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SC820ULTRT Datasheet(PDF) 11 Page - Semtech Corporation |
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SC820ULTRT Datasheet(HTML) 11 Page - Semtech Corporation |
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11 / 21 page  SC820 11 Charger Operation The SC820 is a dual-input stand-alone Li-ion battery charger. The VAD input pin is optimized for a charging adapter. The VUSB pin is optimized for charging from the USB Vbus supply. The device is independently pro- grammed for battery-capacity-dependent currents (adapter fast-charge current and termination current) using the IPRGM pin. Charging currents from the USB Vbus supply, which has a maximum load specification, are programmed using the IPUSB pin. When an input supply is first detected, a charge cycle is initiated and the STATB open-drain output goes low. If the battery voltage is less than the pre-charge threshold voltage, the pre-charge current is supplied. Pre-charge current is 20% of the programmed fast-charge current for the selected input. When the battery voltage exceeds the pre-charge thresh- old, typically within seconds for a standard battery with a starting cell voltage greater than 2V, the fast-charge Constant Current (CC) mode begins. The charge current soft-starts in three steps (20%, 60%, and 100% of pro- grammed fast-charge current) to reduce adapter load transients. CC current is programmed by the IPRGM resis- tance to ground when the VAD input is selected and by the IPUSB resistance to ground when the VUSB input is selected. The charger begins Constant Voltage (CV) regulation when the battery voltage rises to the fully-charged single- cell Li-ion regulation voltage (V CV ), nominally 4.2V. In CV regulation, the output voltage is regulated, and as the battery charges, the charge current gradually decreases. The STATB output goes high when I BAT drops below the termination current threshold, which is 10% of the IPRGM pin programmed fast-charge current regardless of the input selected. This is known as charge termination. Optional Float-charging or Monitoring Depending on the state of the ENB input, upon termina- tion the SC820 either operates indefinitely as a voltage regulator (float-charging) or it turns off its output. If the output is turned off upon termination, the device enters the monitor state. In this state, the output remains off until the BAT pin voltage decreases by the re-charge threshold (VT ReQ). A re-charge cycle then begins auto- matically and the process repeats. A forced re-charge cycle can also be periodically commanded by the pro- cessor to keep the batter y topped- off without float-charging. See the Monitor State section for details. Re-charge cycles are not indicated by the STATB pin. Charging Input Selection The SC820 has two charging supply input pins. VAD is optimized for adapter charging. VUSB is optimized for charging from the USB Vbus power supply. The inputs differ in selection rising and deselection falling thresh- olds, their behavior when overloading their respective charging sources, and in which current programming pin determines the fast-charge and pre-charge current. Both use the same Over-Voltage Protection (OVP) threshold. Glitch filtering is performed on the VAD and VUSB inputs, so an applied input voltage that is ringing across its selec- tion threshold will not be selected until the ringing has ceased. When both inputs exceed their respective UVLO thresholds, VAD is selected even when VAD voltage is applied while already charging from the VUSB input. VAD is also selected in the case that the VAD voltage exceeds its OVP threshold, so that an excessive VAD voltage will disable charging despite the presence of a valid VUSB input voltage. When a valid input (defined as greater than its selection threshold and less than the OVP threshold) is first selected, a charge cycle is initiated and the STATB output is asserted. When a new input selection is made (when VAD is applied or removed while VUSB is present), the charge cycle is immediately halted and re-initiated with the newly selected input. There is a momentary (approxi- mately 1ms) interruption in output current and a release and re -asser tion of the STATB pin during input reselection. If the VAD input charging current loads the adapter beyond its current limit, the VAD input voltage will be pulled down to just above the battery voltage. The adapter input deselection falling threshold is set close to the battery voltage pre-charge threshold to permit low- dissipation charging from a current limited adapter. The VUSB input provides a higher deselection falling threshold appropriate to the USB specification. The USB Applications Information |
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