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MAX630CJA Datasheet(PDF) 4 Page - Maxim Integrated Products |
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MAX630CJA Datasheet(HTML) 4 Page - Maxim Integrated Products |
4 / 14 page Detailed Description The operation of the MAX630 can best be understood by examining the voltage regulating loop of Figure 1. R1 and R2 divide the output voltage, which is com- pared with the 1.3V internal reference by comparator COMP1. When the output voltage is lower than desired, the comparator output goes high and the oscillator out- put pulses are passed through the NOR gate latch, turning on the output N-channel MOSFET at pin 3, LX. As long as the output voltage is less than the desired voltage, pin 3 drives the inductor with a series of pulses at the oscillator frequency. Each time the output N-channel MOSFET is turned on, the current through the external coil, L1, increases, storing energy in the coil. Each time the output turns off, the voltage across the coil reverses sign and the volt- age at LX rises until the catch diode, D1, is forward biased, delivering power to the output. When the output voltage reaches the desired level, 1.31V x (1 + R1 / R2), the comparator output goes low and the inductor is no longer pulsed. Current is then supplied by the filter capacitor, C1, until the output volt- age drops below the threshold, and once again LX is switched on, repeating the cycle. The average duty cycle at LX is directly proportional to the output current. Output Driver (LX Pin) The MAX630/MAX4193 output device is a large N-channel MOSFET with an on-resistance of 4 Ω and a peak current rating of 525mA. One well-known advan- tage that MOSFETs have over bipolar transistors in switching applications is higher speed, which reduces switching losses and allows the use of smaller, lighter, less costly magnetic components. Also important is that MOSFETs, unlike bipolar transistors, do not require base current that, in low-power DC-DC converters, often accounts for a major portion of input power. The operating current of the MAX630 and MAX4193 increases by approximately 1µA/kHz at maximum power output due to the charging current required by the gate capacitance of the LX output driver (e.g., 40µA increase at a 40kHz operating frequency). In compari- son, equivalent bipolar circuits typically drive their NPN LX output device with 2mA of base drive, causing the bipolar circuit’s operating current to increase by a fac- tor of 10 between no load and full load. Oscillator The oscillator frequency is set by a single external, low- cost ceramic capacitor connected to pin 2, CX. 47pF sets the oscillator to 40kHz, a reasonable compromise between lower switching losses at low frequencies and reduced inductor size at higher frequencies. CMOS Micropower Step-Up Switching Regulator 4 _______________________________________________________________________________________ Pin Description PIN NAME FUNCTION 1 LBR Low-Battery Detection Comparator Input. The LBD output, pin 8, sinks current whenever this pin is below the low-battery detector threshold, typically 1.31V. 2CX An external capacitor connected between this terminal and ground sets the oscillator frequency. 47pF = 40 kHz. 3LX This pin drives the external inductor. The internal N-channel MOSFET that drives LX has an output resistance of 4 Ω and a peak current rating of 525mA. 4 GND Ground 5+VS The positive supply voltage, from 2.0V to 16.5V (MAX630). 6IC The MAX630/MAX4193 shut down when this pin is left floating or is driven below 0.2V. For normal operation, connect IC directly to +VS or drive it high with either a CMOS gate or pullup resistor connected to +VS. The supply current is typically 10nA in the shutdown mode 7VFB The output voltage is set by an external resistive divider connected from the converter output to VFB and ground. The MAX630/MAX4193 pulse the LX output whenever the voltage at this terminal is less than 1.31V. 8 LBD The Low-Battery Detector output is an open-drain N-channel MOSFET that sinks up to 600 μA (typ) whenever the LBR input, pin 1, is below 1.31V. |
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