LTC4150CMS datasheet(7/12 Pages) LINER

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LTC4150

4150fa

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, 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:

SENSE

MAX

(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 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 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.

APPLICATIO S I FOR ATIO

Coulomb Counting

The LTC4150’s transfer function is quantified as a voltage

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

Coulombs

SENSE

•

(5)

Battery capacity is most often expressed in ampere-hours.

1Ah = 3600 Coulombs

(6)

Combining Equations 5 and 6:

One INT

SENSE

3600 •

•

[Ah]

(7)

1Ah = 3600 • GVF • RSENSE Interrupts

(8)

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 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.

Burst Mode is registered trademark of Linear Technology Corporation.

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