LTC4012-3_15 Datasheet, PDF (20/28 Pages) Linear Technology – High Efficiency, Multi-Chemistry Battery Charger with PowerPath Control

LTC4012-3
Applications Information
C/10 CHRG Indicator
The value chosen for R_PROG has a strong influence on
charge current monitoring and the accuracy of the C/10
charge indicator output (CHRG). The LTC4012-3 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 accurate low charge current indi-
cation of C/10 as long as R_PROG = 26.7k, independent of
all other current programming considerations. However,
it may sometimes be necessary to deviate from this value
to satisfy other application design goals.
If R_PROG is greater than 26.7k, the actual level at which
low charge current is detected will be less than C/10. The
highest value of R_PROG that can be used while reliably
indicating low charge current before reaching final V_BAT
is 30.1k. R_PROG can safely be set to values higher than
this, but low current indication will be lost.
If R_PROG is less than 26.7k, low charge current detection
occurs at a level higher than C/10. More importantly, the
LTC4012-3 becomes increasingly sensitive to reverse cur-
rent. The lowest value of R_PROG that can be used without
the risk of erroneous boost operation detection at end of
charge is 26.1k. Values of R_PROG less than this should not
be used. See the Operation section for more information
about reverse current.
The nominal fractional value of I_MAX at which C/10 indica-
tion occurs is given by:
( ) I_C10
( ) ^I_MAX
=
400mV – R_PROG • 11.67µA
1.2085V – R_PROG • 11.67µA
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 micro-
processor 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 4.
Table 4. Digital Read Back State (IN, Figure 10)
LTC4012-3
CHARGER STATE
OUT STATE
Hi-Z
1
Off
1
1
C/10 Charge
0
1
Bulk Charge
0
0
Input and Output Capacitors
In addition to typical input supply bypassing (0.1µF) on
DCIN, the relatively high ESR of aluminum electrolytic ca-
pacitors is helpful for reducing ringing when hot-plugging
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.
INTV_DD 17
100k
LTC4012-3
CHRG 7
V_LOGIC
Q1
TP0610T
100k
Q2
2N7002
100k
C/10
CHRG
Q3
2N7002
100k
4012-3 F09
Figure 9. Digital C/10 Indicator
3.3V
LTC4012-3
CHRG 7
200k
33k
V_DD
µP
OUT
IN
4012-3 F10
Figure 10. Microprocessor Status Interface
40123fb
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