Diodes ZXCT1010 - Manual (Page 6)

ISSUE 10 - JULY 2007
S E M I C O N D U C T O R S
ZXCT1010
6
Li-Ion Charger Circuit
The above figure shows the ZXCT1010 supporting
the Benchmarq bq2954 Charge Management IC.
Most of the support components for the bq2954 are
omitted for clarity. This design also uses the Zetex
FZT789A high current Super-
␤ PNP as the switching
transistor in the DC-DC step down converter and the
FMMT451 as the drive NPN for the FZT789A. The
circuit can be configured to charge up to four Li-Ion
cells at a charge current of 1.25A. Charge can be
terminated on maximum voltage, selectable
minimum current, or maximum time out. Switching
frequency of the PWM loop is approximately 120kHz.
100 Ω
0.2 Ω
100 Ω
FZT789A
BC81725 1k Ω
BAS16
10µH
FMMT451
140µH
ZHCS1000
220 Ω
SNS pin
MOD pin
Charger Input To Battery +
bq2954
5V
ZXCT1010
support components omitted for clarity
+ -
Vin Load
Iout
APPLICATIONS INFORMATION (Continued) Bi-Directional Current Sensing
The ZXCT1010 can be used to measure current
bi-directionally, if two devices are connected as
shown below.
If the voltage V1 is positive with respect to the
voltage V2 the lower device will be active, delivering
a proportional output current to Rout. Due to the
polarity of the voltage across Rsense, the upper
device will be inactive and will not contribute to the
current delivered to Rout. When V2 is more positive
than V1, current will be flowing in the opposite
direction, causing the upper device to be active
instead.
Non-linearity will be apparent at small values of
Vsense due to offset current contribution. Devices
can use separate output resistors if the current
direction is to be monitored independently.
Bi-directional Transfer Function
-400 -200 0 200 400
0
1
2
3
4
5
Output Current (mA)
Sense Voltage (mV)
Output Current v Sense Voltage