LT3003

6

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The LT3003 is an easy-to-use 3-channel LED ballaster.

It allows three strings of LEDs to be driven in parallel

with accurate LED current matching. A PWM pin is also

provided allowing LED currents to be turned on and off at

very low duty cycles for very wide LED dimming ranges.

For reliable system thermal management, two output flags,

OT1 and OT2, are provided to indicate when the junction

temperature exceeds 125°C.

The Block Diagram in Figure 1 best illustrates the features

of LT3003. The LT3003 internal bias circuitry is turned on

active in each channel, the PWM pin must exceed 0.5V.

application (see various application modes of boost, buck,

buck-boost in the Typical Applications section).

The LT3003 ballasting feature is achieved by using the

current monitored in the LED2 channel to control the

current in channels LED1 and LED3. A servo loop exists

for each channel LED1 and LED3 that compares the cur-

rent in each channel to the LED2 reference channel. The

current in LED2 channel is determined by the LED current

programmed by the partner LED driver. Details of how LED

current can be programmed by the partner IC are covered

in the Typical Applications section using various ICs as

LED drivers. Maximum fault current in each of the LT3003

LED pins is internally limited to 550mA. If any LED string

experiences an open-circuit fault, all LED string currents

are turned off.

The LT3003 PWM pin has unique level-shifting circuitry to

allow a simple logic-level PWM signal to turn each LED pin

allows very simple PWM dimming control of LED current

without any need for external level-shifting components

in buck mode and buck-boost mode applications. Care is

mizes leakage currents in each application to maximize

PWM dimming ratio. Details of PWM dimming and criti-

cal parameters are given in the Applications Information

section PWM Dimming.

The LT3003 incorporates internal junction temperature

sensing and provides two open-collector outputs, OT1

and OT2, which become active low when junction tem-

perature exceeds 125°C. OT1 is sized to pull 100µA and

can be used as an input to the microprocessor for system

thermal management. OT2 is sized to pull 300µA to defeat

switching for most of LTC’s LED drivers by pulling down

LED current state” by a special internal protection circuit

when junction temperature reaches 150°C.

To calculate the LT3003 junction temperature, see Thermal

Calculations in the Applications Information section.

OPERATION