Simple timeout protection for high-current flash LED drivers in cameras

A flash function to support built-in cameras is now a very common feature in mobile phones. This light source is based on high-luminosity LEDs. However, those LEDs are still expensive, and can accept a very high current only for a limited duration (typically, one second). If the LED current is sustained at this maximum level for a long time, the resultant overheating will degrade the luminosity, and after a certain time, the LED will completely burn out.

Therefore, to guarantee a long life-cycle for these high luminosity LEDs, it is important to manage the time control for the flash function. This protection feature discussed here enables reliable, high-power LED operation over a long life-cycle and prevents the burnout of the LED in case of a software problem.

The timeout function consists of a simple supervisor circuit with capacitor-adjustable delay. More generally, this hardware protection function can be implemented very simply, every time a peripheral needs to be protected against software activation problems.

In Figure 1, the MAX8607 boost-converter is optimized for camera flash/strobe applications using high-current white LEDs.

Figure 1: By adding the MAX6421's adjustable reset circuit to the LED boost converter, you can prevent flash LED overheating and extend LED life.
(Click to Enlarge Image)

The IC, which integrates a 1-MHz PWM boost converter, a 1.5-A low-dropout (LDO) current regulator, and logic control circuitry, is the core of a space-saving subsystem for LED flash control.

In Flash Mode, the current can reach up to 1.5A, with the maximum value programmed by an external resistor (R₁). Maximum V_IH and V_IL of the MAX8607 EN₁ and EN₂ inputs (1.4 V and 0.4 V, respectively) allow designers to use the 1.8 V peripheral CMOS I/O pins to control the LED modes.

To limit the timeout of the EN control signals, a MAX6421 supervisory circuit ties into the MAX8607. This supervisor circuit monitors system voltages from 1.6 to 5V. Its internal, laser-trimmed resistors are factory preset to set the threshold at any of 34 voltage predefined values; in this example we will use the version with a 1.8-V threshold. When the 1.8-V high level from the EN1 signal appears on the MAX6421's V_cc input pin, the IC keeps the logic level at low on its active-low push-pull RESET output pin during the given timeout delay.

The low-level timeout is externally set by a capacitor (C_srt) to provide more flexibility (around 900 ms with C = 330 nF). Alternatively, by selecting a MAX6421 with a 1.6-V preset threshold (which is lower than the EN₁ signal voltage), you can ensure getting a high logic level after the timeout delay on the MAX6421's reset pin.

When the EN₁ and EN₂ pins go high, the flash function is activated on the MAX8607, and it is nominally active for a few hundred milliseconds. The LED current in flash mode is defined with an external resistor (I_LED = [0.6V * 5000]/R1. If the EN1 pin is maintained at a high level continuously, the MAX6421 will automatically disable the Flash function by controlling flash current in the current set-up resistor (R₁ by raising the RESET output to its logic high value (Figure 2a).

That situation disconnects the current source that is programmed by R₁ and decreases the LED current to few milliamps (Figure 2b).

Figure 2a and 2b: When EN₁ goes high, the timeout period of the reset circuit is initiated; when timeout occurs it reduces the LED current by bringing its reset output high (Figure 2a). When the flash pin on the MAX8607 goes high, current to the LED is reduced, thus preventing overheating (Figure 2b). (Here, the current is set to 0.75A with R₁=3.9 KΩ, and the delay to 900 ms with C_srt=330nF.) (Click to Enlarge Images)

Alternately, it can cause the MAX8607 to switch into its I Movie Mode (programmed by R₂) if EN₂ is still kept high. (In the example for the figures, the current is set to 0.75 A with R₁ = 3.9 kΩ and the delay is set to 900 ms with C_srt = 330 nF.)

Author biography

Marc Regnier is a senior field applications engineer at Maxim Integrated Products (France), which he joined in 2005. He received an Electronic Engineering degree in 1990 from Ecole Centrale d'Electronique in 1990 and has 15 years of experience in analog and digital systems design. Prior to joining Maxim, he spent five years each at Mitsubishi (France) and Alcatel.