Prototyping of White LED Drivers

Description: The circuit illuminates a high-brightness LED even at a voltage of 0.8V if the battery can supply 93mA at this voltage. The average step-up efficiency from 0.8V to 1.5V is 75%, with a peak efficiency of 83% occurring around 1.1V. The circuit is designed with minimal components and achieves a step-up efficiency of 70% if the drive transistor is selected carefully. In Figure 2, a practical application of the circuit is illustrated, which includes a Schottky diode (D2) and a smoothing capacitor (C1) added between transistor Q1 and the white LED (WLED). This addition is necessary to mitigate significant ringing on the collector of Q1 that exceeds the absolute maximum forward voltage of the WLED. To construct the inductor, use insulated wire of AWG30 (diameter: 0.2546mm) and wind it eight turns around TDK's axial inductor SP0406-101J (100 µH), securing the windings with glue. The handmade transformer is depicted in Photo 2, with the gold color band indicating inductance tolerance set for the primary winding polarity; winding should commence from the brown color band side for the secondary winding. It is noted that the WLED current does not reach 20mA if resistor R1 in Figure 2 is 820 ohms, which is intended for use with Panasonic's Oxilide battery, having an initial voltage of 1.7V. The current would exceed the absolute maximum for the WLED if a 680-ohm resistor is used with this battery. Implementing the current limiting circuit shown in Figure 4 may be advisable. It is important to connect a WLED before powering the circuit to avoid damaging the WLED due to excessive voltage across C1. Temporarily connecting D3 may help prevent damage to the WLED. The described circuits can drive three WLEDs (20mA class) when utilizing a pulse transformer developed by the author, as shown in Photos 3 and 4. The MAX8901A/MAX8901B step-up converters can drive two to six series-connected WLEDs with constant current, providing uniform intensity for LCD backlighting in devices like cell phones and PDAs. They operate with a supply voltage range of 2.6V to 5.5V and feature direct PWM dimming (MAX8901A) and a 32-step 1-wire serial dimming (MAX8901B). The devices are compact, measuring 2mm x 2mm. Competitive options include Intersil's EL7630 and Linear Technology's LT1932, with the LT1932 functioning at a supply voltage as low as 1.0V when fewer WLEDs are used. The MAX8901A/B has a 2.6V input undervoltage lockout and is designed for use with lithium-ion cells, making it unsuitable for applications using three Ni-MH batteries in series, as this could lead to over-discharge and reduced battery lifespan. Initially, the author intended to provide instructions for creating a "6-LED Flashlight," but the compact size proved challenging for amateur builders. The MAX8901B model features a one-shot multivibrator for dimming control, while the MAX8901A model includes a PWM controller for dimming. The step-up efficiency measured between supply voltages of 2.6V to 4.5V (excluding power consumption of the pulse generation circuit) exceeds 84%. Figure 6 illustrates the typical application circuit of MAX8901B, where pressing the tact switch dims the output by 0.75mA.

The circuit operates as a compact and efficient LED driver, employing a step-up converter to elevate low battery voltages to levels suitable for powering high-brightness LEDs. The design emphasizes component selection to achieve optimal performance, particularly concerning the drive transistor and current limiting resistors. The inclusion of a Schottky diode and smoothing capacitor serves to protect the LED from voltage spikes, ensuring reliable operation. The use of a handmade transformer allows for customization of inductance values, which is critical for achieving the desired step-up efficiency. The circuit's ability to drive multiple WLEDs highlights its versatility in applications requiring high-intensity illumination, such as backlighting for handheld devices. The integration of advanced features like PWM dimming and serial dimming enhances user control over brightness levels, making the circuit suitable for a range of consumer electronics. The careful consideration of battery chemistry and voltage limits ensures that the circuit remains within safe operating parameters, thereby prolonging the lifespan of both the batteries and the LEDs. Overall, this circuit exemplifies an effective approach to LED driving, balancing efficiency, component count, and functionality in a compact design.It lights on the high-brightness LED even at 0. 8V if a battery can feed the circuit 93mA at 0. 8V. The step-up efficiency from 0. 8V to 1. 5V is 75% in average and the peak efficiency (83%) is located at a voltage around 1. 1V. The circuit consists of very few components and it gives a step-up efficiency of 70% if the drive transistor is carefully chosen. Fig. -2 is the circuit for a practical application. It is not an intention to increase a number of components used but in the Fig. -2, a Schotkey diode D2 and a smoothing capacitor C1 are added between Q1 and a WLED. The reason is that there is a large ringing on the collector Q1 and it exceeds the absolute maximum forward voltage of the WLED and need to be smoothed by the additional components. Using a insulated wire of AWG30(wire diameter: 0. 2546mm) or similar wire, wind it 8 turns on the TDK`s axial inductor SP0406-101J (100 µH) and fix the windings by a glue.

Photo-2 is the handmade transformer. The gold color band indicating an inductance tolerance is set for the polarity of primary winding and start to wind a wire from the side of brown color band as for the polarity of secondary winding. By the way, the WLED current doesn`t reach 20mA if R1 in Fig. -2 is 820 ohms. The reason is to stand by for a use of Panasonic`s Oxilide battery which has a initial voltage of 1. 7V and the current exceeds the absolute maximum current of WLED if 680 ohms resistor and the battery is used.

It may be an idea to implement the current limiting circuit (inside the red dotted line) shown in Fig. -4. Note that don`t power up the circuit without connecting a WLED, otherwise a WLED is burned out because C1 is charged up to much higher voltage than the absolute maximum forward voltage of WLED.

It may be an advisable to connect temporally D3 to prevent the WLED`s damage. The circuits described in the above can drive 3 WLEDs (20mA class) if a pulse transformer developed by the author is used. Its prototype is shown in Photo-3 and 4. The MAX8901A/MAX8901B step-up converters drive from two to six series-connected white LEDs (WLEDs) with constant current to provide uniform WLED intensity for LCD backlighting in cell phones, PDAs, and other handheld devices.

The supply voltage is from 2. 6V to 5. 5V and the devices have a direct PWM dimming (MAX8901A) and a 32-Step 1-Wire serial dimming (MAX8901B). It is packaged such small as 2mm x 2mm. There are some competitive devices such as Intersil`s EL7630 or Linear Technology`s LT1932, and in the case of LT1932, it functions with the supply voltage of 1.

0V if the number of WLED is reduced. MAX8901A/B has a 2. 6V input undervoltage lockout and it is intended to be used with a Li+ cell. Thus it can not fit to an application of 3 Ni-MH batteries in series, because at the lockout voltage, each cell is discharged further than the empty voltage (1. 0V) and it may shorten a lifetime of cells. At the beginning of application circuit development, the author planned to provide an article for "How to Make 6-LED Flashlight" but the idea was changed since the package is too small to handle for amateurs.

@is the MAX8901B model with one-shot multivibrator for dimming control, A is the MAX8901B model with astable multivibrator for dimming control, B is the MAX8901A model with PWM controller for dimming control, and C is B plus LED select (6 WLEDs and 3 Red LEDs) / power switch. The step-up efficiency measured between supply voltage of 2. 6V to 4. 5V (excluding the power consumption of each pulse generation circuit) is higher than 84%. In the Fig. -6, the right side circuit from C4 is the typical application circuit of MAX8901B. One push of the tact switch dims 0. 75mA and it returns t