Boost regulator for small-format LED backlights
The National Semiconductor LM3503SQX-16 is a step-up (boost) DC DC regulator designed to drive LED strings in backlight applications. It integrates two outputs with a 400 mA switch, switching at 1 MHz to keep the inductor and output capacitors small enough for thin-profile displays. Output voltage is regulated to 16 V, which supports a string of roughly four to five white LEDs per channel depending on forward voltage. Dimming is handled either through an analog control voltage or a PWM signal, giving the system designer flexibility between smooth brightness adjustment and wider dimming range.
16 V output ceiling and dimming options
The 16 V maximum output sets a practical limit on series LED count. For a typical white LED, four in series reaches 16 V plus headroom for the current-sense resistor — five is marginal. Both analog and PWM dimming modes are available. Analog dimming adjusts LED current continuously via a DC voltage; PWM dimming switches the output on and off at a frequency above visible flicker for a wider contrast ratio. The 1 MHz switching rate keeps the inductor ripple low even at high PWM dimming ratios.
No official replacement or pin-compatible successor is listed on the record.
Package and supply range
The LM3503SQX-16 is supplied in a 16-lead WFQFN with exposed pad, measuring 4 mm × 4 mm.