LTC4150

7

4150fc

OPERATION

Charge is the time integral of current. The LTC4150 mea-

sures battery current by monitoring the voltage developed

across a sense resistor and then integrates this information

in several stages to infer charge. The Block Diagram shows

the stages described below. As each unit of charge passes

into or out of the battery, the LTC4150 INT pin interrupts

an external microcontroller and the POL pin reports the

polarity of the charge unit. The external microcontroller

then resets INT with the CLR input in preparation for the

next interrupt issued by the LTC4150. The value of each

charge unit is determined by the sense resistor value and

LTC4150.

Power-On and Start-Up Initialization

When power is first applied to the LTC4150, all internal

circuitry is reset. After an initialization interval, the LTC4150

the voltage across the sense resistor but will be at least

5ms. It may take an additional 80ms for the LTC4150 to

accurately track the sense voltage. An internal undervoltage

Asserting SHDN low also resets the LTC4150’s internal

circuitry and reduces the supply current to 1.5μA. In this

condition, POL and INT outputs are high impedance. The

LTC4150 resumes counting after another initialization

interval. Shutdown minimizes battery drain when both

the charger and load are off.

CHARGE COUNTING

changes in current arising from ripple, noise and spikes

in the load or charging current.

Second, the filter’s output is applied to an integrator with

the amplifier and 100pF capacitor at its core. When the

integrator output ramps to REFHI or REFLO levels, switches

S1 and S2 reverse the ramp direction. By observing the

condition of S1 and S2 and the ramp direction, polarity is

determined. The integrating interval is trimmed to 600μs

at 50mV full-scale sense voltage.

Third, a counter is incremented or decremented every

time the integrator changes ramp direction. The counter

effectively increases integration time by a factor of 1024,

greatly reducing microcontroller overhead required to

service interrupts from the LTC4150.

At each counter under or overflow, the INT output latches

low, flagging a microcontroller. Simultaneously, the POL

output is latched to indicate the polarity of the observed

charge. With this information, the microcontroller can

total the charge over long periods of time, developing

an accurate estimate of the battery’s condition. Once the

interrupt is recognized, the microcontroller resets INT with

a low going pulse on CLR and awaits the next interrupt.

Alternatively, INT can drive CLR.