LTC4012-3_15 Datasheet, PDF (18/28 Pages) Linear Technology – High Efficiency, Multi-Chemistry Battery Charger with PowerPath Control

LTC4012-3
Applications Information
Programming Input Current Limit
To set the input current limit, I_LIM, the minimum wall
adapter current rating must be known. To account for the
tolerance of the LTC4012-3 input current sense circuit, 5%
should be subtracted from the adapter’s minimum rated
output. Refer to Figure 6 and program the input current
limit function with the following equation:
R_CL
=
100mV
I_LIM
where I_LIM is the desired maximum current draw from
the DC (adapter) input, including adjustments for tolerance,
if any.
Table 3. Common R_CL Values
ADAPTER
RATING
(A)
R_CL VALUE (1%)
(Ω)
1.00
0.100
1.25
0.080
1.50
0.068
1.75
0.056
2.00
0.050
2.50
0.039
3.00
0.033
3.50
0.027
4.00
0.025
R_CL POWER
DISSIPATION
(W)
0.100
0.125
0.150
0.175
0.200
0.244
0.297
0.331
0.400
R_CL POWER
RATING
(W)
0.25
0.25
0.25
0.25
0.25
0.50
0.50
0.50
0.50
FROM DC
POWER
INPUT
C_DC
10k
R_CL
C_F 0.1µF
R_F
5.1k
TO
REMAINDER
OF SYSTEM
2
1
CLP
CLN
LTC4012-3
3 INFET
4012-3 F06
Figure 6. Programming Input Current Limit
Often an AC adapter will include a rated current output
margin of at least +10%. This can allow the adapter cur-
rent limit value to simply be programmed to the actual
minimum rated adapter output current. Table 3 shows
some common R_CL current limit programming values.
A lowpass filter formed by R_F (5.1k) and C_F (0.1µF) is
required to eliminate switching noise from the LTC4012-3
PWM and other system components. If input current limit-
ing is not desired, CLN should be shorted to CLP while
CLP remains connected to power.
Figure 7 shows an optional circuit that can influence
the parameters of the input current limit in two ways.
The first option is to lower the power dissipation of R_CL at
the expense of accuracy without changing the input current
limit value. The second is to make the input current limit
value programmable.
FROM INFET
CLP 2
LTC4012-3
CLN 1
C_F
0.22µF
R_CL
R_F
1%
2.49k 1%
TO REMAINDER
OF SYSTEM
INTV_{DD 17}
R2
Q1
IMX1
R1
1%
Q2
2SC2412
R3 = R1
1%
4012-3 F07
Figure 7. Adjusting Input Current Limit
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