Datasheet

V =xV
N OUT
10.5 Vm
RMS
V
TPS79901-Q1, TPS79912-Q1, TPS79915-Q1
TPS79918-Q1, TPS79925-Q1, TPS79927-Q1, TPS79933-Q1
SBVS097E –MARCH 2008–REVISED JANUARY 2012 www.ti.com
Noise can be referred to the feedback point (FB pin) such that with C
NR
= 0.01 μF, total noise is approximately
given by Equation 1:
(1)
The TPS79901 adjustable version does not have the noise-reduction pin available, so ultra-low noise operation is
not possible. Noise can be minimized according to the previous recommendations.
Board Layout Recommendations to Improve PSRR and Noise Performance
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 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 TPS799xx internal current limit helps protect the regulator during fault conditions. During current limit, the
output sources a fixed amount of current that is largely independent of output voltage. For reliable operation, the
device should not be operated in current limit for extended periods of time.
The PMOS pass element in the TPS799xx has a built-in body diode that conducts current when the voltage at
OUT exceeds the voltage at IN. This current is not limited, so if extended reverse voltage operation is
anticipated, external limiting may be appropriate.
Shutdown
The enable pin (EN) is active high and is compatible with standard and low voltage TTL-CMOS levels. When
shutdown capability is not required, EN can be connected to IN.
Dropout Voltage
The TPS799xx uses a PMOS pass transistor to achieve low dropout. When (V
IN
– V
OUT
) is less than the dropout
voltage (V
DO
), the PMOS pass device is in its linear region of operation and the input-to-output resistance is the
R
DS, ON
of the PMOS pass element. Because the PMOS device behaves like a resistor in dropout, V
DO
scales
approximately with output current.
As with any linear regulator, PSRR and transient response are degraded as (V
IN
– V
OUT
) approaches dropout.
This effect is shown in Figure 9 through Figure 11 in the Typical Characteristics section.
Startup
Fixed voltage versions of the TPS799xx use a quick-start circuit to fast-charge the noise reduction capacitor,
C
NR
, if present (see Functional Block Diagrams, Figure 1). This allows the combination of very low output noise
and fast start-up times. The NR pin is high impedance so a low leakage C
NR
capacitor must be used; most
ceramic capacitors are appropriate in this configuration.
Note that for fastest startup, V
IN
should be applied first, then the enable pin (EN) driven high. If EN is tied to IN,
startup is somewhat slower. The quick-start switch is closed for approximately 135 μs. To ensure that C
NR
is fully
charged during the quick-start time, a 0.01 μF or smaller capacitor should be used.
Transient Response
As with any regulator, increasing the size of the output capacitor reduces over/undershoot magnitude but
increase duration of the transient response. In the adjustable version, adding C
FB
between OUT and FB
improves stability and transient response. The transient response of the TPS799xx is enhanced by an active
pulldown that engages when the output overshoots by approximately 5% or more when the device is enabled.
When enabled, the pulldown device behaves like a 350-Ω resistor to ground.
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Product Folder Links: TPS79901-Q1 TPS79912-Q1 TPS79915-Q1 TPS79918-Q1 TPS79925-Q1 TPS79927-Q1
TPS79933-Q1