LTC4012-2 Linear Technology Corporation, LTC4012-2 Datasheet - Page 13

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

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OPERATION
the end of soft-start will depend on which loop (current,
voltage or adapter limit) is in control of the PWM. If this
current is below that required by the ITH start-up threshold,
the resulting charge current transient duration depends on
loop compensation but is typically less than 100μs.
Bulk Charge
When soft-start is complete, the LTC4012 begins sourcing
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
section, the LT4012 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 voltage,
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
11
2
LTC4012
CLP
BAT
OSCILLATOR
CLOCK
S
R
CC
D
Q
+
–
Figure 1. PWM Circuit Diagram
EA
LOGIC
PWM
+
–
–
–
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
capacitors to be used in the application.
Charge Current Monitoring
When the LTC4012 is charging, the voltage on the PROG pin
varies in direct proportion to the charge current. Referring
to Figure 1, the nominal PROG voltage is given by
Voltage tolerance on PROG is limited by the charge
current accuracy specifi ed in the Electrical Characteristics
table. Refer to the Applications Information section on
programming charge current for additional details.
FROM ICL
1.2085V
WATCHDOG
V
R1
PROG
TIMER
CA
=
I
LTC4012-1/LTC4012-2
+
–
CHRG
BGATE
TGATE
PROG
CSP
CSN
V
•
ITH
FB
R
SENSE
19
16
15
14
13
12
9
R
4012 F01
IN
SYSTEM
COMPENSATION
POWER
R
PROG
•
R
LOOP
PROG
R
R
IN
IN
L1
C
PROG
+
LTC4012/
R
SENSE
11 67
+
–
.
+
V
μA R
I
SENSE
CHRG
•
13
PROG
4012f