ZXCT1009
SEMICONDUCTORS
ISSUE 4 - JULY 2003
6
Li-Ion Charger Circuit
The above figure shows the ZXCT1009 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
frequencyofthePWMloopisapproximately120kHz.
The ZXCT1009 is intended as a direct functional
replacement for the ZDS1009, which is featured in a
complete design from Unitrode/Texas Instruments
on the Li-Ion charger circuit shown above.
Reference: DVS2954S1H Li-Ion Charger
Development System.
100Ω
0.2Ω
100Ω
FZT789A
FMMT3904 1kΩ
FMMD914
10µH
FMMT451
140µH
ZHCS1000
220Ω
SNS pin
MOD pin
Charger Input To Battery +
bq2954
5V
ZXCT1009
support components omitted for clarity
+
-
Vin Load
Iout
APPLICATIONS INFORMATION (Continued) Transient Protection
An additional resistor, Rlim can be added in series
with Rout (figure 1.0), to limit the current from Iout.
Anycircuitconnectedto Vout will be protectedfrom
input voltage transients. This can be of particular
use in automotive applications where load dump
and other common transients need to be
considered.
Figure 1.0
ZXCT1009 with additional current limiting Resistor
Rlim.
Assuming the worst case condition of Vout = 0V;
providing a low impedance to a transient, the
minimum value of Rlim is given by:-
R(min) VV
I
lim pk max
pk
=−
Vpk = Peak transient voltage to be
withstood
Vmax = Maximum working Voltage = 20V
Ipk = Peak output current = 40mA
The maximum value of Rlim is set by Vin(min),
Vout(max) and the dropout voltage (see transfer
characteristic on page 3) of the ZXCT1009 :-
RR [V V V (max)
V
lim out in dp out
out
(max) (min) ( )]
(max)
=−+
Vin(min) = Minimum Supply Operating
Voltage
Vdp =Dropout Voltage
Vout (max)= Maximum Operating Output
Voltage
Vin To Load
Vout
Rsense
R
Rlim
ZXCT
1009