LTC4150 Linear Technology, LTC4150 Datasheet - Page 7

• Manufacturer Part Number: LTC4150
• Description: Coulomb Counter/ Battery Gas Gauge
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
SENSE VOLTAGE INPUT AND FILTERS
Since the overall integration time is set by internally
trimming the LTC4150, no external timing capacitor or
trimming is necessary. The only external component that
affects the transfer function of interrupts per coulomb of
charge is the sense resistor, R
range for the SENSE
a maximum differential voltage range of ±50mV. SENSE
is normally tied to V
when SENSE
relative to SENSE
Choose R
or discharge current, whichever is greater. Calculate R
from:
The sense input range is small (±50mV) to minimize the
loss across R
connections at R
The external filter capacitor C
chip resistance of 4k to form a lowpass filter that averages
battery current and improves accuracy in the presence of
noise, spikes and ripple. 4.7µF is recommended for gen-
eral applications but can be extended to higher values as
long as the capacitor’s leakage is low. A 10nA leakage is
roughly equivalent to the input offset error of the integra-
tor. Ceramic capacitors are suitable for this use.
Switching regulators are a particular concern because
they generate high levels of current ripple which may flow
through the battery. The V
the charger and load should be bypassed by at least 4.7µF
at the LTC4150 if a switching regulator is present.
The LTC4150 maintains high accuracy even when Burst
Mode
levels must be within the specified differential input volt-
age range of 50mV as measured at C
accurate charge information, the LTC4150 must remain
enabled during Burst Mode operation. If the LTC4150
shuts down or V
charge information is lost.
R
SENSE
®
switching regulators are used. Burst pulse “on”
SENSE
=
–
50
I
MAX
SENSE
operates within the 50mV differential limit
to provide 50mV drop at maximum charge
mV
DD
SENSE
+
.
DD
drops below 2.5V, the part resets and
U
+
. To preserve accuracy, use Kelvin
and SENSE
, so there is no common mode issue
.
U
DD
F
and SENSE
operates against a total on-
SENSE
–
pins is V
W
. The common mode
F
+
and C
+
DD
connection to
±60mV, with
F
–
. To retain
U
SENSE
(1)
+
Coulomb Counting
The LTC4150’s transfer function is quantified as a voltage
to frequency gain G
number of interrupts per second and input voltage is the
differential drive V
number of interrupts per second will be:
where
Therefore,
Since I • t = Q, coulombs of battery charge per INT pulse
can be derived from Equation 4:
Battery capacity is most often expressed in ampere-hours.
Combining Equations 5 and 6:
or
The charge measurement may be further scaled within the
microcontroller. However, the number of interrupts, cou-
lombs or Ah all represent battery charge.
The LTC4150’s transfer function is set only by the value of
the sense resistor and the gain G
selected using Equation 1, the charge per interrupt can be
determined from Equation 5 or 7.
Note that R
between ampere-hours of battery charge and number of
interrupts issued by the LTC4150. Rather, R
chosen to keep the maximum sense voltage equal to or
less than the LTC4150’s 50mV full-scale sense input.
Burst Mode is registered trademark of Linear Technology Corporation.
f = G
V
f = G
1Ah = 3600 Coulombs
1Ah = 3600 • G
One INT
One INT
SENSE
VF
VF
• ⏐V
• ⏐I
= I
SENSE
=
=
BATTERY
BATTERY
G
3600 •
SENSE
VF
SENSE
VF
is not chosen to set the relationship
•
⏐
R
1
• R
VF
• R
G
SENSE
• R
, where output frequency is the
VF
SENSE
across SENSE
SENSE
1
SENSE
•
R
Coulombs
SENSE
Interrupts
⏐
[Ah]
VF
+
. Once R
LTC4150
and SENSE
SENSE
SENSE
–
. The
4150fa
7
(2)
(3)
(4)
(5)
(6)
(7)
(8)
is
is