Light Up 12 LEDs from a Single-Cell Li-Ion Battery via Highly Integrated 3mm × 2mm Dual-LED-String Driver

Introduction

The LT3497 is a dual step-up converter capable of driving up to 12 white LEDs from a single-cell Li-Ion input. The device is capable of driving asymmetric LED strings with independent dimming and shutdown control, perfect for driving backlight circuits in battery-powered portable devices, such as cellular phones, MP3 players, PDAs, digital cameras, and portable GPS devices.

The LT3497 directly regulates LED current, providing consistent brightness for all LEDs regardless of variations in their forward voltage drop. Important features including internal compensation, open-LED protection, DC/PWM dimming control, a 35V power switch and a 35V Schottky diode are all integrated into the part, making the LT3497 LED driver an ideal solution for space-constrained portable devices. In addition, the 2.3MHz switching frequency allows the use of tiny inductors and capacitors. Figure 1 shows a typical 12-white-LED application. Figure 2 shows the efficiency of the circuit.

Figure 1. Li-Ion powered driver for twelve white LEDs.

Figure 2. Efficiency of the circuit in Figure 1.

Features

High Side Sense

The LT3497 features a unique high side LED current sense that enables the part to function as a 1-wire current source—the cathode side of the bottom LED in the string can be returned to ground anywhere, allowing a simple 1-wire LED connection. Traditional LED drivers use a grounded resistor to sense LED current requiring a 2-wire connection to the LED string. High side sense moves the sense resistor to the top of the LED string. In addition, high side sense allows the LT3497 to operate in unique applications (Buck-Mode or Buck-Boost Mode) where traditional LED drivers cannot be used.

Dimming & Shutdown Control

The LT3497 features single pin shutdown and dimming control for each converter. The LED current in the two drivers can be set independently by modulating the CTRL1 and CTRL2 pins. There are three different types of dimming methods: DC voltage dimming, filtered PWM signal dimming and direct PWM dimming.

The LED currents are proportional to the DC voltages at the CTRL1 and CTRL2 pins, so DC voltage dimming is achieved by reducing the voltage on the CTRL pin. The dimming range of the part extends from 1.5V at the CTRL pin for full LED current down to 100mV. The CTRL pin directly controls the regulated sense voltage across the sense resistor that sets the LED current (see Figure 3).

Figure 3. LED sense voltage versus CTRL pin voltage.

Filtered PWM dimming works similarly to DC voltage dimming except that the DC voltage input to the CTRL pins comes from an RC-filtered PWM signal. The corner frequency of the R1 and C1 should be much lower than the frequency of the PWM signal for proper filtering. Filtered PWM dimming is shown in Figure 4.

Direct PWM dimming is typically used because it achieves a much wider dimming range compared to using a filtered PWM or a DC voltage. Direct PWM dimming uses a MOSFET in series with the LED string to quickly connect and disconnect the LED string. Figure 5 displays direct PWM dimming in a Li-Ion to a 4-and-4 white LED application. A PWM signal is applied to the CTRL pin and MOSFET where the PWM signal controls both the turn-on and turn-off of the part. Figure 6 shows the linearity of PWM dimming. The available dimming range depends on the settling time of the application and the PWM frequency used. The application in Figure 5 achieves a dimming range of 250:1 using a 100Hz PWM frequency. Figure 7 shows the available dimming ranges for different PWM frequencies.

Figure 5. Li-Ion to eight white LEDs with direct PWM dimming.

Figure 6. Linearity of PWM Dimming of Figure 5 at 100Hz.

Figure 7. Dimming Ratio Range vs Frequency.

Applications

Li-Ion Powered Driver for 12 White LEDs

Figure 1 highlights the LT3497’s impressive input and output voltage range. This circuit is capable of driving two strings of six LEDs each with 20mA of constant current. As shown in Figure 1, the circuit works from a single Li-Ion (3V) battery or 5V wall adapter. Figure 2 shows efficiency with a 3.6V input.

Li-Ion to a 2-LED and 6-LED Display

Figure 8 (Buck-Boost/Boost configuration) shows a white LED driver used to backlight two displays: a 6-LED main and a 2-LED sub display. This design generates a constant 20mA in each white LED string from a Li-Ion (3V~4.2V) or 5V adapter input. Two independent dimming and shutdown controls (CTRL1 and CTRL2) simplify power management and extend battery life. Figure 9 shows the efficiency of the circuit.

Figure 9. Efficiency of the circuit in Figure 8.

Conclusion

The LT3497 is a dual channel white LED driver capable of driving up to 12 white LEDs from a single cell Li-Ion input. The device features 35V internal power switches, internal Schottky diodes, DC or PWM dimming control, open LED protection and optimized internal compensation. The LT3497 is an ideal solution for a wide range of applications including multipanel LCD backlighting, camera flash or space constrained portable applications such as cellular phones, MP3 players, PDAs and digital cameras.