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LT3498

3498f

The time it takes for the LED current to reach its pro-

grammed value sets the achievable dimming range for a

given PWM frequency. For example, the settling time of

the LED current in Figure 6 is approximately 40µs for a

3V input voltage. The achievable dimming range for this

application and 100Hz PWM frequency can be determined

using the following method.

Example:

fHz t

SETTLE

PERIOD

100

001

m Range

Min Du

PERIOD

SETTLE

001

250 1

tty Cycle

SETTLE

PERIOD

•

100

001

100

.. %

%. %

100

0 4

100

Duty Cycle Range

=→

The calculations show that for a 100Hz signal the dimming

range is 250:1. In addition, the minimum PWM duty cycle

of 0.4% ensures that the LED current has enough time to

settle to its ﬁnal value. Figure 8 shows the dimming range

achievable for three different frequencies with a settling

time of 40µs.

The dimming range can be further extended by changing

the amplitude of the PWM signal. The height of the PWM

signal sets the commanded sense voltage across the sense

resistor through the CTRL1 pin. In this manner both analog

dimming and direct PWM dimming extend the dimming

range for a given application. The color of the LEDs no

longer remains constant because the forward current of

the LED changes with the height of the CTRL1 signal. For

the four LED application described above, the LEDs can

be dimmed ﬁrst, modulating the duty cycle of the PWM

signal. Once the minimum duty cycle is reached, the height

of the PWM signal can be decreased below 1.5V down to

125mV. The use of both techniques together allows the

average LED current for the four LED application to be

varied from 20mA down to less than 20µA. Figure 9 shows

the application for dimming using both analog dimming

and PWM dimming. A potentiometer must be added to

ensure that the gate of the NMOS receives a logic-level

signal, while the CTRL1 signal can be adjusted to lower

amplitudes.

Figure 8. Dimming Ratio vs Freqeuncy

APPLICATIONS INFORMATION— LED DRIVER

PWM FREQUENCY (Hz)

100

1000

10000

100

1000

10000

3498 F08

PULSING MAY BE VISIBLE

Figure 9. Li-Ion to Four White LEDs with

Both PWM Dimming and Analog Dimming

CAP1 SW1

CAP2 VOUT2

VIN

LT3498

GND1

GND2

SW2

CIN

µH

RSENSE1

Ω

Si2304BDS

3498 F09

COUT1

µF

VIN 3V TO 5V

µH

LED1

CTRL1

CTRL2

FB2

100k

PWM

FREQ

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