Datasheet

LTC4055/LTC4055-1
14
4055fb
USB CURRENT LIMIT AND CHARGE CURRENT
CONTROL
The current limit and charger control circuits of the
LTC4055/LTC4055-1 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
R
V
V
R
CL
CLPROG
CLPROG
C
=
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
=
49 000 49 000,
•
,
LLPROG
The programmed battery charge current, I
CHG
, is defi ned as:
I
R
V
V
R
CHG
PROG
PROG
PROG
=
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
=
48 500 48 500,
•
,
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 LTC4055/LTC4055-1
can and should be confi gured to limit current to 500mA
for USB applications (selectable using the HPWR pin and
programmed using the CLPROG pin).
When programmed for 500mA current limit and 500mA
or more of charging current, powered from IN1/IN2
and battery charging is active, control circuitry within
OPERATION
the LTC4055/LTC4055-1 reduces the battery charging
current such that the sum of the battery charge current
and the load current does not exceed 500mA (100mA
when HPWR is low, see Figure 2) The battery charging
current goes to zero when load current exceeds 500mA
(80mA 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 (shaded region in Figure 2).
PROGRAMMING CURRENT LIMIT
The formula for programming current limit is:
II
V
R
CL CLPROG
CLPROG
CLPROG
==•, •,49 000 49 000
where V
CLPROG
is the CLPROG pin voltage and R
CLPROG
is
the total resistance from the CLPROG pin to ground.
For example, if typical 490mA current limit is required,
calculate:
R
V
mA
k
CLPROG
==
1
490
49 000 100•,
In USB applications, the minimum value for R
CLPROG
should
be 105k. This will prevent the application current from
I
LOAD
(mA)
0
CURRENT (mA)
300
400
600
500
I
IN
400
4055 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
4055 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
4055 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
(2a) High Power Mode/Full Charge
(R
PROG
= R
CLPROG
= 97.6k)
(2b) Low Power Mode/Full Charge
(R
PROG
= R
CLPROG
= 97.6k)
(2c) High Power Mode with
I
CL
= 500mA and I
CHG
= 250mA
(R
PROG
= 196k, R
CLPROG
= 97.6k)
Figure 2. Input and Battery Currents as a Function of Load Current