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ISL9518AHRTZ Datasheet(PDF) 11 Page - Intersil Corporation |
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ISL9518AHRTZ Datasheet(HTML) 11 Page - Intersil Corporation |
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11 / 25 page  11 FN6775.0 December 8, 2008 Theory of Operation Introduction The ISL9518 differs from the ISL9518A only in the default states of the internal registers at power-up. ISL9518 defaults are for systems with an 8.4V (2-cell) battery and ISL9518A defaults are for systems with a 12.6V battery (3-cell). Unless otherwise noted, all specifications and descriptions of ISL9518 refer to both the ISL9518 and ISL9518A. A high efficiency synchronous buck converter is used to control the system voltage up to 19.2V and charging current up to 8A. The ISL9518 also has input current limiting up to 8.064A (or higher with lower values of sense resistor). The Input current limit, charge current limit, minimum and maximum system voltage are set by internal registers written with SMBus. The ISL9518 “Typical Application Circuit” is shown in Figure 2. The ISL9518 charges the battery with constant charge current, set by the ChargeCurrent register, until the battery voltage rises to a voltage set by the MaxSystemVoltage register. The charger will then operate at a constant voltage. The adapter current is monitored and if the adapter current rises to the limit set by the InputCurrent register, system voltage and battery charge current are reduced to limit adapter current. If battery voltage is below the min system voltage, the trickle charge system is activated. The ISL9518 features two voltage regulation loops and two current regulation loops. The max system voltage loop controls the voltage at CSON with a precision voltage divider to the voltage error amplifier GM2. The min system voltage prevents the system voltage from dropping below a minimum value even if a deeply discharged battery is inserted that is below the minimum. The Charge Current regulation loop limits the battery charging current delivered to the battery to ensure that it never exceeds the current set by the ChargeCurrent register. The Input Current regulation loop limits the current drawn from the AC-adapter to ensure that it never exceeds the limit set by the InputCurrent register to prevent adapter overload. PWM Control The ISL9518 employs a fixed frequency pulse width modulator (PWM) with feed forward. The switching frequency can be reduced with an SMBus command for improved light load efficiency AC-adapter Detection AC-adapter voltage is connected through a resistor divider to ADET to detect when AC power is available, as shown in Figure 2. ADPR is an open-drain output and is active low when ADET is less than Vth,fall, and high Z when ADET is above Vth,rise. The ADET rising threshold is 3.2V (typ) with 57mV hysteresis. ADET must be above the threshold to Enable the output voltage. Current Measurement ACMON is an output voltage that is proportional to the adapter current being sensed across CSIP and CSIN. The output voltage range is 0.1V to 3.2V. The voltage of ACMON is given by Equation 1: where IINPUT is the DC current drawn from the AC-adapter. A capacitor is required at the ACMON output to stabilize the ACMON amplifier and to minimize switching noise. VDD Regulator VDD provides a 5.1V supply voltage from the internal LDO regulator from DCIN and can deliver up to 30mA of continuous current. VDD also supplies power to VDDP through a low pass filter as shown in the “Typical Application Circuit” in Figure 2. The MOSFET drivers are powered by VDDP. Bypass VDDP and VDD with a 1µF capacitor. VSMB Supply The VSMB input provides power to the SMBus interface. Connect an external supply to VSMB to keep the SMBus interface active while the supply to DCIN is removed. When VSMB is biased, the internal registers are maintained. Bypass VSMB to AGND with a 0.1µF or greater ceramic capacitor. SGATE Function If ADET > 3.2V and VDD > 4.5V and ISOLATE_ADAPTER bit is 0 (default state) then SGATE will be ON (meaning SGATE will be driven to ground turning on the inrush limit and the adapter isolation FETs ON). In all other cases, SGATE is OFF (meaning the chip will not pull-down SGATE and the off chip resistor will pull the gates of the in-rush limit and adapter isolation FETs to their sources, turning them OFF). BGATE Function The BGATE pin drives the gate of an external PFET to control the minimum system voltage. If a battery is connected that is discharged below the value set in the MinSystemVoltage register, BGATE controls the system voltage at the value set in the MinSystemVoltage register. Trickle Charging If a battery that is discharged below the value set in the MinSystemVoltage register is connected to the system, the trickle charge system is activated. In trickle charge mode, the charge current is reduced to 256mA. The value in the ChargeCurrent register is not changed. The BGATE FET is controlled in a linear mode to regulate the system voltage at min system voltage and to drop voltage between the min system voltage and the battery. This state is communicated to the host system by the trickle bit in the control register and a low state on the TRKLN pin. When the battery is charged to the min system voltage, the BGATE FET becomes fully enhanced and BGATE is pulled more than 5V below the system voltage. This changes the ACMON 20 I INPUT RS1 ⋅⋅ = (EQ. 1) ISL9518, ISL9518A |
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