BQ24742 Datasheet, PDF (25/35 Pages) Texas Instruments – Li-Ion or Li-Polymer Battery Charger with Low Iq and Accurate Trickle Charge

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bq24741, bq24742
SLUS875B – MARCH 2009 – REVISED OCTOBER 2009
A cost effective and small size solution is shown in Figure 31. The R1 and C1 is composed of a damping RC
network to damp the hot plug-in oscillation. As a result the over voltage spike is limited to a safe level. D1 is used
for reverse voltage protection for PVCC pin. C2 is PVCC pin decoupling capacitor and it should be place to
PVCC pin as close as possible. C2 value should be much 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. R1 has high inrush current. R1 package must be sized enough to handle inrush
current power loss according to resistor manufacturer’s datasheet. The filter components value always need to
be verified with real application and minor adjustments may need to fit in the real application circuit.
Adapter
connector
D1
R1
2W
C1
2.2 mF
(2010)
R2 ⁽¹²⁰⁶⁾
4.7 -30W
PVCC pin
C2
0.1-1 mF
Figure 31. Input Filter
bq24741/2 Design Guideline
The bq24741/2 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 to toggle the CE pin or IC power on reset. Figure 32 shows the
bq24741/2 short circuit protection block diagram.
Adapter
ACP
SCP1
RAC ^ACN RPCB
BTST
High-Side
MOSFET
L
SW
R _SR
REGN
Battery
COMP1
COMP2
SCP2
Low-Side
MOSFET
Charge
Enable
Function
Count to 7
CLR
Latch off
Charger
Figure 32. Block Diagram of bq24741/2 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. the bq24741/2 senses low side switch voltage drop by SW pin and AGND
pin.
The high-side FET short is detected by monitoring the voltage drop between ACP and SW. 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.
Copyright © 2009, Texas Instruments Incorporated
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