LP2953QML Datasheet, PDF (15/24 Pages) Texas Instruments – Adjustable Micropower Low-Dropout Voltage Regulators

* See Application Hints
** Drive with TTL-low to shut down
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FIGURE 2. Adjustable Regulator
DROPOUT VOLTAGE
The dropout voltage of the regulator is defined as the mini-
mum input-to-output voltage differential required for the out-
put voltage to stay within 100 mV of the output voltage
measured with a 1V differential. The dropout voltage is inde-
pendent of the programmed output voltage.
DROPOUT DETECTION COMPARATOR
This comparator produces a logic “LOW” whenever the output
falls out of regulation by more than about 5%. This figure re-
sults from the comparator's built-in offset of 60 mV divided by
the 1.23V reference (refer to block diagrams on page 1). The
5% low trip level remains constant regardless of the pro-
grammed output voltage. An out-of-regulation condition can
result from low input voltage, current limiting, or thermal lim-
iting.
Figure 3 gives a timing diagram showing the relationship be-
tween the output voltage, the ERROR output, and input volt-
age as the input voltage is ramped up and down to a regulator
programmed for 5V output. The ERROR signal becomes low
at about 1.3V input. It goes high at about 5V input, where the
output equals 4.75V. Since the dropout voltage is load de-
pendent, the input voltage trip points will vary with load
current. The output voltage trip point does not vary.
The comparator has an open-collector output which requires
an external pull-up resistor. This resistor may be connected
to the regulator output or some other supply voltage. Using
the regulator output prevents an invalid “HIGH” on the com-
parator output which occurs if it is pulled up to an external
voltage while the regulator input voltage is reduced below
1.3V. In selecting a value for the pull-up resistor, note that
while the output can sink 400 μA, this current adds to battery
drain. Suggested values range from 100 kΩ to 1 MΩ. This
resistor is not required if the output is unused.
When V_IN ≤ 1.3V, the error flag pin becomes a high
impedance, allowing the error flag voltage to rise to its pull-up
voltage. Using V_OUT as the pull-up voltage (rather than an ex-
ternal 5V source) will keep the error flag voltage below 1.2V
(typical) in this condition. The user may wish to divide down
the error flag voltage using equal-value resistors (10 kΩ sug-
gested) to ensure a low-level logic signal during any fault
condition, while still allowing a valid high logic level during
normal operation.
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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 3. ERROR Output Timing
OUTPUT ISOLATION
The regulator output 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
regulator can occur if the output is pulled up by an external
voltage source.
REDUCING OUTPUT NOISE
In reference applications it may be advantageous to reduce
the AC noise present on the output. One method is to reduce
regulator bandwidth by increasing output capacitance. 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 2). The formula for selecting
the capacitor to be used is:
This gives a value of about 0.1 μF. When this is used, the
output capacitor must be 6.8 μF (or greater) to maintain sta-
bility. The 0.1 μF capacitor reduces the high frequency 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 high output voltages.
AUXILIARY COMPARATOR
The LP2953 contains an auxiliary comparator whose invert-
ing input is connected to the 1.23V reference. The auxiliary
comparator has an open-collector output whose electrical
characteristics are similar to the dropout detection compara-
tor. The non-inverting input and output are brought out for
external connections.
SHUTDOWN INPUT
A logic-level signal will shut off the regulator output when a
“LOW” (<1.2V) is applied to the Shutdown input.
To prevent possible mis-operation, the Shutdown input must
be actively terminated. If the input is driven from open-collec-
tor logic, a pull-up resistor (20 kΩ to 100 kΩ recommended)
should be connected from the Shutdown input to the regulator
input.
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