LTC4012-2 Linear Technology Corporation, LTC4012-2 Datasheet - Page 14

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

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LTC4012/
LTC4012-1/LTC4012-2
OPERATION
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 refl ect 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 nonsynchronous
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 amplifi er to
begin to assert control over the PWM loop.
Charger Status Indicator Outputs
The LTC4012 open-drain indicator outputs provide valuable
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.
14
BOTTOM FET
TOP FET
INDUCTOR
CURRENT
OFF
OFF
ON
ON
Figure 2. PWM Waveforms
t
OFF
Table 1. LTC4012 Open-Drain Indicator Outputs
PWM Controller
The LTC4012 uses a synchronous step-down architecture
to produce high operating effi ciency. The nominal operating
frequency of 550kHz allows use of small fi lter components.
The following conceptual discussion of basic PWM
operation references Figure 1.
The voltage across the external charge current sense
resistor R
instantaneous current (V
pin where it is averaged by an external capacitor and
converted to a voltage by the programming resistor R
between PROG and GND. The PROG voltage becomes
the average charge current input signal to error amplifi er
EA. EA also receives loop control information from the
battery voltage feedback input V
current limit circuit.
ACP
Off
On
On
On
On
On
Off
SENSE
CHRG
On or
25μA
25μA
25μA
Off
Off
On
On
is measured by current amplifi er CA. This
On or
ICL
Off
Off
Off
Off
On
On
Off
SENSE
CHARGER STATE
No DC Input (Shutdown)
Shutdown or Reverse Current
Bulk Charge
Low Current Charge or Initial
DCIN – BAT <500mV
Input Current Limit During Bulk
Charge
Input Current Limit During Low
Current Charge
Indicated Charge with DCIN - BAT
< 300mV. Bulk charge may be less
than programmed value.
/R
FB
IN
) is fed to the PROG
and the adapter input
THRESHOLD
SET BY ITH
VOLTAGE
4012 F02
PROG
4012f