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SC804EVB Datasheet(PDF) 8 Page - Semtech Corporation |
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SC804EVB Datasheet(HTML) 8 Page - Semtech Corporation |
8 / 21 page 8 © 2007 Semtech Corp. www.semtech.com SC804 PRELIMINARY POWER MANAGEMENT Fast-Charge Mode (CC) The fast-charge CC (Constant Current) mode is active when the battery voltage is above VT PreQ and less than VCV. The fast-charge current can be set to a maximum of 1.5A and is selected by the program resistor on the IPRGM pin. The voltage on this pin will represent the current through the battery, enabling a microprocessor via an analog-to-digital converter (ADC) to monitor battery current by sensing the voltage on the IPRGM pin. The equation to set the fast- charge current is given by: The superior fast-charge current accuracy of the SC804 is obtained by use of a patented* polarity-switched (i.e., chopped) current sense amplifier to nullify current mea- surement offset errors. Compliance with the absolute maximum output current I VOUTMAX, allowing for current regulation tolerance, requires that R IPRGM be no smaller than 1.05kΩ nominal. R IPRGM can be as large as 11.5k Ω, for a nominal FCI as small as 130mA, but must exceed PCI by at least 80mA. Note that for a given program resistor the current through the battery in CV mode can be determined by replacing V IPRGM_Typ with the actual voltage on the IPRGM pin in the above equation. The CC current can also be modified by applying an analog voltage to the AFC pin as described below. Analog Fast Charge (AFC Pin) Many applications require more than one current setting for fast-charge. This behavior is obtained in the SC804 using the AFC function. When the AFC pin is connected to VCC the device behaves as described in the previous section. When the AFC pin is driven by an analog voltage between 0V and (V VCC-1.0)V, the SC804 automatically uses this pin voltage to set the maximum fast-charge current according to the following equation: General Operation The SC804 can be configured independently with respect to fast-charge and termination current, output voltage, and timing, depending on the application. A typical charging cycle is described below. Details on alternative applications and output programmability are covered in the individual sections. The charging cycle begins when the power adapter is connected to the device. The SC804 performs glitch filtering on the VCC input and initiates a charge cycle when V VCC is greater than the under-voltage lockout (UVLO) rising threshold voltage. If the battery voltage is less than the pre-charge threshold level, the SC804 will output the pre- charge current. Once the pre-charge threshold voltage is exceeded, the SC804 enters fast-charge constant current (CC) mode. When the battery voltage reaches its final value, the charger enters the constant voltage (CV) mode. In this mode the output current decreases as the battery continues to charge until the termination current level is reached. The CHRGB output turns off when IOUT drops below the termination current. If the charge timer is active, the SC804 continues to hold the battery in CV charge mode until the timer expires. When the timer expires the charger enters the monitor mode where the output remains off until the voltage at VOUT drops by VT ReQ. At this point a new charge cycle is initiated. Pre-Charge Mode Pre-charge mode is automatically enabled whenever the battery voltage is below the pre-charge threshold voltage, VT PreQ. It is used to limit the power dissipation and precondition the battery for fast charging. The pre-charge current value is determined by the resistor on the ITERM pin. The pre-charge current is programmable from 50mA to 350mA. The equation to select the pre-charge current is given by: Where V ITERM_Typ designates the typical value of VITERM. When the timer is enabled there is also a maximum allowed pre- charge duration. If the pre-charge time exceeds 25% of the total charge cycle the charger will turn off due to a pre-charge fault. This fault is cleared when VCC is toggled or the output voltage rises above VT PreQ. PCI = V ITERM_Typ × 100 R ITERM V IPRGM_Typ × 1000 FCI = R IPRGM FCI = V AFC × 1000 R IPRGM Applications Information *US Patent 6,836,095. |
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