Datasheet

V
IN
V
OUT
2.2 Fm
0.01 Fm
2.2 Fm
IN
EN
GND
OUT
NR
TPS786xx
TPS786xx
SLVS389L –SEPTEMBER 2002–REVISED OCTOBER 2010 www.ti.com
APPLICATION INFORMATION
order for the regulator to operate properly, the current
The TPS786xx 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, typically), and 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 to ensure that it is
when the regulator is turned off.
fully charged during the quickstart time provided by
the internal switch shown in the functional block
A typical application circuit is shown in Figure 24.
diagram.
For example, the TPS78630 exhibits only 48 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 due to the RC time
constant at the bypass pin that is created by the
internal 250-kΩ resistor and external capacitor.
Figure 24. Typical Application Circuit
BOARD LAYOUT RECOMMENDATIONS TO
IMPROVE PSRR AND NOISE PERFORMANCE
EXTERNAL CAPACITOR REQUIREMENTS
To improve ac measurements like PSRR, output
A 2.2-mF or larger ceramic input bypass capacitor,
noise, and transient response, it is recommended that
connected between IN and GND and located close to
the board be designed with separate ground planes
the TPS786xx, is required for stability and improves
for V
IN
and V
OUT
, with each ground plane connected
transient response, noise rejection, and ripple
only at the ground pin of the device. In addition, the
rejection. A higher-value input capacitor may be
ground connection for the bypass capacitor should
necessary if large, fast-rise-time load transients are
connect directly to the ground pin of the device.
anticipated and the device is located several inches
from the power source.
REGULATOR MOUNTING
Like most low-dropout regulators, the TPS786xx
The tab of the SOT223-6 package is electrically
requires an output capacitor connected between OUT
connected to ground. For best thermal performance,
and GND to stabilize the internal control loop. The
the tab of the surface-mount version should be
minimum recommended capacitor is 1 mF. Any 1 mF
soldered directly to a circuit-board copper area.
or larger ceramic capacitor is suitable.
Increasing the copper area improves heat dissipation.
The internal voltage reference is a key source of
Solder pad footprint recommendations for the devices
noise in an LDO regulator. The TPS786xx has an NR
are presented in Application Report SBFA015, Solder
pin which is connected to the voltage reference
Pad Recommendations for Surface-Mount Devices,
through a 250-kΩ internal resistor. The 250-kΩ
available from the TI web site at 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
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