QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 756
COULOMB COUNTER/BATTERY GAS GAUGE
3
OPERATION
BATTERY, LOAD, CHARGER
A battery is connected to V
BAT
on the DC756 and the
load or battery charger is connected to V
L/CHRG
. N-
Channel MOSFETs are used to interface the LTC4150
with the shift register. The outputs of the LTC4150
drive the gates of the MOSFETs pulled up to the mi-
croprocessor supply to the shift register. This design
on the DC756 allows for a battery and charger voltage
range from 2.7V to 9V. Example batteries the
LTC4150 is intended for are 1-cell or 2-cell Li-Ion and
3-cell to 6-cell NiCd or NiMH.
RSENSE
R
SENSE
is selected such that:
I
MAX
= 50mV / R
SENSE
On the DC756, up to 3.5A can pass through the 0.01
Ohm, 1/8W R
SENSE
. The equivalent value resistor in
the next larger package is rated for 1/4W and can
pass through the calculated maximum current of 5A.
INTERRUPT AND CLEAR
Every time a packet of Coulombs passes through the
sense resistor, the LTC4150 pulls low on its /INT pin
and remains low until the /CLR pin is pulled low. On
the DC756, the /INT pin can be tied to the /CLR pin so
the LTC4150 can self-clear the interrupt with JP2 or
separated for microprocessor clear.
POLARITY AND LED INDICATOR
The LTC4150 monitors the current polarity across the
sense resistor and indicates the direction with its POL
pin. When POL is high, current is flowing into the
battery and when POL is low, current is flowing out of
the battery. On the DC756, POL is also used to drive
the bi-colored LED and bit one of the shift register.
The LED turns red to indicate the battery is discharg-
ing, while a green light indicates the battery is charg-
ing.
SHUTDOWN
Shutdown sets the LTC4150 into a low current shut-
down mode and disables the LED. Set JP1 to Run for
normal operation or to /SHDN to shutdown.
VOLTAGE SPECIFIC APPLICATIONS
Typically the LTC4150 is directly interfaced with a mi-
croprocessor. The battery and charger are a known
set voltage. In the final application, if the bat-
tery/charger supply voltage is greater than the micro-
processor’s supply, then the LTC4150 input and out-
put pins can be pulled up to the microprocessor’s
supply voltage. (See Figure 2)
If the battery/charger voltage is less than the micro-
processor’s supply, the LTC4150 output pins may be
pulled up to the microprocessor supply. But a voltage
divider is implemented on the input pins to insure the
voltage does not exceed V
DD
. Refer to Figure 3 and
select the resistor values according to the following
equations:
V
DD
• V
CC
• R1 / (R1 + R2 + R
L
)
V
DD
• V
CC
• R3 / (R3 + R4)
