Datasheet

LTC3577/LTC3577-1
35
3577fa
OPERATION
and stability the output capacitor for step-down switching
regulators should retain at least 4F of capacitance over
operating temperature and bias voltage. Each switching
regulator input supply should be bypassed with a 2.2F
capacitor. Consult with capacitor manufacturers for de-
tailed information on their selection and specifi cations
of ceramic capacitors. Many manufacturers now offer
very thin (<1mm tall) ceramic capacitors ideal for use in
height-restricted designs. Table 4 shows a list of several
ceramic capacitor manufacturers.
Table 4. Ceramic Capacitor Manufacturers
AVX www.avxcorp.com
Murata www.murata.com
Taiyo Yuden www.t-yuden.com
Vishay Siliconix www.vishay.com
TDK www.tdk.com
LED BACKLIGHT/BOOST OPERATION
Introduction
The LED driver uses a constant frequency, current mode
boost converter to supply power to up to 10 series LEDs.
As shown in Figure 16 the series string of LEDs is con-
nected from the output of the boost converter (BOOST) to
the I
LED
pin. Under normal operation the boost converter
BOOST output will be driven to a voltage where the I
LED
pin regulates at 300mV. The I
LED
pin is a constant-current
sink that is programmed via I
2
C “LED DAC register”. The
LED can be further controlled using I
2
C to program bright-
ness levels and soft turn-on/turn-off effects. See the “I
2
C
Interface” section for more information on programming
the I
LED
current. The boost converter also includes an
overvoltage protection feature to limit the BOOST output
voltage as well as variable slew rate control of the SW pin
to reduce EMI.
LED Boost Operation
The LED boost converter is designed for very high duty
cycle operation and can boost from 3V to 40V for load
currents up to 20mA. The boost converter also features
an overvoltage protection feature to protect the output in
case of an open circuit in the LED string. The overvoltage
protection threshold is set by adjusting R1 in Figure 16
such that:
BOOST(MAX) = 800mV •
R1
10 •R2
+LED_OV
where LED_OV is about 1.0V.
In the case of Figure 16 BOOST(MAX) is set to 40V for a
10-LED string.
Capacitor C3 provides soft-start, limiting the inrush cur-
rent when the boost converter is fi rst enabled. C3 provides
feedback to the I
LED
pin. This feedback limits the rise time
of output voltage and the inrush current while the output
capacitor, C2, is charging.
The boost converter will be operated in either continuous
conduction mode, discontinuous conduction mode or
pulse-skipping mode depending on the inductor current
required for regulation.
C2
1µF
50V
C3
22nF
50V
D12
ZLLS400
C1
22µF
R1
10M
D1
R2
20k
L1
10µH
LPS4018-103ML
39
18
BOOST
19
20
3577 F16
V
OUT
9
LED_OV
SW
SW
3
22
I
LED_FS
I
LED
SW
LTC3577
D2 D3 D4 D5
D10 D9 D8 D7 D6
Figure 16. LED Boost Application Circuit
LED Constant Current Sink
The LED driver uses a precision current sink to regulate the
LED current up to 20mA. The current sink is programmed
via I
2
C “LED DAC Register” and utilizes a 6-bit 60dB expo-
nential DAC. This DAC provides accurate current control
from 20A to 20mA with approximately 1dB per step for
I
LED(FS)
= 20mA. The LED current can be approximated
by the following equations:
I
LED
=I
LED(FS)
•10
3 •
DAC – 63
63
⎛
⎝
⎜
⎞
⎠
⎟
I
LED(FS)
=
0.8V
R2
• 500
(1)