Datasheet

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APPLICATION INFORMATION
GNDEN NR
IN OUT
V
IN
V
OUT
0.01
µ
F
2.2
µ
F
1 µF
TPS794xx
BOARD LAYOUT RECOMMENDATION TO
EXTERNAL CAPACITOR REQUIREMENTS
REGULATOR MOUNTING
TPS794xx
SLVS349E – NOVEMBER 2001 – REVISED DECEMBER 2005
order for the regulator to operate properly, the
The TPS794xx family of low-dropout (LDO)
current flow out of the NR pin must be at a minimum,
regulators has been optimized for use in
because any leakage current creates an IR drop
noise-sensitive equipment. The device features
across the internal resistor, thus creating an output
extremely low dropout voltages, high PSRR, ultralow
error. Therefore, the bypass capacitor must have
output noise, low quiescent current (265 µ A
minimal leakage current. The bypass capacitor
typically), and an enable input to reduce supply
should be no more than 0.1- µ F in order to ensure
currents to less than
that it is fully charged during the quickstart time
1 µ A when the regulator is turned off.
provided by the internal switch shown in the
Functional Block Diagram .
A typical application circuit is shown in Figure 22 .
For example, the TPS79430 exhibits only 33 µ V
RMS
of output voltage noise using a 0.1- µ F ceramic
bypass capacitor and a 10- µ F ceramic output
capacitor. Note that the output starts up slower as
the bypass capacitance increases because of the RC
time constant at the bypass pin that is created by the
internal 250-k Ω resistor and external capacitor.
IMPROVE PSRR AND NOISE
Figure 22. Typical Application Circuit
PERFORMANCE
To improve ac measurements such as PSRR, output
noise, and transient response, it is recommended
that the board be designed with separate ground
A 1- µ F or larger ceramic input bypass capacitor,
planes for V
IN
and V
OUT
, with each ground plane
connected between IN and GND and located close
connected only at the ground pin of the device. In
to the TPS794xx, is required for stability and
addition, the ground connection for the bypass
improves transient response, noise rejection, and
capacitor should connect directly to the ground pin of
ripple rejection. A higher-value input capacitor may
the device.
be necessary if large, fast-rise-time load transients
are anticipated and the device is located several
inches from the power source.
The tab of the SOT223-6 package is electrically
Like most low-dropout regulators, the TPS794xx
connected to ground. For best thermal performance,
requires an output capacitor connected between
the tab of the surface-mount version should be
OUT and GND to stabilize the internal control loop.
soldered directly to a circuit-board copper area.
The minimum recommended capacitance is 1 µ F.
Increasing the copper area improves heat
Any
dissipation.
1 µ F or larger ceramic capacitor is suitable.
Solder pad footprint recommendations for the
The internal voltage reference is a key source of
devices are presented in Application Report
noise in an LDO regulator. The TPS794xx has an
SBFA015 , Solder Pad Recommendations for
NR pin which is connected to the voltage reference
Surface-Mount Devices, available from the TI web
through a 250-k Ω internal resistor. The 250-k Ω
site (www.ti.com ).
internal resistor, in conjunction with an external
bypass capacitor connected to the NR pin, creates a
low-pass filter to reduce the voltage reference noise
and, therefore, the noise at the regulator output. In
8
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