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LTC4150IMS

Coulomb Counter/Battery Gas Gauge

Linear Technology 

LTC4150

APPLICATIONS INFORMATION

SENSE VOLTAGE INPUT AND FILTERS

Since the overall integration time is set by internally trim-

ming 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, RSENSE. The common mode range

for the SENSE+ and SENSE– pins is VDD ±60mV, with a

maximum differential voltage range of ±50mV. SENSE+ is

normally tied to VDD, so there is no common mode issue

when SENSE– operates within the 50mV differential limit

relative to SENSE+.

Choose RSENSE to provide 50mV drop at maximum charge

or discharge current, whichever is greater. Calculate

RSENSE from:

RSENSE

=

50mV

IMAX

(1)

The sense input range is small (±50mV) to minimize the

loss across RSENSE. To preserve accuracy, use Kelvin

connections at RSENSE.

The external filter capacitor, CF, operates against a total

on-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 recom-

mended for general 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 integrator. 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 VDD and SENSE+ connection to

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® switching regulators are used. Burst pulse “on”

levels must be within the specified differential input volt-

age range of 50mV as measured at CF+ and CF–. To retain

accurate charge information, the LTC4150 must remain

enabled during Burst Mode operation. If the LTC4150

shuts down or VDD drops below 2.5V, the part resets and

charge information is lost.

8

Coulomb Counting

The LTC4150’s transfer function is quantified as a volt-

age to frequency gain GVF, where output frequency is the

number of interrupts per second and input voltage is the

differential drive VSENSE across SENSE+ and SENSE–. The

number of interrupts per second will be:

f = GVF • ⏐VSENSE⏐

(2)

where

VSENSE = IBATTERY • RSENSE

(3)

Therefore,

f = GVF • ⏐IBATTERY • RSENSE⏐

(4)

Since I • t = Q, coulombs of battery charge per INT pulse

can be derived from Equation 4:

One INT =

1

Coulombs

(5)

GVF • RSENSE

Battery capacity is most often expressed in ampere-

hours.

1Ah = 3600 Coulombs

(6)

Combining Equations 5 and 6:

One INT =

1

[Ah]

(7)

3600 • GVF • RSENSE

or

1Ah = 3600 • GVF • RSENSE Interrupts

(8)

The charge measurement may be further scaled within

the microcontroller. However, the number of interrupts,

coulombs or Ah all represent battery charge.

The LTC4150’s transfer function is set only by the value

of the sense resistor and the gain GVF. Once RSENSE is

selected using Equation 1, the charge per interrupt can

be determined from Equation 5 or 7.

Note that RSENSE is not chosen to set the relationship

between ampere-hours of battery charge and number of

interrupts issued by the LTC4150. Rather, RSENSE is chosen

to keep the maximum sense voltage equal to or less than

the LTC4150’s 50mV full-scale sense input.

4150fc

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