TPS737XX Datasheet, PDF (11/18 Pages) Texas Instruments – 1A Low-Dropout Regulator with Reverse Current Protection

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BOARD LAYOUT RECOMMENDATION TO
IMPROVE PSRR AND NOISE
PERFORMANCE
To improve ac performance such as PSRR, output
noise, and transient response, it is recommended
that the printed circuit board (PCB) be designed with
separate ground planes for V_IN and V_OUT, with each
ground plane connected only at the GND pin of the
device. In addition, the ground connection for the
bypass capacitor should connect directly to the GND
pin of the device.
INTERNAL CURRENT LIMIT
The TPS737xx internal current limit helps protect the
regulator during fault conditions. Foldback helps to
protect the regulator from damage during output
short-circuit conditions by reducing current limit when
V_OUT drops below 0.5V.
SHUTDOWN
The Enable pin is active high and is compatible with
standard TTL-CMOS levels. V_EN below 0.5V (max)
turns the regulator off and drops the ground pin
current to approximately 10nA. When shutdown
capability is not required, the Enable pin can be
connected to V_IN. When a pull-up resistor is used,
and operation down to 1.8V is required, use pull-up
resistor values below 50kΩ.
DROPOUT VOLTAGE
The TPS737xx 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
TPS737xx requires a larger voltage drop from V_IN to
V_OUT to avoid degraded transient response. The
boundary of this transient dropout region is
approximately twice the dc dropout. Values of V_IN –
V_OUT above this line ensure normal transient
response.
Operating in the transient dropout region can cause
an increase in recovery time. The time required to
recover from a load transient is a function of the
magnitude of the change in load current rate, the
rate of change in load current, and the available
headroom (V_IN to V_OUT voltage drop). Under
worst-case conditions [full-scale instantaneous load
change with (V_IN – V_OUT) close to dc dropout levels],
the TPS737xx can take a couple of hundred
microseconds to return to the specified regulation
accuracy.
TPS737xx
SBVS067C – JANUARY 2006 – REVISED AUGUST 2006
TRANSIENT RESPONSE
The low open-loop output impedance provided by the
NMOS pass element in a voltage follower
configuration allows operation without a 1.0µF output
capacitor. As with any regulator, the addition of
additional capacitance from the output pin to ground
reduces undershoot magnitude but increases
duration. In the adjustable version, the addition of a
capacitor, C_FB, from the output to the adjust pin will
also improve the transient response.
The TPS737xx does not have active pull-down when
the output is over-voltage. This architecture allows
applications that connect higher voltage sources,
such as alternate power supplies, to the output. This
architecture also results in an output overshoot of
several percent if the load current quickly drops to
zero when a capacitor is connected to the output.
The duration of overshoot can be reduced by adding
a load resistor. The overshoot decays at a rate
determined by output capacitor C_OUT and the
internal/external load resistance. The rate of decay is
given by:
(Fixed voltage version)
dV ₊
dT C_OUT
V_OUT
80kW ø R_LOAD
(3)
(Adjustable voltage version)
dV
dT
+
C_OUT
V _OUT
80kW ø (R₁ ) R₂) ø R_LOAD
(4)
REVERSE CURRENT
The NMOS pass element of the TPS737xx provides
inherent protection against current flow from the
output of the regulator to the input when the gate of
the pass device is pulled low. To ensure that all
charge is removed from the gate of the pass
element, the enable pin must be driven low before
the input voltage is removed. If this is not done, the
pass element may be left on because of stored
charge on the gate.
After the enable pin is driven low, no bias voltage is
needed on any pin for reverse current blocking. Note
that reverse current is specified as the current
flowing out of the IN pin because of voltage applied
on the OUT pin. There will be additional current
flowing into the OUT pin as a result of the 80kΩ
internal resistor divider to ground (see Figure 1 and
Figure 2).
For the TPS73701, reverse current may flow when
V_FB is more than 1.0V above V_IN.
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