Datasheet

I (mA)=C ( F) 0.07(V/ s)+I (mA)m m
LOAD
SOFTSTART OUT
´
TPS727xx
SBVS128C –JUNE 2009–REVISED JANUARY 2011 www.ti.com
APPLICATION INFORMATION
I
LIMIT
× R
LOAD
. The PMOS pass transistor dissipates
The TPS727xx family belongs to a family of new
(V
IN
– V
OUT
) × I
LIMIT
until thermal shutdown is
generation LDO regulators that consume extremely
triggered and the device is turned off. As the device
low quiescent current while simulatenously delivering
cools down, it is turned on by the internal thermal
excellent PSRR with very little headroom (V
IN
– V
OUT
shutdown circuit. If the fault condition continues, the
differential voltage), and very good transient
device cycles between current limit and thermal
response. These features, combined with low noise
shutdown. See the Thermal Information section for
without a noise reduction pin in an ultrasmall
more details.
package, make this device ideal for portable
applications. This family of regulators offers The PMOS pass element in the TPS727xx has a
sub-bandgap output voltages, current limit and built-in body diode that conducts current when the
thermal protection, and is fully specified from –40°C voltage at the OUT pin exceeds the voltage at the IN
to +125°C. pin. This current is not limited, so if extended reverse
voltage operation is anticipated, external limiting to
5% of rated output current is recommended.
INPUT AND OUTPUT CAPACITOR
REQUIREMENTS
SOFT START
Although an input capacitor is not required for
stability, it is good analog design practice to connect The startup current is given by Equation 1:
a 0.1mF to 1.0mF low equivalent series resistance
(1)
(ESR) capacitor across the IN pin and GND input of
the regulator. This capacitor counteracts reactive
This equation shows that soft-start current is directly
input sources and improves transient response, noise
proportional to C
OUT
.
rejection, and ripple rejection. A higher-value
The output voltage ramp rate is independent of C
OUT
capacitor may be necessary if large, fast rise-time
and load current, and has a typical value of 0.07V/ms.
load transients are anticipated, or if the device is not
located close to the power source. If source
The TPS727xx automatically adjusts the soft-start
impedance is not sufficiently low, a 0.1mF input
current to supply both the load current and the C
OUT
capacitor may be necessary to ensure stability.
charge current. For example, if I
LOAD
= 0mA upon
enabling the LDO, I
SOFT START
= 1mF × 0.07 V/ms +
The TPS727xx is designed to be stable with standard
0mA = 70mA, the current that charges the output
ceramic capacitors with values of 1.0mF or larger at
capacitor.
the output. X5R- and X7R-type capacitors are best
because they have minimal variation in value and
If I
LOAD
= 200mA, I
SOFT START
= 1mF × 0.07V/ms +
ESR over temperature. Maximum ESR should be less
200mA = 270mA, the current required for charging
than 200mΩ.
output capacitor and supplying the load current.
If the output capacitor and load are increased such
BOARD LAYOUT RECOMMENDATIONS TO
that the soft-start current exceeds the output current
IMPROVE PSRR AND NOISE PERFORMANCE
limit, it is clamped at the typical current limit of
To improve ac performance such as PSRR, output
400mA. For example, if C
OUT
= 10mF and I
OUT
=
noise, and transient response, it is recommended that
200mA, 10mF × 0.07V/ms + 200mA = 900mA is not
the board be designed with separate ground planes
supplied. Instead, it is clamped at 400mA.
for V
IN
and V
OUT
, with the ground plane connected
only at the GND pin of the device. In addition, the
SHUTDOWN
ground connection for the output capacitor should
The enable pin (EN) is active high and is compatible
connect directly to the GND pin of the device. High
with standard and low voltage, TTL-CMOS levels.
ESR capacitors may degrade PSRR.
When shutdown capability is not required, EN can be
connected to the IN pin.
INTERNAL CURRENT LIMIT
The TPS727xx internal current limit helps protect the
regulator during fault conditions. During current limit,
the output sources a fixed amount of current that is
largely independent of output voltage. In such a case,
the output voltage is not regulated, and is V
OUT
=
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