Diodes ZXCT1041 - Manual (Page 9)

ZXCT1041
Issue 2 - January 2008 9 www.zetex.com
© Zetex Semiconductors plc 2008
Application information
The ZXCT1041 uses two current monitors in anti-parallel to provide bidirectional current
measurement. The integrated resistors while having a broad actual value variance provide very
good matching to one another; this provides very tight gain matching from forward current
measurement to reverse current management and removes the need to trim the resistor values.
The internal transconductance setting resistors have a nominal value of 1.5k ⍀ thereby setting the
internal transconductance to 0.67mA/V of V
SENSE-
. The outputs of both current monitors (current)
are summed into an internal common gain-setting resistor of 15k
⍀. This sets the overall gain to
10 which has a very small variance due to the very good matching of internal transistors.
To improve accuracy the offset of amplifier 1 is trimmed.
The direction of measured current flow is determined by comparing the voltages applied to the
bases of transconductance transistors (Q1 and Q2). For maximum versatility the flag output uses
an open collector; this allows the ZXCT1041 to monitor rails at a much higher potential than what
the flag output is interfacing to.
A common application for micro-power current monitors is measuring the discharge current of a
rechargeable lithium ion/polymer battery. The ZXCT1041 enables measuring both the charge and
discharge current into the battery and with its wide operating voltage of 2.5 to 20V enables it to
measure the currents in to/ out of up to 4 cells connected in series.
S+ S-
1.5k 1.5k
OUT
Flag
GND
15k