rc2951mt Fairchild Semiconductor, rc2951mt Datasheet - Page 5

• Manufacturer Part Number: rc2951mt
• Description: Adjustable Micropower Voltage Regulator
• Manufacturer: Fairchild Semiconductor

PRODUCT SPECIFICATION
Applications Discussion
The RC2951 regulator is designed with internal current lim-
iting and thermal shutdown. It is not internally compensated
and requires a 1.0µF (or greater) capacitor between the out-
put terminal and ground for stability. At lower output volt-
ages, more capacitance is required (2.2µF or more is
recommended for 3V and 3.3V versions) for stability. Most
types of aluminum, tantalum or multilayer ceramic capaci-
tors will perform adequately. Solid tantalums or appropriate
multilayer ceramic capacitors are suggested for operation
below 25°C. At lower values of output current, less capaci-
tance is needed to maintain stability at output. The capacitor
at the output can be reduced to 0.33µF for currents less that
10mA, or 0.1µF for currents below 1.0mA.
Using the adjustable versions at voltages below 5V runs the
error amplifier at lower gains so that more output capaci-
tance is needed. For the worst-case situation of a 100 mA
load at 1.23V output (Output shorted to Feedback) a
3.3 µF (or greater) capacitor should be used.
When setting the output voltage of the RC2951 versions with
external resistors, a minimum load of 1 µA is recommended.
A 1 µF tantalum or aluminum electrolytic capacitor should
be placed from the RC2951 input to ground if there is more
than 10 inches of wire between the input and the AC filter
capacitor or if a battery is used as the input.
Stray capacitance to the RC2951 Feedback terminal can
cause instability. This may especially be a problem when
using high value external resistors to set the output voltage.
Adding a 100 pF capacitor between Output and Feedback
and increasing the output capacitor to at least 3.3 µF will fix
this problem.
Error Detection Comparator Output
The comparator switches to a logic low whenever the
RC2951 output falls out of regulation by more than approxi-
mately 5%. This value is the comparator’s built-in offset of
about 60 mV divided by the 1.235 internal reference voltage.
This trip level remains “5% below normal” regardless of the
value of the output voltage. For example, the error flag trip
level is typically 4.75V for a 5V output or 11.4V for a 12V
output. The out of regulation condition may be due either to
low input voltage, current limiting, or thermal limiting.
REV. 1.0.1 4/8/02
Figure 1 is a timing diagram showing the ERROR signal and
the regulated output voltage as the RC2951 input is ramped
up and down. For 5V versions, the ERROR signal becomes
valid (low) at about 1.3V input. It goes high at about 5V
input (the input voltage at which V
RC2951’s dropout voltage is load-dependent (see curve in
typical performance characteristics), the input voltage trip
point (about 5V) will vary with the load current. The output
voltage trip point (approx. 4.75V) does not vary with load.
The error comparator has an open-collector output which
requires an external pullup resistor. This resistor may be
returned to the output or some other supply voltage depend-
ing on system requirements. In determining a value for this
resistor, note that while the output is rated to sink 400 µA,
this sink current adds to battery drain in a low battery condi-
tion. Suggested values range from 100k to 1MΩ. The resistor
is not required if this output is unused.
Programming the Output Voltage (RC2951)
The RC2951 may be pin-strapped for the nominal fixed
output voltage using its internal voltage divider by tying the
output and sense pins together, and also tying the feedback
and V
for any output voltage between its 1.235V reference and its
30V maximum rating. As seen in Figure 2 an external pair of
resistors is required.
TAP
Voltage
Voltage
Output
pins together. Alternatively, it may be programmed
Error*
Input
Figure 1. ERROR Output Timing
4.75V
1.3V
5V
OUT
= 4.75.) Since the
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RC2951
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