LTC4012-3_15 Datasheet, PDF (13/28 Pages) Linear Technology – High Efficiency, Multi-Chemistry Battery Charger with PowerPath Control

LTC4012-3
Operation
LTC4012-3
CLP
2
BAT
11
CLOCK
OSCILLATOR
SQ
R_D
WATCHDOG
TIMER
PWM
LOGIC
SYSTEM
POWER
TGATE
19
L1
BGATE
16
CSP
15
R_IN
+
CC
CA
R1
CSN ₁₄
R_IN
R_{SENSE VSENSE}
–
PROG
13
R_PROG
C_PROG
I_CHRG
FROM ICL AMP
EA
1.2085V
Figure 1. PWM Circuit Diagram
V_FB
9
ITH
12
LOOP
COMPENSATION
+
4012-3 F01
Bulk Charge
When soft-start is complete, the LTC4012-3 begins sourc-
ing the current programmed by the external components
connected to CSP, CSN and PROG. Some batteries may
require a small conditioning trickle current if they are heavily
discharged. As shown in the Applications Information sec-
tion, the LT4012-3 can address this need through a variety
of low current circuit techniques on the PROG pin. Once
a suitable cell voltage has been reached, charge current
can be switched to a higher, bulk charge value.
End of Charge and CHRG Output
As the battery approaches the programmed output volt-
age, charge current will begin to decrease. The open-drain
CHRG output can indicate when the current drops to 10%
of its programmed full-scale value by turning off the strong
pull-down (open-drain FET) and turning on a weak 25µA
pull-down current. This weak pull-down state is latched
until the part enters shutdown or the sensed current rises
to roughly C/6. C/10 indication will not be set if charge
current has been reduced due to adapter input current
limiting. As the charge current approaches 0A, the PWM
continues to operate in full continuous mode. This avoids
generation of audible noise, allowing bulk ceramic capaci-
tors to be used in the application.
Charge Current Monitoring
When the LTC4012-3 is charging, the voltage on the PROG
pin varies in direct proportion to the charge current. Refer-
ring to Figure 1, the nominal PROG voltage is given by:
V_PROG
=
I_CHRG
• R_SENSE
R_IN
• R_PROG
+
11.67µA
• R_PROG
Voltage tolerance on PROG is limited by the charge current
accuracy specified in the Electrical Characteristics table.
Refer to the Applications Information section on program-
ming charge current for additional details.
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