BQ24707 Datasheet, PDF (26/36 Pages) Texas Instruments – 1-4 Cell Li+ Battery SMBus Charge Controller With Independent Comparator and Advanced Circuit Protection

bq24707
bq24707A
SLUSA78B – JULY 2010 – REVISED MARCH 2011
D1
R1(2010)
Adapter ^2Ω
connector _C1
2.2μF
R2(1206)
10-20 Ω
VCC pin
C2
0.47-1μF
Figure 20. Input Filter
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IC Design Guideline
The IC has a unique short circuit protection feature. Its cycle-by-cycle current monitoring feature is achieved
through monitoring the voltage drop across Rdson of the MOSFETs after a certain amount of blanking time. In
case of MOSFET short or inductor short circuit, the over current condition is sensed by two comparators and two
counters will be triggered. After seven times of short circuit events, the charger will be latched off. The way to
reset the charger from latch-off status is reconnect adapter. Figure 21 shows the IC short circuit protection block
diagram.
Adapter
ACP
SCP1
R_AC ^ACN _RPCB
BTST
High-Side
MOSFET
PHASE
REGN
L
R_DC
Battery
COMP1
COMP2 ^SCP2
Low-Side
C
MOSFET
Adapter
Plug in
Count to 7
CLR
Latch off
Charger
Figure 21. Block Diagram of IC Short Circuit Protection
In normal operation, low side MOSFET current is from source to drain which generates negative voltage drop
when it turns on, as a result the over current comparator can not be triggered. When high side switch short circuit
or inductor short circuit happens, the large current of low side MOSFET is from drain to source and can trig low
side switch over current comparator. IC senses low side switch voltage drop by PHASE pin and GND pin.
The high-side FET short is detected by monitoring the voltage drop between ACP and PHASE. As a result, it not
only monitors the high side switch voltage drop, but also the adapter sensing resistor voltage drop and PCB trace
voltage drop from ACN terminal of RAC to charger high side switch drain. Usually, there is a long trance between
input sensing resistor and charger converting input, a careful layout will minimize the trace effect.
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