Datasheet

V
OUT
C
OUT
10 Fm
TPS74901
GND
EN
FB
IN PG
BIAS
SS
OUT
V
IN
R
1
R
2
R
3
C
IN
1 Fm
C
SS
V
BIAS
C
BIAS
1 Fm
V =0.8
OUT
´ 1+
R
1
R
2
)(
TPS749xx
www.ti.com SBVS082G –JUNE 2007–REVISED NOVEMBER 2010
APPLICATION INFORMATION
The TPS749xx belongs to a family of low dropout
INPUT, OUTPUT, AND BIAS CAPACITOR
regulators that feature soft-start capabilities. These
REQUIREMENTS
regulators use a low current bias input to power all
The device is designed to be stable for all available
internal control circuitry, allowing the NMOS pass
types of and values of output capacitors ≥ 2.2mF. The
transistor to regulate very low input and output
device is also stable with multiple capacitors in
voltages.
parallel, which can be of any type or value.
The use of an NMOS-pass FET offers several critical
The capacitance required on the IN and BIAS pin
advantages for many applications. Unlike a PMOS
strongly depends on the input supply source
topology device, the output capacitor has little effect
impedance. To counteract any inductance in the
on loop stability. This architecture allows the
input, the minimum recommended capacitor for V
INTPS749xx to be stable with any capacitor type of
and V
BIAS
is 1mF. If V
IN
and V
BIAS
are connected to
value 2.2mF or greater. Transient response is also
the same supply, the recommended minimum
superior to PMOS topologies, particularly for low V
IN
capacitor for V
BIAS
is 4.7mF. Good quality, low ESR
applications.
capacitors should be used on the input; ceramic X5R
The TPS749xx features a programmable
and X7R capacitors are preferred. These capacitors
voltage-controlled soft-start circuit that provides a
should be placed as close the pins as possible for
smooth, monotonic start-up and limits startup inrush
optimum performance.
currents that may be caused by large capacitive
loads. A power-good (PG) output is available to allow
TRANSIENT RESPONSE
supply monitoring and sequencing of other supplies.
The TPS749xx is designed to have excellent transient
An enable (EN) pin with hysteresis and deglitch
response for most applications with a small amount of
allows slow-ramping signals to be used for
output capacitance. In some cases, the transient
sequencing the device. The low V
IN
and V
OUT
response may be limited by the transient response of
capability allows for inexpensive, easy-to-design, and
the input supply. This limitation is especially true in
efficient linear regulation between the multiple supply
applications where the difference between the input
voltages often present in processor intensive
and output is less than 300mV. In this case, adding
systems.
additional input capacitance improves the transient
Figure 25 illustrates the typical application circuit for
response much more than just adding additional
the TPS749xx adjustable output device.
output capacitance would do. With a solid input
supply, adding additional output capacitance reduces
R
1
and R
2
can be calculated for any output voltage
undershoot and overshoot during a transient event;
using the formula shown in Figure 25. Refer to
refer to Figure 22 in the Typical Characteristics
Table 1 for sample resistor values of common output
section. Because the TPS749xx is stable with output
voltages. In order to achieve the maximum accuracy
capacitors as low as 2.2mF, many applications may
specifications, R
2
should be ≤ 4.99kΩ.
need very little capacitance at the LDO output. For
these applications, local bypass capacitance for the
powered device may be sufficient to meet the
transient requirements of the application. This design
reduces the total solution cost by avoiding the need
to use expensive high-value capacitors at the LDO
output.
Figure 25. Typical Application Circuit for the
TPS749xx (Adjustable)
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