Diodes ZXCT1009 - Manual (Page 5)

ZXCT1009
HIGH-SIDE CURRENT MONITOR
ZXCT1009
Document number: DS33441 Rev. 12 - 2
5 of 8
www.diodes.com
April 2011
© Diodes Incorporated
Application Information (cont.)
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 Diodes FZT789A high current
Super-ß 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.
Transient Protection
An additional resistor, R
LIM
can be added in series with R
OUT
(as below), to limit the current from I
OUT
. Any circuit
connected to V
OUT
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.
ZXCT1009 with additional current limiting Resistor R
LIM
.
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
V
IN
R
SENSE To load
V
OUT
R
OUT
1
2 3
ZXCT1009
V
SENSE+
V
SENSE-
I
OUT
R
LIM
V
IN
R
SENSE To load
V
OUT
R
OUT
1
2 3
ZXCT1009
V
SENSE+
V
SENSE-
I
OUT
R
LIM