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參數(shù)資料
| 型號： | TPS79285 |
| 廠商： | Texas Instruments, Inc. |
| 英文描述： | 200mA Low Quiescent Current, Ultra-Low Noise, High PSRR, Low Dropout Linear Regulator |
| 中文描述： | 200mA低靜態(tài)電流，超低噪聲，高PSRR，低壓差線性穩(wěn)壓 |
| 文件頁數(shù)： | 12/18頁 |
| 文件大?。?/td> | 377K |
| 代理商： | TPS79285 |
www.ti.com
Output Noise
In most LDOs, the bandgap is the dominant noise source. If a noise reduction capacitor (C
NR
) is used with the
TPS799xx, the bandgap does not contribute significantly to noise. Instead, noise is dominated by the output
resistor divider and the error amplifier input. To minimize noise in a given application, use a 0.01μF noise
reduction capacitor; for the adjustable version, smaller value resistors in the output resistor divider reduce noise.
A parallel combination that gives 2μA of divider current will have the same noise performance as a fixed voltage
version. To further optimize noise, equivalent series resistance of the output capacitor can be set to
approximately 0.2
. This configuration maximizes phase margin in the control loop, reducing total output noise
by up to 10%.
V
N
10.7 V
RMS
V
V
OUT
(1)
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 will source 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.
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
will
approximately scale with output current.
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.
TPS79901, TPS79915, TPS79916, TPS79918
TPS79925, TPS79927, TPS79928, TPS799285
TPS79929, TPS79930, TPS79932, TPS79933
SBVS056C–JANUARY 2005–REVISED MAY 2005
Noise can be referred to the feedback point (FB pin) such that with C
NR
= 0.01μF total noise is approximately
given by
Equation 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 above recommendations.
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.
As with any linear regulator, PSRR and transient response are degraded as (V
IN
– V
OUT
) approaches dropout.
This effect is shown in
Figure 18
through
Figure 20
in the
Typical Characteristics
section.
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 will be somewhat slower. Refer to
Figure 25
and
Figure 26
in the
Typical Characteristics
section. The
quick-start switch is closed for approximately 135μs. To ensure that C
NR
is fully charged during the quick-start
time, a 0.01μF or smaller capacitor should be used.
12
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