Datasheet

LTC4012/
LTC4012-1/LTC4012-2
14
4012fa
Adapter Input Current Limit
The LTC4012 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 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 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 three indicator outputs as a
function of LTC4012 operation.
operation
Table 1. LTC4012 Open-Drain Indicator Outputs
ACP CHRG ICL CHARGER STATE
Off Off Off No DC Input (Shutdown)
On Off Off Shutdown or Reverse Current
On On Off Bulk Charge
On 25µA Off Low Current Charge or Initial
DCIN – BAT <500mV
On On On Input Current Limit During Bulk
Charge
On 25µA On Input Current Limit During Low
Current Charge
Off On or
25µA
On or
Off
Indicated Charge with DCIN - BAT
< 300mV. Bulk charge may be less
than programmed value.
PWM Controller
The LTC4012 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 aver-
age 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.
Figure 2. PWM Waveforms
ON
OFF
OFF
INDUCTOR
CURRENT
TOP FET
BOTTOM FET
ON
t
OFF
THRESHOLD
SET BY ITH
VOLTAGE
4012 F02