LTC4150

6

4150fa

OPERATIO

Charge is the time integral of current. The LTC4150

measures battery current by monitoring the voltage devel-

oped 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 the sense voltage to interupt frequency

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 undervolt-

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 effec-

tively 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.