Chips Simplify Driving of High-Brightness LEDs

Nov 1, 2006 12:00 PM
By David Morrison, Editor, Power Electronics Technology

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A pair of LED drivers from Maxim Integrated Products provides highly compact and simplified designs for driving the high-brightness (HB) LEDs that are now being used in automotive, architectural and ambient lighting applications. The MAX16819/MAX16820 HB LED drivers require just one external MOSFET and only a few passive components to power LEDs needing 1.5 A of drive current (see the figure). The part is capable of delivering more than 25 W (max) of output power while driving as many as six white LEDs connected in series.

Small size and component-count reduction are key advantages of these LED drivers. Each driver comes in a 3-mm × 3-mm × 0.8-mm, 6-pin TDFN package and enables the use of a very small external inductor and capacitors, thanks to a switching frequency that may be as high as 2 MHz.

The MAX16819/MAX16820 are intended to supplant the standard buck regulators currently being used to power HB LEDs. Unlike the most commonly used bucks, which typically employ fixed-frequency PWM control, the new Maxim LED drivers employ hysteretic control. With that control technique, the driver can produce very narrow pulses, which translates to a wide dimming range (5000-to-1) or the ability to dim at very high switching frequencies. With these chips, dimming is controlled via a dedicated pin. Hysteretic control also offers a high level of input-supply rejection, as well as rapid response during LED dimming.

High-side current sensing is another distinction between these drivers and standard buck converters. This approach results in lower power dissipation because the voltage drop across the external sense resistor is just 200 mV, versus a 400-mV drop or more with a typical buck converter. The high-side current sensing and onboard current setting circuitry minimize the number of external components needed, while providing ±5% current accuracy with trim.

The LED drivers operate from an input voltage range of 4.5 V to 28 V, which encompasses the 24-V power bus found in architectural, automotive and industrial applications. The chips provide up to 0.5 A of source and 1 A of sink MOSFET gate-drive capability. The nominal operating frequency can be set up to 2 MHz by appropriate selection of external circuit components.

The MAX16819 produces 30% LED current ripple (hysteretic window) while the MAX16820 produces 10% LED current ripple. Both parts are specified over the automotive -40°C to +125°C operating junction temperature range.

In quantities of 1000, unit pricing for the MAX16819 and MAX16820 is $0.58 each. These chips are currently sampling with mass production quantities expected in December. Maxim is also offering an evaluation board along with an Excel spreadsheet that calculates values of external components. For more information, see www.maxim-ic.com/led.

MAX16819/MAX16820 Applications

• Architectural industrial and ambient lighting
• Automotive lighting
  • Rear combination lamp (RCL)
  • Daytime running lights (DRL)
  • Fog lights
  • Heads-up displays
• Indicators and emergency lighting