LTC4009-2_15 Datasheet, PDF (13/28 Pages) Linear Technology – High Efficiency, Multi-Chemistry Battery Charger

LTC4009
LTC4009-1/LTC4009-2
Operation
be used for a variety of purposes in applications. Table 1
summarizes the state of the three indicator outputs as a
function of LTC4009 operation.
Table 1. LTC4009 Open-Drain Indicator Outputs
ACP
CHRG
ICL CHARGER STATE
Off
Off
Off No DC Input (Shutdown)
On
Off
Off Shutdown, Reverse Current or
DCIN Overvoltage
On
On
Off Bulk Charge
On
25µA
Off Low Current Charge or Initial
CLP-BAT < 100mV
On
On
On Input Current Limit During Bulk
Charge
On
25µA
On Input Current Limit During Low
Current Charge
On
Off
On Input Current Limit During DCIN
Overvoltage
PWM Controller
The LTC4009 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. The ITH output of the error amplifier is a scaled
control voltage for one input of the PWM comparator, CC.
ITH sets a peak inductor current threshold, sensed by R1,
to maintain the desired average current through R_SENSE.
The current comparator output does this by switching the
state of the RS latch at the appropriate time.
At the beginning of each oscillator cycle, the PWM clock
sets the RS latch and turns on the external topside NFET
(bottom-side synchronous NFET off) to refresh the current
carried by the external inductor L1. The inductor current
and voltage across R_SENSE begin to rise linearly. CA buffers
this instantaneous voltage rise and applies it to CC with
gain supplied by R1. When the voltage across R1 exceeds
the peak level set by the ITH output of EA, the top FET
turns off and the bottom FET turns on. The inductor cur-
rent then ramps down linearly until the next rising PWM
clock edge. This closes the loop and sources the correct
inductor current to maintain the desired parameter (charge
current, battery voltage, or input current). To produce a
near constant frequency, the PWM oscillator implements
the equation:
t_OFF
=
CLP – BAT
CLP • 550kHz
Repetitive, closed-loop waveforms for stable PWM opera-
tion appear in Figure 2.
ON
TOP FET
OFF
ON
BOTTOM FET
OFF
INDUCTOR
CURRENT
t_OFF
Figure 2. PWM Waveforms
THRESHOLD
SET BY ITH
VOLTAGE
4009 F02
4009fd
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