AN3425| Application Note
Maxim/Dallas > App Notes > OSCILLATORS/DELAY LINES/TIMERS/COUNTERS Keywords: switching converters, boost converters, high voltage, MOSFETs
Dec 01, 2004
APPLICATION NOTE 3425
Converter Boosts One-Cell Battery Voltage to 50V
Cascading two step-up switching converters boosts a low voltage (as low as 0.9V) to 50V. Handheld instruments often require a voltage far above the internal analog supply rail. The circuit of Figure 1 can derive such a voltage (+50V, in this case) from the output of a single Li-ion cell.
Figure 1. These two boost converters in cascade can produce +50V from a single-cell input. No single IC at this time is capable of converting a single-cell output to 50V, but the 2-stage step-up converter of Figure 1 handles the task. Because the fixed-output, low-power boost converter (U1) accepts voltages in the range 0.9V to 5V, the circuit's power source can include batteries of various chemistries and cell counts--such as one Li-ion cell, one to three alkaline cells, or one to four NiMH cells. U1 provides a fixed 5V output that is stepped up by a low-cost boost controller (U2) to +50V. For input voltages above 1.8V, the circuit generates +50V at load currents above 6mA. For a single alkaline or NiMH cell, it generates +50V with more than 1mA of load current. (Both ICs come in small SOT23 packages.) You should choose a MOSFET with breakdown voltage above 55V and a rated current above 0.8A. A 1N4148 or other low-cost silicon diode serves for the rectifier. The forward voltage drop across that diode is neg
ligible in a +50V system. Because, moreover, the load current is low (single-digit milliampere range), the diode's power dissipation is minuscule. A 47V zener diode (D2) in conjunction with 300k in the feedback loop lowers the circuit's supply current. Quiescent supply current declines as you raise the input voltage (Figure 2).
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