LT3498 データシートの表示（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

APPLICATIONS INFORMATION— LED DRIVER

Direct PWM Dimming

Changing the forward current flowing in the LEDs not only

changes the intensity of the LEDs, it also changes the color.

The chromaticity of the LEDs changes with the change in

forward current. Many applications cannot tolerate any

shift in the color of the LEDs. Controlling the intensity of

the LEDs with a direct PWM signal allows dimming of the

LEDs without changing the color. In addition, direct PWM

dimming offers a wider dimming range to the user.

Dimming the LEDs via a PWM signal essentially involves

turning the LEDs on and off at the PWM frequency. The

typical human eye has a limit of ~60 frames per second.

By increasing the PWM frequency to ~80Hz or higher,

the eye will interpret that the pulsed light source is con-

tinuously on. Additionally, by modulating the duty cycle

(amount of “on-time”), the intensity of the LEDs can be

controlled. The color of the LEDs remains unchanged in

this scheme since the LED current value is either zero or

a constant value.

Figure 5 shows a Li-Ion powered driver for four white

LEDs. Direct PWM dimming method requires an external

NMOS tied between the cathode of the lowest LED in the

string and ground as shown in Figure 5. A simple logic

level Si2304 MOSFET can be used since its source is con-

nected to ground. The PWM signal is applied to the CTRL1

pin of the LT3498 and the gate of the MOSFET. The PWM

RSENSE1

10Ω

VIN 3V TO 5V

CIN

L1

1μH

15μH

signal should traverse between 0V to 5V, to ensure proper

turn-on and -off of the driver and the NMOS transistor Q1.

When the PWM signal goes high, the LEDs are connected

to ground and a current of ILED = 200mV / RSENSE1 flows

through the LEDs. When the PWM signal goes low, the

LEDs are disconnected and turn off. The MOSFET ensures

that the LEDs quickly turn off without discharging the

output capacitor which in turn allows the LEDs to turn on

faster. Figure 6 shows the PWM dimming waveforms for

the circuit in Figure 5.

ILED

20mA/DIV

IL

200mA/DIV

PWM

5V/DIV

VIN = 3V

4 LEDs

2ms/DIV

3498 F06

Figure 6. Direct PWM Dimming Waveforms

80

VIN = 3.6V

4 LEDs

75 100Hz = PWM

70

65

CAP1 SW1

VIN SW2 CAP2 VOUT2

60

COUT1

1μF

Q1

Si2304BDS

100k

LED1

CTRL1 GND1

5V

0V

PWM

FREQ

LT3498

GND2 CTRL2 FB2

3498 F05

55

50

0 2 4 6 8 10 12 14 16 18 20

LED CURRENT (mA)

3498 F07

Figure 7. PWM Dimming Efficiency

Figure 5. Li-Ion to Four White LEDs

with Direct PWM Dimming

3498fa

12

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