ZXCT1009
APPLICATIONS INFORMATION (Continued)
Charger input
FMMT3904
FZT789A
ZHCS100
V
in
FMMD914
5V
Load
To battery +
Transient Protection
An additional resistor, Rlim can be added in series
with Rout (figure 1.0), to limit the current from I
out
.
Any circuit connected to Vout will be protected from
input voltage transients. This can be of particular
use in automotive applications where load dump
and other common transients need to be
considered.
bq2954
MOD pin
FMMT451
I
out
ZXCT1009
SNS pin
Support components omitted for clarity
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
frequency of the PWM loop is approximately
120kHz.
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.
Figure 1.0
ZXCT1009 with additional current limiting Resistor
Rlim.
Assuming the worst case condition of V
out
= 0V;
providing a low impedance to a transient, the
minimum value of R
lim
is given by:-
R
lim
(min)
=
V
pk
−
V
max
I
pk
V
pk
= Peak transient voltage to be withstood
V
max
= Maximum working Voltage = 20V
I
pk
= Peak output current = 40mA
The maximum value of R
lim
is set by V
in
(min),
V
out
(max) and the dropout voltage (see transfer
characteristic on page 3) of the ZXCT1009 :-
R
lim
(max)
=
R
out
[V
in
(min)
−
(V
dp
+
V
out
(max))]
V
out
(max)
V
in
(min) = Minimum Supply Operating Voltage
V
dp
=Dropout Voltage
V
out
(max)= Maximum Operating Output
Voltage
ISSUE 10 - JULY 2007
SEMICONDUCTORS
6
ZETEX [ ZETEX SEMICONDUCTORS ]
