Single-Wire Camera LED Charge Pump Allows Multiple Output Current Levels With Single-Resistor Programmability

Introduction

The number of features in cell phones continues to grow, even as the phones themselves physically shrink, driving a need for space saving circuits to control these features. The LTC3218 is such a device. It can drive a white LED with multiple current levels, requiring only three 0603 ceramic capacitors and one 0402 resistor. Its low profile, 3mm × 2mm, DFN package allows for an application circuit footprint of less than 30mm², making it an ideal driver for a cell phone camera flash. Additionally, due to its single-wire, high side current sensing design, only one high current trace is required to run to the anode of the LED. The cathode of the LED can be grounded locally, eliminating the need for a separate return trace. The LTC3218 can operate from a single-cell Li-Ion battery, with an input voltage range of 2.9V to 4.5V.

The LTC3218 generates the regulated output voltage needed to maintain the desired LED current. By remaining in the current regulated, 1x mode for as much of the battery voltage range as possible, efficiency is maximized. The LTC3218 steps up to 2x mode only when needed. Figure 1 shows the efficiency of the LTC3218 for various current levels.

Figure 1. Efficiency vs V_IN for various LED currents.

To protect the LED from experiencing high currents for long periods of time, the LTC3218 features a built-in timer. This timer shuts down the part if it has been enabled in flash-mode (ENF = HIGH) for more than 2 seconds. The timer is reset by bringing the part into shutdown and re-enabling it.

Multiple Current Ratios

LED drivers often use external resistors to program LED current. The LED current is related to the programming resistor current through a fixed ratio. By employing multiple current ratios, the LTC3218 can be programmed for three different current levels using a single programming resistor. The current ratios are selected using the ENT and ENF pins. Table 1 shows the three different current ratios, and the ENT/ENF settings required to select them. R_SETT refers to the resistor connected between the I_SETT pin and GND, and R_SETF refers to the resistor connected between the I_SETF pin and GND. In the case where single-resistor programming is desired, the I_SETT and I_SETF pins can be shorted together and connected to a resistor to GND. Figure 2 shows an example of this configuration, along with the resulting output current levels.

Figure 2. Typical application, using a single resistor to program LED currents.

Table 1. Output current modes for all ENT and ENF settings
ENF	ENT	I_LED
LOW	LOW	SHUTDOWN
LOW	HIGH	1029/R_SETT
HIGH	LOW	2965/R_SETF
HIGH	HIGH	3993/R_SETF

Dimming and Brightness Control

Figure 3 shows how the LTC3218 can be configured to control LED brightness with just a few external components. By pulse-width modulating the gate of M1, the reference current in resistor R1 can be varied. The maximum LED current is determined by:

where R_SETT = R1 + R2 and the on-resistance of M1 is small compared to R_SETT. Resistor R1 should be greater than 1kΩ to provide adequate isolation between the 1µF capacitor and the internal servo-amplifier.

Figure 3. LED driver uses pulse-width modulation to implement dimming and brightness control.

Conclusion

Due to its small size and low external parts count, the LTC3218 is ideally suited for compact, camera LED applications. Features such as its single resistor programmability, multiple current ratios and 2-second flash timeout make the part simple to use, without the need for complicated control algorithms. Its low shutdown current and high efficiency make it perfect for situations where battery power is at a premium.