LP2956 Datasheet, PDF (12/16 Pages) National Semiconductor (TI) – Dual Micropower Low-Dropout Voltage Regulators

Application Hints (Continued)
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 improve-
ments are more pronounced at higher output voltages.
DS011339-12
*In shutdown mode, ERROR will go high if it has been pulled up to an
external 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 volt-
age 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 volt-
age 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 capaci-
tance. This is relatively inefficient, since large increases in
capacitance 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 select-
ing the capacitor to be used is:
This gives a value of about 0.1µF. 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
DS011339-13
where: V_REF = 1.23V and I_FB = −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.
DS011339-14
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 kΩ to 50 kΩ recommended) must be connected
between the shutdown input and ground in cases where the
input is not actively pulled low.
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