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SP6641BEK-L-5-0-TR Datasheet(PDF) 7 Page - Exar Corporation |
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SP6641BEK-L-5-0-TR Datasheet(HTML) 7 Page - Exar Corporation |
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7 / 14 page  7 Rev. 5/26/05, *Patent Pending SP6641A/6641B 500mA Alkaline DC/DC Boost Regulator in SOT-23 © Copyright 2002 Sipex Corporation The SP6641 feed forward off-time control de- livers more load current than constant off-time control because the input battery voltage drops during its life cycle. The term (IPK – KOFF/2L) is the average current delivered to the output ca- pacitor during the discharge phase. This is con- stant with respect to input and output voltage. With constant off-time control, the average dis- charge current term becomes (IPK-TOFF*(VOUT-VIN)/2L), which decreases as the input voltage drops. Table 1 illustrates the average inductor current delivered to the load during discharge versus the input voltage. The SP6641 feed forward off- time control and the constant off-time control are compared. For purposes of illustration, the off times of each control scheme are normalized at a typical two cell alkaline input voltage of 2.6V. The values used in Table 1 are: IPK = 0.33A L = 22 µH VOUT= 3.3V TOFF (SP6641) = 1.5V*µs/(3.3-VIN) TOFF (constant) = 2.14µs SP6641A Constant TOFF VIN TOFF Avg IL TOFF Avg IL 3.0 5.00 µs 0.30A 2.14 µs 0.32A 2.6 2.14 µs 0.30A 2.14 µs 0.30A 2.0 1.15 µs 0.30A 2.14 µs 0.27A 1.3 0.75 µs 0.30A 2.14 µs 0.23A 1.0 0.65 µs 0.30A 2.14 µs 0.22A Table 1- Average IL vs. Input Voltage The following equation defines the burst mode frequency under heavy load conditions: where: VD = Forward schottky drop, (0.4V, typ) VC = Average charging switch drop, Rnmos*IPK, typically 0.1V Ignoring the conduction losses of VD and VC, the burst frequency equation simplifies to: FBURST = (VOUT – VIN)VIN KOFFVOUT Startup The internal regulator circuitry is bootstrapped to the VOUT pin. This requires a low voltage oscillator and charging switch powered from the VBATT pin to pump up the output voltage until the reference is established. The reference pro- vides a REFREADY signal that determines when output control is handed over to the regulator. REFREADY shuts down the startup circuit and enables the regulator when the reference is valid and VOUT is above +1.9V. Once the regulator is given control it will continue to pump up the output at full power until regulation is reached. For two cell alkaline input voltages and above, the output voltage will be pulled above +1.9V quickly through the rectifying diode before the reference has a chance to establish. In this sce- nario the startup circuit will coarsely regulate around +2.8V until the REFREADY signal as- serts. This keeps the output from overshooting in startup with higher input voltages. Startup is guaranteed at +0.9V at room tempera- ture with a 3k Ω load. Heavier loads will require a higher input voltage. Shutdown/Enable Control Pin 4 of the device is a VBATT referred control pin that shuts down the converter with the pin tied to ground, or enables the converter with the pin tied to VBATT. When the converter is shut- down the power switch is opened and all circuit biasing is extinguished leaving only junction leak- age currents on supply pins 3 and 5. The output voltage will droop to one diode drop below the battery voltage through the rectifying diode. After pin 4 is brought high, the startup circuit is enabled and starts pumping up the output until REFREADY hands over control to the internal regulator. OPERATION Loop Regulation: continued F BURST = (VOUT–VIN)( VIN–VC ) KOFF VOUT + VD –VC |
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