ELE Times December 2016 Electronics News in India | Online Electronics Mag | Page 32

Cover Story
Analog dimming via adjustable voltage on the CTRL pin has always
been easier to implement than the more accurate PWM dimming.
Until now, PWM dimming required an external clock or micro
signal whose duty cycle controlled the brightness via the PWM
input pin. However, the LT3922 features an internally generated
PWM dimming signal that only requires an external voltage on
the PWM pin to set the duty cycle for 128:1 PWM dimming. The
PWM period, such as 122Hz, is set by a single resistor on the RP
pin.
LED current accuracy is a necessity for vehicles with redundant
light clusters. The brightness of both sides must match for
obvious reasons. Identically manufactured LEDs can produce
different brightnesses at the same drive current. The internal
dimming feature of the LT3922 can be used for brightness
trimming near or just below 100% duty cycle and then set to
accurate 10:1 or 100:1 ratios. This can save the light cluster
manufacturer from paying extra for specially binned LEDs.
When higher dimming ratios are needed, the LT3922 can be
externally dimmed in the usual manner. The high bandwidth
400kHz buck mode LED driver in Figure 2 yields a 1000:1 PWM
dimming ratio at 100Hz. The 2MHz boost LED driver in Figure 1
can achieve 2000:1 dimming ratio at 120Hz as shown in Figure 7a.
The same circuit can be set up for internally generated PWM
dimming by placing a 122Hz frequency resistor on the RP pin and
setting the PWM pin voltage between 1.0V and 2.0V for up to
128:1 dimming as shown in Figure 7b. The LT3922 can be set up
to run with up to 5000:1 external PWM dimming in some
applications and PWM dimming can be combined with LT3922’s
analog dimming for over 50,000:1 brightness control.

Machine Vision

Figure 8. Assembly line system overview with machine vision application

performing “maintenance switching” during the PWM off-time
while the LEDs are disconnected by the high side PMOS. During
standard PWM dimming at frequencies above 100Hz, the longest
off-time is 10ms or less, and not much leakage current can be
pulled off the output at that time. Machine vision and strobe
applications can have long off-times between 100ms and 5s (or
longer), allowing for tens to hundreds times more leakage.

Figure 9. Camera flash waveform looks the same regardle ss of idle or
down time. Waveforms show pulse after 10ms and after one hour. The
flash looks the same after sitting idle for one hour as it does after 10ms.
These results are for the circuit shown in Figure 1.

Maintenance switching ensures that the output capacitor
maintains the voltage recorded during the LT3922’s previous
sample cycle. The digital sample of the state of the converter is
stored indefinitely, assuming uninterrupted input power is
provided to the IC. This allows the LT3922 to have a consistent
output current waveform for any given off-time, as demonstrated
in Figure 9.
Conclusion
The LT3922 36V LED driver with internal, synchronous, 2A
switches is a compact and versatile LED driver. It can be easily
used in boost, buck and boost-buck topologies. Regardless of
topology, all of its features are available, including high PWM
dimming capability and internally generated PWM dimming. Low
EMI is easily achievable with its Silent Switcher layout and SSFM.
Its compact and synchronous switches maintain high efficiency,
even at frequencies up to 2MHz. With robust fault protection, this
IC easily meets the requirements of automotive other demanding
applications.

In industrial assembly line applications, machine vision (Figure 8)
provides rapid visual feedback of devices using high speed digital
photography in conjunction with digital image processing. This
helps rapidly identify and isolate defective products with little or
no human inspection. The lighting used for machine vision
systems must be synchronized with the speed of the assembly
line processes while maintaining the ability to produce a
consistent pulse of light for an indefinite period of off-time.
Conventional LED drivers are unable to maintain their output
voltage after the PWM input signal is held low for any sustained
amount of time. This is due to the gradual discharge of the output
capacitor, making generic LED drivers unsuitable for these types
of applications. However, the LT3922 digitally samples the output
state of the converter during the falling edge of the PWM signal.
It then maintains its output voltage during prolonged off-times by

ELE Times | 32 | December, 2016