Datasheet

IN OUT
ADJ
GND
TPS7A4501
R
2
V
IN
V
OUT
R
1
V = 1.21 V
OUT
1
R2
R1
)
(I
ADJ
)(R2)
V = 1.21 V
ADJ
I = 3 µA at 25°C
ADJ
Output range = 1.21 V to 20 V
+
+
)
TPS7A45xx
www.ti.com SLVS720D –JUNE 2008–REVISED AUGUST 2011
Adjustable Operation ceramic capacitors. Ceramic capacitors are
manufactured with a variety of dielectrics, each with
The adjustable version of the TPS7A45xx has an
different behavior over temperature and applied
output voltage range of 1.21 V to 20 V. The output
voltage. The most common dielectrics used are Z5U,
voltage is set by the ratio of two external resistors as
Y5V, X5R and X7R. The Z5U and Y5V dielectrics are
shown in Figure 33. The device maintains the voltage
good for providing high capacitances in a small
at the ADJ pin at 1.21 V referenced to ground. The
package, but exhibit strong voltage and temperature
current in R1 is then equal to (1.21 V/R1), and the
coefficients. When used with a 5-V regulator, a 10-μF
current in R2 is the current in R1 plus the ADJ pin
Y5V capacitor can exhibit an effective value as low as
bias current. The ADJ pin bias current, 3 μA at 25°C,
1 μF to 2 μF over the operating temperature range.
flows through R2 into the ADJ pin. The output voltage
The X5R and X7R dielectrics result in more stable
can be calculated using the formula shown in
characteristics and are more suitable for use as the
Figure 33. The value of R1 should be less than
output capacitor. The X7R type has better stability
4.17 kΩ to minimize errors in the output voltage
across temperature, while the X5R is less expensive
caused by the ADJ pin bias current. Note that in
and is available in higher values.
shutdown the output is turned off, and the divider
current is zero. Voltage and temperature coefficients are not the only
sources of problems. Some ceramic capacitors have
a piezoelectric response. A piezoelectric device
generates voltage across its terminals due to
mechanical stress, similar to the way a piezoelectric
accelerometer or microphone works. For a ceramic
capacitor the stress can be induced by vibrations in
the system or thermal transients.
Overload Recovery
Like many IC power regulators, the TPS7A45xx has
safe operating area protection. The safe area
protection decreases the current limit as
input-to-output voltage increases and keeps the
power transistor inside a safe operating region for all
values of input-to-output voltage. The protection is
Figure 33. Adjustable Operation
designed to provide some output current at all values
of input-to-output voltage up to the device
The adjustable device is tested and specified with the
breakdown.
ADJ pin tied to the OUT pin for an output voltage of
When power is first turned on, as the input voltage
1.21 V. Specifications for output voltages greater than
rises, the output follows the input, allowing the
1.21 V are proportional to the ratio of the desired
regulator to start up into very heavy loads. During
output voltage to 1.21 V: V
OUT
/1.21 V. For example,
start up, as the input voltage is rising, the
load regulation for an output current change of 1 mA
input-to-output voltage differential is small, allowing
to 1.5 A is –3 mV (typ) at V
OUT
= 1.21 V. At V
OUT
= 5
the regulator to supply large output currents. With a
V, load regulation is:
high input voltage, a problem can occur wherein
(5 V/1.21 V)(–3 mV) = –12.4 mV
removal of an output short does not allow the output
voltage to recover. Other regulators also exhibit this
Output Capacitance and Transient Response
phenomenon, so it is not unique to the TPS7A45xx.
The TPS7A45xx regulators are designed to be stable
The problem occurs with a heavy output load when
with a wide range of output capacitors. The ESR of
the input voltage is high and the output voltage is low.
the output capacitor affects stability, most notably
Common situations occur immediately after the
with small capacitors. A minimum output capacitor of
removal of a short circuit or when the shutdown pin is
10 μF with an ESR of 3 Ω or less is recommended to
pulled high after the input voltage has already been
prevent oscillations. Larger values of output
turned on. The load line for such a load may intersect
capacitance can decrease the peak deviations and
the output current curve at two points. If this happens,
provide improved transient response for larger load
there are two stable output operating points for the
current changes. Bypass capacitors, used to
regulator. With this double intersection, the input
decouple individual components powered by the
power supply may need to be cycled down to zero
TPS7A45xx, increase the effective output capacitor
and brought up again to make the output recover.
value.
Extra consideration must be given to the use of
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