LTC4055-1_15 Datasheet, PDF (14/24 Pages) Linear Technology – USB Power Controller and Li-Ion Charger

LTC4055/LTC4055-1
OPERATION
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 defined as:
I_CL
= ^{⎛⎝⎜⎜}_R4C⁹_L^,_P⁰_R⁰_O⁰_G
•
⎞
V_{CLPROG ⎠⎟⎟} =
49,000V
R_CLPROG
The programmed battery charge current, I_CHG, is defined as:
I_CHG
=
⎛
⎝⎜⎜
48,500
R_PROG
•
V_PROG
⎞
⎠⎟⎟
=
48,500V
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 LTC4055/LTC4055-1
can and should be configured 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
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:
I_CL
=
I_CLPROG
•
49,000
=
V_CLPROG
R_CLPROG
•
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_CLPROG
=
1V
490mA
•
49,000
=
100k
In USB applications, the minimum value for R_CLPROG should
be 105k. This will prevent the application current from
600
120
I_IN
500
100
I_IN
400
80
I_LOAD
I_LOAD
300
60
200
I_BAT
40
CHARGING
I_BAT
CHARGING
100
20
600
500
I_IN
400
300
I_BAT = I_CHG
I_LOAD
200
I_BAT
I_BAT = I_CL – I_OUT
100
CHARGING
0
0
0
–100
0
–20
100 200 300 400 500 600
0
I_LOAD (mA)
I_BAT
(IDEAL DIODE)
–100
20 40 60 80 100 120
0 100 200 300 400 500 600
I_LOAD (mA)
I_BAT
(IDEAL DIODE)
I_LOAD (mA)
I_BAT
(IDEAL DIODE)
4055 F02a
4055 F02b
4055 F02c
(2a) High Power Mode/Full Charge
(2b) Low Power Mode/Full Charge
(2c) High Power Mode with
(R_PROG = R_CLPROG = 97.6k)
(R_PROG = R_CLPROG = 97.6k)
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
4055fb
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