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4012f

LTC4012/

LTC4012-1/LTC4012-2

APPLICATIONS INFORMATION

The ﬁrst option is to lower the power dissipation of RCL at

the expense of accuracy without changing the input current

limit value. The second is to make the input current limit

value programmable.

The overall accuracy of this circuit needs to be better

than the power source current tolerance or be margined

such that the worse-case error remains under the power

source limits.

The accuracy of the Figure 7 circuit is a function of the

INTVDD, VBE, RCL, RF, R1 and R3 tolerances. To improve

accuracy, the tolerance of RF should be changed from

5.1k, 5% to a 2.49k 1% resistor. RCL and the programming

resistors R1 and R3 should also be 1% tolerance such

that the dominant error is INTVDD (±3%). Bias resistor R2

can be 5%. When choosing NPN transistors, both need

to have good gain (>100) at 10μA levels. Low gain NPNs

will increase programming errors. Q1 must be a matched

NPN pair. Since RF has been reduced in value by half, the

capacitor value of CF should double to 0.22μF to remain

effective at ﬁltering out any noise.

If you wish to reduce RCL power dissipation for a given

current limit, the programming equation becomes:

LIM

⎛

⎝⎜

⎞

⎠⎟

100

52 49

–

•.

If you wish to make the input current limit programmable,

the equation becomes:

LIM

⎛

⎝⎜

⎞

⎠⎟

100

52 49

–

•.

The equation governing R2 for both applications is based

on the value of R1. R3 should always be equal to R1.

R2 = 0.875 • R1

Figure 8. PROG Voltage Buffer

INTVDD

PROG

<30nA

LTC4012

4012 F08

TO SYSTEM

MONITOR

In many notebook applications, there are situations

where two different ILIM values are needed to allow two

different power adapters or power sources to be used.

In such cases, start by setting RLIM for the high power

ILIM conﬁguration and then use Figure 7 to set the lower

ILIM value. To toggle between the two ILIM values, take

the three ground connections shown in Figure 7, combine

them into one common connection and use a small-signal

NFET (2N7002) to open or close that common connec-

tion to circuit ground. When the NFET is off, the circuit

is defeated (ﬂoating) allowing ILIM to be the maximum

value. When the NFET is on, the circuit will become active

and ILIM will drop to the lower set value.

Monitoring Charge Current

The PROG pin voltage can be used to indicate charge cur-

rent where 1.2085V indicates full programmed current (1C)

and zero charge current is approximately equal to RPROG •

11.67μA. PROG voltage varies in direct proportion to the

charge current between this zero-current (offset) value and

1.2085V. When monitoring the PROG pin voltage, using a

buffer ampliﬁer as shown in Figure 8 will minimize charge

current errors. The buffer ampliﬁer may be powered from

the INTVDD pin or any supply that is always on when the

charger is on.

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