bq24735
www.ti.com
SLUSAK9 –SEPTEMBER 2011
If frequency is reduced, for a fixed inductor the current ripple is increased. Inductor value must be carefully
selected so that it will not trig cycle-by-cycle peak over current protection even for the worst condition such as
higher input voltage, 50% duty cycle, lower inductance and lower switching frequency.
Inductor Short, MOSFET Short Protection
The bq24735 has a unique short circuit protection feature. Its cycle-by-cycle current monitoring feature is
achieved through monitoring the voltage drop across RDS(on) 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 and
ACFET and RBFET are turned off to disconnect adapter from system. BATFET is turned on to connect battery
pack to system. To reset the charger from latch-off status, the IC VCC pin must be pulled below UVLO or the
ACDET pin must be pulled below 0.6V. This can be achieved by removing the adapter and shut down the
operation system. The low side MOSFET short circuit voltage drop threshold can be adjusted via SMBus
command. ChargeOption() bit[7] =0, 1 set the low side threshold 135mV and 230mV respectively. The high side
MOSFET short circuit voltage drop threshold can be adjusted via SMBus command. ChargeOption() bit[8] = 0, 1
disable the function and set the threshold 750mV respectively. During boost function, the low side MOSFET short
circuit protection threshold is used for cycle-by-cycle current limiting, charger will not latch up.
Due to the certain amount of blanking time to prevent noise when MOSFET just turn on, the cycle-by-cycle
charge over-current protection may detect high current and turn off MOSFET first before the short circuit
protection circuit can detect short condition because the blanking time has not finished. In such a case the
charger may not be able to detect short circuit and counter may not be able to count to seven then latch off.
Instead the charger may continuously keep switching with very narrow duty cycle to limit the cycle-by-cycle
current peak value. However, the charger should still be safe and will not cause failure because the duty cycle is
limited to a very short of time and MOSFET should be still inside the safety operation area. During a soft start
period, it may takes long time instead of just seven switching cycles to detect short circuit based on the same
blanking time reason.
Table 8. Component List for Typical System Circuit of Figure 1
PART DESIGNATOR
QTY
6
DESCRIPTION
C1, C2, C3, C13, C14, C16
Capacitor, Ceramic, 0.1µF, 25V, 10%, X7R, 0603
Capacitor, Ceramic, 100pF, 25V, 10%, X7R, 0603
Capacitor, Ceramic, 1µF, 25V, 10%, X7R, 0603
Capacitor, Ceramic, 0.047µF, 25V, 10%, X7R, 0603
Capacitor, Ceramic, 10µF, 25V, 10%, X7R, 1206
Capacitor, Ceramic, 0.01µF, 25V, 10%, X7R, 0603
Capacitor, Ceramic, 2200pF, 25V, 10%, X7R, 0603
Capacitor, Ceramic, 2.2µF, 25V, 10%, X7R, 1210
Capacitor, Electrolytic, 220µF, 25V
C4
1
C5, C6
2
C7
1
C8, C9, C10, C11
4
C15
1
C17
1
Ci
1
Csys
1
D1
1
Diode, Schottky, 30V, 200mA, SOT-23, Fairchild, BAT54
Diode, Dual Schottky, 30V, 200mA, SOT-23, Fairchild, BAT54C
N-channel MOSFET, 30V, 12.5A, SO-8, Fairchild, FDS6680A
N-channel MOSFET, 30V, 12A, PowerPAK 1212-8, Vishay Siliconix, SiS412DN
N-channel MOSFET, 50V, 0.2A, SOT-323, Diodes, BSS138W
Inductor, SMT, 4.7µH, 5.5A, Vishay Dale, IHLP2525CZER4R7M01
Resistor, Chip, 430kΩ, 1/10W, 1%, 0603
D2
1
Q1, Q2, Q5
3
Q3, Q4
2
Q6
1
L1
1
R1
1
R2
1
Resistor, Chip, 66.5kΩ, 1/10W, 1%, 0603
R3, R4, R5
3
Resistor, Chip, 10kΩ, 1/10W, 1%, 0603
R6, R10, R11
3
Resistor, Chip, 4.02kΩ, 1/10W, 1%, 0603
R7
1
Resistor, Chip, 316kΩ, 1/10W, 1%, 0603
R8
1
Resistor, Chip, 100kΩ, 1/10W, 1%, 0603
R9
1
Resistor, Chip, 10Ω, 1/4W, 1%, 1206
R12
R13
1
Resistor, Chip, 1.00MΩ, 1/10W, 1%, 0603
1
Resistor, Chip, 3.01MΩ, 1/10W, 1%, 0603
Copyright © 2011, Texas Instruments Incorporated
Submit Documentation Feedback
27
Product Folder Link(s) :bq24735
TI [ TEXAS INSTRUMENTS ]
