Datasheet

Board Layout Recommendation to Improve
Transient Response
Internal Current Limit
Shutdown
dVńdt +
V
OUT
C
OUT
80 kW ø R
LOAD
(4)
Dropout Voltage
dVńdt +
V
OUT
C
OUT
80 kW ø
(
R
1
) R
2
)
ø R
LOAD
(5)
TPS73601-EP , TPS73615-EP , TPS73618-EP
TPS73625-EP , TPS73630-EP , TPS73632-EP , TPS73633-EP
www.ti.com .................................................................................................................................................. SGLS326C – APRIL 2006 – REVISED FEBRUARY 2009
The TPS73601 adjustable version does not have the avoid degraded transient response. The boundary of
noise-reduction pin available. However, connecting a this transient dropout region is approximately twice
feedback capacitor, C
FB
, from the output to the FB pin the dc dropout. Values of
reduces output noise and improves load transient V
IN
– V
OUT
above this line ensure normal transient
performance. response.
The TPS736xx uses an internal charge pump to Operating in the transient dropout region can cause
develop an internal supply voltage sufficient to drive an increase in recovery time. The time required to
the gate of the NMOS pass element above V
OUT
. The recover from a load transient is a function of the
charge pump generates ~250 µ V of switching noise magnitude of the change in load current rate, the rate
at ~4 MHz; however, charge-pump noise contribution of change in load current, and the available
is negligible at the output of the regulator for most headroom (V
IN
to V
OUT
voltage drop). Under
values of I
OUT
and C
OUT
. worst-case conditions [full-scale instantaneous load
change with (V
IN
– V
OUT
) close to dc dropout levels],
the TPS736xx can take a couple of hundred
microseconds to return to the specified regulation
PSRR and Noise Performance
accuracy.
To improve ac performance such as PSRR, output
noise, and transient response, it is recommended that
the board be designed with separate ground planes
for V
IN
and V
OUT
, with each ground plane connected The low open-loop output impedance provided by the
only at the GND pin of the device. In addition, the NMOS pass element in a voltage-follower
ground connection for the bypass capacitor should configuration allows operation without an output
connect directly to the GND pin of the device. capacitor for many applications. As with any
regulator, the addition of a capacitor (nominal value
1 µ F) from the output pin to ground reduces
undershoot magnitude but increases duration. In the
The TPS736xx internal current limit helps protect the
adjustable version, the addition of a capacitor, C
FB
,
regulator during fault conditions. Foldback helps to
from the output to the adjust pin also improves the
protect the regulator from damage during output
transient response.
short-circuit conditions by reducing current limit when
The TPS736xx does not have active pulldown when
V
OUT
drops below 0.5 V. See Figure 12 for a graph of
the output is overvoltage. This allows applications
I
OUT
vs V
OUT
.
that connect higher voltage sources, such as
alternate power supplies, to the output. This also
results in an output overshoot of several percent if
The enable (EN) pin is active high and is compatible
load current quickly drops to zero when a capacitor is
with standard TTL-CMOS levels. V
EN
below 0.5 V
connected to the output. The duration of overshoot
(max) turns the regulator off and drops the ground-pin
can be reduced by adding a load resistor. The
current to approximately 10 nA. When shutdown
overshoot decays at a rate determined by output
capability is not required, EN can be connected to
capacitor C
OUT
and the internal/external load
V
IN
. When a pullup resistor is used, and operation
resistance. The rate of decay is given by:
down to 1.8 V is required, use pullup resistor values
Fixed-voltage version:
below 50 k Ω .
Adjustable-voltage version:
The TPS736xx uses an NMOS pass transistor to
achieve extremely low dropout. When (V
IN
– V
OUT
) is
less than the dropout voltage (V
DO
), the NMOS pass
device is in its linear region of operation and the
input-to-output resistance is the R
DS-ON
of the NMOS
pass element.
For large step changes in load current, the TPS736xx
requires a larger voltage drop from V
IN
to V
OUT
to
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Product Folder Link(s): TPS73601-EP TPS73615-EP TPS73618-EP TPS73625-EP TPS73630-EP TPS73632-EP
TPS73633-EP