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SC804A Datasheet(PDF) 11 Page - Semtech Corporation |
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SC804A Datasheet(HTML) 11 Page - Semtech Corporation |
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11 / 21 page  11 © 2005 Semtech Corp. www.semtech.com SC804A PRELIMINARY POWER MANAGEMENT DRAFT Applications Information (Cont.) The choice of R CT1 and RCT2 is somewhat arbitrary. The simplest approach is to pick one and compute the other. A good choice here is R CT1 = 115kΩ, and RCT2 = 221kΩ, as these standard 1% tolerance values produce the closest match to the desired voltage divider ratio. With these resistor nominal values, which is, nominally, only 0.2% below the target value of 0.6591×V VCC. The CTO network will present a load of only 15μA to a 5V charging adapter. The nominal impedance presented to the CTO pin is R CT1 || RCT2 = 75.6kΩ. Any impedance on the order of 100k Ω (or less) is acceptable. Remote Kelvin Sensing at the Battery The BSEN pin provides the positive Kelvin sensing voltage feedback to the CV amplifier and should be connected as close to the battery + terminal as possible. Likewise, the RGND pin should be connected directly to the negative terminal of the battery. This allows the designer great flexibility in PCB layout and achieves greater accuracy by sensing the battery voltage directly at the battery terminals. When laying out the PCB, the designer should route the BSEN and RGND trace directly to the battery connection terminals, rather than just to the VOUT and GND pins on the device. Dropout Voltage Dropout voltage is the smallest achievable difference voltage between VCC and VOUT under a particular operating condition. Dropout voltage is encountered during CC charging whenever the current limit of the charging adapter is less than the SC804A FCI programmed current. In this case, the adapter voltage (the SC804A input voltage) will be pulled down to the battery voltage (the SC804A output voltage) plus the dropout voltage. Dropout voltage is the larger of two values: (1) the I-R component, which is the output current multiplied by the minimum VCC-to-VOUT path resistance (which is highly temperature dependent), and (2) a regulated minimum difference voltage, which is output voltage dependent but is independent of the output current. The regulated minimum dropout voltage results from the collapse of internal voltage references as VOUT pulls VCC down to near, or below, V CV, creating a reduced output regulation voltage approximately 200mV below VCC. Thus VCC cannot be pulled down below VOUT + 200mV. The dropout voltage will be larger than 200mV whenever the minimum path resistance multiplied by the output current exceeds 200mV, but it cannot be smaller than 200mV. This greatest-of-two-limit dropout voltage behavior is evident in the dropout voltage typical performance plot. When operating in Adjust Mode (next section), the regulated minimum dropout voltage depends on the programmed VOUT regulation voltage, and dropout also varies with the actual output voltage during CC charging. See Figure 4 for an illustration of dropout voltage data. Adjust Mode The SC804A can be configured for an output voltage other than V CV using Adjust (ADJ) Mode. In Adjust Mode the output voltage is determined by an external resistor divider from VOUT to BSEN. When BSEN is connected in this fashion, V VOUT (during Constant Voltage (CV) charging) will be controlled such that the voltage at the BSEN pin (V BSEN) is the reference voltage VBSEN-ADJ. The output voltage can be set to any voltage desired by an appropriate choice of divider network resistors, within the following limits. When the SC804A is programmed for adjust mode, V VOUT is required to be 150mV less than VVCC, and V VOUT is required to be 400mV greater than VBSEN. V VOUT within 150mV of VBSEN guarantees normal mode operation. This implies that, for BSEN used as a Kelvin sense of battery voltage, the product of the fast charge current and the charge path resistance from VOUT to the Kelvin sense point should not exceed 150mV to ensure normal mode operation. The SC804A Adjust Mode schematic is shown in Figures 3a and 3b. Referring to these schematics, the equation for setting the output voltage is: The capacitor C3 across R8 in the feedback network introduces zero-pole frequency compensation for stability. Place the zero according to the following equation to ensure stability: V CTO = VCC × R CT2 = 0.6577 × VCC R CT1 + RCT2 R11 × C3 = 1 2 × 100kHz VOUT = V BSEN-ADJ_TYP x ( 1 + R11 ) R12 |
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