
BIAS tied together easily consume huge power. Pay
V
OUT(NOM)
is the nominal set output voltage,
attention not to exceed the power rating of the IC
package.
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.
(1) 3.25V is a test condition of this device and can be adjusted by
referring to Figure 8 .
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