LP2952 National Semiconductor, LP2952 Datasheet - Page 12

• Manufacturer Part Number: LP2952
• Description: Adjustable Micropower Low-Dropout Voltage Regulators
• Manufacturer: National Semiconductor

www.national.com
Application Hints
*
**
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 mea-
sured with a 1V differential. The dropout voltage is
independent of the programmed output voltage.
DROPOUT DETECTION COMPARATOR
This comparator produces a logic “LOW” whenever the out-
put falls out of regulation by more than about 5%. This figure
results 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 5 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 regula-
tor 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
dependent, 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 k
resistor is not required if the output is unused.
When V
ance, allowing the error flag voltage to rise to its pull-up volt-
age. Using V
nal 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 k sug-
gested) to ensure a low-level logic signal during any fault
condition, while still allowing a valid high logic level during
normal operation.
See Application Hints
Drive with TTL-low to shut down
IN
FIGURE 4. Adjustable Regulator
1.3V, the error flag pin becomes a high imped-
OUT
as the pull-up voltage (rather than an exter-
(Continued)
to 1 M . This
DS011127-9
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.
*
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 4 ). The formula for select-
ing 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 improve-
ments are more pronounced at high output voltages.
AUXILIARY COMPARATOR (LP2953 only)
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” (
To prevent possible mis-operation, the Shutdown input must
be actively terminated. If the input is driven from
open-collector logic, a pull-up resistor (20 k
ommended) should be connected from the Shutdown input
to the regulator input.
* Exact value depends on dropout voltage. (See Application Hints)
<
1.2V) is applied to the Shutdown input.
FIGURE 5. ERROR Output Timing
to 100 k
DS011127-10
rec-