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010

Voltage Regulator Assistant

Everyone will be familiar with the problem of

voltage drop across wires, connectors or fil-

tering components such as inductors, which

force the voltage at the regulator output to be

set higher than the circuit operating voltage. If

the current drawn falls then the operating

voltage rises above its nominal value; when

the current rises, the voltage falls.

Assistance here comes in the form of the

four-input MAX1804 Feedback Integrator , a

novel product from Maxim ( www.maxim-

ic.com ). The device can be connected to

your favourite voltage regulator and will inter-

vene in its feedback loop if any of its four

inputs is fed with too low a voltage. The

MAX1804 manages the fine control of the reg-

ulator output voltage by increasing it until the

lowest voltage of those sensed is made cor-

rect. Unused inputs to the MAX1804 are auto-

matically disabled if the voltage present is

below 90 % of the nominal value. The

MAX1804 will therefore not intervene in the feedback loop

if the voltage drop is too great. The device is designed for

supply voltages between +2.7 V and +5.5 V. It is supplied

directly from the output of the controlled regulator.

The OUT connection of the MAX1804 is connected to the

feedback pin of the regulator via R3, which has a rather

higher value than R1 or R2. The OUT pin draws as much

+U OUT1

+2V7...+5V5

+U

R PCB

+U OUT2

IN

IN

OUT

R1

IC1

Regulator

+U OUT3

FB

100n

R2

R3

REF

ON/OFF

10

9

SHDN

IN1

8

1

OUT

2

3

4

IC2

IN2

IN3

IN4

5

ADJ

MAX1804

Regulation

Setpoint

COMP

6

7

C2

470p

014103 - 11

current as necessary to ground to force the regulator to

raise its output voltage to bring the lowest of the MAX1804

inputs to four times the voltage on the ADJ pin. The voltage

on ADJ can be provided by a reference voltage generated

by the regulator, or, if this is not available, by an external

voltage reference or Zener diode. The available control

range can be set via R3: in the limit, R3 is effectively in par-

7-8/2001

Elektor Electronics

27

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SUMMER CIRCUITS COLLECTION

allel with R2.

The COMP pin is used for compensating the regulation

loop. The manufacturer recommends a compensation

capacitor of 470 pF.

The shutdown pin (SHDN) forces all four inputs and the

output into a high-impedance state, so that the MAX1804

can no longer affect the regulator.

(014103-1)

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