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SP6644 Datasheet(PDF) 10 Page - Sipex Corporation |
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SP6644 Datasheet(HTML) 10 Page - Sipex Corporation |
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10 / 15 page  10 Rev:B Date 4/13/04 SP6644/6645 High Efficiency Boost Regulator © Copyright 2004 Sipex Corporation Internal Bootstrap Circuitry The internal bootstrap circuitry contains a low-voltage start-up oscillator that pumps up the output voltage to approximately 1.9V so the main DC-DC converter can function. At lower battery supply voltages, the circuitry can start up with low-load conditions. Designers can reduce the load as needed to allow start-up with input voltages below 1V. Refer to Figures 10 to 13. Once started, the output voltage can maintain the load as the battery voltage decreases below the initial start-up voltage. The start-up oscillator is powered by V BATT driving a charge pump and NMOS switch. During start-up, the P-channel synchronous rectifier remains off and either its body diode or an external diode is used as an output rectifier. BATTLO Circuitry The SP6644 device has an internal comparator for low-battery detection. If V BATT drops below 1V, BATTLO will sink current. BATTLO is an open-drain output. The SP6645 operates in the same manner with a threshold voltage of 2V. Shutdown for the SP6644 A logic LOW at SHDN will drive the SP6644 into a shutdown mode where BATTLO goes into a high-impedance state, the internal switching MOSFET turns off, and the synchronous rectifier turns off to prevent reverse current from flowing from the output back to the input. Designers should note that in shutdown, the output can drift to one diode drop below V BATT because there is still a forward current path through the synchronous-rectifier body diode from the input to the output. To disable the shutdown feature, designers can connect SHDN to V BATT. Adjustable Output Voltage Driving FB to ground (logic LOW) will drive the output voltage to the fixed-voltage operation of +3.3V + 4%. Connecting FB to a voltage divider between V OUT and ground will select an adjustable output voltage between +2V and +5.5V. Refer to Figure 28. FB regulates to +1.25V. Since the FB leakage current is 10nA maximum, designers should select the feedback resistor R2 in the 100k Ω to 1MΩ range. R1 can be determined with the following equation: R1 = R2 x -1 VOUT VREF where R1 [ Ω] and R2 [Ω] are the feedback resistors in Figure 29, V OUT [V] is the output voltage, and V REF [V] is 1.25V. Battery Reversal Protection The SP6644/6645 devices will tolerate single- cell battery reversal up to the package power- dissipation limits noted in the ABSOLUTE MAXIMUM RATINGS section. An internal diode in series with an internal 5 Ω resistor limits any reverse current to less than 220mA preventing damage to the devices. Prolonged operation above 220mA reverse-battery current can degrade performance of the devices. The Inductor The programmable peak inductor current feature of the SP6644/6645 devices affords a great deal of flexibility in choosing an inductor. The most important point to consider when choosing an inductor is to insure that the peak inductor current is programmed below the saturation rating of the inductor. If the inductor goes into saturation, the internal switches and the inductor will be stressed due to current peaking, potentially leading to reliability problems with the application circuit. The peak inductor current is programmed by putting a resistor between the R LIM pin and ground. The usable current range is between 150mA and 560mA. This is defined by: where I PEAK [A] is the peak inductor current, and R LIM [Ω] is the value of the resistor connected from pin R LIM to ground. I PEAK = 1400 R LIM |
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