LT3003 查看數據表（PDF） - Linear Technology

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LT3003

3-Channel LED Ballaster with PWM

Linear Technology 

LT3003

APPLICATIONS INFORMATION

Input Capacitor Selection

The LT3003 is typically driven from the same input voltage

used for the partner LED driver IC. The LED driver and

inductor govern the requirements for the input capacitor

of the application. A ceramic input capacitor in the range of

1µF to 10µF works for most applications. In cases where

the LT3003 input voltage is derived separately from the

LED driver, a 1µF input capacitor works well.

LED Current Matching

An LED driver programs the LED current and LT3003 ac-

tively ballasts three separate strings of LEDs. The currents

in all three channels will be matched to better than ±3%.

To achieve best current matching, (VIN – VEE) should be

between 3V and 10V. A low (VIN – VEE) also minimizes

LT3003 internal power dissipation (see Thermal Calcula-

tion section for more information).

10

ILED2 = 350mA

∆VLED = 1.4V

8

6

4

2

0

3 6 9 12 15 18 21 24 27 30 33 36

VIN – VEE (V)

3003 F02

Figure 2. ILED Matching vs (VIN – VEE)

LED Pin Current Range

The steady-state operational current range for each LED pin

is between 100mA and 350mA. Internal protection circuitry

limits absolute maximum pin current to 550mA.

LED Open-Circuit Protection

If any LED string is open, then currents in all three chan-

nels reduce to zero. The driver chip, which supplies LED

current, should have an overvoltage clamp to protect the

LT3003 from high LED pin voltages.

PWM Dimming

The LT3003 has a wide PWM dimming range for constant

color LED dimming. PWM dimming is superior to analog

dimming as it preserves true color quality. PWM dimming

control with the LT3003 is achieved using a simple ground

referenced PWM signal with a 0.5V on/off threshold. The

LEDs operate at either programmed or zero current but

their brightness changes with the PWM signal duty cycle.

When PWM is low, LED strings are completely discon-

nected. In addition, the LT3003 switches to low power

standby mode ~10µs after PWM low edge, resulting in

higher system power efficiency.

For the widest dimming range, the PWM signal should be

100Hz. The human eye is typically sensitive to flickering

below ~80Hz. Operating the PWM higher than 100Hz results

in a reduced PWM dimming ratio.

Achieving high PWM dimming ratios require attention to

circuit leakages, such as reverse bias leakage currents

through the external Schottky. Hence, for high PWM dim-

ming ratios, components should be chosen to minimize

leakage currents.

If an application does not require PWM dimming, the PWM

pin can be left open (unconnected) and an internal 10µA

source current pulls PWM high.

Boost PWM Dimming

The LT3003 supports up to a 3000:1 PWM dimming ratio

with a 100Hz PWM dimming frequency. To achieve such

high PWM ratios, leakages of the LED driver and other

external components should be minimal.

Buck Mode: PWM Dimming

The LT3003 supports up to a 3000:1 PWM dimming ratio in

buck mode. The PWM dimming in buck mode is achieved

by an architecture that allows the LT3003 power ground

(VEE) to move with output capacitor voltage. PWM dim-

ming control is achieved by a simple ground referenced

PWM signal, eliminating the need for any external level-

shift component.

High PWM dimming ratios require very low VEE and

VMAX pin currents during the PWM off state. The LT3003

3003fa

7

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