Datasheet

R1(2010)
2Ω
C1
2.2μF
D1
C2
0.47-1μF
R2(1206)
10-20 Ω
Adapter
connector
VCC pin
PHASE
Battery
Adapter
ACP
ACN
R
AC
BTST
REGN
High-Side
MOSFET
Low-Side
MOSFET
CLR
Count to 7
Adapter
Plug in
R
DC
L
Latch off
Charger
SCP1
SCP2
COMP1
COMP2
R
PCB
C
bq24725A
SLUSAL0 –SEPTEMBER 2011 www.ti.com
Figure 19. Input Filter
bq24725A Design Guideline
The bq24725A has a unique short circuit protection feature. Its cycle-by-cycle current monitoring feature is
achieved through monitoring the voltage drop across R
DS(on)
of the MOSFETs after a certain amount of blanking
time. For a MOSFET short or inductor short circuit, the over current condition is sensed by two comparators, and
two counters are triggered. After seven occurrences of a short circuit event, the charger will be latched off. To
reset the charger from latch-off status, reconnect the adapter. Figure 20 shows the bq24725A short circuit
protection block diagram.
Figure 20. Block Diagram of bq24725A Short Circuit Protection
In normal operation, the low side MOSFET current is from source to drain which generates a negative voltage
drop when it turns on, as a result the over current comparator can not be triggered. When the 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. bq24725A senses the low side switch voltage drop through the
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 R
AC
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.
32 Submit Documentation Feedback Copyright © 2011, Texas Instruments Incorporated
Product Folder Link(s) :bq24725A