Datasheet

R
AC
To ACP
To ACN
SystemPathPCB Trace
Systemcurrent
Chargerinputcurrent
ChargerInputPCB Trace
ACP ACN
R
AC
Charger
R
PCB
I
CHRGIN
I
BAT
I
SYS
I
DPM
(a)PCBLayout (b)EquivalentCircuit
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
Not Recommended for New Designs
bq24725
SLUS702A –JULY 2010–REVISED NOVEMBER 2010 www.ti.com
Figure 21. Block Diagram of bq24725 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. bq24725 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.
To prevent unintentional charger shut down in normal operation, MOSFET R
DS(on)
selection and PCB layout is
very important. Figure 22 shows a improvement PCB layout example and its equivalent circuit. In this layout, the
system current path and charger input current path is not separated, as a result, the system current causes
voltage drop in the PCB copper and is sensed by the IC. The worst layout is when a system current pull point is
after charger input; as a result all system current voltage drops are counted into over current protection
comparator. The worst case for IC is when the total system current and charger input current sum equals the
DPM current. When the system pulls more current, the charger IC tries to regulate the R
AC
current as a constant
current by reducing the charging current.
Figure 22. Need improve PCB layout example.
Figure 23 shows the optimized PCB layout example. The system current path and charge input current path is
separated, as a result the IC only senses charger input current caused PCB voltage drop and minimized the
possibility of unintentional charger shut down in normal operation. This also makes PCB layout easier for high
system current application.
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