Datasheet

V
OUT
C
OUT
10 Fm
TPS74801
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
)(
TPS74801
www.ti.com SBVS074J –JANUARY 2007– REVISED JANUARY 2012
APPLICATION INFORMATION
The TPS74801 belongs to a family of low dropout
R
1
and R
2
can be calculated for any output voltage
regulators that feature soft-start capability. These
using the formula shown in Figure 25. Refer to
regulators use a low current bias input to power all
Table 1 for sample resistor values of common output
internal control circuitry, allowing the NMOS pass
voltages. In order to achieve the maximum accuracy
transistor to regulate very low input and output
specifications, R
2
should be ≤ 4.99kΩ.
voltages.
The use of an NMOS-pass FET offers several critical
INPUT, OUTPUT, AND BIAS CAPACITOR
advantages for many applications. Unlike a PMOS
REQUIREMENTS
topology device, the output capacitor has little effect
The device is designed to be stable for all available
on loop stability. This architecture allows the
types and values of output capacitors ≥ 2.2μF. The
TPS74801 to be stable with any capacitor type of
device is also stable with multiple capacitors in
value 2.2μF or greater. Transient response is also
parallel, which can be of any type or value.
superior to PMOS topologies, particularly for low V
IN
applications.
The capacitance required on the IN and BIAS pins
strongly depends on the input supply source
The TPS74801 features a programmable
impedance. To counteract any inductance in the
voltage-controlled soft-start circuit that provides a
input, the minimum recommended capacitor for V
IN
smooth, monotonic start-up and limits startup inrush
and V
BIAS
is 1μF. If V
IN
and V
BIAS
are connected to
currents that may be caused by large capacitive
the same supply, the recommended minimum
loads. A power good (PG) output is available to allow
capacitor for V
BIAS
is 4.7μF. Good quality, low ESR
supply monitoring and sequencing of other supplies.
capacitors should be used on the input; ceramic X5R
An enable (EN) pin with hysteresis and deglitch
and X7R capacitors are preferred. These capacitors
allows slow-ramping signals to be used for
should be placed as close the pins as possible for
sequencing the device. The low V
IN
and V
OUT
optimum performance.
capability allows for inexpensive, easy-to-design, and
efficient linear regulation between the multiple supply
voltages often present in processor-intensive TRANSIENT RESPONSE
systems.
The TPS74801 was designed to have excellent
Figure 25 illustrates the typical application circuit for transient response for most applications with a small
the TPS74801 adjustable output device. amount of output capacitance. In some cases, the
transient response may be limited by the transient
response of the input supply. This limitation is
especially true in applications where the difference
between the input and output is less than 300mV. In
this case, adding additional input capacitance
improves the transient response much more than just
adding additional output capacitance would do. With
a solid input supply, adding additional output
capacitance reduces undershoot and overshoot
during a transient event; refer to Figure 22 in the
Typical Characteristics section. Because the
TPS74801 is stable with output capacitors as low as
2.2μF, many applications may then need very little
Figure 25. Typical Application Circuit for the
capacitance at the LDO output. For these
TPS74801 (Adjustable)
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.
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