White-LED driver operates down to 1.2V supply voltage
This circuit beats commercially available LED drivers, which bottom out at 2.5V input voltage, by operating at voltages as low as 1V.
Dave Wuchinich, Modal Mechanics, Yonkers, NY; Edited by Charles H Small and Fran Granville -- EDN, September 3, 2007
Many LED drivers, using both charge pumps and inductors, are available to boost the 1.2 to 2.4V available from single- and dual-cell NiMH (nickel-metal-hydride) batteries to the 3.6V that white LEDs require. However, most of these circuits, such as the Maxim MAX1595, require a minimum input voltage of approximately 2.5V to operate properly. The MAX1595 works with an input voltage of 2.4V but does not ensure an adequate output until the input voltage reaches approximately 3V. Furthermore, as the battery voltage decreases to the threshold level, the output becomes erratic. The circuit in Figure 1 uses a flip-flop to generate flux in an inductor, which then charges a capacitor in the common boost configuration. US Patent 4,068,149 describes the flip-flop’s operation in an application for operating an incandescent safety lamp’s flasher (Reference 1).
In Figure 1, R1 provides a path for starting current through the base-emitter junctions of Q1 and Q2. Q2 thus turns on and, in so doing, turns on Q1, rapidly forcing both transistors into saturation. However, C1 charges through R2 to the battery voltage minus the base-emitter drop of Q1 and the saturated collector-emitter voltage of Q2, eventually causing Q1 to turn off and thereby also turning off Q2. C1 then discharges through R1 and R2 and the forward-biased base-collector junction of Q2. The R2C1 time constant determines the turn-on time, and (R1+R2)(C2) determines the turn-off time. C2 acts as the capacitive input filter for the current flowing from L1 when Q2 is off and provides a substantially constant voltage to power D2, a standard white LED. The output voltage is proportional to the battery voltage.
With the component values in Figure 1 and with L1, a Coilcraft MSS7341-104MLB, the operating frequency is approximately 60 kHz. With a battery voltage of 2.36V from two NiMH cells, approximately 20 mA of current flows through the LED. In tests simultaneously driving two LEDs, each with its own current-limiting resistor, R3, the energy-conversion efficiency of the circuit at this battery voltage is approximately 80%. Operation continues with battery voltages of slightly more than 1V, and the delivered current diminishes but still provides usable illumination.
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Hello,
This note is for the boost circuit that works down
to 1.2v .
The efficiency of this circuit seems to be horrible
for use with a white LED, and maybe with any LED.
With a 270 ohm resistor in series with the LED the
resistor dissipates 0.1 watt while the LED only 0.07
watts. This in itself wastes a lot of battery energy
and battery energy determines run time on any given
battery type.
The efficiency with 2v input and an LED current of
22ma is as low as 20 percent (yikes). It's not
much better for other voltages either.
Replace the 270 ohm resistor with a 10 ohm resistor
and change R1 to 37k and the efficiency jumps up
more than three times to about 64 percent.
The trick in these kinds of designs is to keep the
resistor in series with the LED as low as possible to
ensure the longest run time with a given battery type.
Al Getz - 2007-11-10 02:12:00 PDT -
As FYI in regards to my previous post:
My article “Single NiCd cells drives op amp” appeared in printed version of EDN Mag in 1996 and it's also available online at EDN’s Web portal. As their policy prohibits adding HTML/links directly, then I was unable to provide you with direct link. Instead, I would recommend to Google™ on "Single NiCD EDN" search phrase to get to the link (it should appear on the top of the list).
Btw, the key design issue addressed in my old DI was to build DC-DC converter with high quality DC output (low ripples) and low EMI, powered by extremely low operating Voltage. Efficiency was a secondary concern, so inductorless topology was selected. In case of just driving LCD from low Voltage power source design criteria and selected topology would be different; switching inductor circuit would provide higher efficiency. Based on my experiments with switching regulators tracing back to 80s, DC converter driving LED could operate starting from 0.8 … 1.0 V using Silicon Bipolar Transistors. With certain Germanium stuff operating Voltage could go even lower, though the practical aspect of such circuit is not quite clear.
Alexander Bell - 2007-20-9 12:47:00 PDT -
Voltage booster that operates from 0.7V
This DI could be further extended by adding the Voltage booster described in my EDN article: Single NiCd cells drives op amp, EDN 12/05/96. It is actually capable of driving LED starting from operating voltage as low as 0.9 … 1V.
Regards,
Alexander Bell
Alexander Bell - 2007-20-9 09:06:00 PDT -
Yes, it would, on the face of it, seem so but the base-emitter junction of Q2 presents a finite resistance which is far greater than that presented by the inductive load in the present application or in that of the patent cited. Hence, while the current flowing through the emitter-collector junction of Q1 and the base-emitter junction of Q2 compromises efficiency, most of the current flows from the battery through the collecor emitter junction of Q2.
Dave Wuchinich - 2007-19-9 16:59:00 PDT -
There seems to be an error in the the schematic in figure 1. The base-emitter junction of Q1 is connected in series with Q2 and across the battery potential.
When Q1 saturates, Q2 will be destroyed for battery voltage above 1.4 Volts.
Paul Jacobs - 2007-7-9 08:37:00 PDT
