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LT3478

4.5A Monolithic LED Drivers with True Color PWM Dimming

Linear Technology 

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OPERATIO

The LT3478/LT3478-1 are high powered LED drivers with

a 42V, 4.5A internal switch and the ability to drive LEDs

with up to 1050mA for LT3478-1 and up to 105mV/RSENSE

for LT3478.

The LT3478/LT3478-1 work similarly to a conventional

current mode boost converter but use LED current (instead

of output voltage) as feedback for the control loop. The

Block Diagram in Figure 1 shows the major functions of

the LT3478/LT3478-1.

For the part to turn on, the VIN pin must exceed 2.8V and

the ⎯S⎯H⎯D⎯N pin must exceed 1.4V. The ⎯S⎯H⎯D⎯N pin threshold

allows programming of an undervoltage lockout (UVLO)

threshold for the system input supply using a simple

resistor divider. A 10µA current flows into the ⎯S⎯H⎯D⎯N pin

before part turn on and is removed after part turn on. This

current hysteresis allows programming of hysteresis for

the UVLO threshold. See “Shutdown Pin and Programming

Undervoltage Lockout” in the Applications Information

Section. For micropower shutdown the ⎯S⎯H⎯D⎯N pin at 0V

reduces VIN supply current to approximately 3µA.

Each LED driver is a current mode step-up switch-

ing regulator. A regulation point is achieved when the

boosted output voltage VOUT across the output LED(s) is

high enough to create current in the LED(s) equal to the

programmed LED current. A sense resistor connected in

series with the LED(s) provides feedback of LED current

to the converter loop.

The basic loop uses a pulse from an internal oscillator to set

the RS flip-flop and turn on the internal power NPN switch

Q1 connected between the switch pin, SW, and ground.

Current increases in the external inductor until switch

current limit is exceeded or until the oscillator reaches

its maximum duty cycle. The switch is then turned off,

causing inductor current to lift the SW pin and turn on an

external Schottky diode connected to the output. Inductor

current flows via the Schottky diode charging the output

capacitor. The switch is turned back on at the next reset

cycle of the internal oscillator. During normal operation

LT3478/LT3478-1

the VC voltage controls the peak switch current limit and

hence the inductor current available to the output LED(s).

As with all current mode converters, slope compensation

is added to the control path to ensure stability.

The CTRL1 pin is used to program maximum LED current

via Q2. The CTRL2 pin can be used to program a decrease

in LED current versus temperature for maximum reliability

and utilization of the LED(s). A CTRL2 voltage with negative

temperature coefficient can be created using an external

resistor divider from VREF with temperature dependant

resistance. Unused CTRL2 is tied to VREF.

For True Color PWM dimming, the LT3478/LT3478-1

provide up to a 3000:1 wide PWM dimming range by al-

lowing the duty cycle of the PWM pin (connected to the

IC and an external N-channel MOSFET in series with the

LED(s)) to be reduced from 100% to as low as 0.033%

for a PWM frequency of 100Hz. Dimming by PWM duty

cycle, allows for constant LED color to be maintained over

the entire dimming range.

For robust operation, the LT3478/LT3478-1 monitor system

performance for any of the following faults : VIN or ⎯S⎯H⎯D⎯N

pin voltages too low and/or inductor current too high

and/or boosted output voltage too high. On detection of

any of these faults, the LT3478/LT3478-1 stop switching

immediately and a soft-start latch is set discharging the

SS pin (see Timing Diagram for SS pin in Figure 11). All

faults are detected internally and do not require external

components. When all faults no longer exist, an internal

12µA supply charges the SS pin with a timing programmed

using a single external capacitor. A gradual ramp up of SS

pin voltage limits switch current during startup.

For optimum component sizing, duty cycle range and ef-

ficiency the LT3478/LT3478-1 allow for a separate inductor

supply VS and for switching frequency to be programmed

from 200kHz up to 2.25MHz using a resistor from the RT

pin to ground. The advantages of these options are covered

in the Applications Informations section.

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