ELE Times December 2016 Electronics News in India | Online Electronics Mag | Page 28

Cover Story
Boost
The LT3922 can power LEDs up to 34V when operated as a boost
converter, leaving some headroom, below 40V, for open LED
overshoot. The 2MHz, 4V to 28V boost LED driver shown in Figure
1 powers a 330mA string of LEDs at up to 34V. It can be externally
PWM dimmed at 120Hz to a 2000:1 ratio or it can be internally
dimmed to 128:1 ratio with an analog input voltage on the PWM
pin.

The input voltage can be as high as 36V and a string of LEDs can
be driven at up to 1.5A when the LT3922 is used in a buck mode
topology, as shown in Figure 2. The high side ISP and ISN current
sense input and PWMTG PMOS driver are easily moved to the
high side of the LEDs, which is connected to the input in buck
mode. LED− is connected directly to the inductor and not to
ground. When driving two, 1A LEDs at 6.5V, the synchronous buck
mode efficiency is as high as 94% at 12V VIN and remains as high
as 89% at 36V VIN. The high bandwidth of the buck mode
converter allows it to work with a 1000:1 PWM dimming ratio at
100Hz.

Boost-Buck

Figure 1. 2MHz regular boost schematic with 2000:1 PWM dimming at
120Hz

Figure 3. 2MHz boost-buck LED driver with low input and output ripple.
This solution passes CISPR 25 Class 5.

It survives open LED and LED+-to-ground short-circuits and
reports these faults by asserting its FAULT pin. The output current
can be monitored via the ISMON pin, even during PWM dimming.
At 2MHz switching frequency, its fundamental EMI harmonic
resides above the AM band, but its EMI is still low. Spread
spectrum frequency modulation (SSFM) can be added to spread
the switching frequency between 2MHz to 2.5MHz and reduce
the EMI at the fundamental and its many harmonics. The
efficiency of the 2MHz boost converter remains as high as 91% at
12VIN due to the integrated synchronous switches. At lower VIN,
when the peak inductor current hits its limit, the output current is
gracefully reduced without flicker while the LEDs remain on.

The LT3922 boost-buck topology in Figure 3 supports an input
voltage range extending above and below the LED string voltage.
The sum of the LED string voltage and the input voltage must
remain below 35V to keep the ISP and ISN voltage below the 40V
absolute maximum.
This patented low EMI topology features a boost-type low ripple
input inductor and a buck mode-type low ripple output-facing
inductor. A 4V–18V automotive input or multiple battery
chemistry input (5V, 12V and 19V) boost-buck converter can drive
an LED string voltage anywhere between 3V and 16V.
As in the other topologies, the PWMTG driver simplifies PWM
dimming MOSFET connection. Open- and short-circuit protection
are not compromised in the floating LED topologies. An optional
diode on LED− protects against LED—to-GND short-circuits.
The 2MHz converter in Figure 3 features 85% efficiency (87%
without EMI filters) at 12V VIN, 15V VLED, 330mA ILED and up to
2000:1 PWM dimming ratio at 120Hz. This solution fits the
requirements of an automotive daytime running light, signal light,
or tail light LED driver, due to its size, versatility and low EMI.

Buck

Automotive Lighting

Figure 2. 400kHz buck mode LED driver with 1000:1 100Hz PWM
dimming brightness control

So much about LEDs make them ideal for use in automotive
lighting. There is a visual appeal of LED tail and daytime running
lights. Efficient LED headlights are robust, with lifetimes orders of
magnitude longer than their relatively burn-out-prone filamentbased predecessors. Drivers are small and efficient with wide
input and output voltage ranges, and should feature low EMI.
The tiny LT3922 LED driver features low EMI, high efficiency and
fault protection required in automotive environments. It can be

ELE Times | 28 | December, 2016