Datasheet

TPS795xx
GNDEN NR
IN OUT
V
IN
V
OUT
1 Fm
0.01 Fm
1 Fm
TPS795xx
www.ti.com SLVS350H –OCTOBER 2002–REVISED AUGUST 2010
APPLICATION INFORMATION
order for the regulator to operate properly, the current
The TPS795xx family of low-dropout (LDO) regulators
flow out of the NR pin must be at a minimum,
has been optimized for use in noise-sensitive
because any leakage current creates an IR drop
equipment. The device features extremely low
across the internal resistor, thus creating an output
dropout voltages, high PSRR, ultralow output noise,
error. Therefore, the bypass capacitor must have
low quiescent current (265 mA typical), and an enable
minimal leakage current. The bypass capacitor
input to reduce supply currents to less than 1 mA
should be no more than 0.1-mF in order to ensure that
when the regulator is turned off.
it is fully charged during the quickstart time provided
by the internal switch shown in the Functional Block
A typical application circuit is shown in Figure 23.
Diagram.
For example, the TPS79530 exhibits only 33 mV
RMS
of output voltage noise using a 0.1-mF ceramic
bypass capacitor and a 10-mF 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-kΩ resistor and external capacitor.
Figure 23. Typical Application Circuit
BOARD LAYOUT RECOMMENDATION TO
IMPROVE PSRR AND NOISE PERFORMANCE
EXTERNAL CAPACITOR REQUIREMENTS
To improve ac measurements such as PSRR, output
Although not required, it is good analog design
noise, and transient response, it is recommended that
practice to place a 0.1mF to 2.2mF capacitor near the
the board be designed with separate ground planes
input of the regulator to counteract reactive input
for V
IN
and V
OUT
, with each ground plane connected
sources. A higher-value input capacitor may be
only at the ground pin of the device. In addition, the
necessary if large, fast-rise-time load transients are
ground connection for the bypass capacitor should
anticipated and the device is located several inches
connect directly to the ground pin of the device.
from the power source.
Like most low-dropout regulators, the TPS795xx
REGULATOR MOUNTING
requires an output capacitor connected between OUT
The tab of the SOT223-6 package is electrically
and GND to stabilize the internal control loop. The
connected to ground. For best thermal performance,
minimum recommended capacitor is 1 mF. Any 1 mF
the tab of the surface-mount version should be
or larger ceramic capacitor is suitable.
soldered directly to a circuit-board copper area.
The internal voltage reference is a key source of
Increasing the copper area improves heat dissipation.
noise in an LDO regulator. The TPS795xx has an NR
Solder pad footprint recommendations for the devices
pin which is connected to the voltage reference
are presented in application report SBFA015, Solder
through a 250-kΩ internal resistor. The 250-kΩ
Pad Recommendations for Surface-Mount Devices,
internal resistor, in conjunction with an external
available from the TI web site (www.ti.com).
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
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