LTC4012-2 Linear Technology Corporation, LTC4012-2 Datasheet - Page 20

• Manufacturer Part Number: LTC4012-2
• Description: Multi-Chemistry Battery Charger
• Manufacturer: Linear Technology Corporation

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APPLICATIONS INFORMATION
LTC4012/
LTC4012-1/LTC4012-2
C/10 CHRG Indicator
The value chosen for R
on charge current monitoring and the accuracy of the
C/10 charge indicator output (CHRG). The LTC4012
uses the voltage on the PROG pin to determine when
charge current has dropped to the C/10 threshold.
The nominal threshold of 400mV produces an accu-
rate low charge current indication of C/10 as long as
R
gramming considerations. However, it may sometimes
be necessary to deviate from this value to satisfy other
application design goals.
If R
low charge current is detected will be less than C/10. The
highest value of R
indicating low charge current before reaching fi nal V
is 30.1k. R
this, but low current indication will be lost.
If R
occurs at a level higher than C/10. More importantly, the
LTC4012 becomes increasingly sensitive to reverse cur-
rent. The lowest value of R
the risk of erroneous boost operation detection at end of
charge is 26.1k. Values of R
be used. See the Operation section for more information
about reverse current.
The nominal fractional value of I
tion occurs is given by:
Direct digital monitoring of C/10 indication is possible
with an external application circuit like the one shown in
Figure 9.
By using two different value pull-up resistors, a micropro-
cessor can detect three states from this pin (charging, C/10
and not charging). See Figure 10. When a digital output port
(OUT) from the microprocessor drives one of the resistors
and a second digital input port polls the network, the charge
state can be determined as shown in Table 5.
20
PROG
I
I
PROG
MAX
PROG
C
10
= 26.7k, independent of all other current pro-
=
is less than 26.7k, low charge current detection
is greater than 26.7k, the actual level at which
1 2085
PROG
400
.
mV
can safely be set to values higher than
V
PROG
–
–
(
R
(
R
PROG
PROG
that can be used while reliably
PROG
PROG
PROG
•
• •
11 67
MAX
11 67μA
has a strong infl uence
that can be used without
less than this should not
.
.
at which C/10 indica-
μA
)
)
BAT
Table 5. Digital Read Back State (IN, Figure 10)
Input and Output Capacitors
In addition to typical input supply bypassing (0.1μF) on
DCIN, the relatively high ESR of aluminum electrolytic
capacitors is helpful for reducing ringing when hot pluging
the charger to the AC adapter. Refer to LTC Application
Note 88 for more information.
The input capacitor between system power (drain of top
FET, Figure 1) and GND is required to absorb all input PWM
ripple current, therefore it must have adequate ripple current
rating. Maximum RMS ripple current is typically one-half
of the average battery charge current. Actual capacitance
value is not critical, but using the highest possible voltage
rating on PWM input capacitors will minimize problems.
Consult with the manufacturer before use.
CHARGER STATE
C/10 Charge
Bulk Charge
LTC4012
Off
LTC4012
Figure 10. Microprocessor Status Interface
INTV
CHRG
DD
LTC4012
Figure 9. Digital C/10 Indicator
17
7
CHRG
100k
7
3.3V
200k
Hi-Z
1
0
0
Q1
TP0610T
100k
33k
4012 F09
OUT STATE
V
OUT
IN
LOGIC
V
100k
Q2
2N7002
Q3
2N7002
μP
DD
4012 F10
100k
C/10
CHRG
1
1
1
0
4012f