Datasheet

dVńdt +
V
OUT
C
OUT
80kW ø
(
R
1
) R
2
)
dVńdt +
V
OUT
C
OUT
80kW
TPS73201-Q1
TPS73225-Q1
SGLS303E –MAY 2005–REVISED AUGUST 2013 www.ti.com
Dropout Voltage
The TPS732xx 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 TPS732xx 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 TPS732xx can take a couple of
hundred microseconds to return to the specified regulation accuracy.
Transient Response
The low open-loop output impedance provided by the NMOS pass element in a voltage follower configuration
allows operation without an output capacitor for many applications. As with any regulator, the addition of a
capacitor (nominal value 1 μF) from the output pin to ground will reduce undershoot magnitude but increase
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 TPS732xx does not have active pulldown when the output is over-voltage. This allows applications 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 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)
(4)
(Adjustable voltage version)
(5)
Reverse Current
The NMOS pass element of the TPS732xx 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 due to 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 due to voltage applied on the OUT pin. There
will be additional current flowing into the OUT pin due to the 80-k Ω internal resistor divider to ground (see
Figure 2 and Figure 3).
For the TPS73201, reverse current may flow when V
FB
is more than 1 V above V
IN
.
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