Datasheet

ManualsBrandsTEXAS INSTRUMENTS ManualsPMICsPMIC - LDO voltage regulator Positive, fixed SC 70 5

Output Noise Dropout Voltage

Startup

V =

11.5

xV

OUT

RMS

(2)

Board Layout Recommendations to Improve

Internal Current Limit

t =160 s+(540xC )

START

nF sm

(3)

Shutdown

Transient Response

TPS717xx

SBVS068G – FEBRUARY 2006 – REVISED APRIL 2009 ..................................................................................................................................................

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In most LDOs, the bandgap is the dominant noise The TPS717xx uses a PMOS pass transistor to

source. If a noise reduction capacitor (C

) is used achieve low dropout. When (V

– V

OUT

) is less than

with the TPS717xx, the bandgap does not contribute the dropout voltage (V

), the PMOS pass device is

significantly to noise. Instead, noise is dominated by in its linear region of operation and the input-to-output

the output resistor divider and the error amplifier resistance is the R

DS(ON)

of the PMOS pass element.

input. To minimize noise in a given application, use a V

will approximately scale with output current

0.01 µ F (minimum) noise reduction capacitor; for the because the PMOS device behaves like a resistor in

adjustable version, smaller value resistors in the dropout.

output resistor divider reduce noise. A parallel

As with any linear regulator, PSRR and transient

combination that gives 2.5 µ A of divider current has

response are degraded as (V

– V

OUT

) approaches

the same noise performance as a fixed voltage

dropout. This effect is shown in Figure 21 through

version.

Figure 23 in the Typical Characteristics section.

Equation 2 approximates the total noise referred to

the feedback point (FB pin) when C

= 0.01 µ F, total

noise is approximately given by Equation 2 :

Fixed voltage versions of the TPS717xx use a

quick-start circuit to fast-charge the noise reduction

capacitor, C

, if present (see Functional Block

Diagrams , Figure 1 ). This circuit allows the

combination of very low output noise and fast start-up

times. The NR pin is high impedance, so a low

PSRR and Noise Performance

leakage C

capacitor must be used; most ceramic

To improve ac performance such as PSRR, output

capacitors are appropriate in this configuration.

noise, and transient response, it is recommended that

Note that for fastest startup, V

should be applied

the board be designed with separate ground planes

first, then the enable pin (EN) driven high. If EN is

for V

and V

OUT

, with each ground plane connected

tied to IN, startup will be somewhat slower. Refer to

only at the GND pin of the device. In addition, the

Figure 31 in the Typical Characteristics section. The

ground connection for the bypass capacitor should

quick-start switch is closed for approximately 135 µ s.

connect directly to the GND pin of the device.

To ensure that C

is fully charged during the

quick-start time, a 0.01 µ F or smaller capacitor should

be used.

The TPS717xx internal current limit helps protect the

For output voltages below 1.6V, a voltage divider on

regulator during fault conditions. During current limit,

the bandgap reference voltage is employed to

the output sources a fixed amount of current that is

optimize output regulation performance for lower

largely independent of output voltage. For reliable

output voltages. This configuration results in an

operation, the device should not be operated in a

additional resistor in the quick-start path and

current limit state for extended periods of time.

combined with the noise reduction capacitor (C

)

The PMOS pass element in the TPS717xx has a

results in slower start-up times for output voltages

built-in body diode that conducts current when the

below 1.6V.

voltage at OUT exceeds the voltage at IN. This

Equation 3 approximates the start-up time as a

current is not limited, so if extended reverse voltage

function of C

for output voltages below 1.6V:

operation is anticipated, external limiting may be

appropriate.

The enable pin (EN) is active high and is compatible

with standard and low voltage, TTL-CMOS levels.

As with any regulator, increasing the size of the

When shutdown capability is not required, EN can be

output capacitor reduces over/undershoot magnitude

connected to IN.

but increases duration of the transient response.