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參數(shù)資料
| 型號(hào)： | LT3587EUD#TRPBF |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 穩(wěn)壓器 |
| 英文描述： | SWITCHING REGULATOR, 1200 kHz SWITCHING FREQ-MAX, PQCC20 |
| 封裝： | 3 X 3 MM, LEAD FREE, PLASTIC, QFN-20 |
| 文件頁數(shù)： | 8/24頁 |
| 文件大?。?/td> | 395K |
| 代理商： | LT3587EUD#TRPBF |
LT3587
16
3587fc
Figure 12. Dimming Using a DAC and a Resistor
APPLICATIONS INFORMATION
Besides acting as a fault output indicator, the Fault pin
is also an input pin. If this pin is externally forced low
below 400mV, the LT3587 behaves as if a fault event has
been detected and all the channels turn off. In order to
turn the part back on, remove the external voltage that
forces the pin low and reset the part. Figure 11 shows the
waveforms when the Fault pin is externally forced low and
the subsequent resetting of the part.
Since the programmed VOUT3 current is proportional to
the current through RIFB3, the LED current can be adjusted
according to the following formula:
IVOUT3 = (0.8V – VDAC-OUT) 200/RIFB3
A higher DAC output voltage level results in lower LED
current and hence lower overall brightness. Conversely,
a lower DAC output voltage results in higher LED current
and higher brightness. Note that the DAC output impedance
should be low enough to be able to sink approximately
1/200 of the desired maximum LED current without any
appreciable error for accurate dimming control.
Note also that the maximum output current is limited by
the output disconnect current limit to 110mA (typ).
PWM Dimming
Changing the forward current owing in the LEDs not
only changes the brightness 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. Control-
ling 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.
10μH
3587 F12
VOUT3
CAP3
IFB3
LT3587
RIFB3
8.06k
EN/SS3
SW3
VIN
LED DRIVER
1μF
VVIN
2.5V TO 5V
VDAC-OUT
DAC
LTC2630
Figure 11. Waveforms When the
Fault Pin is Externally Forced Low
3587 F11
ENSS1/ENSS3
5V/DIV
VNEG
10V/DIV
VVOUT3
20V/DIV
VVOUT1
10V/DIV
VFLT
5V/DIV
100ms/DIV
PART RESET
FLT FORCED LOW
Dimming Control For Boost3 Current Regulator as an
LED Driver
As shown on the front page application and the Block Dia-
gram, one of the most common applications for the Boost3
channel when congured as a boost current regulator is
a backlight LED driver. In an LED driver application, there
are two different ways to implement a dimming control of
the LED string. The LED current can be adjusted either by
using a digital to analog converter (DAC) with a resistor
RIFB3 or by using a PWM signal.
Using a DAC and a Resistor
For some applications, the preferred method of brightness
control is using a DAC and a resistor. The Boost3 congura-
tion for using this method is shown in Figure 12.
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