MIC2251
High-Efficiency Low EMI
Boost Regulator
MLF and MicroLeadFrame are registered trademarks of Amkor Technology, Inc.
Micrel Inc. • 2180 Fortune Drive • San Jose, CA 95131 • USA • tel +1 (408) 944-0800 • fax + 1 (408) 474-1000 • http://www.micrel.com
April 2011 M9999-041811-B
General Description
The MIC2251 is a general purpose DC/DC boost switching
regulator that features low noise, EMI reduction circuitry,
and high efficiency across a wide output current range.
The MIC2251 is optimized for noise-sensitive hand held
battery powered applications. A proprietary control method
allows low ripple across the output voltage and current
ranges. The MIC2251 incorporates a pseudo-random
dithering function to reduce EMI levels up to 10dB enabled
by the DITH pin.
The MIC2251 is designed for use with inductor values from
4.7µH to 22µH, and is stable with ceramic capacitors from
1µF to 22µF.
The MIC2251 attains a high peak efficiency up to 90% at
100mA and excellent light load efficiency of 80% at 1mA.
High power density is achieved with the MIC2251’s
internal 40V/2A rated switch, allowing it to power large
loads in a tiny footprint.
The MIC2251-1 is available in a 5-pin Thin SOT-23
package with dithering disabled. While the MIC2251-2 is
available in a 5-pin Thin SOT-23 package with dithering
enabled. The MIC2251 is available in a 8-pin 2mm x 2mm
MLF® leadless package option with an operating junction
temperature range of –40°C to +125°C.
Data sheets and support documentation can be found on
Micrel’s web site at: www.micrel.com.
Features
• Over 80% efficient for a 300:1 load range
• 2.5V to 5.5V input voltage range
• Output voltage adjustable to 37V
• 52µA (typ) quiescent current
• Constant peak current control reduces output ripple
• EMI reduction circuitry
• Stable with small ceramic capacitors
• <1µA shutdown current
• UVLO and thermal shutdown
• 8-pin 2mm x 2mm leadless MLF® package (MIC2251)
• 5-pin Thin SOT-23 package (MIC2251-1 & -2)
• –40°C to +125°C junction temperature range
Applications
• LCD/OLED display bias supply
• CCD bias supply
• Mobile Phones, PDA, Media Players, GPS PND
• Haptic displays
• Local 5V, 15V, 24V rail
___________________________________________________________________________________________________________
Typical Application
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Ordering Information
Part Number Marking(1) Junction Temp. Range Package(3) Lead Finish
MIC2251YML AAD (2) –40° to +125°C 8-Pin 2mm x 2mm MLF® Pb-Free
MIC2251-1YD5 DZ1(2) –40° to +125°C 5-Pin Thin SOT-23 Pb-Free
MIC2251-2YD5 DZ2(2) –40° to +125°C 5-Pin Thin SOT-23 Pb-Free
Note:
1. Pin 1 identifier = “•”.
2. Over/Underbar ( /__ ) may not be to scale.
3. MLF® is GREEN RoHs compliant package. Lead finish is NiPdAu. Mold compound is Halogen Free.
Pin Configur ation
8-Pin 2mm x 2mm MLF® (ML)
(Top View) 5-Pin Thin SOT-23 (D5)
(Top View)
Pin Description
Pin Name Pin Number
MIC2251YML Pin Number
MIC2251-1YD5 Pin Number
MIC2251-2YD5 Pin Function
FB 1 3 3 Feedback (Input): 1.24V output voltage sense node.
VOUT = 1.24V (1 + R1/R2)
AGND 2 ― ― Analog Ground. Connect to power ground.
PGND 3 2 2 Power Ground.
SW 4 1 1 Switch Node (Input): Internal power NMOS drain.
NC 5 ― ― Not Internally Connected.
VIN 6 5 5 Supply (Input): 2.5V to 5.5V input voltage.
DITH 7 ― 4 Frequency Dithering (Input): Connect this pin high to
enable pseudo-random on-time dithering to reduce EMI.
Connect this pin-to-ground to disable this function.
EN 8 4 4 Enable (Input): Logic high enables the regulator. Logic
low shuts down the regulator. Do not leave floating.
HS PAD EP ― ― Ground (Return): Exposed backside pad. Connect to
power ground.
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Absolute Maximum Ratings(1)
Supply Voltage (VIN).........................................................6V
Switch Voltage (VSW)....................................... –0.3V to 40V
Enable Voltage (VEN)......................................... –0.3V to VIN
FB Voltage (VFB)...............................................................6V
Switch Current (ISW) ......................................................3.5A
Ambient Storage Temperature (Ts) ..........–65°C to +150°C
ESD Rating(4)................................................. ESD Sensitive
Operating Ratings(2)
Supply Voltage (VIN)......................................... 2.5V to 5.5V
Enable Voltage (VEN).............................................. 0V to VIN
Junction Temperature (TJ)(3) .....................–40°C to +125°C
Junction Thermal Resistance
2mm x 2mm MLF®-8 (θJA)..................................90°C/W
Electrical Characteristics(5)
VIN = VEN = 3.6V; VDITH = 0V; VOUT = 15V; IOUT = 40mA; TA = 25°C, unless otherwise noted. Bold values indicate
–40°C ≤ TJ ≤ +85°C.
Symbol Parameter Condition Min Typ Max Units
VIN Input Voltage Range 2.5 5.5 V
VULVO Under-voltage Lockout VIN rising 1.8 2 2.4 V
IQ Quiescent Current VFB = 1.5V (not switching) 70 120 µA
ISD Shutdown Current VEN = 0V, Note 6 0.1 1 µA
1.20 1.24 1.277 V VFB Feedback Voltage
–40°C ≤ TJ ≤ +85°C 1.19 1.29 V
IFB Feedback Input Current VFB = 1.24V 10 nA
PFM Operation
Tss Soft Start time 1 ms
tSW Switch Off-time VIN = 3.6V 1.6 µs
DMAX Maximum Duty Cycle 75 87 %
tDITH Off-time Dithering VDITH = 3.6V. Percentage from nominal. ±20 %
Line Regulation 3V ≤ VIN ≤ 5V 0.3 2 %
Load Regulation 1mA ≤ IOUT ≤ 40mA 0.1 2 %
ISW Switch Current Limit Note 7 0.9 1.6 A
RON Switch ON-resistance ISW = 200mA 0.7 1 Ω
ISW Switch Leakage Current VEN = 0V, VSW = 10V 0.01 5 µA
Turn ON 1.5 V VEN,
VDITH
Logic Input Thresholds
Turn OFF 0.4 V
IEN Enable Pin Current VEN = VIN = 5.0V 0.1 2 µA
130 °C T Thermal Shutdown Threshold
Hysteresis 15 °C
Notes:
1. Absolute maximum ratings indicate limits beyond which damage to the component may occur.
2. The device is not guaranteed to function outside its operating rating.
3. The maximum allowable power dissipation is a function of the maximum junction temperature, TJ(Max), the junction-to-ambient thermal resistance, θ JA,
and the ambient temperature, TA. The maximum allowable power dissipation will result in excessive die temperature, and the regulator will go into
thermal shutdown.
4. Devices are ESD sensitive. Handling precautions recommended. Human body model, 1.5kΩ in series with 100pF.
5. Specification for packaged product only.
6. ISD = IVIN.
7. Guaranteed by design.
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Typical Characteristics
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Functional Characteristics
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Functional Characteristics (continued)
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Functional Diagram
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Functional Description
VIN
The input supply (VIN) provides power to the internal
MOSFETs and control circuitry for the switch mode
regulator. The operating input voltage range is from 2.5V
to 5.5V. An input capacitor with a minimum voltage
rating of 6.3V is recommended. Refer to the layout
recommendations for details.
EN
A logic level input of 1.5V or higher enables the
regulator. A logic input of 0.4V or less places the
regulator in shutdown mode which reduces the supply
current to less than 1µA. The MIC2251 features built-in
soft start circuitry that reduces in-rush current and
prevents the output voltage from overshooting during
startup. Do not leave the Enable pin floating.
SW
The MIC2251 has an internal MOSFET switch that
connects directly to one end of the inductor (SW pin) and
provides a current path to ground during switching
cycles. The source of the internal MOSFET connects
through a current sense resistor to ground.
PGND
The power ground pin is the high current path to ground.
The current loop for the power ground should be as
small as possible and separate from the analog ground
(AGND). Refer to the layout recommendations for more
details.
AGND
Analog ground (AGND) is the ground path for the biasing
and control circuitry. The current loop for the signal
ground should be separate from the power ground
(PGND) loop. Refer to the layout recommendations for
more details.
DITH
The DITH function is a frequency dithering technique
that reduces EMI noise by spreading the boost
regulators’ noise spectrum. This technique reduces the
EMI peaks by distributing the switching frequency across
a wider spectrum. Connect this pin high to enable the
pseudo-random on-time dithering. Connect this pin to
ground to disable this function.
FB
The feedback pin (FB) allows the regulated output
voltage to be set by applying an external resistor divider
network. The internal reference voltage is 1.24V. The
output voltage is calculated from the following equation:
⎟
⎠
⎞
⎜
⎝
⎛+= R2
R1
11.24VVOUT
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Application Information
Overview
The MIC2251 Boost Regulator utilizes a combination of
PFM & Current Mode Control to achieve very high
efficiency over a wide range of output load. This
innovative design is the basis for the regulator’s high
efficiency, excellent stability, and self compensation
technique. The boost regulator performs a power
conversion that results in an output voltage that is
greater than the input. Operation starts with activating an
internal MOSFET switch which draws current through
the inductor (L1). While one end of the inductor is fixed
at VIN, the other end is switched up and down. While the
switch is on, the current through the inductor increases.
When the switch is off the inductor current continues to
flow through the output diode.
The current flow imposes a voltage across the inductor,
which is added to VIN to produce a higher voltage VOUT.
At low power levels (typically less than 1W), the period
varies between switching cycles, indicative of Pulse
Frequency Modulation (PFM). As the output power
increases beyond approximately 1W, the period between
switching cycles continues to decrease and the power
(switch current) delivered with each cycle increases
indicative of Current Mode control.
PFM Regulation
The error amplifier compares the regulator’s reference
voltage with the feedback voltage obtained from the
output resistor voltage divider network. The resulting
error voltage acts as a correction input signal to the
control block. The control block generates two signals
that turn on and off the output MOSFET switch. An
increase in load current causes VOUT and VFB to
decrease in value. The control loop then changes the
switching frequency to increase the energy transferred to
the output capacitor to regulate the output voltage. A
reduction in load causes VOUT and VFB to increase. Now
the control loop compensates by reducing the effective
switching frequency, thus reducing the amount of energy
delivered to the output capacitor in order to keep the
output voltage within regulation.
Current Mode Regulation
The control block’s oscillator starts the cycle by setting
the MOSFET switch control flip flop. The switch then
turns on. This flip flop is reset when the switch current
ramp reaches the threshold set by the error amplifier. If
the error amplifier indicates that VFB is either too high or
too low, then the threshold for the comparator measuring
the switch current is appropriately adjusted to bring VOUT
back to within regulation limits. The level of the error
signal also sets the off time of the switch. A higher error
signal (output voltage is low) will reduce off time to
increase energy transfer to the output. A lower error
signal (output voltage is high) will conversely, increase
off time to reduce energy transfer to the output.
Component Selection
Resistors
An external resistive divider network (R1 and R2) with its
center tap connected to the feedback pin sets the output
voltage. The appropriate R1 and R2 values for the
desired output voltage are calculated by:
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛−
=
1
1.24V
V
R1
R2
OUT
Large resistor values are recommended to reduce light
load operating current, and improve efficiency. The table
below gives a good compromise between quiescent
current and accuracy. Additionally, a feedforward
capacitor (CFF) (placed in parallel with R1) may be added
to improve transient performance. Recommended values
are suggested below:
VOUT Suggested R1 CFF
5V to 10V 100k 4.7nF
10V to 15V 240k 2.2nF
15V to 37V 1M 470pF
Figure 1. Typical Application Circuit
Inductor
Inductor selection is a balance between efficiency,
stability, cost, size, and rated current. For most
applications, inductors in the range 4.7uH to 22uH are
recommended. Larger inductance values reduce the
peak-to-peak ripple current, thereby reducing both the
DC losses and the transition losses for better efficiency.
The inductor’s DC resistance (DCR) also plays an
important role. Since the majority of the input current
(minus the MIC2251 operating current) is passed
through the inductor, higher DCR inductors will reduce
efficiency at higher load currents. Figure 2 shows the
comparison of efficiency between a 140mΩ DCR, 4.7uH
inductor and a 190mΩ DCR, 10uH inductor. The switch
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current limit for the MIC2251 is typically 2A. The
saturation current rating of the selected inductor should
be 20-30% higher than the 2A specification for proper
operation.
Figure 2. Efficiency Comparison between Lower
and Higher Inductor Values
Input Capacitor
The boost converter exhibits a triangular current
waveform at its input, so an input capacitor is required to
decouple this waveform and thereby reduce the input
voltage ripple. A 10uF to 22uF ceramic capacitor should
be sufficient for most applications. A minimum input
capacitance of 1uF is recommended. The input capacitor
should be as close as possible to the inductor and the
MIC2251, with short PCB traces for good noise
performance.
Output Capacitor
Output capacitor selection is also a trade-off between
performance, size, and cost. Increasing COUT will lead to
an improved transient response however the size and
cost also increase. X5R and X7R ceramic capacitors are
recommended. For most applications, 2.2uF to 22uF
should be sufficient.
Diode
The MIC2251 requires an external diode for operation.
The diode must be rated for the peak inductor current,
and its reverse voltage rating must be greater than the
output voltage. A Schottky diode is recommended for
lower output voltages due to its lower forward voltage
drop and reverse recovery time. However, at higher
output voltages (>10V), a high speed diode such as
LS4148 can be more efficient as it has the advantage of
considerably lower leakage currents, especially at higher
temperatures. This will greatly improve light load
efficiency when compared to a Schottky diode.
For example: At 70oC ambient temperature, VIN = 2.5V,
VOUT= 24V at no load.
Input current (Vishay SL04 Schottky) = 2.1mA
Input current (Generic LS4148) = 0.37mA
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MIC2251 Schematic
Bill of Materials
Item Part Number Manufacturer Description Qty.
C2012X5R0J106K TDK(1)
VJ0805G106KXYAT Vishay Vitramon(2)
08056D106KAT AVX(3)
C1
GRM21BR60J106M Murata(4)
Capacitor, 10µF, 6.3V, X5R 1
C2012X5R1E225K TDK(1)
08053C225MAT AVX(3)
C3
GRM21R61E225KE36D Murata(4)
Capacitor, 2.2µF, 25V, X5R 1
LS4148 Vishay(2) High Speed Diode, 75V, 300mA
D1 LS04 Vishay(2) Schottky Diode, 40V, 1A 1
VLF5012ST-100M1R0 TDK(1) 10µH
LPS4018-100 Coilcraft(5) 10µH, 10%
L1
CDRH4D28NP-100NC Sumida(6) 10µH, 1.26A
1
R1 CRCW06031004FKEYE3 Vishay Dale(2) Resistor, 1M, 1%. 1/16W, Size 0603 1
R2 CRCW06039012FKEYE3 Vishay Dale(2) Resistor, 90.1k, 1%. 1/16W, Size 0603 1
U1 MIC2251YML Micrel, Inc.
(7) High-Efficiency Low EMI Boost Regulator 1
Notes:
1. TDK: www.tdk.com.
2. Vishay: www.vishay.com.
3. AVX: www.avx.com.
4. Murata: www.murata.com.
5. Coilcraft: www.coilcraft.com.
6. Sumida: www.sumida.com.
7. Micrel, Inc.: www.micrel.com.
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PCB Layout Recommendations
Top Layer
Bottom Layer
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Package Information
8-Pin 2mm x 2mm MLF® (ML)
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5-Pin Thin SOT-23 (D5)
MICREL, INC. 2180 FORTUNE DRIVE SAN JOSE, CA 95131 USA
TEL +1 (408) 944-0800 FAX +1 (408) 474-1000 WEB http://www.micrel.com
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