LP2952 Datasheet, PDF (16/24 Pages) National Semiconductor (TI) – Adjustable Micropower Low-Dropout Voltage Regulators

Application Hints (Continued)
down the error flag voltage using equal-value resistors
(10 kΩ suggested) 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
01112709
FIGURE 4. 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 in-
dependent 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
programmed output voltage. An out-of-regulation condition
can result from low input voltage, current limiting, or thermal
limiting.
Figure 5 gives a timing diagram showing the relationship
between the output voltage, the ERROR output, and input
voltage 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 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 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 imped-
ance, allowing the error flag voltage to rise to its pull-up
voltage. Using V_OUT as the pull-up voltage (rather than an
external 5V source) will keep the error flag voltage below
1.2V (typical) in this condition. The user may wish to divide
01112710
* 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 5. ERROR Output Timing
OUTPUT ISOLATION
The regulator output can be left connected to an active
voltage 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
stability. 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.
15
www.national.com