Datasheet ZXCT1009 (Diodes) - 5

ZXCT1009 HIGH-SIDE CURRENT MONITOR Application Information (cont.) Li-Ion Charger Circuit Transient Protection
An additional resistor, RLIM can be added in series with ROUT (as below), to limit the current from IOUT. 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.
V R V SENSE IN To load 2 3 V V SENSE+ SENSE- SENSE+ ZXCT1009 CT10 IOUT OU 1
The above figure shows the ZXCT1009 supporting the Benchmarq bq2954 Charge Management IC. Most of the
RLIM LI
support components for the bq2954 are omitted for clarity.
V
This design also uses the Diodes FZT789A high current
OUT
Super-ß PNP as the switching transistor in the DC-DC step down converter and the FMMT451 as the drive NPN for the
ROUT
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 ZXCT1009 with additional current limiting Resistor RLIM. PWM loop is approximately 120kHz. Assuming the worst case condition of V The ZXCT1009 is intended as a direct functional OUT = 0V; providing a low impedance to a transient, the minimum value of R replacement for the ZDS1009, which is featured in a LIM is given by:- complete design from Unitrode/Texas Instruments on the Li-Ion charger circuit shown above. Reference: DVS2954S1H Li-Ion Charger Development System. RLIM(min) = (VPK – VMAX)/IPK V PK = 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 :- RLIM(MAX) = ROUT[VIN(MIN) – (VDP + VOUT(MAX))]/VOUT(MAX) VIN(MIN) = Minimum Supply Operating Voltage VDP = Dropout Voltage VOUT(MAX) = Maximum Operating Output Voltage ZXCT1009 5 of 8 April 2011 Document number: DS33441 Rev. 12 - 2
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