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MAX653MJA Datasheet(PDF) 8 Page - Maxim Integrated Products |
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MAX653MJA Datasheet(HTML) 8 Page - Maxim Integrated Products |
8 / 12 page Figure 2 shows what happens to the ideal circuit of Figure 1 if the switch turns on with a 66% duty cycle and V+ = 3/2 VOUT. The inductor current rises more slowly than it falls because the magnitude of the voltage applied during tON is less than that applied during tOFF. Varying the duty cycle and switching frequency keeps the peak current constant as input voltage varies. The MAX639/MAX640/MAX653 control the switch (tON and tOFF) according to the following equations: Equation (1) tON = 50µsV / (V+ - VOUT) Equation (2) tOFF ≥ 50µsV / VOUT Equation (3) IPEAK = 50µsV / L These three equations ensure constant peak currents for a given inductor value, across all input voltages (ignoring the voltage drop across the diode (D1) and the resistive losses in the switch and inductor). The variable duty cycle also ensures that the current through the inductor discharges to zero at the end of each pulse. Figure 3 shows the MAX639/MAX640/MAX653 block dia- gram and a typical connection in which 9V is converted to 5V (MAX639), 3.3V (MAX640), or 3.0V (MAX653). The sequence of events in this application is as follows: When the output dips: (1) The error comparator switches high. (2) The internal oscillator starts (15µs start-up time) and connects to the gate of the LX output driver. (3) LX turns on and off according to tON and tOFF, charging and discharging the inductor, and sup- plying current to the output (as described above). When the output voltage recovers: (1) The comparator switches low. (2) LX turns off. (3) The oscillator shuts down to save power. Fixed or Adjustable Output For operation at the preset output voltage, connect VFB to GND; no external resistors are required. For other output voltages, use an external voltage divider. Set the output voltage using R3 and R4 as determined by the following formula: R3 = R4 [(VOUT / VFB Threshold) - 1] where R4 is any resistance in the 10k Ω to 1MΩ range (typ- ically 100k Ω), and the VFB threshold is typically 1.28V. 5V/3.3V/3V/Adjustable, High-Efficiency, Low IQ, Step-Down DC-DC Converters 8 _______________________________________________________________________________________ MAX639 MAX640 MAX653 VARIABLE FREQUENCY AND DUTY-CYCLE OSCILLATOR ERROR COMPARATOR LOW-BATTERY COMPARATOR +1.28V BANDGAP REFERENCE MODE-SELECT COMPARATOR COUT 100 µF 1N5817 L = 100 µH 5V, 3.3V OR 3.0V AT 100mA 5 1 LX VOUT 50mV GND 4 VFB 7 R2 2 3 LBO LBI R1 CIN 33 µF INPUT, +5.5V TO +11.5V (MAX639), +3.8V TO +11.5V (MAX640), +3.5V TO +11.5V (MAX653) 8 6 SHDN V+ Figure 3. Block Diagram |
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