LP2956 datasheet(14/20 Pages) NSC

14 / 20 page

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*In shutdown mode, ERROR will go high if it has been pulled up to an ex-

ternal supply. To avoid this invalid response, pull up to regulator output

**Exact value depends on dropout voltage. (See Application Hints)

FIGURE 4. ERROR Output Timing

If a single pull-up resistor is used to the regulator output, the

error flag may briefly rise up to about 1.3V as the input voltage

ramps up or down through the 0V to 1.3V region.

In some cases, this 1.3V signal may be mis-interpreted as a

false high by a

μP which is still “alive” with 1.3V applied to it.

To prevent this, the user may elect to use two resistors which

are equal in value on the error output (one connected to

ground and the other connected to the regulator output).

If this two-resistor divider is used, the error output will only be

pulled up to about 0.6V (not 1.3V) during power-up or power-

down, so it can not be interpreted as a high signal. When the

regulator output is at 5V, the error output will be 2.5V, which

is still clearly a high signal.

OUTPUT ISOLATION

The regulator outputs can be left connected to an active volt-

age source (such as a battery) with the regulator input power

shut off, as long as the regulator ground pin is connected to

ground. If the ground pin is left floating, damage to the regu-

lator can occur if the output is pulled up by an external voltage

source.

REDUCING MAIN OUTPUT NOISE

In reference applications it may be advantageous to reduce

the AC noise present on the main output. One method is to

reduce regulator bandwidth by increasing output capacitance.

This is relatively inefficient, since large increases in capaci-

tance are required to get significant improvement.

Noise can be reduced more effectively by a bypass capacitor

placed across R1 (refer to Figure 3 ). The formula for selecting

the capacitor to be used is:

This gives a value of about 0.1

μF. When this is used, the out-

put capacitor must be 6.8

μF (or greater) to maintain stability.

The 0.1

μF capacitor reduces the high frequency noise gain

of the circuit to unity, lowering the output noise from 260

μV

to 80

μV using a 10 Hz to 100 kHz bandwidth. Also, noise is

no longer proportional to the output voltage, so improvements

are more pronounced at higher output voltages.

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where: V

REF = 1.23V and IFB = −10 nA (typical)

FIGURE 5. Auxiliary Adjustable Regulator

AUXILIARY LDO OUTPUT

The LP2956 has an auxiliary LDO regulator output (which can

source up to 75 mA) that is adjustable for voltages from 1.23V

to 29V.

The output voltage is set by an external resistive divider, as

shown in Figure 5. The maximum output current is 75 mA,

and the output requires 10

μF from the output to ground for

stability, regardless of load current.

SHUTDOWN INPUT

The shutdown input equivalent circuit is shown in Figure 6.

The main regulator output is shut down when the NPN tran-

sitor is turned ON.

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FIGURE 6. Shutdown Circuitry

The current into the input should be at least 0.5

μA to assure

the output shutdown function. A resistor may be placed in se-

ries with the input to minimize current draw in shutdown mode,

provided this minimum input current requirement is met.

IMPORTANT:

The shutdown input must not be left floating: a pull-down re-

sistor (10 k

Ω to 50 kΩ recommended) must be connected

between the shutdown input and ground in cases where the

input is not actively pulled low.

www.national.com

11339 Version 8 Revision 4

Print Date/Time: 2009/12/07 17:53:15

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