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LT3498

3498f

LED CURRENT (mA)

3498 F07

VIN = 3.4V

4 LEDs

100Hz = PWM

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

connected to ground. The PWM signal is applied to the

CTRL1 pin of the LT3498 and the gate of the MOSFET.

The PWM 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.

APPLICATIONS INFORMATION— LED DRIVER

Figure 7. PWM Dimming Efﬁciency

Figure 5. Li-Ion to Four White LEDs

with Direct PWM Dimming

Figure 6. Direct PWM Dimming Waveforms

CAP1 SW1

CAP2 VOUT2

VIN

LT3498

GND1

GND2

SW2

CIN

µH

RSENSE1

Ω

Si2304BDS

3498 F05

COUT1

µF

VIN 3V TO 5V

µH

LED1

CTRL1

CTRL2

FB2

100k

PWM

FREQ

2ms/DIV

ILED

20mA/DIV

200mA/DIV

PWM

5V/DIV

3498 F06

VIN = 3V

4 LEDs

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