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PRELIMINARY INFORMATION describes products that are not in full production at the time of printing. Specifications are based on design goals
and limited characterization. They may change without notice. Contact Raytheon for current information.

www.fairchildsemi.com

REV. 1.0.1 4/8/02

Features

· High accuracy output voltage
· Guaranteed 100 mA output current
· Extremely low quiescent current
· Extremely tight load and line regulation
· Requires only a 1.0µF output capacitor for stability
· Internal Current and Thermal Limiting
· Error flag warns of output dropout
· Logic-controlled electronic shutdown
· Output programmable from 1.24 to 29V
· Fixed 3.3V version available
· -25°C to +85°C operating range
· 8 lead SOIC package

Description

The RC2951 is a voltage regulator specifically designed to
maintain proper regulation with a very low dropout voltage
(Typ. 40mV at light loads and 380 mV at 100mA). It has a
low quiescent bias current of 75

µA and is capable of supply-

ing output currents in excess of 100mA. It has internal cur-
rent and thermal limiting protection. The output can be
programmed from 1.24V to 29V with two external resistors.
A fixed output voltage (3.3V) is also available.

The error flag output can be used as power-on reset for
warning of a low output voltage. The Shutdown input feature
allows a logic level signal to turn on and off the regulator
output. The RC2951 is ideally suited for battery powered
computer, consumer, and industrial equipment where an
extension of useful battery life is desirable. The RC2951 is
available in an 8-pin SOIC package.

Block Diagram

Unregulated DC

Shut

Down

Feed-

Back

Error
Amplifier

1.23V
Reference

From

CMOS

or TTL

To CMOS
or TTL

Input

Output

Sense

330k

60 mV

TAP

100 mA

OUT

See Application

Discussion

Error Detection Comparator

Ground

Error

RC2951

Adjustable Micropower Voltage Regulator

RC2951

PRODUCT SPECIFICATION

REV. 1.0.1 4/8/02

Pin Assignments

Absolute Maximum Ratings

Notes:
1. Junction to ambient thermal resistance for the S.O. (M) package is 160

°C/W.

May exceed input supply voltage.

3. When used in dual-supply systems where the output terminal sees loads returned to a negative supply, the output voltage

should be diode-clamped to ground.

Power Dissipation

Internally Limited

Lead Temp. (Soldering, 5 seconds)

260

°C

Storage Temperature Range

° to +150°C

Operating Junction Temperature Range

° to +150°C

Input Supply/Voltage

0.3 to +30V

Feedback Input Voltage

2,3

1.5 to +30V

Shutdown Input Voltage

0.3 to +30V

Error Comparator Output Voltage

0.3 to +30V

Electrical Characteristics

The

· denotes that the limits apply at temperature extremes.

Parameter

Conditions

Typ.

Tested Limit

Units

Output Voltage
(RC2951M)

= 25

°C

5.0

5.025
4.975

V max

V min

-25

°C T

85°C

5.0

5.06
4.94

V max

V min

Output Voltage
(RC2951M)

100

µA I

100mA

JMAX

5.0

5.075
4.925

V max

V min

Output Voltage
(RC2951M-3.3)

= 25

°C

3.3

3.317
3.284

V max

V min

-25

°C T

85°C

3.3

3.340
3.260

V max

V min

Output Voltage
(RC2951M-3.3)

100

µA I

100mA

JMAX

3.3

3.346
3.254

V max

V min

Output Voltage Temperature
Coefficient

120

ppm/

°C

Line Regulation

9, 10

NOM + 1)V

20V

0.03

0.1
0.5

% max
% max

Load Regulation

100

µA I

100 mA

0.08

0.2
0.4

% max
% max

Output

Sense

Shutdown

Ground

Input

Feedback

TAP

Error

Top View

8 Lead SOIC Package

PRODUCT SPECIFICATION

RC2951

REV. 1.0.1 4/8/02

Dropout Voltage

= 100

µA

150

mV max
mV max

= 100 mA

380

500
700

mV max
mV max

Ground Current

= 100

µA

120
140

µA max

= 100 mA

14
15

mA max
mA max

Dropout Ground Current

= (V

NOM - 0.5)V

= 100

µA

110

250
300

µA max

Current Limit

out

= 0

160

200
220

mA max
mA max

Thermal Regulation

0.05

0.2

%/W max

Output Noise,
10 Hz to
100 KHz

= 1

µF (5V Only)

430

µV rms

= 200

µF

160

µV rms

= 3.3

µF

(Bypass = 0.01

µF

Pins 7 to 1 (RC2951)

100

µV rms

Reference Voltage

1.235

1.25
1.26
1.22

1.2

V max
V max

V min
V min

Reference Voltage

1.27
1.19

V max

V min

Feedback Pin Bias Current

40
60

nA max
nA max

Reference Voltage
Temperature Coefficient

ppm/

°C

Feedback Pin Bias Current
Temperature Coefficient

0.1

nA/

°C

Error Comparator

Output Leakage Current

= 30V

0.01

1
2

µA max

Output Low Voltage

= (V

NOM - 0.5)V

= 400

µA

150

250
400

mV max
mV max

Upper Threshold Voltage

40
25

mV min
mV min

Lower Threshold Voltage

140

mV max
mV max

Hysteresis

Electrical Characteristics

(continued)

The

· denotes that the limits apply at temperature extremes.

Parameter

Conditions

Typ.

Tested Limit

Units

RC2951

PRODUCT SPECIFICATION

REV. 1.0.1 4/8/02

Notes:

1. Unless otherwise specified all limits guaranteed for T

= 25

°C, V

= (V

NOM + 1)V, I

= 100

µA and C

= 1

µF for 5V

versions, and 2.2

µF for 3V and 3.3V versions. Additional conditions for the 8-pin versions are Feedback tied to V

TAP

Output tied to Output Sense and V

shutdown

< 0.8V.

2. Guaranteed and 100% production tested.

3. Dropout Voltage is defined as the input to output differential at which the output voltage drops 100 mV below its nominal value

measured at 1V differential. At very low values of programmed output voltage, the minimum input supply voltage of 2V (2.3V
over temperature) must be taken into account.

4. Comparator thresholds are expressed in terms of a voltage differential at the Feedback terminal below the nominal

reference voltage measured at V

= (V

NOM + 1)V. To express these thresholds in terms of output voltage change, multiply

by the error amplifier gain = V

out

ref

= (R1 + R2)/R2. For example, at a programmed output voltage of 5V, the Error output

is guaranteed to go low when the output drops by 95 mV x 5V/1.235V = 384 mV. Thresholds remain constant as a percent
of V

out

as V

out

is varied, with the dropout warning occurring at typically 5% below nominal, 7.5% guaranteed.

5. V

ref

< V

out

1V), 2.3V

30V, 100µA I

100 mA, T

JMAX

6. V

shutdown

2V, V

30V, V

out

= 0, Feedback pin tied to V

TAP

7. Output or reference voltage temperature coefficient is defined as the worst case voltage change divided by the total

temperature range.

8. Thermal regulation is defined as the change in output voltage at a time T after a change in power dissipation is applied,

excluding load or line regulation effects. Specifications are for a 50 mA load pulse at V

= 30V (1.25W pulse) for

T = 10 ms.

9. Regulation is measured at constant junction temperature, using pulse testing with a low duty cycle. Changes in output voltage

due to heating effects are covered under the specification for thermal regulation.

10. Line regulation for the RC2951 is tested at 150

°C for I

= 1 mA. For I

= 100

µA and T

= 125

°C, line regulation is guaranteed

by design to 0.2%. See Typical Performance Characteristics for line regulation versus temperature and load current.

Shutdown Input

Input Logic Voltage

Low (Regulator ON)
High (Regulator OFF)

1.3

0.6
2.0

V max

V min

Shutdown Pin Input Current

shutdown

= 2.4V

100

µA max

shutdown

= 30V

450

600
750

µA max

Regulator Output
Current in Shutdown

10
20

µA max

Electrical Characteristics

(continued)

The

· denotes that the limits apply at temperature extremes.

Parameter

Conditions

Typ.

Tested Limit

Units

PRODUCT SPECIFICATION

RC2951

REV. 1.0.1 4/8/02

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.

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

OUT

= 4.75.) Since the

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.

Figure 1. ERROR Output Timing

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 1M

. 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

TAP

pins together. Alternatively, it may be programmed

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.

Output

Voltage

Input

Voltage

1.3V

4.75V

2950-10

Error*

Part Number RC2951