LTC4012 datasheet(13/18 Pages) AD

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LTC2959

Rev. 0

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APPLICATIONS INFORMATION

Internal Registers

The LTC2959 register map is shown above. The LTC2959

integrates current through a sense resistor, measures

battery charge, voltage, current and temperature (and,

optionally, GPIO), and stores the results in internal reg-

isters accessible via I2C. Charge is stored in a 32-bit reg-

ister, all other quantities are stored in 16-bit registers.

High and low thresholds can be programmed for each

measured quantity. The voltage and current ADC channels

also include minimum and maximum trackers.

After each voltage, current, temperature or GPIO con-

version, the ADC conversion result is compared to the

respective threshold registers. If a value in the threshold

registers is exceeded, the corresponding bit in the status

enabled, the GPIO pin pulls low.

Analog inputs that exceed the ADC input range will cause

overflow or underflow. The corresponding ADC conver-

sion result will be the most negative (underflow) or most

positive (overflow) output code. Overflow or underflow

will also cause the corresponding status alert bit to be set.

If alert mode is enabled, the GPIO pin pulls low.

The minimum and maximum tracking registers will hold

the lowest and the highest value measured since their ini-

tialization, respectively. These registers are updated after

every voltage ADC or current ADC conversion.

The minimum and maximum trackers initialize to the high-

est and lowest possible conversion result, respectively,

and they can be set via I2C.

The accumulated charge register can be configured by

writing a target 32-bit value to it via I2C, or by asserting

charge complete (CC). When GPIO is configured in digital

input mode and it is pulled low externally, CC is asserted

and the ACR will be set to 32’hFFFFFFFF. Coulomb count-

ing only resumes when CC is no longer asserted.

Do not simultaneously assert charge complete and

attempt to write the ACR to a target value over I2C.

The ACR is compared to the charge thresholds every

time the coulomb counter increments or decrements it.

If the ACR value exceeds the threshold register values,

the corresponding bits A[3] or A[2] are set. Bit A[5] is

set if the ACR overflows or underflows. At each overflow

or underflow, the ACR rolls over and resumes integration.

All status register bits are cleared after being read by the

host but may be reasserted after the next ADC conversion

or charge integration, if the corresponding alert condition

is still fulfilled.

Processing Digital Results

The LTC2959 can measure charge, voltage, current and

temperature.

To calculate the amount of charge represented by the

ACR, read out registers D, E, F and G in a single sequen-

tial read. Their combined output yields an unsigned 32-bit

number, which should be multiplied by the ACR LSB size,

QLSB = 533nAh, to obtain the total charge.

The result of the 16-bit ADC conversion of the voltage

at either VDD or SENSEN (depending on the ADC control

settings) is stored in the voltage registers (P, Q). This data

is unsigned. From the result of the 16-bit voltage register

RESULT[15:0] = P[7:0]Q[7:0], the measured voltage can

be calculated as:

VBAT = 62.6V •

RESULTdec

65536

The values in the voltage threshold and min/max track-

ing registers, registers R, S, T, U, V, W, X ansd Y, are

also stored in unsigned notation. As an example, to

set the voltage low threshold to 3.72V, write registers

T[7:0]U[7:0] to 0F37h.

The result of the 16-bit ADC conversion of the current

is stored in the current registers (Z, AA). The ADC mea-

sures battery current by converting the voltage, VSENSE,

across the sense resistor RSENSE. Depending on whether

the battery is being charged or discharged, the measured

voltage drop on RSENSE is positive or negative.

The result in registers Z and AA is stored in signed, two’s

complement notation. Bit Z[7] is the sign bit of the result.

The battery current can be obtained from the two-byte

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