LT3498 Просмотр технического описания (PDF) - Linear Technology

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LT3498

20mA LED Driver and OLED Driver with Integrated Schottky in 3mm x 2mm DFN

Linear Technology 

LT3498

OPERATION— LED DRIVER

The LED portion of the LT3498 uses a constant-frequency,

current mode control scheme to provide excellent line

and load regulation. Operation can be best understood

by referring to the Block Diagram.

At power-up, the capacitor at the CAP1 pin is charged up

to VIN (input supply voltage) through the inductor and the

internal Schottky diode. If CTRL1 is pulled higher than

125mV, the bandgap reference, the start-up bias and the

oscillator are turned on. At the start of each oscillator cycle,

the power switch Q1 is turned on. A voltage proportional

to the switch current is added to a stabilizing ramp and the

resulting sum is fed into the positive terminal of the PWM

comparator, A2. When this voltage exceeds the level at the

negative input of A2, the PWM logic turns off the power

switch. The level at the negative input of A2 is set by the

error amplifier A1, and is simply an amplified version of

the difference between the VCAP1 and VLED1 voltage and

the bandgap reference. In this manner the error amplifier,

A1, sets the correct peak current level in inductor L1 to

keep the output in regulation. The CTRL1 pin is used to

adjust the LED current. The LED Driver is shutdown when

CTRL1 is pulled lower than 75mV.

Minimum Output Current

The LED Driver of the LT3498 can drive a 4-LED string at

2mA LED current, without pulse-skipping, using the same

external components shown in the application circuit on

the front page of this data sheet. As current is further

reduced, the device will begin skipping pulses. This will

result in some low frequency ripple, although the average

LED current remains regulated down to zero. The photo in

Figure 1 details circuit operation driving four white LEDs

at 2mA load. Peak inductor current is less than 60mA and

the regulator operates in discontinuous mode, meaning

the inductor current reaches zero during the discharge

phase. After the inductor current reaches zero, the SW1

pin exhibits ringing due to the LC tank circuit formed

by the inductor in combination with the switch and the

diode capacitance. This ringing is not harmful; far less

spectral energy is contained in the ringing than in the

switch transitions.

IL

50mA/DIV

VSW

10V/DIV

VIN = 4.2V

ILED = 2mA

4 LEDs

200ns/DIV

3498 F01

Figure 1. Switching Waveforms with

Four White LEDs at 2mA Load

3498fa

8

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