BQ24735_17 Datasheet, PDF (31/46 Pages) Texas Instruments – 1- to 4-Cell Li+ Battery SMBus Charge Controller for Supporting Turbo Boost Mode With N-Channel Power MOSFET Selector

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bq24735
SLUSAK9B – SEPTEMBER 2011 – REVISED APRIL 2015
10.2.2.8 Input Filter Design
During adapter hot plug-in, the parasitic inductance and input capacitor from the adapter cable form a second
order system. The voltage spike at VCC pin maybe beyond IC maximum voltage rating and damage IC. The
input filter must be carefully designed and tested to prevent overvoltage event on VCC pin.
There are several methods of damping or limiting the overvoltage spike during adapter hot plug-in. An electrolytic
capacitor with high ESR as an input capacitor can damp the overvoltage spike well below the IC maximum pin
voltage rating. A high current capability TVS Zener diode can also limit the overvoltage level to an IC safe level.
However, these two solutions may not have low cost or small size.
A cost-effective and small-size solution is shown in Figure 17. R1 and C1 are composed of a damping RC
network to damp the hot plug-in oscillation. As a result the overvoltage spike is limited to a safe level. D1 is used
for reverse voltage protection for VCC pin. C2 is VCC pin decoupling capacitor and it should be place to VCC pin
as close as possible. C2 value should be less than C1 value so R1 can dominant the equivalent ESR value to
get enough damping effect. R2 is used to limit inrush current of D1 to prevent D1 getting damage when adapter
hot plug-in. R2 and C2 should have 10-us time constant to limit the DV/DT on VCC pin to reduce inrush current
when adapter hot plug in. R1 has high inrush current. R1 package must be sized enough to handle inrush current
power loss according to resistor manufacturer’s data sheet. The filter components value always must be verified
with real application and minor adjustments may need to fit in the real application circuit.
D1
R1(2010)
Adapter ^2Ω
connector _C1
2.2μF
R2(1206)
10-20 Ω
VCC pin
C2
0.47-1μF
Figure 17. Input Filter
10.2.2.9 bq24735 Design Guideline
The bq24735 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 overcurrent 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 18 shows the bq24735 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 18. Block Diagram of bq24735 Short-Circuit Protection
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