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YB1508
Step-up DC-DC Converter , White LED Driver
Description
Features
The YB1508 is a step-up DC-DC converter,
acting as current source to drive up to 8
white LEDs in series. Series connecting of
the LEDs provides identical LED currents
resulting in uniform brightness and
eliminating the need for ballast resistors.
The light intensity of these LEDs is
proportional to the current passing through
them. Additional features include output over
voltage protection to prevent device damage
due to faulty LED or open circuit caused by
abnormal conditions, and one-pin shutdown
and multiple methods of dimming control.
The YB1508 switches at a fixed frequency of
1MHz, allowing the use of tiny external
components and low input noise and ripple.
To control LED brightness, the LED current
can be pulsed by applying a PWM (pulse
width modulated) signal with a frequency
range of 100Hz to 50KHz to the CTRL pin.
The YB1508 is available in a low profile 5-lead
SOT-23 package.
Typical Application Circuit
„ 2.7V to 9V input range
„ Adjustable output voltage up to 40V
„ Built in internal switch
„ Inherently matched LED current
„ Fixed frequency of 1MHz PWM
operation
„ Open LED protection
„ Low profile 5-Lead SOT-23 package
„ High efficiency: over 80% at 3.6V
input voltage
„ Drives up to 8 white LEDs
Applications
„ LCD Display Module
„ OLED panel/sub-panel
„ White LED Backlighting
„ Digital Camera
„ Cellular Phone
„ Portable Application
YB1508 MRev1.4
Figure 1Typical Application Circuit
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Pin Description
YB1508
Step-up DC-DC Converter , White LED Driver
Figure 2YB1508 SOT23-5
Pin Designator
Table 1 :
Pin Name
1 SW
2 GND
3 FB
4 CTRL
5 VIN
Description
Switching Pin. This is the collector of the internal NPN power switch. Connect to
inductor and diode. Minimize the metal trace area connected to this pin to reduce
EMI.
Ground Pin. Connect directly to local ground plane.
Feedback Pin. Reference voltage is 200mV. Connect LEDs and a resistor at this
pin. LED current is determined by the resistance and CTRL voltage.
Shutdown Pin and Dimming Control pin.
A. VCTRL > 1.8V generates full-scale LED current
B. VCTRL < 0.4V chip is off
C. Switching from 0.4V to 2.0V, PWM duty cycle controls the LED current
Input Supply Pin. Bypass this pin with a capacitor as close to the device as
possible.
Ordering Information
Table 2:
Order Number
High Ovp YB1508
Low Ovp YB1508P
Package Type
SOT23-5
SOT23-5
Supplied as
3000 units Tape & Reel
3000 units Tape & Reel
Package Marking
Y58
Y58P
YB1508 MRev1.4
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YB1508
Step-up DC-DC Converter , White LED Driver
Absolute Maximum Ratings
SW voltage
All other pins
40V
10V
Maximum Junction Temp,Tj
Lead Temperature (Soldering 10
sec)
Thermal Resistance
150 °C
300 °C
195 °C
Electricity Characteristics
Table 3(TA=25°C, Vin=3.3V Cin=1uF Cout=4.7uF unless otherwise noted)
Function Parameter
Input Voltage Range
Feedback Voltage
FB Pin Bias current
Supply current
Switching Frequency
Maximum Duty Cycle
Minimum duty cycle
Switch Vcesat
Switch Current Limit
Switch Leakage Current
VCTRL for Full LED current
CTRL Pin Bias Current
Chip turn off delay
Over Voltage Protection
Conditions
Iout =20mA, Vout = 12.5V
Circuit of figure 1. (L=22uH
Cout =4.7uF)
Rfb = 10 ohms
Vfb = 0.3V
CTRL = 0V
At Isw = 200mA
40% duty cycle
Ctrl = 0.3V
Ctrl = 2V
YB1508
YB1508P
Min
2.7
180
2
0.7
20
1.6
Typ
200
10
3.5
1
0.8
25
200
250
1
1.7
20
36
21
Max
9
Units
V
220 mV
20 μA
mA
μA
1.3 MHz
93 %
35 %
mV
mA
μA
1.8 V
μA
30 μS
V
YB1508 MRev1.4
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YB1508
Step-up DC-DC Converter , White LED Driver
Functional Block Diagram
Figure 3. Block Diagram
Application Information
Operation
The operation of YB1508 can be
understood by referring to the block
diagram in Figure 3. A voltage
proportional to the switch current is added
to a ramp output and the resulting sum is
fed into the positive terminal of the PWM
comparator. When this voltage exceeds
the level of the comparator negative input,
the peak current has been reached, and
the SR latch is reset and turns off the
power switch. The voltage at the negative
input of the comparator comes from the
output of the error amplifier. 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 22uH inductor is recommended for most
YB1508 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 YB1508
applications. X5R and X7R types are
recommended because they retain their
capacitance over wider voltage and
temperature ranges than other types.
Diode selection
To maintain high efficiency, the average
current rating of the schottky diode
should be larger than the peak inductor
current, IPK. Schottky diode with a low
forward drop and fast switching speeds
are ideal for increasing efficiency in
portable applications. The reverse
breakdown voltage of the schottky diode
must be larger than the output voltage.
YB1508 MRev1.4
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YB1508
Step-up DC-DC Converter , White LED Driver
Inrush Current
The maximum switch current is limited to
about 300 mA by the chip. Typically, this
large current will occur for about 40us
during start up. A larger Cout will increase
the duration of high current. However,
when Vin is first connected, an even
larger current can flow into Cout. This
occurs when the SW pin is open circuit
and and the Cout voltage changes from
0V to Vin in a few microseconds. Because
the duration of this large current is short, it
will usually not cause problems.
LED Current and Dimming Control
The LED current can be reduced by pulse
width modulating the CTRL voltage from
0.4V to 1.8V. Using the PWM mode, the
LED current can be controlled from 0% to
100%.
Open Circuit Protection
The YB1508 has an internal open-circuit
protection circuit. When the LEDs are
disconnected from the circuit or fail
open, the YB1508 will then shutdown
until Vin goes low or CTRL goes lower
than 0.4V. Different maximum Vout
can be requested from YoBon.
Board Layout Consideration
To maximize efficiency, rise and fall
times of switch should be made as
short as possible. Proper layout of the
high frequency switching path is
important to prevent electromagnetic
interference (EMI) problems. Place
COUT close to the VOUT pin. A
ground plane under the switching
regulator is needed to minimize
interplane coupling. In addition, the
feedback resistor Rs should be tied
directly to the GND Pin without any
other connection to ensure a clean,
noise-free connection.
Figure 4. Dimming Control Using a DC Voltage
Figure 5. Dimming Control Using a Filtered PWM
Signal
YB1508 MRev1.4
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