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MAX1680 Datasheet(PDF) 6 Page - Maxim Integrated Products |
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MAX1680 Datasheet(HTML) 6 Page - Maxim Integrated Products |
6 / 12 page _______________Detailed Description The MAX1680/MAX1681 switched-capacitor voltage converters either invert or double the input voltage. They have low output resistance (3.5 Ω) and can deliver up to 125mA output current. These devices operate at one of two selectable frequencies: 125kHz/250kHz (MAX1680) and 500kHz/1MHz (MAX1681). This pro- vides the flexibility to optimize capacitor size, operating supply current, and overall circuit efficiency. Frequency selection also allows for minimizing coupling into other sensitive circuits. These devices contain no internal divider; the oscillator frequency equals the switching frequency. The devices can easily be cascaded to pro- duce a higher output voltage, or paralleled to deliver more current. The MAX1680/MAX1681 feature a shutdown mode that reduces supply current to <1µA (SHDN = high). OUT, in the inverter configuration, pulls to ground in shut- down mode. Shutdown is not available in the doubler configuration; connect SHDN to OUT. __________Applications Information Voltage Inverter A simple voltage inverter is the most common MAX1680/MAX1681 application. It requires three exter- nal capacitors (including the input bypass capacitor) as shown in the Typical Operating Circuits (inverter configuration). Although the output is not regulated, low output resistance produces a typical drop of only 0.44V with a 125mA load. This low output resistance makes the devices fairly insensitive to changes in load (see the graphs for Output Source Resistance vs. Temperature and Supply Voltage in the Typical Operating Characteristics section). Voltage Doubler The MAX1680/MAX1681 can be configured as a volt- age doubler with two external capacitors as shown in the Typical Operating Circuits (doubler configuration). When loaded, the output voltage drop is similar to that of the voltage inverter. The minimum input supply range is slightly higher than in the inverter configuration. Calculate ripple voltage using the equation in the Capacitor Selection section. Frequency Control A frequency-control pin, FSEL, provides design flexibili- ty. Each device has two selectable frequencies: 125kHz/250kHz (MAX1680) and 500kHz/1MHz (MAX1681). This allows optimization of capacitor size and supply current for a given output load. Table 1 summarizes the frequency options . Table 1. Nominal Switching Frequencies 125mA, Frequency-Selectable, Switched-Capacitor Voltage Converters 6 _______________________________________________________________________________________ ______________________________________________________________Pin Description Selects operating frequency. MAX1680: 250kHz when FSEL is low, 125kHz when FSEL is high. MAX1681: 1MHz when FSEL is low, 500kHz when FSEL is high. FSEL 1 Positive Charge-Pump Capacitor Connection CAP+ 2 Negative Charge-Pump Capacitor Connection CAP- 4 Power-Supply Ground Input GND 3 Logic Voltage Input. Connect LV to GND. LV 6 Power-Supply Positive Voltage Input IN 8 SHDN 7 Negative Voltage Output OUT 5 Power-Supply Positive Voltage Input Power-Supply Ground Connection Connect LV to OUT. Not available; connect to OUT. Shutdown Input. Driving SHDN high disables the charge pump, and the output goes to 0V. SHDN is a CMOS input. Positive Voltage Output MAX1680 250 FSEL = LV 125 FSEL = IN FSEL CONNECTION MAX1681 1000 500 FREQUENCY (kHz) INVERTER DOUBLER PIN FUNCTION NAME |
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