HBLED Driver Controls Up to 18 LEDs In Series

Nov 1, 2009 12:00 PM
SAM DAVIS, Editor in Chief

A controller IC drives high-brightness LEDs for a variety of indoor, outdoor, and automotive applications. Its LED system design is simplified by the use of an online program.

Housed in a 20-pin thermally enhanced TSSOP package, the LM3424 drives up to 18 high-brightness LEDs (HBLEDs) in series with an output current above 2 A in typical applications. The LED driver can regulate currents based on buck, boost, SEPIC, flyback, and buck-boost topologies, and enables improved reliability for LED systems.

Operating over a 4.5- to 75-V input range allows regulation of a variety of LED loads. The device's PWM controller provides high-speed capabilities, including an oscillator frequency range that can be synchronized to 2.0 MHz. Additional characteristics include programmable soft-start, and PWM and analog dimming. Protection includes low-power and thermal shutdown, and cycle-by-cycle current limit.

LM3424 FEATURES

The MOSFET-based LM3424 offers high-side differential current-sensing with a low adjustable threshold voltage that regulates output current while maintaining efficiency. Peak current-mode control provides noise immunity and an inherent cycle-by-cycle current limit. The adjustable current-sense threshold enables amplitude-dimming (analog) of the LED current.

The output enable/disable function, coupled with an internal dimming drive circuit, provides high-speed PWM dimming with an external MOSFET at the LED load. Maximum LED current is not internally limited because the LM3424 is a controller. Instead, current is a function of the system's operating point, components, and switching frequency.

Peak current-mode control uses a series resistor in the LED path to sense LED current. The driver can use either a series resistor in the MOSFET path or MOSFET R_DS(ON) for cycle-by-cycle current limit and input-voltage feed forward.

Fig. 1 shows an LM3424 configured as a buck-boost controller that drives a series string of LEDs. The MOSFET on the input voltage source (Q1) stores energy in the inductor (L1) while the output capacitor (C _OUT) provides energy to the LED load. When Q1 turns off, recirculating diode (D1) becomes forward-biased and L1 provides energy to both COUT and the LED load. The average output LED current (I_LED) is proportional to the average inductor current; therefore, if the average current is tightly controlled, the I_LED is well-regulated. As the system changes input voltage or output voltage, the ideal duty cycle is varied to regulate the average current and, ultimately, I_LED.

THERMAL FOLDBACK

In extreme temperatures, thermal foldback is necessary because if LED temperatures rise above a safe threshold, their lifetime and efficacy decreases. Thermal foldback ensures the warranty period of fixtures in applications such as headlights, warehouse lighting, and street lights.

The circuit enclosed by the dashed lines in Fig. 1 provides thermal foldback. The circuit includes a resistor network with an NTC thermistor. The T_SENSE pin sets a temperature-dependent voltage. As thermistor temperature increases, its resistance decreases (non-linearly). Therefore, as temperature increases, the T_SENSE voltage decreases, causing the LED current to decrease. The reduced current dims the LED to a programmed range, where it remains until it returns to a safe operating temperature.

Generally, the foldback circuit requires two functions: a temperature breakpoint (TBK) — after which the nominal operating current needs to be reduced — and a slope corresponding to LED current that decreases as temperature increases (Fig. 2). Here, the reciprocal of R _GAIN sets the slope and the resistor network sets the temperature breakpoint. View Table

SWITCHING FREQUENCY

An external resistor (RT) from the RT/SYNC pin to ground sets the LM3424's switching frequency. Alternatively, an external PWM signal can be applied to the RT/SYNC pin — through a filter and an ac coupling capacitor — to synchronize the part to an external clock.

If the external PWM signal is applied at a frequency higher than the base frequency set by the RT resistor, the internal oscillator is bypassed and the switching frequency becomes the synchronized frequency. An external synchronization signal should have a pulse-width of 100 ns, an amplitude between 3 and 6 V, and should be ac coupled to the RT/SYNC pin with a 100-pF ceramic capacitor.

The internal programmable oscillator may cause current-mode instability at duty cycles higher than 50%. To avoid this, the IC adds an artificial ramp (slope compensation) to the control signal. The slope is programmable to allow a wide range of component choices.

ONLINE DESIGNER

National's WEBENCH® LED Designer (Fig. 3) helps to identify the ideal LED temperature threshold or foldback breakpoint. Traditionally, resistor values are calculated by hand to identify temperature foldback breakpoints.

WEBENCH allows the designer to enter the temperature foldback and slope points, and then visualize the behavior of the design at different LED temperatures. An interactive temperature slider shows the results graphically. The driver design automatically updates with the entry of new temperature breakpoint requirements.

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