lt3498eddb-trpbf Linear Technology Corporation, lt3498eddb-trpbf Datasheet - Page 13

• Manufacturer Part Number: lt3498eddb-trpbf
• Description: 20ma Led Driver And Oled Driver With Integrated Schottky In 3mm X 2mm Dfn
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION— LED DRIVER
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:
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 fi nal value. Figure 8 shows the dimming range
achievable for three different frequencies with a settling
time of 40µs.
Di
Min Du
Duty Cycle Range
f
t
t
t
PERIOD
SETTLE
PERIOD
m m Range
100
10000
1000
100
t t y Cycle
Hz t
10
Figure 8. Dimming Ratio vs Freqeuncy
1
•
10
,
100
1
f
SETTLE
t
t
100
PERIOD
SETTLE
1
PULSING MAY BE VISIBLE
PWM FREQUENCY (Hz)
0 01
40
100
.
100
0 01
40
s
s
.
•
100 0. . %
0 01
%
40
s
s
.
1000
s
s
0 4
. %
250 1
4
3498 F08
10000
at
:
100
Hz
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 fi rst, 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.
C
OUT1
1 F
Si2304BDS
R
SENSE1
Figure 9. Li-Ion to Four White LEDs with
Both PWM Dimming and Analog Dimming
10
Q1
100k
PWM
FREQ
5V
0V
LED1
CTRL1
CAP1 SW1
GND1
15 H
L1
GND2
LT3498
V
V
IN
IN
3V TO 5V
SW2
CTRL2
LT3498
CAP2 V
FB2
C
1 H
13
OUT2
IN
3498 F09
3498f