LM26 datasheet(4/6 Pages) NSC

4 / 6 page

Electrical Characteristics (Continued)

Note 5: Adapter Enable input is used to change the state of Disconnect, Direction Sense input is used to change the state of Backfeed.

Fall time is measured as the time it takes Backfeed or Disconnect voltage to go from a high level (approx. VADAPTER − 0.5V) to (VADAPTER − 4V) after the signal

is initiated at Adapter Enable or Direction Sense.

Rise time is measured as the time it takes Backfeed or Disconnect voltage to go from a low level (approx. 0.5V) to (VADAPTER − 2V) after the signal is initiated

at Adapter Enable or Direction Sense.

Typical Application

10130903

Notes

SENSE value should be selected to guarantee that a

programmed over current will cause a voltage drop

across R

SENSE of approx. 135mV. If a higher value of

SENSE has to be selected then R1/R3 resistor divider

should be used to scale the voltage drop down.

Internal and external circuits associated with Direction

Sense and Backfeed FET pins allow to emulate a Schot-

tky diode functionality with much lower forward voltage

drop and, therefore, with much lower power dissipation.

R2/R4 resistor divider programs the backfeed current

protection threshold. The backfeed FET Q1 turns off

when the backfeed current pulse level causes more than

9 mV (typical value) voltage drop between V

adapter pin

and Direction Sense pin.

Voltage difference (V

DIRECTION SENSE -VADAPTER) must

be higher than 20mV to reliably isolate the adapter in

case of leakage.

R5 and R6 prevent the FETs from turning on unless

driven by LM2601.

DELAY value can be calculated from the following for-

mula:

DELAY(nF) = TDELAY(µs)/90 at 25˚C

DELAY temperature coefficient equals 4000ppm/˚C. See

also the Typical Delay Time vs. Temperature graph.

Pin Master Power Source (MPS) should be connected to

System Management Controller’s (SMC) and battery

packs’ GPIOs. When SMC detects a powered AC

adapter via active signals at Charger Present or Adapter

Present outputs, it asserts Adapter Enable signal and

isolates the battery pack(s). Adapter Enable signal turns

the Disconnect FET on. When the AC adapter is un-

plugged or powered off, SMC enables one of the system

battery packs. The battery packs’ embedded controller

starts driving the logic MPS signal high that turns the

Disconnect FET off.

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