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Alfa-MOS
constant current. The device can drive up to 27
Additional feature include
Technology
General Description Features
The AF1937K is a step-up DC/DC converter
specifically designed to drive white LEDs with a
LEDs from a 5V supply.
output voltage limiting when LEDs are
disconnected.
White LED Step UP Converter
Inherently Matched LED Current
Drives Up to 27 LEDs from a 5V Supply
36V Rugged Bipolar Switch
Fast 1.2MHz Switching Frequency
V
OUT(MAX)
=30V
Application
Battery Power Equipment
Notebook Computers
PDA
Cellular Phone
Digital Cameras
MP3 Players
GPS Receivers
AF1937K
30V
Pin Define
Marking Information
SOT-23-6L
SOT-23-6L
©Alfa-MOS Technology Corp.
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Technology
Block Diagram
Typical Application Circuit
White LED Step UP Converter
AF1937K
30V
Pin Description ( SOT-23-6L )
Pin Symbol Description
1 SW Switch Pin.
2 GND Ground Pin
3 FB Feedback Pin.
4 SHDN Shutdown Pin. Active-low enable
5 Vout Voltage Output
6 Vin Supply Voltage Input
Ordering Information
Part Number Package Part Marking Unit Quantity
AF1937KS26RG SOT-23-6L 97KYW Tape & Reel 3000 EA
Y : Year Code
W : Week Code : A ~ Z ( 1 ~ 26 ) ; a ~ z ( 27 ~ 52 )
AF1937KS26RG : Tape Reel ; 7” Pb- Free ; Halogen- Free
©Alfa-MOS Technology Corp.
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Alfa-MOS
AF1937K
30V
Technology
White LED Step UP Converter
Absolute Maximum Ratings
(TA=25 Unless otherwise noted)
Parameter Symbol Value Unit
DC Supply Voltage Vin 20 V
SW Voltage VSW
FB Voltage VFB 10 V
SHDN Voltage V
Operating Temperature
Maximum Junction Temperature T
Storage Temperature
Note : Absolute Maximum Ratings are those values beyond which the life of the device may be impaired.
Electrical Characteristics
(TA=25, Vin = 5V, V
/SHDN
36 V
SHDN
10 V
OPR
T
J(Max)
125
T
S
= 5V, unless otherwise noted. Unless otherwise noted)
0
70
150
-65
©Alfa-MOS Technology Corp.
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Alfa-MOS
AF1937K
30V
Technology
White LED Step UP Converter
Application Note
OPERATION
The AF1937K uses a constant frequency, current mode control scheme to provide excellent line and
load regulation. Operation can be best understood by referring to the block diagram. At the start of each
oscillator cycle, the RS latch is set, which turns on the power switch Q1. A voltage proportional to the
switch current is added to a stabilizing ramp and the resulting sum is fed into the positive terminal of the
PWM comparator A2. When this voltage exceeds the level at the negative input of A2, the RS latch is
reset turning off the power switch. The level at the negative input of A2 is set by the error amplifier A1,
and is simply an amplified version of the difference between the feedback voltage and the reference
voltage of 95mV. In this manner, the error amplifier sets the correct peak current level to keep the output
in regulation. If the error amplifier’s output increases, more current is delivered to the output; if it
decreases, less current is delivered.
Inductor Selection
A 10µH inductor is recommended for most AF1937K applications. Although small size and high
efficiency
are major concerns, the inductor should have low core losses at 1.2MHz and low DCR (copper wire
resistance).
Capacitor Selection
The small size of ceramic capacitors makes them ideal for AF1937K applications. X5R and X7R types
are recommended because they retain their capacitance over wider voltage and temperature ranges
than ther types such as Y5V or Z5U. A 4,7µF input capacitor and a 4,7Μf output capacitor are sufficient
for most AF1937K application.
Diode Selection
Schottky diodes, with their low forward voltage drop and fast reverse recovery, are the ideal choices for
AF1937Kapplications. The forward voltage drop of a Schottky diode represents the conduction losses in
the diode, while the diode capacitance (CT or CD) represents the switching losses. For diode selection,
both forward voltage drop and diode capacitance need to be considered. Schottky diodes with higher
current ratings usually have lower forward voltage drop and larger diode capacitance, which can cause
significant switching losses at the 1.2MHz switching frequency of the AF1937K. A Schottky diode rated
at 1000mA t is sufficient for most AF1937K application.
©Alfa-MOS Technology Corp.
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Alfa-MOS
Technology
Package Information ( SOT-23-6L )
White LED Step UP Converter
AF1937K
30V
©2010 Alfa-MOS Technology Corp.
2F, No.80, Sec.1, Cheng Kung Rd., Nan Kang Dist., Taipei City 115, Taiwan (R.O.C.)
Tel : 886 2) 2651 3928
Fax : 886 2) 2786 8483
©http://www.alfa-mos.com
©Alfa-MOS Technology Corp.
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