LTC3577EUFF#PBF Linear Technology, LTC3577EUFF#PBF Datasheet - Page 35

• Manufacturer Part Number: LTC3577EUFF#PBF
• Description: IC PWR MANAGEMENT HANDHELD 44QFN
• Manufacturer: Linear Technology

Specifications of LTC3577EUFF#PBF

• Applications: Handheld/Mobile Devices
• Voltage - Supply: 4.35 V ~ 5.5 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 44-QFN
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Supply: -

OPERATION
and stability the output capacitor for step-down switching
regulators should retain at least 4μF of capacitance over
operating temperature and bias voltage. Each switching
regulator input supply should be bypassed with a 2.2μF
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
Murata
Taiyo Yuden
Vishay Siliconix
TDK
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
BOOST output will be driven to a voltage where the I
pin regulates at 300mV. The I
sink that is programmed via I
LED can be further controlled using I
ness levels and soft turn-on/turn-off effects. See the “I
Interface” section for more information on programming
the I
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
LED
LED
pin. Under normal operation the boost converter
current. The boost converter also includes an
LED
www.avxcorp.com
www.murata.com
www.t-yuden.com
www.vishay.com
www.tdk.com
2
C “LED DAC register”. The
pin is a constant-current
2
C to program bright-
LED
2
C
protection threshold is set by adjusting R1 in Figure 16
such that:
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
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.
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
nential DAC. This DAC provides accurate current control
from 20μA to 20mA with approximately 1dB per step for
I
by the following equations:
LED(FS)
I
I
BOOST(MAX) = 800mV •
LED
LED(FS)
2
C “LED DAC Register” and utilizes a 6-bit 60dB expo-
=I
LTC3577
= 20mA. The LED current can be approximated
LED(FS)
LED_OV
I
Figure 16. LED Boost Application Circuit
=
LED_FS
V
I
3577 F16
OUT
0.8V
SW
SW
SW
LED
R2
LTC3577/LTC3577-1
LED
39
18
19
20
9
3
22
• 10
10M
20k
• 500
R1
R2
pin. This feedback limits the rise time
L1
10μH
LPS4018-103ML
ZLLS400
⎛
⎝ ⎜
D12
22μF
3 •
C1
DAC – 63
C2
1μF
50V
10 •R2
BOOST
63
R1
C3
22nF
50V
⎞
⎠ ⎟
+LED_ OV
D10 D9
D1
D2
D3
D8
D4
D7
35
D5
D6
3577fa
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