LTC4009-2_15 Datasheet, PDF (12/28 Pages) Linear Technology – High Efficiency, Multi-Chemistry Battery Charger

LTC4009
LTC4009-1/LTC4009-2
Operation
LTC4009
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
I_CHRG
R_PROG
C_PROG
FROM ICL
EA
1.2085V
+
V_FB
10
ITH
12
LOOP
COMPENSATION
4009 F01
Figure 1. PWM Circuit Diagram
Charge Current Monitoring
When the LTC4009 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
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.
Adapter Input Current Limit
The LTC4009 can monitor and limit current from the input
DC supply, which is normally an AC adapter. When the
programmed adapter input current is reached, charge
current is reduced to maintain the desired maximum input
current. The ITH and PROG pins will reflect the reduced
charge current. This limit function avoids overloading the
DC input source, allowing the product to operate at the
same time the battery is charging without complex load
12
management algorithms. The battery will automatically be
charged at the maximum possible rate that the adapter will
support, given the application’s operating condition. The
LTC4009 can only limit input current by reducing charge
current, and in this case the charger uses nonsynchro-
nous PWM operation to prevent boosting if the average
charge current falls below about 25% of the maximum
programmed current. Note that the ICL indicator output
becomes active (low) at an adapter input current level just
slightly less than that required for the internal amplifier to
begin to assert control over the PWM loop.
If system load current equals or exceeds the input adapter
current limit for more than a few milliseconds, the bootstrap
capacitor between BOOST and SW can fully discharge due
to normal pin leakage currents. In this case, the PWM will
not restart until the system current has dropped to about
85% of the programmed input adapter limit value.
Charger Status Indicator Outputs
The LTC4009 open-drain indicator outputs provide valu-
able information about the IC’s operating state and can
4009fd