BQ24700_14 Datasheet, PDF (14/32 Pages) Texas Instruments – NOTEBOOK PC BATTERY CHARGE CONTROLLER

bq24700, bq24701
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NOTEBOOK PC BATTERY CHARGE CONTROLLER
AND SELECTOR WITH DPM
SLUS452B – APRIL 2001 – REVISED NOVEMBER 2002
APPLICATION INFORMATION
ACDET sense point
The ACDET function senses adapter voltage via a resistor divider (refer to the Application Circuit). The location
of the ACDET sense node depends on the maximum adapter voltage capability. For operation with V_ADPT < 18
V, the ACDET sense node can be at the anode of the input blocking diode. Since the V_ADPT voltage does not
exceed the absolute maximum rating of the SRN pin, SRN stays within safe operating range. For operation with
V_ADPT ≥ 18 V, the ACDET sense node should be at the cathode of the input blocking diode. Moving the ACDET
sense point to the cathode of the input diode ensures that the bq24700/bq24701 remains active after adapter
power is lost until the load capacitors have discharged to a safe level to protect the SRN and SRP pins. In either
case, it is assumed that the ACDET level is set for V_ADPT < 17 V.
alternative method
Alternatively, the battery select MOSFET and its associated gate drive protection circuitry could be replaced
with a Schottky. The Schottky allows the ACDET sense point to be moved to the anode side of the input diode,
for V_ADPT ≥ 18 V, since it blocks the system voltage from the SRN and SRP pins. The bq24700/bq24701 would
retain all functionality with fewer components at the expense of lower battery efficiency and a higher drop-out
voltage.
battery charger operation
The bq24700/bq24701 fixed-frequency, PWM controller is designed to provide closed-loop control of battery
charge-current (I_CH) based on three parameters, battery-float voltage (V_BAT), battery-charge current, and
adapter charge current (I_ADPT). The bq24700/bq24701 is designed primarily for control of a buck converter
using a high side P-channel MOSFET device (SW, refer to Figure 3).
The three control parameters are voltage programmable through resistor dividers from the bq24700/bq24701
precision 5-V reference, an external or internal precision reference, or directly via a DAC interface from a
keyboard controller.
Adapter and battery-charge current information is sensed and fed back to two transconductance (g_m ) amplifiers
via low-value-sense resistors in series with the adapter and battery respectively. Battery voltage information is
sensed through an external resistor divider and fed back from the battery to a third g_m amplifier.
NOTE:The ENABLE pin should be asserted high only after ACDET has been asserted high and
V_REF has been established. When ac is lost, and the bq24700/bq24701 drives ACPRES low, the
host must assert the ENABLE low.
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