LTC4007-1_15 Datasheet, PDF (11/24 Pages) Linear Technology – 4A, High Efficiency, Li-Ion Battery Charger

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OPERATIO
LTC4007-1: State Diagram
MASTER
SHUTDOWN
2
1
ANY
SHUTDOWN
4
5, 6, 7
CONDITION
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3, 8,
9, 10
3, 17, 18,
19, 20, 21
CHARGE
LTC4007-1
12, 13, 14, 15, 16
40071 TBL01
gate of the input FET is driven to a voltage sufficient to keep
a low forward voltage drop from drain to source. If the
voltage between DCIN and CLN drops to less than 25mV,
the input FET is turned off slowly. If the voltage between
DCIN and CLN is ever less than – 25mV, then the input FET
is turned off in less than 10µs to prevent significant
reverse current from flowing in the input FET. In this
condition, the ACP pin is driven low and the charger is
disabled.
Battery Charger Controller
The LTC4007-1 charger controller uses a constant off-
time, current mode step-down architecture. During nor-
mal operation, the top MOSFET is turned on each cycle
when the oscillator sets the SR latch and turned off when
the main current comparator I_CMP resets the SR latch.
While the top MOSFET is off, the bottom MOSFET is turned
on until either the inductor current trips the current com-
parator I_REV or the beginning of the next cycle. The
oscillator uses the equation:
t_OFF
=
V_DCIN –
V_DCIN •
V_BAT
f_OSC
to set the bottom MOSFET on time. The result is a nearly
constant switching frequency over a wide input/output
voltage range. This activity is diagrammed in Figure 1.
The peak inductor current, at which I_CMP resets the SR
latch, is controlled by the voltage on ITH. ITH is in turn
controlled by several loops, depending upon the situation
at hand. The average current control loop converts the
voltage between CSP and BAT to a representative current.
Error amp CA2 compares this current against the desired
current programmed by R_PROG at the PROG pin and
OFF
TGATE
ON
ON
BGATE
OFF
INDUCTOR
CURRENT
t_OFF
TRIP POINT SET BY ITH VOLTAGE
Figure 1
40071 F01
40071f
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