LTC4012-3_15 Datasheet, PDF (14/28 Pages) Linear Technology – High Efficiency, Multi-Chemistry Battery Charger with PowerPath Control

LTC4012-3
Operation
Adapter Input Current Limit
The LTC4012-3 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
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
LTC4012-3 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.
Charger Status Indicator Outputs
The LTC4012-3 open-drain indicator outputs provide valu-
able information about the IC’s operating state and can
be used for a variety of purposes in applications. Table 1
summarizes the state of the indicator outputs as a function
of LTC4012-3 operation.
Table 1. LTC4012-3 Open-Drain Indicator Outputs
CHRG
ICL
CHARGER STATE
Off
Off
No DC Input (Shutdown)
or Reverse Current
On
Off
Bulk Charge
25µA
Off
Low Current Charge or Initial
DCIN – BAT <500mV
On
On
Input Current Limit During Bulk
Charge
25µA
On
Input Current Limit During Low
Current Charge
PWM Controller
The LTC4012-3 uses a synchronous step-down architec-
ture to produce high operating efficiency. The nominal
operating frequency of 550kHz allows use of small filter
components. The following conceptual discussion of basic
PWM operation references Figure 1.
The voltage across the external charge current sense
resistor R_SENSE is measured by current amplifier, CA. This
instantaneous current (V_SENSE/R_IN) is fed to the PROG pin
where it is averaged by an external capacitor and converted
to a voltage by the programming resistor R_PROG between
PROG and GND. The PROG voltage becomes the average
charge current input signal to error amplifier, EA. EA also
receives loop control information from the battery voltage
feedback input, V_FB, and the adapter input current limit
circuit.
ON
TOP FET
OFF
ON
BOTTOM FET
OFF
INDUCTOR
CURRENT
t_OFF
Figure 2. PWM Waveforms
THRESHOLD
SET BY ITH
VOLTAGE
4012-3 F02
40123fb
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