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參數(shù)資料
| 型號： | LQH2MCN150K02 |
| 廠商： | Linear Technology Corporation |
| 英文描述： | 20mA LED Driver and OLED Driver with Integrated Schottky in 3mm x 2mm DFN |
| 中文描述： | 20mA的LED驅(qū)動器和3毫米的OLED驅(qū)動器集成肖特基x 2mm DFN封裝 |
| 文件頁數(shù)： | 13/24頁 |
| 文件大?。?/td> | 449K |
| 代理商： | LQH2MCN150K02 |
LT3498
13
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:
f
Hz t
s
t
f
s
Di
SETTLE
PERIOD
=
=
=
=
=
100
40
1
1
100
t
t
SETTLE
=
40
0 01
0 01
.
,
μ
mRange
s
s
MinDu
t
t
PERIOD
Duty CycleRange
PERIOD
=
=
=
0 01
40
250 1
.
:
μ
ty Cycle
s
s
SETTLE
=
=
.
100
100
0
μ
. %
4
%
0. %
100
100
at
Hz
=
→
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)
10
P
100
1000
10000
100
1000
10000
3498 F08
1
10
PULSING MAY BE VISIBLE
Figure 9. Li-Ion to Four White LEDs with
Both PWM Dimming and Analog Dimming
CAP1
S
W1
CAP2 V
OUT2
V
IN
LT349
8
GND1
GND2
S
W2
C
IN
1
μ
H
R
S
EN
S
E1
10
Q1
S
i2304BD
S
349
8
F09
C
OUT1
F
V
IN
3V TO 5V
L1
15
μ
H
LED1
CTRL1
CTRL2
FB2
100k
0V
PWM
FREQ
5V
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