Datasheet

t (s)=76,000xC
STR NR
(F)
TPS7A80xx
SBVS135H –JUNE 2010–REVISED JANUARY 2013 www.ti.com
The TPS7A80xx implements an innovative internal Dropout Voltage
compensation circuit that does not require a feedback
The TPS7A80xx uses a PMOS pass transistor to
capacitor across R
2
for stability. A feedback capacitor
achieve low dropout. When (V
IN
– V
OUT
) is less than
should not be used for this device.
the dropout voltage (V
DO
), the PMOS pass device is
in its linear region of operation and the input-to-output
Output Noise
resistance is the R
DS(ON)
of the PMOS pass element.
V
DO
scales approximately with output current
In most LDOs, the bandgap is the dominant noise
because the PMOS device in dropout behaves the
source. If a noise reduction capacitor (C
NR
) is used
same way as a resistor.
with the TPS7A80xx, the bandgap does not
contribute significantly to noise. Instead, noise is
As with any linear regulator, PSRR and transient
dominated by the output resistor divider and the error
response are degraded as (V
IN
– V
OUT
) approaches
amplifier input. To minimize noise in a given
dropout. This effect is shown in Figure 21 and
application, use a 0.01μF (minimum) noise-reduction
Figure 22 in the Typical Characteristics section.
capacitor.
Equation 2 approximates the total noise when Startup
C
NR
= 0.01μF:
Through a lower resistance, the bandgap reference
V
N
= 12.6 × V
OUT
+ 13.5 (µV
RMS
) (2)
can quickly charge the noise reduction capacitor
(C
NR
). The TPS7A80xx has a quick-start circuit to
Board Layout Recommendations to Improve
quickly charge C
NR
, if present; see the Functional
PSRR and Noise Performance
Block Diagrams. At startup, this quick-start switch is
closed, with only 33kΩ of resistance between the
To improve ac performance such as PSRR, output
bandgap reference and the NR pin. The quick-start
noise, and transient response, it is recommended that
switch opens approximately 2ms after any device
the board be designed with separate ground planes
enabling event, and the resistance between the
for V
IN
and V
OUT
, with each ground plane connected
bandgap reference and the NR pin becomes higher in
only at the GND pin of the device. In addition, the
value (approximately 250kΩ) to form a very good low-
ground connection for the bypass capacitor should
pass (RC) filter. This low-pass filter achieves very
connect directly to the GND pin of the device.
good noise reduction for the reference voltage.
Internal Current Limit
Inrush current can be a problem in many applications.
The 33kΩ resistance during the startup period is
The TPS7A80xx internal current limit helps protect
intentionally put there to slow down the reference
the regulator during fault conditions. During current
voltage ramp up, thus reducing the inrush current.
limit, the output sources a fixed amount of current
For example, the capacitance of connecting the
that is largely independent of output voltage. For
recommended C
NR
value of 0.01μF along with the
reliable operation, the device should not be operated
33kΩ resistance causes approximately 1ms RC
in a current limit state for extended periods of time.
delay. Startup time with the other C
NR
values can be
The PMOS pass element in the TPS7A80xx has a
calculated as:
built-in body diode that conducts current when the
(3)
voltage at OUT exceeds the voltage at IN. This
current is not limited, so if extended reverse voltage
Note that Equation 3 is valid up to t
STR
= 2ms or C
NR
operation is anticipated, external limiting may be
= 26nF, whichever is smaller.
appropriate.
Although the noise reduction effect is nearly saturated
at 0.01μF, connecting a C
NR
value greater than
Shutdown
0.01μF can help reduce noise slightly more; however,
The enable pin (EN) is active high and is compatible
startup time will be extremely long because the quick-
with standard and low voltage, TTL-CMOS levels.
start switch opens after approximately 2ms. That is, if
When shutdown capability is not required, EN can be
C
NR
is not fully charged during this 2ms period, C
NR
connected to IN.
finishes charging through a higher resistance of
250kΩ, and takes much longer to fully charge.
Note that a low leakage C
NR
should be used; most
ceramic capacitors are suitable.
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