Datasheet

To ACP To ACN
Systemcurrent
Chargerinputcurrent
SinglepointconnectionatR
AC
SystemPathPCB Trace
ChargerInputPCB Trace
R
AC
ACP ACN
Charger
R
AC
R
PCB
I
CHRGIN
I
BAT
I
SYS
I
DPM
(a)PCBLayout (b)EquivalentCircuit
Not Recommended for New Designs
bq24725
www.ti.com SLUS702A –JULY 2010–REVISED NOVEMBER 2010
Figure 23. Optimized PCB layout example.
The total voltage drop sensed by IC can be express as the following equation.
V
top
= R
AC
x I
DPM
+ R
PCB
x (I
CHRGIN
+ (I
DPM
- I
CHRGIN
) x k) + R
DS(on)
x I
PEAK
(15)
where the R
AC
is the AC adapter current sensing resistance, I
DPM
is the DPM current set point, R
PCB
is the PCB
trace equivalent resistance, I
CHRGIN
is the charger input current, k is the PCB factor, R
DS(on)
is the high side
MOSFET turn on resistance and I
PEAK
is the peak current of inductor. Here the PCB factor k equals 0 means the
best layout shown in Figure 23 where the PCB trace only goes through charger input current while k equals 1
means the worst layout shown in Figure 22 where the PCB trace goes through all the DPM current. The total
voltage drop must below the high side short circuit protection threshold to prevent unintentional charger shut
down in normal operation.
The low side MOSFET short circuit voltage drop threshold is fixed to typical 110mV. The high side MOSFET
short circuit voltage drop threshold can be adjusted via the SMBus command. ChargeOption() bit[8:7] = 00, 01,
10, 11 sets the threshold at 300mV, 500mV, 700mV and 900mV respectively. For a fixed PCB layout, the host
should set the proper short circuit protection threshold level to prevent unintentional charger shut down in normal
operation.
Copyright © 2010, Texas Instruments Incorporated Submit Documentation Feedback 31
Product Folder Links :bq24725