Datasheet

LTC4085-3/LTC4085-4
14
408534fb
USB Current Limit and Charge Current Control
The current limit and charger control circuits of the
LTC4085 are designed to limit input current as well as
control battery charge current as a function of I
OUT
. The
programmed current limit, I
CL,
is defi ned as:
I
CL
=
1000
R
CLPROG
•V
CLPROG
⎛
⎝
⎜
⎞
⎠
⎟
=
1000V
R
CLPROG
The programmed battery charge current, I
CHG
, is defi ned as:
I
CHG
=
50,000
R
PROG
•V
PROG
⎛
⎝
⎜
⎞
⎠
⎟
=
50,000V
R
PROG
Input current, I
IN
, is equal to the sum of the BAT pin output
current and the OUT pin output current:
I
IN
= I
OUT
+ I
BAT
The current limiting circuitry in the LTC4085 can and should
be confi gured to limit current to 500mA for USB applica-
tions (selectable using the HPWR pin and programmed
using the CLPROG pin).
The LTC4085 reduces battery charge current such that the
sum of the battery charge current and the load current
does not exceed the programmed input current limit (one-
fi fth of the programmed input current limit when HPWR
is low, see Figure 2). The battery charge current goes to
zero when load current exceeds the programmed input
current limit (one-fi fth of the limit when HPWR is low). If
the load current is greater than the current limit, the output
voltage will drop to just under the battery voltage where
the ideal diode circuit will take over and the excess load
current will be drawn from the battery.
(2a) High Power Mode/Full Charge
R
PROG
= 100k and R
CLPROG
= 2k
(2b) Low Power Mode/Full Charge
R
PROG
= 100k and R
CLPROG
= 2k
(2c) High Power Mode with
I
CL
= 500mA and I
CHG
= 250mA
R
PROG
= 100k and R
CLPROG
= 2k
Figure 2: Input and Battery Currents as a Function of Load Current
OPERATION
I
LOAD
(mA)
0
CURRENT (mA)
300
400
600
500
I
IN
400
408534 F02a
200
100
–100
0
100
200
300
600500
I
LOAD
I
BAT
CHARGING
I
BAT
(IDEAL DIODE)
I
LOAD
(mA)
0
CURRENT (mA)
60
80
120
100
I
IN
80
408534 F02b
40
20
–20
0
20
40
60
120100
I
LOAD
I
BAT
CHARGING
I
BAT
(IDEAL DIODE)
I
LOAD
(mA)
0
CURRENT (mA)
300
400
600
500
I
IN
400
408534 F02c
200
100
–100
0
100
200
300
600500
I
LOAD
I
BAT
CHARGING
I
BAT
(IDEAL DIODE)
I
BAT
= I
CHG
I
BAT
= I
CL
– I
OUT