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Al Dutcher, Al Labs, West Deptford, NJ -- EDN, 7/5/01

                 The circuit in Figure 1 allows you to light any type of LED
                 from a single cell whose voltage ranges from 1 to 1.5V. This
                 range accommodates alkaline, carbon-zinc, NiCd, or NiMH
                 single cells. The circuit's principal application is in LED-based
                 flashlights, such as a red LED in an astronomer's flashlight,
                 which doesn't interfere with night vision. White LEDs make
                 handy general-purpose flashlights. You can use the circuit in
                 Figure 1 with LEDs ranging from infrared (1.2V) to blue or
                 white (3.5V). The circuit is tolerant of the varying LED voltage
                 requirements and delivers relatively constant power. It provides
                 compensation for varying battery voltage. The circuit is an
                 open-loop, discontinuous, flyback boost converter. Q2 is the
                 main switch, which charges L1 with the energy to deliver to the
                 LED. When Q2 turns off, it allows L1 to dump the stored
                 energy into the LED during flyback.

                 Q1, an inverting amplifier, drives Q2, an inverting switch. R4,
                 R5, and R2 provide feedback around the circuit. Two
                 inversions around the loop equal noninversion, so regeneration
                 (positive feedback) exists. If you replace L1 with a resistor, the
                 circuit would form a classic bistable flip-flop. L1 blocks dc
                 feedback and allows it only at ac. Thus, the circuit is astable,
                 meaning it oscillates. Q2's on-time is a function of the time it
                 takes L1's current to ramp up to the point at which Q2 can no
                 longer stay in saturation. At this point, the circuit flips to the off
                 state for the duration of the energy dump into the LED, and the
                 process repeats. Because inductors maintain current flow, they
                 are essentially current sources as long as their stored energy
                 lasts. An inductor assumes any voltage necessary to maintain
                 its constant-current flow. This property allows the circuit in
                 Figure 1 to comply with the LED's voltage requirement.

                 Constant-voltage devices, such as LEDs, are happiest when
                 they receive their drive from current sources. The LED in
                 Figure 1 receives pulses at a rapid rate. The inductor size is
                 relatively unimportant, because it determines only the
                 oscillation frequency. If, in the unlikely case the inductor value
                 is too large, the LED flashes too slowly, resulting in a
                 perceivable flicker. If the inductor value is too small, switching
                 losses predominate, and efficiency suffers. The value in Figure
                 1 produces oscillation in the 50-kHz neighborhood, a
                 reasonable compromise. D1 provides compensation for varying
                 cell voltage. By the voltage-division action at Node 4, D1
                 provides a variable-clipping operation. The higher the supply
                 voltage, the higher the clipping level, and the result is
                 correspondingly less feedback. Q1 inverts this clipping level to
                 reduce the turn-on bias to Q2 at higher cell voltages. We chose
                 2N3904s, but any small-signal npn works. Q2 runs at high
                 current at the end of the charging ramp. Internal resistance
                 causes its base-voltage requirement to rise. The R2-R1 divider
                 at Q1's base raises the collector voltage to match that
                 requirement and thus controls Q2's final current.

                 The LED's drive current is a triangular pulse of approximately
                 120 mA peak, for an average of approximately 30 mA to a red
                 LED and 15 mA to a white one. These levels give a reasonable
                 brightness to a flashlight without unduly stressing the LED. The
                 supply current for the circuit is approximately 40 mA. A
                 1600-mAhr NiMH AA cell lasts approximately four hours. L1
                 must be able to handle the peak current without saturating. The
                 total cost of the circuit in Figure 1 is less than that of a white
                 LED. You can use higher current devices and larger cells to run
                 multiple LEDs. In this case, you can connect the LEDs in
                 series. If you connect them in parallel, the LEDs need
                 swamping (ballast) resistors. You can also rectify and filter the
                 circuit's output to provide a convenient, albeit uncontrolled, dc
                 supply for other uses.