Semtech adds PWM dimming interface to charge pump platform for LED backlighting in handheld devices

Semtech Corp., has introduced two LED backlight charge pump drivers as new members of the SC65x charge pump platform.

The company claims the new devices are the industry's first LED drivers to integrate digital low-pass filters that convert a PWM dimming signal's duty-cycle into DC current for LCD backlighting with white LEDs.

The SC652 and SC656 are optimized for single-cell Li-ion battery handheld applications. The SC652 supports up to five LEDs with one PWM dimming input. The SC656 supports up to seven LEDs with one or two independent PWM dimming inputs for end products that have multiple zones of lighting.

Recently, PWM dimming has become more popular due to its flexibility, cross-platform firmware compatibility, and extensive use in advanced power saving schemes such as ambient light sensing (ALS) and content adaptive brightness control (CABC). However, most competing PWM dimming solutions simply turn on and off the LED current in synchronization to the PWM signal, resulting in three distinct negative effects:

1) increased voltage ripple on the input and outputs of the LED driver, which can couple noise into nearby circuits,

2) decreased efficiency because the LED forward voltage remains high even when dimming, and

3) potential display flicker, banding, or herringbone pattern noise due to the LED light interfering with the LCD refresh rate.

External R-C filters have been used to convert the logic-level PWM dimming signal into an analog signal, which can be injected into a feedback or setting node at the LED driver IC. Beyond the need for the added external components, the weakness of such solutions is that the PWM logic high and low levels, which are not accurate, will influence the backlight brightness, causing large errors that are especially visible at dim levels.

Semtech claims the SC652 and SC656 are the first devices to fully solve these negative issues without the addition of any external components. Semtech's proprietary digital low-pass filter samples the PWM dimming signal to extract the duty cycle information and maps this into the current setting DACs. The PWM input accepts 200 Hz to 50 kHz PWM signals with logic level of 1.4 V and above. LED current is linearly proportional to duty-cycle from 1 percent to 100 percent, while 0 percent duty-cycle places the IC into 0.1 uA shutdown. Under all conditions, LED current remains DC to minimize input/output ripple, reduce LED forward voltage to allow the 1x/1.5x/2x charge pump to operate in the most efficient mode possible, and to eliminate any possibility of interference with the LCD refresh rate.

In addition to these critical benefits, the PWM dimming filter inherently responds to sudden changes in duty-cycle with an aesthetically pleasing automatic fade up/down in LED brightness to add a high-quality feel to the end product without needing any special software subroutines in the host system processor.

The SC652 is available in 2 x 2 mm lead-free 14-pin MLPQ-UT package. The SC656 is available in 3 x 3 mm lead-free 20-pin MLPQ-UT package. Both devices are halogen-free and comply with the European Union's Restriction on Hazardous Substances in Electrical and Electronic Equipment (RoHS) directive to meet worldwide environmental and regulatory requirements.

Key Features of the SC652 and SC656

  • Input supply voltage range – 2.9 V to 5.5 V

  • Charge pump modes – 1x, 1.5x and 2x

  • Five (SC652) or seven (SC656) adjustable current sinks

  • PWM dimming with integrated digital low-pass filter for DC LED current

  • Resistor programmable full-scale current range up to 25 mA/LED

  • Current accuracy ±1.5 percent typical

  • Current matching ±0.5 percent typical

  • Two optional zones/banks of LED current settings for SC656

  • Automatic fade-in/out for smooth transitions when changing LED brightness

  • Ultra-thin MLPQ package – 2 x 2 x 0.6 mm (SC652) or 3 x 3 x 0.6 mm (SC656)

Availability and Pricing

The SC652 is priced at $0.55 each in 3,000-piece lots. The SC656 is priced at $0.71 each in 3,000-piece lots. Each device is available immediately in production quantities. Evaluation boards (SC652EVB and SC656EVB) are also available.



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