LTC4012CUF-3-PBF LINER [Linear Technology], LTC4012CUF-3-PBF Datasheet - Page 19

• Manufacturer Part Number: LTC4012CUF-3-PBF
• Description: High Efficiency, Multi-Chemistry Battery Charger with PowerPath Control
• Manufacturer: LINER [Linear Technology]

applicaTions inForMaTion
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
INTV
accuracy, the tolerance of R
5.1k, 5% to a 2.49k 1% resistor. R
resistors R1 and R3 should also be 1% tolerance such
that the dominant error is INTV
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 R
capacitor value of C
effective at filtering out any noise.
If you wish to reduce R
current limit, the programming equation becomes:
If you wish to make the input current limit programmable,
the equation becomes:
The equation governing R2 for both applications is based
on the value of R1. R3 should always be equal to R1.
I
R
R2 = 0.875 • R1
LIM
CL
DD
, V
=
=
100
100
BE
, R
mV
mV
CL
, R
F
–
–
R
I
has been reduced in value by half, the
LIM

 

 
F
CL
F
5 2 49
5 2 49
, R1 and R3 tolerances. To improve
should double to 0.22µF to remain
• .
• .
CL
R
R
1
1
power dissipation for a given
k
k
F

 

 
DD
should be changed from
CL
(±3%). Bias resistor R2
and the programming
In many notebook applications, there are situations
where two different I
different power adapters or power sources to be used.
In such cases, start by setting R
I
I
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 I
value. When the NFET is on, the circuit will become active
and I
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 R
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 INTV
charger is on.
LIM
LIM
configuration and then use Figure 7 to set the lower
value. To toggle between the two I
LIM
DD
will drop to the lower set value.
LTC4012-3
pin or any supply that is always on when the
INTV
PROG
Figure 8. PROG Voltage Buffer
DD
17
13
LIM
<30nA
values are needed to allow two
–
+
LIM
4012-3 F08
LIM
LTC4012-3
to be the maximum
for the high power
TO SYSTEM
MONITOR
LIM
values, take

PROG
40123fb
•