LTC1734 데이터 시트보기 (PDF) - Linear Technology
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LTC1734 Datasheet PDF : 12 Pages
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LTC1734
APPLICATIONS INFORMATION
Programming Constant Current
When in the constant current mode, the full-scale charge
current (C) is programmed using a single external resistor
between the PROG pin and ground. This charge current
will be 1000 times greater than the current through the
program resistor. The program resistor value is selected
by dividing the voltage forced across the resistor (1.5V)
by the desired resistor current.
The LTC1734 is designed for a maximum current of
approximately 700mA. This translates to a maximum
PROG pin current of 700µA and a minimum program
resistor of approximately 2.1k. Because the PROG pin is in
a closed-loop signal path, the pole frequency must be kept
high enough to maintain adequate AC stability by avoiding
excessive capacitance on the pin. See the Stability section
for more details.
The minimum full-scale current that can be reliably pro-
grammed is approximately 50mA, which requires a pro-
gram resistor of 30k. Limiting capacitive loading on the
program pin becomes more important when high value
program resistors are used. In addition, the current
monitoring accuracy can degrade considerably at very
low current levels. If current monitoring is desired, a
minimum full-scale current of 200mA is recommended.
Different charge currents can be programmed by various
means such as by switching in different program resistors
as shown in Figures 2 and 3. A voltage DAC connected
through a resistor to the PROG pin or a current DAC
connected in parallel with a resistor to the PROG pin can
also be used to program current (the resistor is required
with the IDAC to maintain AC stability as discussed in the
Stability section). Another means is to use a PWM output
from a microcontroller to duty cycle the charger into and
out of shutdown to create an average current (see Manual
Shutdown section for interfacing examples). Because
chargers are generally slow to respond, it can take up to
approximately 300µs for the charger to fully settle after a
shutdown is deasserted. This delay must be accounted for
unless the minimum PWM low duration is about 3ms or
more. Shutdown occurs within a few microseconds of a
shutdown command. The use of PWM can extend the
average current to less than the normal 200mA minimum
constant current.
OPTIONAL FILTER
CHARGE
CURRENT
MONITOR
(FILTERED)
1k
PIN 4
0.1µF TO
0.5µF
CHARGE
CURRENT
MONITOR
(UNFILTERED)
VIN
5V
1µF
3k
Q1
2N7002
3
VCC
1
ISENSE
2 LTC1734 6
GND DRIVE
4
5
PROG BAT
7.5k
Q2
2N7002
FZT549
IBAT
10µF
CHARGE CURRENT
0
200mA
500mA
700mA
CONTROL 1
LOW
LOW
HIGH
HIGH
CONTROL 2
LOW
HIGH
LOW
HIGH
SINGLE
Li-Ion
BATTERY
1734 F02
CONTROL 1 CONTROL 2
Figure 2. Logic Control Programming of Output Current to 0mA, 200mA, 500mA or 700mA
8
VIN
5V
1µF
3
VCC
1
ISENSE
2 LTC1734 6
GND DRIVE
FZT549*
3k
Q1
2N7002
4
PROG
7.5k
Q2
2N7002
5
BAT
ILOAD
LOAD
1734 F03
CURRENT
0
200mA
500mA
700mA
CONTROL 1
LOW
LOW
HIGH
HIGH
CONTROL 2
LOW
HIGH
LOW
HIGH
*OBSERVE MAXIMUM TEMPERATURE
CONTROL 1 CONTROL 2
Figure 3. Programmable Current Source with Output Current of 0mA, 200mA, 500mA or 700mA
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