Minilogic ML0308A Tiny package, high efficiency, white led driver and step-up dc/dc converter Datasheet

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
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.
TYPICAL APPLICATIONS
Figure 1: One-cell Li-ion to 15V Converter and White LED Driver
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ML0308 Preliminary
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ABSOLUTE MAXIMUM RATINGS
VIN, FB, CE voltage
PACKAGE/ORDER INFORMATION
6.0V
ORDER PART
NUMBER
LX Voltage
18.0V
Junction Temperature
125°C
ML0308AMRG
-30°C to 85°C
ML0308BMRG
Operating Temperature
PART MARKING
(Note 2)
Storage Temperature
-40°C to 150°C
L30C (ML0308AMRG)
300°C
Lead Temperature
L36A (ML0308BMRG)
(soldering 10 seconds)
ELECTRICAL CHARACTERISTICS The “X” denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. VIN = 3.0V, VCE = 3.0V unless otherwise noted.
PARAMETER
CONDITIONS
MIN
Input Voltage
Quiescent Current
TYP
2.7
Non-switching
100
VCE = 0V
Feedback Voltage
MAX
UNITS
5.5
V
150
uA
0.1
uA
ML0308A
X
186
198
210
mV
ML0308B
X
1.205
1.230
1.255
V
0.1
1.0
uA
Lx Leakage Current
VCE = 0V , VLX = 5.5V
Lx ON Resistance
VIN = 3.0V, IDS=100mA
Ω
Lx Current Limit
180
250
mA
Lx Voltage Limit (Note 3)
16.5
17.5
V
0.9
1.2
1.5
MHz
72
75
78
%
Switching Frequency
Duty Ratio
X
CE Pin Input High Voltage
1.5
CE Pin Input LOW Voltage
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 0°C to 70°C. Specifications over the –40°C to 85°C 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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Rev. B, July 2005
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
Ground. Tie this pin directly to the local ground plane.
(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 =
× 0.198V (ML0308A) or VOUT =
× 1.23V (ML0308B)
R2
R2
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 =
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.
R1 + R 2
× 1.23V (ML0308B)
R2
When the inductor current reaches the current
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Rev. B, July 2005
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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
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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(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 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
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
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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
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.
Rev. B, July 2005
ML0308 Preliminary
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TYPICAL APPLICATIONS
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Rev. B, July 2005
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ML0308 PACKAGE OUTLINE - SOT23-5L
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ML0308 Preliminary
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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.
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