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ISL88731AHRZ-T Datasheet(PDF) 9 Page - Intersil Corporation |
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ISL88731AHRZ-T Datasheet(HTML) 9 Page - Intersil Corporation |
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9 / 23 page  9 FN6738.1 January 7, 2009 Theory of Operation Introduction The ISL88731A includes all of the functions necessary to charge 1-to-4 cell Li-ion and Li-polymer batteries. A high efficiency synchronous buck converter is used to control the charging voltage up to 19.2V and charging current up to 8A. The ISL88731A also has input current limiting up to 11A. The Input current limit, charge current limit and charge voltage limit are set by internal registers written with SMBus. Figure 2 shows the ISL88731A “Typical Application Circuit”. The ISL88731A charges the battery with constant charge current, set by the ChargeCurrent register, until the battery voltage rises to a voltage set by the ChargeVoltage 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, battery charge current is reduced so the charger does not reduce the adapter current available to the system. The ISL88731A features a voltage regulation loop (VCOMP) and two current regulation loops (ICOMP). The VCOMP voltage regulation loop monitors VFB to limit the battery charge voltage. The ICOMP 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 ICOMP current regulation loop also limits the input current drawn from the AC-adapter to ensure that it never exceeds the limit set by the InputCurrent register, and to prevent a system crash and AC-adapter overload. PWM Control The ISL88731A employs a fixed frequency PWM control architecture with a feed-forward function. The feed-forward function maintains a constant modulator gain of 11 to achieve fast line regulation as the input voltage changes. The duty cycle of the buck regulator is controlled by the lower of the voltages on ICOMP and VCOMP. The voltage on ICOMP and VCOMP are inputs to a Lower Voltage Buffer (LVB) who’s output is the lower of the two inputs. The output of the LVB is compared to an internal 400kHz ramp to produce the Pulse Width Modulated signal that controls the UGATE and LGATE drivers. An internal clamp holds the higher of the two voltages (0.3V) above the lower voltage. This speeds the transition from voltage loop control to current loop control or vice versa. The ISL88731A can operate up to 99.6% duty cycle if the input voltage drops close to or below the battery charge voltage (drop out mode). The DC/DC converter has a timer to prevent the frequency from dropping into the audible frequency range. To prevent boosting of the system bus voltage, the battery charger drives the lower FET in a way that prevents negative inductor current. An adaptive gate drive scheme is used to control the dead time between two switches. The dead time control circuit monitors the LGATE output and prevents the upper side MOSFET from turning on until 20ns after LGATE falls below 1V VGS, preventing cross-conduction and shoot-through. The same occurs for LGATE turn on. In order for the dead time circuit to work properly, there must be a low resistance, low inductance path from the LGATE driver to MOSFET gate, and from the source of MOSFET to PGND. An internal Schottky diode between the VDDP pin and BOOT pin keeps the bootstrap capacitor charged. AC-Adapter Detection Connect the AC-adapter voltage through a resistor divider to ACIN to detect when AC power is available, as shown in Figure 2. ACOK is an open-drain output and is active low when ACIN is less than Vth,fall, and high when ACIN is above Vth,rise. The ACIN rising threshold is 3.2V (typ) with 57mV hysteresis. Current Measurement Use ICM to monitor the adapter current being sensed across CSSP and CSSN. The output voltage range is 0V to 2.5V. The voltage of ICM is proportional to the voltage drop across CSSP and CSSN, and is given by Equation 1: where IADAPTER is the DC current drawn from the AC-adapter. It is recommended to have an RC filter at the ICM output for minimizing the switching noise. VDDP Regulator VDDP provides a 5.2V supply voltage from the internal LDO regulator from DCIN and can deliver up to 30mA of continuous current. The MOSFET drivers are powered by VDDP. VDDP also supplies power to VCC through a low-pass filter as shown in the “Typical Application Circuit” (see Figure 2) on page 2. Bypass VDDP and VCC with a 1µF capacitor. VDDSMB Supply The VDDSMB input provides power to the SMBus interface. Connect VDDSMB to VCC, or apply an external supply to VDDSMB to keep the SMBus interface active while the supply to DCIN is removed. When VDDSMB is biased the internal registers are maintained. Bypass VDDSMB to GND with a 0.1µF or greater ceramic capacitor. Short Circuit Protection and 0V Battery Charging Since the battery charger will regulate the charge current to the limit set by the ChargeCurrent register, it automatically has short circuit protection and is able to provide the charge current to wake up an extremely discharged battery. Undervoltage trickle charge folds back current if there is a short circuit on the output. ICM 20 I INPUT RS1 ⋅⋅ = (EQ. 1) ISL88731A |
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