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Sheet目录47

3页 4页 5页 6页 [7页]8页 9页 10页

ZXCT1050

Issue 1 - June 2007

www.zetex.com

?Zetex Semiconductors plc 2007

Applications information

The ZXCT1050 is a current output version of the ZXCT1051 and as such uses a separate power

supply pin. All biasing for the internal amplifiers comes from its separate V

input and is not line

powered, unlike the ZXCT1021.

This means that at very small sense voltages the ZXCT1050 draws very little current (<1A) from

the lines being sensed.

The separate V

pin enables the ZXCT1050 to be operated at sense line voltages down to 0V,

where the ZXCT1021 would switch off. This feature enables the ZXCT1050 to be used to sense the

currents flowing through lines that have been shorted to ground.

Basic operation

Load current, I

, from V

RAIL

is drawn through R

SENSE

developing a voltage V

SENSE

across the

sense inputs of the ZXCT1050.

The internal amplifier forces V

SENSE

across external resistance R

(internal on the ZXCT1051)

causing a current to flow through transistor Q1 and out of the output pin, OUT. This current is then

converted to a voltage by a resistor, R

, between OUT and GND.

The overall gain of the ZXCT1050 is determined by the following expression:

A ratio of 1:2 between R

and R

creates the fixed gain of 10 with an output impedance equal to

RG (see electrical characteristics for output current-voltage characteristics).

The ZXCT1050 has both R

and R

external. This allows R

and R

to be varied so that the

required gain can be achieved at the required output impedance.

For low power applications both R

and R

can be increased whereas for driving low impedance

and R

can be decreased.

The maximum recommended value for R

is 40k and the maximum recommended value for

RSH is 10k. Large values of R

start increasing the effective input offset error, while large values

of R

can created load errors and reduce bandwidths.

The maximum differential input voltage, V

SENSE

, is 150mV (I

* R

SENSE

); however voltages up to

500mV will not damage it. This can be increased further by the inclusion of a resistor, R

LIM

between the SENSE+ pin and the rail being sensed, V

RAIL

For best performance R

SENSE

should be connected as close to the SENSE+ and SENSE- pins thus

minimizing any series resistance with R

SENSE

ZXCT1050

SENSE+ (4)

Out

(3)

(5) SENSE-

(1)

RAIL

SENSE

Load

GND (2)

GND

AIN 20

----------

?/DIV>

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