Whether you use them as indicators or to provide illumination, LEDs are hard to beat in efficiency, reliability, and cost. White LEDs are rapidly gaining popularity as sources of illumination, as in LCD backlights, but with forward voltages typically ranging from 3 to 5 V, operating them from a single cell presents obvious difficulties. This design exploits the ultralow operating voltage of a single-gate Schmitt inverter, such as the SN74AUC1G14 or the NC7SP14 (Figure 1). When you first apply battery power, Schottky diode D1 conducts, and the familiar Schmitt-trigger astable multivibrator starts to oscillate at a frequency determined by timing components C2 and R1. When IC1’s output goes high, transistor Q1 turns on, and current begins to ramp up in inductor L1.
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| Figure 1. | This circuit produces dazzling intensity in a white LED from very low battery voltages (a). A modification allows even lower battery voltages (b). |
The maximum, or peak, level of inductor current is
where VBATT is the applied battery voltage, VCE(SAT) is Q1’s saturation voltage, and tON is the duration of the high-level pulse at the Schmitt trigger's output. If Q1’s saturation voltage is, for example, less than 50 mV, you can ignore VCE(SAT) and simplify the expression to
At the end of tON, when the inverter output goes low, Q1 turns off, and the voltage across L1 reverses polarity. The resulting “flyback” voltage immediately raises Q1’s collector voltage above VBATT and forward-biases the LED and D2, which appear in series. This action illuminates the LED with a maximum forward current equal to IL(PEAK) and raises IC1’s supply voltage, VBOOT, to a diode drop above VBATT. D1 is now reverse-biased and remains so for as long as the circuit continues to oscillate. The resulting “bootstrapped” supply voltage for IC1 ensures that the astable multivibrator continues to operate even when VBATT falls to very low levels. You should choose values for C2 and R1 to produce a time constant of microseconds, thereby allowing a small inductance value for L1. For example, a test circuit using values of C2 = 68 pF, R1 = 39 kΩ, and L1 = 47 µH produces an operating frequency of approximately 150 kHz at VBATT = 1 V. The resulting value of tON = 3 µsec leads to a peak inductor current of approximately 65 mA and produces excellent brightness in the white LED. Even with VBATT as low as 500 mV, the corresponding peak current of 33 mA produces reasonable LED intensity.
The inductance value should be as low as possible to maintain a high peak current and, hence, adequate LED brightness at the lowest supply voltage. However, L1 should not be too small, or the peak current could exceed the LED's maximum current rating when VBATT is at a maximum. Remember that the inductor should be adequately rated to ensure it does not saturate at the highest value of peak current. Switching transistor Q1 should have very low saturation voltage to minimize losses and produce the highest possible peak current. The addition of D3 and C4 enables the circuit to generate an auxiliary supply voltage, VAUX, which you can use to drive low-power circuitry without adversely affecting the LED's intensity. With a battery voltage of 1 V, the test circuit produces good light intensity in the white LED and delivers almost 1.5 mA at 4.7 V to the auxiliary load. Even at VBATT = 500 mV, the circuit delivers 340 µA into a 10-kΩ load and maintains reasonable LED brightness. Note that IC1 cannot take power from the auxiliary rail, because VAUX can easily exceed the maximum voltage rating of the two suggested device types.
The minimum start-up voltage depends largely on the device you use for D1. Tests using a high-quality Schottky diode produce a minimum power-up voltage of just 800 mV. You can further reduce this level by replacing D1 with pnp transistor Q2 (Figure 1b). This modification allows the test circuit to start up at just 650 mV at room temperature. Note, however, that Q2’s collector-base junction becomes forward-biased under quiescent conditions, which results in wasted power in its base-bias resistor. Despite its simplicity, the circuit can produce spectacular results with high-brightness LEDs. The Luxeon range of LEDs from Lumileds allows the circuit to demonstrate its prowess. With L1 reduced to 10 µH and VBATT = 1 V, the circuit generates a peak current of 220 mA in a Luxeon LXHL-PW01 white LED, resulting in dazzling light intensity.
