CYT 2506
White LED Step-Up Converter
General Description
The CYT2506 is a step-up DC/DC converter
specifically designed to drive white LEDs with a
constant current. The device can drive two,
three or four LEDs in series from a Li-Ion cell.
Series connection of the LEDs provides
identical LED current resulting in uniform
brightness and eliminating the need for ballast
resistors.
The CYT2506 switches at 1MHz, allowing
the use of any tiny external components. The
output capacitor can be as small as 1µF,
saving space and cost versus alternative
solutions. A low 95mV feedback voltage
minimizes power loss in the current setting
resistor for better efficiency.
CYT2506 is available in low profile
SOT-23-5 packages.
Features
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Inherently Matched LED Current.
High Efficiency: 84% Typical.
Drives Up to Four LEDs from a 3.2V Supply.
Drives Up to Six LEDs from a 5V Supply.
Fast 1.2MHz Switching Frequency.
Uses Tiny 1mm Tall Inductors.
Requires Only 1µF Output Capacitor.
Standard SOT-23-5 package.
Low Profile SOT Packaging.
Applications
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Cellular Phones
PDAs,
Handheld Computuers
Digital Camera
MP3 Players
GPS Receivers
SOT-23-5 Package
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CYT 2506
Pin Configuration
1.SW
2.GND
3.FB
4.EN
5.VIN
SOT-23-5
Typical Application
VIN: 3V to 5V
L1: 22uH
D1
LED1
15mA
VIN
C
IN
: 1uF
ON
OFF
EN
GND
R1: 6.34Ω
SW
FB
LED2
LED3
C
OUT
: 1uF
CYT2506
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CYT 2506
Absolute Maximum Rating
(1)
Parameter
Input Voltage
Switch Voltage
Feedback Voltage
Enable Voltage
Thermal Resistance, Junction-to-mbient
Lead Temperature (Soldering, 10 sec.)
Junction Temperature
Storage Temperature
T
J
T
S
Symbol
V
IN
V
SW
V
FB
V
EN
Θ
JA
Value
+5.0
+30
+5.0
+5.0
250 (SOT-23-5)
260
125
-65 to +150
Units
V
V
V
V
°C/W
°C
°C
°C
Operating Rating
(2)
Parameter
Supply Input Voltage
Junction Temperature
Symbol
V
IN
T
J
Value
+2.7V to +5.0
0 to +125
Units
V
°C
Electrical Characteristics
V
IN
= V
EN
= 3.6V; T
J
= 25°C; unless otherwise specified
Symbol
Parameter
Conditions
V
IN
Input Voltage Range
I
SW
= 100mA, Duty Cycle
V
FB
Feedback Voltage
= 66%
I
FB
FB Pin Bias Current
I
CC
Supply Current
Switching Frequency
Maximum Duty Cycle
I
LIMIT
Switch Current Limit
Switch Leakage Current
Enable Input Threshold
Voltage
Enable Pin Bias Current
V
SW
= 5V
Voltage Raising, Output
Turns On, Logic High
Voltage Falling, Output
Turns Off, Logic Low
1.6
--
--
V
EN
= 0V
Min
2.7
Typ
--
Max
5.0
Unit
V
--
--
--
--
--
--
--
95
45
100
0.1
1.0
85
300
0.01
--
--
300
--
100
--
1
--
--
--
5
--
mV
nA
A
A
MHz
%
mA
A
V
TH(EN)
V
0.4
--
nA
I
EN
Note 1:
Exceeding the absolute maximum rating may damage the device.
Note 2:
The device is not guaranteed to function outside its operating rating.
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CYT 2506
Application Hints
Inductor Selection
A 22µH inductor is recommended for most CYT2506 applications. Although small size and high
efficiency are major concerns, the inductor should have low core losses at 1MHz and low DCR (copper
wire resistance).
Capacitor Selection
The small size of ceramic capacitors makes them ideal for CYT2506 applications. X5R and X7R types
are recommended because they retain their capacitance over wider voltage and temperature ranges than
other types such as Y5V or Z5U. A 1µF input capacitor and a 1µF output capacitor are sufficient for most
CYT2506 applications.
Diode Selection
Schottky diodes, with their low forward voltage drop and fast reverse recovery, are the ideal choices
for CYT2506 applications. The forward voltage drop of a Schottky diode represents the conduction losses
in the diode, while the diode capacitance (C
T
or C
D
) 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 1MHz switching frequency of the CYT2506. A Schottky diode rated at
100mA to 200mA is sufficient for most CYT2506 applications.
LED Current Control
The LED current is controlled by the feedback resistor (R1). The feedback reference is 95mV. The
LED current is 95mV/R1. In order to have accurate LED current, precision resistors are preferred (1% is
recommended). The formula and table for R1 selection are shown below.
R1 = 95mV/I
LED
R1 Resistor Value Selection
I
LED
(mA)
5
10
12
15
20
R1 (Ω)
19.1
9.53
7.87
6.34
4.75
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CYT 2506
Open-Circuit Protection
In the cases of output open circuit, when the LEDs are disconnected from the circuit or the LEDs fail,
the feedback voltage will be zero. The CYT2506 will then switch at a high duty cycle resulting in a high
output voltage, which may cause the SW pin voltage to exceed its maximum 30V rating. A zener diode can
be used at the output to limit the voltage on the SW pin. The zener voltage should be larger than the
maximum forward voltage of the LED string. The current rating of the zener should be larger than 0.1mA.
Dimming Control
(There are two different types of dimming control circuits:)
1. Using a PWM Signal to EN Pin
With the PWM signal applied to the EN pin, the CYT2506 is turned on or off by the PWM signal. The
LEDs operate a either zero or full current. The average LED current increases proportionally with the duty
cycle of the PWM signal. A 0% duty cycle will turn off the CYT2506 and corresponds to zero LED current. A
100% duty cycle corresponds to full current. The typical frequency range o the PWM signal is 1kHz to 5kHz.
The magnitude of the PWM signal should be higher than the minimum EN voltage high. The switching
waveforms of the EN pin PWM control are shown below.
2. Using a DC Voltage
For some applications, the preferred method o brightness control is a variable DC voltage to adjust the
LED current. The dimming control using a DC voltage is shown below. As the DC voltage increases, the
voltage drop on R2 increases and the voltage drop on R1 decreases. Thus, the LED current decreases.
The selection of R2 and R3 will make the current from the variable DC source much smaller than the LED
current and much larger than the FB pin bias current. For Vcc range from 0V to 2V, the selection of
resistors gives dimming control of LED current from 0mA to 15mA
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