AEC-Q100 75V Buck-Boost LED Driver with Programmable Spread Spectrum

AMS, the Analog Mixed-Signal division of ISSI, announced the IS32LT3957 to complement their recent introduction of the IS32LT395x family of switching buck LED drivers for automotive and industrial applications. The IS32LT3957 is an LED driver that operates across the wide range of battery voltages present in a typical automotive environment. It provides a constant LED current even when the battery voltage is above, below or equal to the output LED voltage.

The buck-boost switching capability makes the IS32LT3957 suitable for motorcycle (6V), automotive (12V), and truck (24V) headlight, front and rear fog lights, daytime running lights, and other LED lighting applications. The IS32LT3957 serves as a high-frequency configurable switching controller that outputs a constant LED current independent of the battery voltage in the range of 5V to 75V.

When the battery voltage is lower than the LED string voltage, it is configured for boost mode operation. When the battery voltage is higher than the LED string voltage, it is configured for buck mode operation. When the battery state of charge voltage varies above and below the output LED string voltage, it is configured for buck-boost mode operation. In either operating mode, the LED string brightness is modulated by applying a digital PWM or a dc voltage level signal.

Typical application circuit – Buck-Boost configuration. The IS32LT3957 can also be configured in SEPIC and Boost topologies. (click on schematic to enlarge)

All switching converters such as the IS32LT3957 operate on a fast cycle-by-cycle basis to transfer and convert power from the input to the output. This conversion will create large components of EMI noise at the frequency of operation as well as its harmonics that may interfere with radios and fail EMI compliance for the automotive electronic market.

The IS32LT3957 incorporates programmable spread spectrum technology that dithers and spreads this switching noise over a range of frequencies, thus significantly reducing the radiated peak emission and improving EMI performance.

“LED Headlights are fast replacing halogen bulbs in auto’s and motorcycles. They consume much less power, turn instantly on and are environmentally friendly, key driving factors for rapid growth in the market,” said Ven Shan, VP of AMS Products at ISSI. “The IS32LT3957 has all the optimized features and functions to enable up to a 50W buck boost drive required to provide a very stable, rugged and reliable drive scheme for this safety critical function.”

Extensive protection features of the IS32LT3957 include cycle-by-cycle peak current limit, hiccup mode short-circuit protection, output-short protection, under-voltage lockout, and thermal shutdown. When a fault is detected, a fault output signal is generated to report the fault condition to the system monitor.


  • 5V to 75V input
  • ±3.5% output current accuracy over -40°C to +125°C
  • Fault protection with reporting
  • EMI reduction capabilities
  • AEC-Q100 Qualified

The IS32LT3957 is available in an eTSSOP-16 package with exposed pad for enhanced thermal dissipation.

Integrated Silicon Solution Inc. , AMS
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