ZXCT1012 datasheet(5/10 Pages) DIODES

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ZXCT1012

Issue 1 - December 2006

www.zetex.com

Applications information

The ZXCT1012 current monitor works by converting the voltage developed across a small sense

resistor into a current on the out pin. In reality it is a voltage to current converter. This output

current can be converted into a voltage simply by passing it through a resistor (ROUT) to ground.

The current monitor has a transconductance of 10mA/V. But the overall amplifying conversion is

affected by both the RSENSE and ROUT.

The gain equation of the ZXCT1012 is:

For best performance RSENSE should be connected as close to the SENSE+ (and SENSE-) pins;

which minimizes any series resistance with RSENSE and potential for interference pickup.

When choosing appropriate values for RSENSE a compromise must be reached between in-line

signal loss (including potential power dissipation effects) and small signal accuracy.

Higher values for RSENSE gives better accuracy at low load currents by reducing the inaccuracies

due to internal offsets. For best operation the ZXCT1012 has been designed to provide best

performance with VSENSE of the order of 40mV to 200mV.

Current monitors are single supply devices which means they tend to saturate at very low sense

voltages. However it does mean the output can never go negative. Also the output can never

change direction (monotonic). This is important if the current monitor is used in a control loop.

As the sense voltage is reduced the output will tend to saturate as the input offset voltage starts

to have greater effect. It is recommended to have a minimum sense voltage of 10mV to minimize

linearity errors. Zetex has specified the output voltage at VSENSE of 10mV, 40mV, 100mV and

200mV; which is the recommended sense voltage range.

The maximum differential input voltage, VSENSE, is 2.5V; however this will cause large output

currents to flow increasing power dissipation in the chip. The sense voltage can be increased

further, without damaging the ZXCT1012, by the inclusion of a resistor, RLIM, between SENSE- pin

and the load. Typical values around 10k . See figure below.

If large reverse currents are expected then the resistor, RLIM, will provide protection from

exceeding absolute maximum ratings.

A suitable value for RLIM can be determined from:

Where VSENSE(REV) is the maximum expected reverse sense voltage generated.

OUT

SENSE

OUT

100

---------------

ZXCT1012

SENSE

RLIM

LIM

SENSE REF

()

5mA

-----------------------------------

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