MIC2238-S4YMLTR - Micrel, Inc.

MIC2238

2.5MHz Dual Phase

PWM Buck Regulator

MLF and MicroLead Frame 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 2008

M9999-042108

General Description

The Micrel MIC2238 is dual output 2-phase synchronous

buck (step down) PWM DC/DC switching regulator. Power

conversion efficiencies of above 95% are easily obtainable

for a wide range of applications and loads. An ultra-low

quiescent current of 28µA at light loads assures minimum

current draw in battery powered applications that require

standby modes. The 2.5MHz PWM operation of MIC2238

allows the use of miniature 1µH, 2.2µH or 4.7µH inductor

with the use of 2.2µF ceramic output capacitor.

The MIC2238 operates from 2.5V to 5.5V input and is

available in fixed output voltage versions to save space

and reduce external component count. The MIC2238-AA’s

(adjustable version) output voltage versions can be

programmed as low as 0.8V

For applications that require the lowest noise performance,

the /FPWM pin allows the automatic Trickle mode to be

disabled, remaining in full PWM mode. To maximize

battery life in low-dropout conditions, MIC2238 can

operate with a maximum duty cycle of 100%.

The MIC2238 is available in a space-saving 3mm x 3mm

MLF

-12L package with a junction temperature range from

-40°C to +125°C.

Data sheets and support documentation can be found on

Micrel’s web site at www.micrel.com.

Features

• Input voltage range: 2.5V to 5.5V

• Dual output voltages running out of phase

• 28µA quiescent current

• Fixed output voltage versions

• Adjustable version down to 0.8V

• Low noise 2.5MHz PWM operation

• 800mA output current capability for each channel

• Stable with 2.2µH inductor, 2.2µF ceramic cap

• Automatic switching into light load mode of operation

• /FPWM pin allows low noise all-PWM mode operation

• Power good output with internal 5µA current source

allows sequencing with programmable delay time

• Internal soft-start

• 1µA shutdown current

• Built-in soft-start circuitry

• Current limit protection

• Pb-Free 3mm x 3mm MLF

-12L package

Applications

• Cellular phones

• PDAs

• Digital Cameras

• MP3 Players

___________________________________________________________________________________________________________

Typical Application

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Ordering Information

Part Number V

OUT1

OUT2

Junction

Temperature Range Package Lead Finish

MIC2238-AAYML Adj. Adj. –40°C to +125°C 12-Pin 3mm x 3mm MLF

Pb-Free

MIC2238-521YML 1.28V 1.65V –40°C to +125°C 12-Pin 3mm x 3mm MLF

Pb-Free

MIC2238-G4YML 1.8V 1.2V –40°C to +125°C 12-Pin 3mm x 3mm MLF

Pb-Free

MIC2238-GF9YML 1.8V 1.545V

–40°C to +125°C 12-Pin 3mm x 3mm MLF

Pb-Free

MIC2238-GFHYML 1.8V 1.575V

–40°C to +125°C 12-Pin 3mm x 3mm MLF

Pb-Free

MIC2238-GSYML 1.8V 3.3V –40°C to +125°C 12-Pin 3mm x 3mm MLF

Pb-Free

MIC2238-GWYML 1.8V 1.6V –40°C to +125°C 12-Pin 3mm x 3mm MLF

Pb-Free

MIC2238-J4YML 2.5V 1.2V

–40°C to +125°C 12-Pin 3mm x 3mm MLF

Pb-Free

MIC2238-S4YML 3.3V 1.2V –40°C to +125°C 12-Pin 3mm x 3mm MLF

Pb-Free

MIC2238-SSYML 3.3V 3.3V –40°C to +125°C 12-Pin 3mm x 3mm MLF

Pb-Free

Note:

Other voltages available. Contact Micrel Marketing for details.

MLF

is a GREEN RoHS compliant package. Lead finish is NiPdAu. Mold compound is Halogen Free.

Pin Configur ation

Adjustable

MIC2238-AAYML

12-Pin MLF

(ML)

(Top View)

Fixed

MIC2238-xxYML

12-Pin MLF

(ML)

(Top View)

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Pin Description

Pin Number

Adjustable Pin Number

Fixed

Pin Name Pin Name

1 – FB2

Feedback 2: For adjustable voltage options connect the

external resistor divider network to FB2 to set the output

voltage of regulator 2. Nominal value is 0.8V.

2 2 EN2

Enable 2 input. Logic low powers down regulator 2. Logic

high powers up regulator 2. MIC2238 features built-in soft-

start circuitry that reduces in-rush current and prevents the

output voltage from overshooting at start up.

3 3 AVIN

Analog Supply Voltage: Supply voltage for the analog control

circuitry. Requires bypass capacitor to GND.

4 4 SW2 Switch node for regulator 2, connected to external inductor.

5 5 AGND Analog (signal) ground.

6 6 PGND Power ground.

7 7 /FPWM

Forced PWM Mode Bar. Grounding this pin forces the device

to stay in constant frequency PWM mode only. Pulling this

pin high enables automatic Trickle mode operation.

8 8 SW1 Switch node for regulator 1, connected to external inductor.

9 9 VIN

Supply Voltage: Supply voltage for the internal switches and

drivers.

10 10 PGOOD

Power Good Output. This output is pulled down unless the

regulator 1 output voltage is within +6.25% and -8.5% of

regulation. After the output voltage is in regulation, the output

starts to go high with an internal 5µA current source. A delay

time could be programmed by tying a capacitor to this pin.

11 11 EN1

Enable 1 input. Logic low powers down regulator 1. Logic

high powers up regulator 1. MIC2238 features built-in soft-

start circuitry that reduces in-rush current and prevents the

output voltage from overshooting at start up.

12 – FB1

Feedback 1: For adjustable voltage options connect to the

external resistor divider network to FB1 to set the output

voltage of regulator 1. Nominal value is 0.8V.

– 1 OUT2

Output Voltage 2. For fixed output voltage options connect

OUT2 to the output voltage of regulator 2.

– 12 OUT1

Output Voltage 1. For fixed output voltage options connect

OUT1 to the output voltage of regulator 1.

EP EP GND Ground, backside pad.

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Absolute Maximum Ratings(1)

Supply Voltage (V

).......................................................+6V

Enable 1 Voltage ………………………………………... +6V

Enable 2 Voltage ……………………………………….. +6V

Logic Input Voltage (V

, V

FPWM

)............................ V

to 0V

Storage Temperature (T

) .........................-65°C to +150°C

ESD .............................................................................. 2KV

Operating Ratings(2)

Supply Voltage (V

) ........................................ 2.5V to 5.5V

Junction Temperature (T

) .........................-40°C to +125°C

Package Thermal Resistance (θ

) ..........................60°C/W

Electrical Characteristics(4)

= 25°C with V

= V

EN1

= V

EN2

=3.6V, V

OUT1

, V

OUT2

, L= 2.2µH, C = 2.2µF, unless otherwise specified. Bold values indicate -40ºC ≤ T

≤+125ºC..

Parameter Condition Min Typ Max Units

Supply Voltage and Current

Supply Voltage Range 2.5 5.5 V

UVLO (rising) 2.3 2.4 2.5 V

UVLO Hysteresis 100 mV

PWM Mode Supply

Current /FPWM = Low, I

OUT

= 200mA 560 950 µA

Trickle Mode Supply

Current /FPWM = High, I

OUT

= 100µA 28 50 µA

Shutdown Quiescent

Current V

= 0V 0.1 1 µA

Output Voltage Accuracy

Feedback voltage, V

Adjustable 0.780 0.8 0.820 V

Output voltage, V

OUT

Fixed Output Options

-2.5

+2.5 %

Feedback bias current 10 nA

Output Voltage Line

Regulation 2.5V ≤ V

≤ 5.5V 0.1

0.5 %

Output Voltage Load

Regulation

= 5V, I

OUT

= 10mA to 800mA, /FPWM = 0V

= 3V; I

OUT

= 10mA to 800mA, /FPWM = 0V 0.5 %

Ripple in Trickle Mode Vin=3.6V; Iout = 1mA; C

OUT

= 2.2µF, L = 2.2µH. 40 mV

Logic Inputs

On 0.8 1.2 V

EN Input Threshold

Off 0.3 0.7 V

EN Input Current 0.01 1 µA

On 0.6xV

/FPWM Input Threshold

Off 0.3xV

/FPWM Input Current 0.01 1 µA

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Electrical Characteristics (cont.)(4)

Parameter Condition Min Typ Max Units

Protection

Current Limit Peak Switch Current, V

OUT

= 0V 0.9 1.2 1.8 A

Over Temperature

Shutdown Hysteresis 20

°C

Control

Maximum Duty Cycle V

= 0.7V 100 %

Oscillator

PWM Mode Frequency 2.125 2.5 2.875 MHz

Power Good

Power Good Reset

Threshold

Upper Threshold

Lower Threshold

-14

6.25

-8.5

PGOOD Series

Resistance 1 1.4 kΩ

PGOOD Pull-Up Current Output within 8.5% of regulation 5 µA

Power Switch

Switch On-Resistance I

= 150mA (PFET)

= 150mA (NFET)

0.4

0.35

Ω

Notes:

1. Exceeding the absolute maximum rating may damage the device.

2. The device is not guaranteed to function outside its operating rating.

3. Devices are ESD sensitive. Handling precautions recommended. Human body model, 1.5kΩ in series with 100pF.

4. Specification for packaged product only.

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Typical Characteristics

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Functional Characteristics

VIN = 3.6V, VOUT = 1.8V, L = 2.2µH, /FPWM = 0 VIN = 3.6V, VOUT = 1.8V, L = 2.2µH, /FPWM = 3.6V

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Functional Characteristics (continued)

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Functional Block Diagram

Functional Description

VIN

VIN provides power to the MOSFETs for the switch

mode regulator section, along with the current limiting

sensing. Due to the high switching speeds, a 10µF

capacitor is recommended close to VIN and the power

ground (PGND) pin for bypassing. Please refer to layout

recommendations.

AVIN

Analog V

(AVIN) provides power to the analog supply

circuitry. AVIN and VIN must be tied together. Careful

layout should be considered to ensure high frequency

switching noise caused by VIN is reduced before

reaching AVIN. A 1µF capacitor as close to AVIN as

possible is recommended. See layout recommendations

for detail.

EN1

Enable 1 controls the on and off state of regulator 1. A

high logic on Enable 1 (EN1) activates regulator 1 while

a low logic deactivates regulator 1. MIC2238 features

built-in soft-start circuitry that reduces in-rush current

and prevents the output voltage from overshooting at

start up.

EN2

Enable 2 controls the on and off state of regulator 2. A

high logic on Enable 2 (EN2) activates regulator 2 while

a low logic deactivates regulator 2. MIC2238 features

built-in soft-start circuitry that reduces in-rush current

and prevents the output voltage from overshooting at

start up.

/FPWM

The Forced PWM Mode selects the mode of operation

for this device. Grounding this pin forces the device to

stay in constant frequency PWM mode only. Pulling this

pin high enables automatic selection of Trickle or PWM

mode operation, depending on the load. While /FPWM

is high and the load is below 100mA, the device will go

into Trickle mode. If the load is above 100mA, PWM

mode will automatically be selected. Do not leave this

pin floating.

PGOOD

The Power Good Output is pulled down unless the

regulator 1 output voltage is within +6.25% or -8.5% of

regulation. After the output voltage is in regulation, the

output starts to go high with an internal 5µA current

source. A delay time could be programmed by tying a

Micrel, Inc. MIC2238

April 2008

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capacitor to this pin. A 390pF capacitor at the PGOOD

pin will cause the PGOOD pin voltage to rise from low to

high in around 200µs.

FB1/FB2

The feedback pin (FB) provides the control path to

control the output. For adjustable versions, a resistor

divider connecting the feedback to the output is used to

adjust the desired output voltage. The output voltage is

calculated as follows:

OUT

REF

×R1

R2 +1

⎛

⎝

⎜

⎞

⎠

⎟

where V

REF

is equal to 0.8V.

A feedforward capacitor is recommended for most

designs using the adjustable output voltage option. To

reduce battery current draw, a 100KΩ feedback resistor

is recommended from the output to the FB pin (R1).

Also, a feedforward capacitor should be connected

between the output and feedback (across R1). The large

resistor value and the parasitic capacitance of the FB pin

can cause a high frequency pole that can reduce the

overall system phase margin. By placing a feedforward

capacitor, these effects can be significantly reduced.

Typically, a 22pF small ceramic capacitor is

recommended.

SW1/SW2

The switch (SW) pin connects directly to the inductor

and provides the switching current necessary to operate

in PWM mode. Due to the high speed switching on this

pin, the switch node should be routed away from

sensitive nodes.

PGND

Power ground (PGND) is the ground path for the high

current PWM mode. The current loop for the power

ground should be as small as possible and separate

from the Analog ground (AGND) loop. Refer to the layout

considerations for more details.

AGND

Signal 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 considerations for more

details.

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Applications Information

Input Capacitor

A minimum 2.2µF ceramic is recommended on the VIN

pin for bypassing. X5R or X7R dielectrics are

recommended for the input capacitor. Y5V dielectrics,

aside from losing most of their capacitance over

temperature, they also become resistive at high

frequencies. This reduces their ability to filter out high

frequency noise.

Output Capacitor

The MIC2238 was designed specifically for use with a

2.2µF or greater ceramic output capacitor. The output

capacitor requires either an X7R or X5R dielectric. Y5V

and Z5U dielectric capacitors, aside from the

undesirable effect of their wide variation in capacitance

over temperature, become resistive at high frequencies.

Inductor Selection

Inductor selection will be determined by the following

(not necessarily in the order of importance);

• Inductance

• Rated current value

• Size requirements

• DC resistance (DCR)

The MIC2238 was designed for use with a 1µH, 2.2µH,

or 4.7µH inductor. For a better load transient response,

a 1µH inductor is recommended. For better efficiency, a

4.7µH inductor is recommended.

Maximum current ratings of the inductor are generally

given in two methods; permissible DC current and

saturation current. Permissible DC current can be rated

either for a 40°C temperature rise or a 10% to 20% loss

in inductance. Ensure the inductor selected can handle

the maximum operating current. When saturation current

is specified, make sure that there is enough margin that

the peak current will not saturate the inductor.

The size requirements refer to the area and height

requirements that are necessary to fit a particular

design. Please refer to the inductor dimensions on their

datasheet.

DC resistance is also important. While DCR is inversely

proportional to size, DCR can represent a significant

efficiency loss. Refer to the Efficiency Considerations.

Compensation

The MIC2238 is an internally compensated, current

mode buck regulator. Current mode is achieved by

sampling the peak current and using the output of the

error amplifier to pulse width modulate the switch node

and maintain output voltage regulation.

The MIC2238 is designed to be stable with a 1µH, 2.2µH

or 4.7µH inductor with a 2.2µF ceramic (X5R) output

capacitor.

Feedback

The MIC2238 provides a feedback pin to adjust the

output voltage to the desired level. This pin connects

internally to an error amplifier. The error amplifier then

compares the voltage at the feedback to the internal

0.8V reference voltage and adjusts the output voltage to

maintain regulation. Calculating the resistor divider

network for the desired output is as follows;

⎟

⎠

⎞

⎜

⎝

⎛−

REF

OUT

Where V

REF

is 0.8V and V

OUT

is the desired output

voltage.

A 100KΩ from the output to the feedback is

recommended for R1. Larger resistor values require an

additional capacitor (feed- forward) from the output to

the feedback. The large high side resistor value and the

parasitic capacitance on the feedback pin (~10pF) can

cause an additional pole in the control loop. The

additional pole can create a phase loss at high

frequencies. This phase loss degrades transient

response by reducing phase margin. Adding feed-

forward capacitance negates the parasitic capacitive

effects of the feedback pin. A minimum 100pF capacitor

is recommended for feed-forward capacitance.

Large feedback resistor values increase impedance,

making the feedback node more susceptible to noise

pick-up. A feed-forward capacitor would also reduce

noise pick-up by providing a low impedance path to the

output.

Efficiency Considerations

Efficiency is defined as the amount of useful output

power, divided by the amount of power supplied.

100

_%Efficiency

ININ

OUTOUT

⎟

⎠

⎞

⎜

⎝

⎛

Maintaining high efficiency serves two purposes. It

reduces power dissipation in the power supply, reducing

the need for heat sinks and thermal design

considerations and it reduces consumption of current for

battery powered applications. Reduced current draw

from a battery increases the devices operating time and

is critical in hand held devices.

There are two types of losses in switching converters;

DC losses and switching losses. DC losses are simply

the power dissipation of I

R. Power is dissipated in the

high side switch during the on cycle. Power loss is equal

to the high side MOSFET R

DSON

multiplied by the Switch

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M9999-042108

Current

. During the off cycle, the low side N-channel

MOSFET conducts, also dissipating power. Device

operating current also reduces efficiency. The product of

the quiescent (operating) current and the supply voltage

is another DC loss. The current required driving the

gates on and off at a constant 2.5MHz frequency and the

switching transitions make up the switching losses.

The Figure above shows an efficiency curve. From no

load to 100mA, efficiency losses are dominated by

quiescent current losses, gate drive and transition

losses. By Forcing the MIC2238 into Trickle Mode

(/FPWM=High), the buck regulator significantly reduces

the required switching current by entering into a PFM

(Pulse Frequency Modulation) mode. This significantly

increases efficiency at low output currents.

Over 100mA, efficiency loss is dominated by MOSFET

RDSON and inductor losses. Higher input supply

voltages will increase the Gate to Source threshold on

the internal MOSFETs, reducing the internal RDSON.

This improves efficiency by reducing DC losses in the

device. All but the inductor losses are inherent to the

device. In which case, inductor selection becomes

increasingly critical in efficiency calculations. As the

inductors are reduced in size, the DC resistance (DCR)

can become quite significant. The DCR losses can be

calculated as follows;

DCRIoutL_Pd

×=

From that, the loss in efficiency due to inductor

resistance can be calculated as follows;

100

L_PdIV

1_LossEfficiency

OUTOUT

⎥

⎦

⎤

⎢

⎣

⎡

⎟

⎠

⎞

⎜

⎝

⎛

+×

−=

Efficiency loss due to DCR is minimal at light loads and

gains significance as the load is increased. Inductor

selection becomes a trade-off between efficiency and

size in this case.

Trickle Mode Operation

Trickle Mode operation is achieved by clamping the

minimum peak current to approximately 150mA. This

forces a PFM mode by comparing the output voltage to

the internal reference. If the voltage is less than 0.8V,

the MIC2238 turns on the high side until the peak

inductor current reaches approximately 150mA. A

separate comparator then monitors the output voltage. If

the feedback voltage is greater than 0.8V, the high side

switch is used as a 10µA current source, never turning

completely off. This creates a highly efficient light load

mode by increasing the time it takes for the output

capacitor to discharge, delaying the amount of switching

required and increasing light load efficiency. When the

load current is greater than approximately 100mA, the

MIC2238 automatically switches to PWM mode.

FPWM Operation

In Forced PWM Mode (/FPWM=LOW) the MIC2238 is

forced to provides constant switching at 2.5MHz with

synchronous internal MOSFETs throughout the load

current.

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April 2008 13

M9999-042108

MIC2238 Adjustable Opti on (1.8V, 3.3V)

Bill of Materials

Item Part Number Manufacturer Description Qty

C1 C1608X5R0J106K TDK 10µF Ceramic Capacitor, 6.3V, X5R, Size 0603 1

C2 C1005X5R0J105K TDK 1µF Ceramic Capacitor, 6.3V, X5R, Size 0402 1

C3 C0603Y391KXXA Vishay 390pF Ceramic Capacitor, 25V, X7R, Size 0603 1

C4, C7 0603ZD225MAT AVX 2.2µF Ceramic Capacitor, 6.3V, X5R,. Size 0603 2

C5, C6 VJ0603A220KXXAT Vishay 22pF Ceramic Capacitor, 25V, NPO, Size 0603 2

CDRH2D11/HPNP-2R2NC Sumida 2.2µH, 1.1A Isat., 120mΩ, (1.2mmx3.2mmx3.2mm)

L1, L2 LQH43CN2R2M03 Murata 2.2µH, 900mA Isat., 110mΩ, (2.6mmx3.2mm, 4.5mm) 2

R1 CRCW06031374FT1 Vishay 1.37MΩ, 1%, Size 0603 1

R2, R4 CRCW06034423FT1 Vishay 442kΩ, 1%, Size 0603 2

R3 CRCW06035493FT1 Vishay 549kΩ, 1%, Size 0603 1

R5, R6 CRCW06031002FRT1 Vishay 10kΩ, 1%, Size 0603 2

U1 MIC2238-AAYML Micrel 2.5MHz Dual Phase PWM Buck Regulator 1

1. TDK: www.tdk.com

2. Murata: www.murata.com

3. Sumida: www.sumida.com

4. Vishay-Dale: www.vishay.com

5. AVX: www.avx.com

6. Micrel, Inc: www.micrel.com

Micrel, Inc. MIC2238

April 2008 14

M9999-042108

Layout Recommendations

Top Layer

Bottom Layer

Micrel, Inc. MIC2238

April 2008 15

M9999-042108

Package Information

12-Pin 3mm × 3mm MLF

(ML)

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

The information furnished by Micrel in this data sheet is believed to be accurate and reliable. However, no responsibility is assumed by Micrel for its

use. Micrel reserves the right to change circuitry and specifications at any time without notification to the customer.

Micrel Products are not designed or authorized for use as components in life support appliances, devices or systems where malfunction of a product

can reasonably be expected to result in personal injury. Life support devices or systems are devices or systems that (a) are intended for surgical implant

into the body or (b) support or sustain life, and whose failure to perform can be reasonably expected to result in a significant injury to the user. A

Purchaser’s use or sale of Micrel Products for use in life support appliances, devices or systems is a Purchaser’s own risk and Purchaser agrees to fully

indemnify Micrel for any damages resulting from such use or sale.