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ML0308 Preliminary
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ML0308A/B - Tiny Package, High Efficiency,
White LED Driver and Step-up DC/DC Converter
FEATURES 1.2MHz Switching Frequency Furnishes the Use of Tiny Surface Mount Components High Output Voltage: Up To 16V Automatic Softstart Drives Up To Four White LEDs From 3.6V Supply APPLICATIONS LCD Bias Handheld Computers Battery Backup Digital Cameras TYPICAL APPLICATIONS DESCRIPTION The ML0308 are high efficiency, white LED driver and step-up DC/DC converters in a tiny SOT23 5-lead package. The ML0308 switches at 1.2MHz. High switching frequency permits the use of tiny, low profile inductors and capacitor to minimize footprint and cost in space-conscious portable applications. The 18V switch allows high output voltage up to 16V to be easily generated in a simple boost topology avoiding the use of costly transformers.
Figure 1: One-cell Li-ion to 15V Converter and White LED Driver 1/9 Rev. B, July 2005
ML0308 Preliminary
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ABSOLUTE MAXIMUM RATINGS
VIN, FB, CE voltage LX Voltage Junction Temperature Operating Temperature
(Note 2)
PACKAGE/ORDER INFORMATION
ORDER PART NUMBER ML0308AMRG ML0308BMRG PART MARKING
6.0V 18.0V 125C -30C to 85C -40C to 150C 300C
Storage Temperature Lead Temperature
(soldering 10 seconds)
L30C (ML0308AMRG) L36A (ML0308BMRG)
ELECTRICAL CHARACTERISTICS The "X" denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25C. VIN = 3.0V, VCE = 3.0V unless otherwise noted.
PARAMETER Input Voltage Quiescent Current
CONDITIONS
MIN 2.7
TYP
MAX 5.5
UNITS V uA uA mV V uA
Non-switching VCE = 0V
100
150 0.1
Feedback Voltage
ML0308A ML0308B
X X
186 1.205
198 1.230 0.1
210 1.255 1.0
Lx Leakage Current Lx ON Resistance Lx Current Limit Lx Voltage Limit (Note 3) Switching Frequency Duty Ratio CE Pin Input High Voltage CE Pin Input LOW Voltage
VCE = 0V , VLX = 5.5V VIN = 3.0V, IDS=100mA
180 16.5 0.9 X 72 1.5 1.2 75
250 17.5 1.5 78
mA V MHz % V
0.3
V
Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: The ML0308 is guaranteed to meet performance specifications from 0C to 70C. Specifications over the -40C to 85C operating temperature range are assured by design, characterization and correlation with statistical process controls. Note 3: If Lx pin exceeds the voltage limit, the ML0308 will stop switching for about 40us.
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ML0308 Preliminary
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PIN FUNCTION DESCRIPTION Pin Description
LX Switch Pin. This is the drain of the internal power MOSFET. Minimize the metal trace area (Pin 1) connected to this pin to minimize EMI. GND (Pin 2) FB Feedback Pin. Set the output voltage by selecting values for R1 and R2: (Pin 3) R1 + R 2 R1 + R 2 VOUT = x 0.198V (ML0308A) or VOUT = x 1.23V (ML0308B) R2 R2 Ground. Tie this pin directly to the local ground plane.
CE This pin is the chip-enable Pin. Ground this pin to shutdown the chip. Apply a clock with (Pin 4) different duty ratio to control brightness of the LEDs. VIN The external voltage supply is connected to this pin. A high quality reservoir capacitor must (Pin 5) be connected across pin 5 and pin 2 (ground) to achieve the specified output voltage parameters. A 4.7uF/6.3 V, low ESR capacitor must be connected as close as possible across pin 5 and pin 2 (ground).
BLOCK DIAGRAM - ML0308A
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BLOCK DIAGRAM - ML0308B
Figure 2: Internal Block Diagram of ML0308B
OPERATION
The ML0308 uses a pulse-skipping PFM, current mode control scheme to provide excellent line and load regulation. Its operation can be best explained with reference to figure 2 and figure 1. The oscillator generates a clock of 1.2MHz with 75% duty ratio for the PFM controller. When CE pin goes HIGH, the power MOSFET switches at 1.2 MHz to charge the inductor and boost the output voltage to the target level specified by R1 and R2. The output voltage is set as follow:
VOUT = R1 + R 2 x 1.23V (ML0308B) R2
limit, comparator A3 will toggle and the control circuit will turn off the MOSFET immediately. So the inductor current is limited. Therefore, soft-start is achieved and potential hazards like current and voltage overshoot are avoided. When output voltage reaches and rises over the regulation point, amplifier A2 will amplify the error. The chip will adjust its pulse-skipping rate to monitor the output voltage.
When the inductor current reaches the current
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APPLICATION INFORMATION
Choosing an Inductor Several recommended inductors that work well
with the ML0308. Consult each manufacturer for more detailed information and for their entire selection of related parts. Many different sizes and shapes are available. Use the equations and recommendations in the next few sections to find the correct inductance value for your design.
Inductor Selection--Boost Regulator The formula below calculates the appropriate inductor value to be used for a boost regulator using the ML0308 (or at least provides a good starting point). This value provides a good tradeoff in inductor size and system performance. Pick a standard inductor close to this value. A larger value can be used to slightly increase the available output current, but limit it to around twice the value calculated below, as too large of an inductance will increase the output voltage ripple without providing much additional output current. A smaller value can be used (especially for systems with output voltages greater than 12V) to give a smaller physical size. Inductance can be calculated as: VOUT - VIN ( MIN ) + VD L= tOFF I LIMIT
(see Current Limit Overshoot section). For higher output voltages, the formula above will give large inductance values. For a 3V to 16V converter (typical LCD Bias application), a 22uH inductor is called for with the above equation, but a 10uH inductor could be used without excessive reduction in maximum output current.
Current Limit Overshoot For a fixed duty ratio PFM control scheme of the ML0308, the power switch is turned off after 3/4 of a cycle or the 180mA current limit is reached. There is a 100ns delay between the time when the current limit is reached and when the switch actually turns off. During this delay, the inductor current exceeds the current limit by a small amount. The peak inductor current can be calculated by
VIN ( MAX ) - VDSSAT I PEAK = I LIMIT + L
t d
where VD = 0.4V (Schottky diode voltage), ILIMIT = 180mA, and tOFF = 250ns; for designs with varying VIN such as battery powered applications, use the minimum VIN value in the above equation. For most systems with output voltages below 7V, a 4.7uH inductor is the best choice, even though the equation above might specify a smaller value. This is due to the inductor current overshoot that occurs when very small inductor values are used
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Where VDSSAT = 0.25V (switch saturation voltage) and td = 100ns. The current overshoot will be most evident for systems with high input voltages and for systems where smaller inductor values are used. This overshoot can be beneficial as it helps increase the amount of available output current for smaller inductor values. This will be the peak current seen by the inductor (and the diode) during normal operation. For designs using small inductance values (especially at input voltages greater than 5V), the current limit overshoot can be quite high. Although it is internally current limited to 180mA, the power switch of the ML0308 can handle larger currents without problem, but the overall efficiency will suffer.
Rev. B, July 2005
ML0308 Preliminary
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Best results will be obtained when IPEAK is kept below 300mA.
Capacitor Selection Low ESR (Equivalent Series Resistance) capacitors should be used at the output to
speed, are the best match for the ML0308. Many different manufacturers make equivalent parts, but make sure that the component is rated to handle at least 0.35A.
Lowering Output Voltage Ripple Using low ESR capacitors will help minimize the
minimize the output ripple voltage. Multilayer ceramic capacitors are the best choice, as they have a very low ESR and are available in very small packages. Their small size makes them a good companion to the ML0308 SOT23 package. Solid tantalum capacitors can be used, but they will occupy more board area than a ceramic and will have a higher ESR. Always use a capacitor with a sufficient voltage rating. Ceramic capacitors also make a good choice for the input decoupling capacitor, which should be placed as close as possible to the ML0308. A 4.7uF input capacitor is sufficient for most applications.
Diode Selection For most ML0308 applications, the Motorola MBR0520 surface mount Schottky diode (0.5A, 20V) is an ideal choice. Schottky diodes, with their low forward voltage drop and fast switching
output ripple voltage, but proper selection of the inductor and the output capacitor also plays a big role. The ML0308 provides energy to the load in bursts by ramping up the inductor current, then delivering that current to the load. If too large of an inductor value or too small of a capacitor value is used, the output ripple voltage will increase because the capacitor will be slightly overcharged each burst cycle. To reduce the output ripple, increase the output capacitor value or add a 4.7pF feed-forward capacitor in the feedback network of the ML0308 (see the circuits in the Typical Applications section). Adding this small, inexpensive 4.7pF capacitor will greatly reduce the output voltage ripple.
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TYPICAL APPLICATIONS
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ML0308 PACKAGE OUTLINE - SOT23-5L
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TAPE DIMENSIONS
REEL DIMENSIONS
DISCLAIMER: The information presented in this document does not form part of any quotation or contract, is believed to be accurate and reliable and may be changed without notice. No liability will be accepted by the publisher for any consequence of its use. 9/9 Rev. B, July 2005

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