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February 2001

MIC4681

Micrel

MIC4681

2A-Peak SuperSwitcherTM SOP-8 Buck Regulator

Final Information

General Description

The MIC4681 SuperSwitcherTM is an easy-to-use step-down
(buck) voltage-mode switching regulator. The 200kHz
MIC4681 achieves over 1A of continuous output current over
a 4V to 30V input range in an 8-lead SOPpackage . The
MIC4681 features a high 2.1A minimum current limit, making
the device ideal for pulsed current applications such as GSM
and TDMA cell phone battery chargers and power supplies.
The MIC4681 sustains an output of 4.2V/2A within a typical
GSM charging environment.

The MIC4681 has an input voltage range of 4V to 30V, with
excellent line, load, and transient response. The regulator
performs cycle-by-cycle current limiting and thermal shut-
down for protection under fault conditions. In shutdown
mode, the regulator draws less than 6

A of standby current.

The MIC4681 SuperSwitcherTM regulator requires a mini-
mum number of external components and can operate using
a standard series of inductors and capacitors. Frequency
compensation is provided internally for fast transient re-
sponse and ease of use.

The MIC4681 is available in the 8-lead SOP with a
40

C to +125

C junction temperature range.

Typical Applications

GND

SHDN

C2
220

10V

R1
3.01k

R2
2.94k

2.5V/1A

MIC4681BM

35V

+5V to +30V

SHUTDOWN

ENABLE

Power
SOP-8

D1
B340A or
SS36

Adjustable Regulator Circuit

Features

· SO-8 package with over 1A continuous output current
· Capable of 2A pulse charging for GSM applications
· All surface mount solution
· Only 4 external components required
· Fixed 200kHz operation
· Output adjustable down to 1.25V
· Internally compensated with fast transient response
· Wide 4V to 30V operating input voltage range
· Less than 6

A typical shutdown-mode current

· Up to 90% efficiency
· Thermal shutdown
· Overcurrent protection

Applications

· Cellular phone battery charger
· Cellular phone power supply
· Simple 1A continuous high-efficiency step-down (buck)

regulator

· Replacement of a TO-220 and TO-263 designs
· Positive-to-negative converter (inverting buck-boost)
· Negative boost converter
· Higher output current regulator using external FET

Micrel, Inc. · 1849 Fortune Drive · San Jose, CA 95131 · USA · tel + 1 (408) 944-0800 · fax + 1 (408) 944-0970 · http://www.micrel.com

SuperSwitcher is a trademark of Micrel, Inc.

February 2001

MIC4681

Micrel

Absolute Maximum Ratings

(Note 1)

Supply Voltage (V

), Note 3 ...................................... +34V

Shutdown Voltage (V

SHDN

) .......................... 0.3V to +34V

Steady-State Output Switch Voltage (V

) .................. 1V

Feedback Voltage [Adjustable] (V

) .......................... +12V

Storage Temperature (T

) ....................... 65

C to +150

ESD, Note 5

Operating Ratings

(Note 2)

Supply Voltage (V

) ....................................... +4V to +30V

Junction Temperature (T

) ...................................... +125

Package Thermal Resistance (

), Note 6 ............ 63

C/W

(

), Note 6 ........... 20

C/W

Electrical Characteristics

= 12V; I

LOAD

= 500mA; T

= 25

C, bold values indicate 40

+125

C, Note 7; unless noted.

Parameter

Condition

Min

Typ

Max

Units

Feedback Voltage

(

1%)

1.217

1.230

1.243

(

2%)

1.205

1.255

30V, 0.1A

LOAD

1A, V

OUT

= 5V

1.193

1.230

1.267

1.180

1.280

Maximum Duty Cycle

= 1.0V

Output Leakage Current

= 30V, V

SHDN

= 5V, V

= 0V

500

= 30V, V

SHDN

= 5V, V

= 1V

Quiescent Current

= 1.5V

Frequency Fold Back

110

kHz

Oscillator Frequency

180

200

220

kHz

Saturation Voltage

OUT

= 1A

1.4

1.8

Short Circuit Current Limit

= 0V, see Test Circuit V

= 30V (Note 8)

2.2

3.4

4.5

Standby Quiescent Current

SHDN

= 5V (regulator off)

100

SHDN

= V

Shutdown Input Logic Level

regulator off

1.4

regulator on

1.25

0.8

Shutdown Input Current

SHDN

= 5V (regulator off)

0.5

SHDN

= 0V (regulator on)

1.5

Thermal Shutdown @ T

160

Note 1.

Exceeding the absolute maximum rating may damage the device.

Note 2.

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

Note 3.

Absolute maximum rating is intended for voltage transients only, prolonged dc operation is not recommended.

Note 4.

IN(min)

= V

OUT

+ 2.5V or 4V whichever is greater.

Note 5.

Devices are ESD sensitive. Handling precautions recommended.

Note 6.

Measured on 1" square of 1 oz. copper FR4 printed circuit board connected to the device ground leads.

Note 7.

Test at T

= +85

C, guaranteed by design, and characterized to T

= +125

Note 8.

Short circuit protection is guaranteed to 30V max.

February 2001

MIC4681

Micrel

4.95

4.97

4.99

5.01

5.03

5.05

0.5

1.0

1.5

2.0

OUTPUT VOLTAGE (V)

OUTPUT CURRENT (A)

Load Regulation

Amb = 25

= 12V

Typical Characteristics

4.95

4.97

4.99

5.01

5.03

5.05

OUTPUT VOLTAGE (V)

INPUT VOLTAGE (V)

Line Regulation

Amb = 25

100

SHUTDOWN CURRENT (

INPUT VOLTAGE (V)

Shutdown Current

vs. Input Voltage

SHDN

= 5V

Amb = 25

SHUTDOWN CURRENT (

INPUT VOLTAGE (V)

Shutdown Current

vs. Input Voltage

SHDN

= V

Amb = 25

5.8

6.2

6.4

6.6

-40 -20 0

20 40 60 80 100 120

SHUTDOWN CURRENT (

TEMPERATURE (

Shutdown Current

vs. Temperature

= 12V

SHDN

= V

0.5

1.5

2.5

OUTPUT VOLTAGE (V)

OUTPUT CURRENT (A)

Current Limit

Characteristics

= 12V

Amb = 25

195

197

199

201

203

205

FREQUENCY (KHz)

INPUT VOLTAGE (V)

MIC4681BM Frequency

vs. Supply Voltage

OUT

= 100mA

Amb = 25

185

190

195

200

205

210

215

-40 -20 0

20 40 60 80 100 120

FREQUENCY (KHz)

TEMPERATURE (

Frequency

vs. Temperature

= 12V

1.222

1.224

1.226

1.228

1.230

1.232

1.234

1.236

1.238

-40 -20 0

20 40 60 80 100 120

FEEDBACK VOLTAGE (V)

TEMPERATURE (

Feedback Voltage

vs. Temperature

= 12V

0.5

1.5

2.5

EFFICIENCY (%)

OUTPUT CURRENT (A)

MIC4681 3.3V Output

Efficiency

24Vin

6Vin

12Vin

Amb = 25

100

0.5

1.0

1.5

2.0

EFFICIENCY (%)

OUTPUT CURRENT (A)

5V Output Efficiency

24Vin

7Vin

12Vin

Amb = 25

100

0.5

1.5

EFFICIENCY (%)

OUTPUT CURRENT (A)

12V Output Efficiency

24Vin

15Vin

Amb = 25

February 2001

MIC4681

Micrel

Bode Plots

The following bode plots show that the MIC4681 is stable over all conditions using a 68

F inductor (L) and a 220

F output

capacitor (C

OUT

). To assure stability, it is a good practice to maintain a phase margin of greater than 35

No-Load Stability

Phase Margin = 94

= 7V

OUT

= 5.0V

OUT

= 0A

Amb = 22

L = 68

OUT

= 220

Full-Load Stability

Phase Margin = 74

= 7V

OUT

= 5.0V

OUT

= 1.1A

Amb = 22

L = 68

OUT

= 220

No-Load Stability

Phase Margin = 102

= 12V

OUT

= 5.0V

OUT

= 0A

Amb = 22

L = 68

OUT

= 220

Full-Load Stability

Phase Margin = 59

= 30V

OUT

= 5.0V

OUT

= 1.1A

Amb = 22

L = 68

OUT

= 220

No-Load Stability

Phase Margin = 118

= 30V

OUT

= 5.0V

OUT

= 0A

Amb = 22

L = 68

OUT

= 220

Full-Load Stability

Phase Margin = 53

= 12V

OUT

= 5.0V

OUT

= 1.4A

Amb = 22

L = 68

OUT

= 220

February 2001

MIC4681

Micrel

Functional Description

The MIC4681 is a variable duty cycle switch-mode regulator
with an internal power switch. Refer to the block diagrams.

Supply Voltage

The MIC4681 operates from a +4V to +30V unregulated
input. Highest efficiency operation is from a supply voltage
around +12V. See the efficiency curves on page 6.

Enable/Shutdown

The shutdown (

SHDN

) input is TTL compatible. Ground the

input if unused. A logic-low enables the regulator. A logic-
high shuts down the internal regulator which reduces the
current to typically 35

A when V

SHDN

= V

= 12V and 6

when V

SHDN

= 5V. See "Shutdown Input Behavior: Shutdown

Hysteresis."

Feedback

Require an external resistive voltage divider from the output
voltage to ground, center tapped to the

pin. See Figure 1b

for recommended resistor values.

Duty Cycle Control

A fixed-gain error amplifier compares the feedback signal
with a 1.23V bandgap voltage reference. The resulting error
amplifier output voltage is compared to a 200kHz sawtooth
waveform to produce a voltage controlled variable duty cycle
output.

A higher feedback voltage increases the error amplifier
output voltage. A higher error amplifier voltage (comparator

inverting input) causes the comparator to detect only the
peaks of the sawtooth, reducing the duty cycle of the com-
parator output. A lower feedback voltage increases the duty
cycle. The MIC4681 uses a voltage-mode control architec-
ture.

Output Switching

When the internal switch is ON, an increasing current flows
from the supply V

IN,

through external storage inductor L1, to

output capacitor C

OUT

and the load. Energy is stored in the

inductor as the current increases with time.

When the internal switch is turned OFF, the collapse of the
magnetic field in L1 forces current to flow through fast
recovery diode D1, charging C

OUT

Output Capacitor

External output capacitor C

OUT

provides stabilization and

reduces ripple. See "Bode Plots" for additional information.

Return Paths

During the ON portion of the cycle, the output capacitor and
load currents return to the supply ground. During the OFF
portion of the cycle, current is being supplied to the output
capacitor and load by storage inductor L1, which means that
D1 is part of the high-current return path.

Applications Information

Adjustable Regulators

Adjustable regulators require a 1.23V feedback signal. Rec-
ommended voltage-divider resistor values for common out-
put voltages are included in Figure 1b.

For other voltages, the resistor values can be determined
using the following formulas:

R1 R2

1.23V

OUT

REF

OUT

REF

GND

SHDN

OUT

MIC4681BM

SHUTDOWN

ENABLE

Figure 1a. Adjustable Regulator Circuit

)

(

Figure 1b. Recommended Components for Common Ouput Voltages

MIC4681

Micrel

MIC4681

February 2001

BIENT

printed circuit board

ground plane

heat sink area

SOP-8

Figure 2. Power SOP-8 Cross Section

Thermal Considerations

The MIC4681 SuperSwitcherTM features the power-SOP-8.
This package has a standard 8-lead small-outline package
profile, but with much higher power dissipation than a stan-
dard SOP-8. Micrel's MIC4681 SuperSwitcherTM family are
the first dc-to-dc converters to take full advantage of this
package.

The reason that the power SOP-8 has higher power dissipa-
tion (lower thermal resistance) is that pins 5 through 8 and the
die-attach paddle are a single piece of metal. The die is
attached to the paddle with thermally conductive adhesive.
This provides a low thermal resistance path from the junction
of the die to the ground pins. This design significantly im-
proves package power dissipation by allowing excellent heat
transfer through the ground leads to the printed circuit board.

One limitation of the maximum output current on any MIC4681
design is the junction-to-ambient thermal resistance (

) of

the design (package and ground plane).

Examining

in more detail:

= (

)

where:

= junction-to-case thermal resistance

= case-to-ambient thermal resistance

is a relatively constant 20

C/W for a power SOP-8.

is dependent on layout and is primarily governed by the

connection of pins 5 though 8 to the ground plane. The
purpose of the ground plane is to function as a heat sink.

is ideally 63

C/W, but will vary depending on the size of

the ground plane to which the power SOP-8 is attached.

Determining Ground-Plane Heat-Sink Area

There are two methods of determining the minimum ground
plane area required by the MIC4681.

Quick Method

Make sure that MIC4681 pins 5 though 8 are connected to a
ground plane with a minimum area of 6cm

. This ground

plane should be as close to the MIC4681 as possible. The
area may be distributed in any shape around the package or
on any pcb layer

as long as there is good thermal contact to

pins 5 though 8. This ground plane area is more than
sufficient for most designs.

Minimum Copper/Maximum Current Method

Using Figure 3, for a given input voltage range, determine the
minimum ground-plane heat-sink area required for the
application's maximum continuous output current. Figure 3
assumes a constant die temperature of 75

C above ambient.

0.5

1.0

1.5

CONTINUOUS OUTPUT CURRENT (A)

AREA (cm

)

12V

= 30V

24V

= 50

Figure 3. Output Current vs. Ground Plane Area

When designing with the MIC4681, it is a good practice to
connect pins 5 through 8 to the largest ground plane that is
practical for the specific design.

Checking the Maximum Junction Temperature:

For this example, with an output power (P

OUT

) of 5W, (5V

output at 1A maximum with V

= 12V) and 65

C maximum

ambient temperature, what is the maximum junction tem-
perature?

Referring to the "Typical Characteristics: 5V Output Effi-
ciency" graph, read the efficiency (

) for 1A output current at

= 12V or perform you own measurement.

= 79%

The efficiency is used to determine how much of the output
power (P

OUT

) is dissipated in the regulator circuit (P

P =

OUT

P =

0.79

= 1.33W

A worst-case rule of thumb is to assume that 80% of the total
output power dissipation is in the MIC4681 (P

D(IC)

) and 20%

is in the diode-inductor-capacitor circuit.

D(IC)

= 0.8 P

D(IC)

= 0.8

1.33W

D(IC)

= 1.064W

Calculate the worst-case junction temperature:

= P

D(IC)

+ (T

) + T

A(max)

where:

= MIC4681 junction temperature

D(IC)

= MIC4681 power dissipation

= junction-to-case thermal resistance.

The

for the MIC4681's power-SOP-8 is approximately

C/W.

= "pin" temperature measurement taken at the

entry point of pins 6 or 7

February 2001

MIC4681

Micrel

Load

GND

SHDN

OUT

MIC4681BM

GND

+4V to +30V

Power
SOP-8

Figure 5. Critical Traces for Layout

GND

SHDN

D1
B340A
or
SS36

OUT

GND

U1 MIC4681BM

0.1

50V

35V

4V to +30V

GND

SOP-8

JP1

OFF

C4
220

10V

C3* 1800pF / 50V
optional

C5
0.1

50V

R1
3.01k

R2
6.49k

JP2a
1.8V

R6
optional

R3
2.94k

R4
1.78k

R5
976

JP2b
2.5V

JP2c
3.3V

JP2d
5.0V

* C3 can be used to provide additional stability

and improved transient response.

Figure 6a. Evaluation Board Schematic Diagram

= ambient temperature

A(max)

= maximum ambient operating temperature

for the specific design.

Calculating the maximum junction temperature given a
maximum ambient temperature of 65

= 1.064

C/W + (45

C 25

C) + 65

= 106.3

This value is within the allowable maximum operating junc-
tion temperature of 125

C as listed in "Operating Ratings."

Typical thermal shutdown is 160

C and is listed in "Electrical

Characteristics."

Layout Considerations

Layout is very important when designing any switching regu-
lator. Rapidly changing currents through the printed circuit
board traces and stray inductance can generate voltage
transients which can cause problems.

To minimize stray inductance and ground loops, keep trace
lengths, indicated by the heavy lines in Figure 5, as short as
possible. For example, keep D1 close to pin 3 and pins 5
through 8, keep L1 away from sensitive node FB, and keep

close to pin 2 and pins 5 though 8. See "Applications

Information: Thermal Considerations" for ground plane lay-
out.

The feedback pin should be kept as far way from the switching
elements (usually L1 and D1) as possible.

A circuit with sample layouts are provided. See Figures 6a
though 6e. Gerber files are available upon request.

MIC4681

Micrel

MIC4681

February 2001

Printed Circuit Board Layouts

Figure 6b. Top-Side Silk Screen

Figure 6c. Top-Side Copper

Figure 6d. Bottom-Side Silk Screen

Figure 6e. Bottom-Side Copper

Abbreviated Bill of Material (Critical Components)

Reference

Part Number

Manufacturer

Description

Qty

593D226X035E2T

Vishay Dale

F / 35V

594D227X0010D2

Vishay Dale

220

F / 10V

C2,C5

VJ0805Y104KXXMB

Vishay Dale

0.1 / 50V

340A

Diodes Inc.

Schottky Diode 3A, 40V

CDRH127-680MC

Sumida

H, I

SAT

2.1A, shielded

MIC4681BM

Micrel Semiconductor

200kHz Super SwitcherTMSOIC 8 pin

Vishay Dale, Inc., tel: 1 877-847-4291,

http://www.vishay.com

Diodes Inc, tel: (805) 446-4800,

http://www.diodes.com

Sumida, tel: (408) 982-9960,

http://www.sumida.com

Micrel, tel: (408) 944-0800,

http://www.micrel.com

February 2001

MIC4681

Micrel

Applications Circuits*

For continuously updated circuits using the MIC4681, see

Application Hint 37 at www.micrel.com.

100

GND

SHDN

C3
220

10V

R2
3.01k

R3
976

1N4148

R4
16.2k

R5
221k

10k

0.100

10nF

D1
MMBR140LT3

1N4148

C5
220nF

800mA

GND

U1 MIC4681

100nF

35V

+30V max.

GND

SOP-8

S1
NKK G12AP

OFF

LM4041DIM3-1.2

U3
MIC6211BM5

R7
4.99k

Cellular

lephone

Figure 7. Constant Current and Constant Voltage Battery Charger

GND

SHDN

C1
68

20V

D1
ES1B
1A 100V

GND

U1 MIC4681

12V/150mA

C2
0.1

C4
68

20V

C5
33

35V

+12V

GND

SOP-8

R1
8.87k

R2
1k

C3
0.022

50V

Figure 8. +12V to 12V/150mA Buck-Boost Converter

L1*

GND

SHDN

470

6.3V

C3
470

6.3V

3.01k

C5
0.1

1N4148

R5
16k
1%

16k 1%

C4
1000pF

20m

R3
1k
1%

R4
1k
1%

1.78k

R1
1k
1%

3.3V/5A

GND

U1 MIC4681

MIC4417BM4

Si4425DY

100

20V

C6
0.1

50V

+4.5V to +17V

SHUTDOWN

ENABLE

SOP-8

* I

SAT

= 7.5A

U3
MIC6211BM5

Figure 9. 5V to 3.3V/5A Power Supply

* See Application Hint 37 at www.micrel.com for bills of material.

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