November 2000 1 MIC4043

MIC4043 Micrel

MIC4043Low-Voltage Secondary-Side Shunt Regulator

Final Information

General DescriptionThe MIC4043 is a shunt regulator optimized for secondary-side regulation in low-voltage power supplies. Featuring anoutput stage guaranteed to swing within 400mV of ground,the MIC4043 can be used in power supplies operating downto 1.8V, even with optoisolators requiring greater than 1.2V ofheadroom.

In power supply applications, the MIC4043 normally drivesthe LED of an optically isolated feedback circuit. The MIC4043monitors a resistively-divided output voltage and sinks errorcurrent through the optoisolator’s LED (secondary side); theoptoisolator’s transistor (primary side) provides this signal tothe controller’s feedback input. The MIC4043 is also practicalfor other voltage-monitoring applications requiring an open-collector output.

The MIC4043 replaces conventional ’431-type shunt regula-tors to allow low-voltage applications where there is inad-equate headroom for a 2.5V regulator in series with anoptoisolator. Replacing ’431-type devices requires only aminor change to the way that the resistive-divider values arecalculated.

Typical Application

VOUT

MIC4043

MIC38HC43BN

VDD

VREF

GND

COMP

VOUT

RT/CT

FB

ISNS

2

1

4

3

8

5

7

6

Return

1

2

7

6

4

3

FBGND

SNKIN

VIN

OPTICALISOLATION

COMPENSATION

MIC4043Low-Side Feedback

Control

PR

IMA

RY

SID

E

SE

CO

ND

AR

Y S

IDE

R1

R2

V 1.245VR2R1

1OUT = +

200kHz DC-DC Flyback Converter

Features• Ideal for 1.8V switching converters• Low-voltage operation

400mV maximum saturation overoperating temperature range

• Easy to usevoltage in, current out

• 2% voltage tolerance over operating temperature range

Applications• Optically isolated low-volage power supplies• Low-voltage discrete regulator control

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

MIC4043 Micrel

MIC4043 2 November 2000

Pin DescriptionPin Number Pin Name Pin Function

1 IN Input: Supply voltage input.

2 SNK Sink (Output): NPN open collector output.

3 GND Ground

4 FB Feedback (Input): Feedback input from external voltage-divider network.

Pin Configuration

34

1 2

FB GND

SNK

PartIdentification

IN

RBxx

MIC4043

Ordering Information Part Number Marking Voltage Tolerance Configuration Temperature Range Package Lead-Finish

MIC4043BM4 RB1D 1.245V 1% Open Collector –40°C to +85°C SOT-143 Leaded MIC4043YM4 RB1D 1.245V 1% Open Collector –40°C to +85°C SOT-143 Pb-Free

Absolute Maximum Ratings (Note 1)

Input Voltage (VIN) ......................................................+15VOutput Voltage (VSNK) ................................................+15VStorage Temperature (TS) ....................... –65°C to +150°CESD Rating, Note 3

human body model .................................................... 2kVmachine model ........................................................ 200V

Operating Ratings (Note 2)

Input Voltage (VIN) ......................................................+10VOutput Voltage (VSNK) ................................................+10VMaximum Output Current (ISNK) ................................ 15mATemperature Range (TA) ........................... –40°C to +85°C

November 2000 3 MIC4043

MIC4043 Micrel

Electrical CharacteristicsTA = 25°C, bold values indicate –40°C ≤ TA ≤ +85°C; unless noted

Parameter Condition Min Typ Max Units

Reference Voltage, Note 4 1.245 V

Reference Voltage Tolerance ±1 %±2 %

Supply Current ISNK = 0mA 35 65 µA70 µA

Transconductance 1mA < ISNK < 15mA 3.5 150 S∆ISINK/∆VIN 2 S

Output Transistor ISNK = 15mA 160 250 mVSaturation Voltage 400 mV

Output Leakage VSNK = 5V, output transistor off 0.5 µA1 µA

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. Devices are ESD sensitive. Handling precautions recommended. Human body model, 1.5k in series with 100pF. Machine model, 200pF.

Note 4. Reference voltage is not referenced to ground. The reference is between pins IN and FB.

Test Circuits

IN

R

A

1kR1

R233k

C1*

50ΩR3

GND FB

SNKOUT

RETURN

MIC4043

FloatingBenchSupply

R

R

OUTPUT

RA

A

A* Compensation element

Do Not Ground!

Analyzer

V TUO 1R 2R

V8.1 k27 k33

V5.2 k33 k33

V3.3 k02 k33

3R

k02

k04

k015

1C

0.001µF

0.001µF

0.001µF

Test Circuit 1. Compensation (Bode Plot) Circuit

IN1k

R1

R233k

R3 C1

GND FB

SNKOUT

RETURN

MIC4043Supply

* Compensation components

Test Circuit 2. Transient Response Circuit

MIC4043 Micrel

MIC4043 4 November 2000

Transient Response 1a.1.8V Output

VIN

VREG1.8VACCoupled

VOUT

5.0VACCoupled

6.0V

Overshoot 25mV

Turn On Transient Response 1b.1.8V Output

VIN

VREG1.8V

VOUT

Transient Response 2a.1.8V Output

VIN

VREG2.5VACCoupled

VOUT

5.0VACCoupled

6.0V

Overshoot 56mV

Turn On Transient Response 2b.1.8V Output

VIN

VREG2.5V

VOUT

Transient Response 2a.1.8V Output

VIN

VREG3.3VACCoupled

VOUT

5.0VACCoupled

6.0V

Overshoot 64mV

Turn On Transient Response 3b.1.8V Output

VIN

VREG3.3V

VOUT

November 2000 5 MIC4043

MIC4043 Micrel

Functional Diagram

SNK

FB

IN

VREF

1.245V

GNDMIC4043

Functional DescriptionThe MIC4043 combines a Gm amplifier, precision 1.245Vreference, and a pass transistor in a single package.

The operation of the MIC4043 is similar to conventional shuntregulators such as the industry standard ’431. In a closedloop system, the MIC4043 maintains the desired feedbackvoltage at the FB pin by sinking current onto the SNK pinproportional to the error voltage at the FB pin. The ratio of sinkcurrent to error voltage is the transconductance of the device.

Reference

The MIC4043 uses a high-side reference. External voltagedividers providing feedback to the MIC4043 will be invertedwhen compared to those used with ’431-equivalent devices.

Behavior

The external feedback voltage is compared to the internalhigh-side 1.245V reference.

If the feedback voltage, VFB, is less than VIN – VREF, theamplifier provides no drive to the sink transistor. If thefeedback voltages is greater than VIN – VREF, the amplifierdrives the pass transistor which sinks current to ground.

MIC4043 Micrel

MIC4043 6 November 2000

Applications InformationReplacement of ’431-Type Devices

Since the MIC4043 uses a high-side reference, externalvoltage dividers providing the feedback voltage will beinverted when compared to those used with ’431-equiva-lent devices.The industry-standard ’431 is also typically used in series withan opto-isolator LED. This configuration has a voltage drop ofat least 2.5V for the ’431 plus 1.4V for the LED (3.9V). Morerecent lower-voltage shunt regulators require at least 1.25Vof headroom in addition to the 1.4V for the opto isolator, fora total of 2.65V.

The MIC4043 removes the need to place the shunt referencein parallel with the opto-isolator. The MIC4043 combines a1.245V reference in conjunction with an error amplifier thatdrives an NPN output transistor. The NPN transistor isconnected in series with the opto-isolator and regulates thedrive current in the opto-isolator. Unlike conventional shuntregulators, the MIC4043 does not have to connect the shuntreference in series with the opto-isolator. Only the NPNoutput stage is in series with the opto-isolator, so the voltagedrop is just the saturation voltage or one transistor, typucally160mV at full load

Compensation

The noninverting side of the error amplifier is connected to thehigh-side reference; the reference is connected to the IN pin.The inverting side of the error amplifier is brought out to theFB pin. For some applications, no compensation is needed,but for most, some resistor capacitor network is necessarybetween the FB pin and GND pin. The value of the feedbackcapacitance is application specific, but for most applications100pF to 3000pF is all that is needed. Changing the feedbackcapacitor changes the loop response; that is, phase and gainmargin. An empirical way to check overall system loop

response, if a network analyzer is not available, is to step loadthe output of the systems from 10% to 100% of nominal load.The resultant small signal response at the output of thesystems will provide an idea of which direction to go based onthe overshoot and settling time of VOUT.

Voltage Detector

IN

R1

LogicOutput

RPULL-UP

R233k

GND FB

SNK

VOUT(FROMPOWERSUPPLY)

MIC4043

DISABLEDENABLED

V 1.245VR2R1

1TRIP = +

Figure 1. Voltage Detector

Figure 1 shows a simple voltage threshold detector with alogic output.

High-Current Regulator

IN

40Ω 1000pF

Q1RBIASIBIAS

≥500µA

R233k

GND FB

SNK

MIC4043

VOUT = 2.5VVIN

V 1.245VR2R1

1

R1 33k, R2 33k

OUT = +

∴ = =

Figure 2. High-Current Regulator

For the high-current regulator shown in Figure 2, headroomis equal to the saturation voltage of Q1 plus the saturationvoltage of the MIC4043 (VSAT(min) = 200mV).

November 2000 7 MIC4043

MIC4043 Micrel

Off-Line 1.8V/2A Power Supply

R13

10Ω 1%

C111200µF10V

R14200Ω1%

VOUT+1.8V/2A

D412CTQ045 L2

5µH

C12220µF10V

U2b2501

U3

249Ω1000pF

MIC4043

R1072k1%

R1133k1%

U1MIC38HC43BN

VDD

VREF

GND

COMP

VOUT

RT/CT

FB

ISNS

2

1

4

3

8

5

7

6

R2332k1%

R3332k1%

D11N4448

R434Ω1%

C50.1µF50/63V

D218V

C7470pF63V

C6470pF

63V

Q1IRFIBE30G

L1

R5

1.21k 1%

R61.21k1%

BR1DBR1

C447µF400V

U2a2501

F1

85 to 264Vac50/60Hz

Hot

Neutral

Return

T1

1

2

7

6

4

3

1AGround

R81.9Ω1/4W

1%

R9470Ω1/2W

D3UF4005

C9100pF1kVR1

1k 1%

R714k 1%

C32200pF400V

C22200pF

400V

C10.1µF250V

C100.1µF

50/63V80T 3T

20mH

C822µF25V

10T

FBGND

SNKIN

V 1.245VR2R1

1TRIP = +

Figure 3. Off-Line 1.8V/2A Power Supply

Figure 3a. 1.8V/1A Bode Plot(θ margin = 102°)

Figure 3b. 1.8V/2A Bode Plot(θ margin = 87°)

MIC4043 Micrel

MIC4043 8 November 2000

Off-Line 2.5V/2A Power Supply

R13

10Ω 1%

C111200µF10V

R14200Ω1%

VOUT+2.5V/2A

D412CTQ045 L2

5µH

C12220µF10V

U2b2501

U3

249Ω1000pF

MIC4043

R1072k1%

R1133k1%

U1MIC38HC43BN

VDD

VREF

GND

COMP

VOUT

RT/CT

FB

ISNS

2

1

4

3

8

5

7

6

R2332k1%

R3332k1%

D11N4448

R434Ω1%

C50.1µF50/63V

D218V

C7470pF63V

C6470pF

63V

Q1IRFIBE30G

L1

R5

1.21k 1%

R61.21k1%

BR1DBR1

C447µF400V

U2a2501

F1

85 to 264Vac50/60Hz

Hot

Neutral

Return

T1

1

2

7

6

4

3

1AGround

R81.9Ω1/4W

1%

R9470Ω1/2W

D3UF4005

C9100pF1kVR1

1k 1%

R714k 1%

C32200pF400V

C22200pF

400V

C10.1µF250V

C100.1µF

50/63V80T 3T

20mH

C822µF25V

10T

FBGND

SNKIN

V 1.245VR2R1

1TRIP = +

Figure 4. Off-Line 2.5V/2A Power Supply

Figure 4a. 2.5V/1A Bode Plot(θ margin = 83°)

Figure 4b. 2.5V/2A Bode Plot(θ margin = 83°)

November 2000 9 MIC4043

MIC4043 Micrel

Off-Line 3.3V/2A Power Supply

R13

10Ω 1%

C111200µF10V

R14200Ω1%

VOUT+3.3V/2A

D412CTQ045 L2

5µH

C12220µF10V

U2b2501

U3

249Ω1000pF

MIC4043

R1072k1%

R1133k1%

U1MIC38HC43BN

VDD

VREF

GND

COMP

VOUT

RT/CT

FB

ISNS

2

1

4

3

8

5

7

6

R2332k1%

R3332k1%

D11N4448

R434Ω1%

C50.1µF50/63V

D218V

C7470pF63V

C6470pF

63V

Q1IRFIBE30G

L1

R5

1.21k 1%

R61.21k1%

BR1DBR1

C447µF400V

U2a2501

F1

85 to 264Vac50/60Hz

Hot

Neutral

Return

T1

1

2

7

6

4

3

1AGround

R81.9Ω1/4W

1%

R9470Ω1/2W

D3UF4005

C9100pF1kVR1

1k 1%

R714k 1%

C32200pF400V

C22200pF

400V

C10.1µF250V

C100.1µF

50/63V80T 3T

20mH

C822µF25V

10T

FBGND

SNKIN

V 1.245VR2R1

1TRIP = +

Figure 5. Off-Line 3.3V/2A Power Supply

Figure 5a. 3.3V/1A Bode Plot(θ margin = 82°)

Figure 5b. 3.3V/2A Bode Plot(θ margin = 80°)

MIC4043 Micrel

MIC4043 10 November 2000

Off-Line 5V/2A Power Supply

R13

10Ω 1%

C111200µF10V

R14200Ω1%

VOUT+5.0V/2A

D412CTQ045 L2

5µH

C12220µF10V

U2b2501

U3

249Ω1000pF

MIC4043

R1072k1%

R1133k1%

U1MIC38HC43BN

VDD

VREF

GND

COMP

VOUT

RT/CT

FB

ISNS

2

1

4

3

8

5

7

6

R2332k1%

R3332k1%

D11N4448

R434Ω1%

C50.1µF50/63V

D218V

C7470pF63V

C6470pF

63V

Q1IRFIBE30G

L1

R5

1.21k 1%

R61.21k1%

BR1DBR1

C447µF400V

U2a2501

F1

85 to 264Vac50/60Hz

Hot

Neutral

Return

T1

1

2

7

6

4

3

1AGround

R81.9Ω1/4W

1%

R9470Ω1/2W

D3UF4005

C9100pF1kVR1

1k 1%

R714k 1%

C32200pF400V

C22200pF

400V

C10.1µF250V

C100.1µF

50/63V80T 3T

20mH

C822µF25V

10T

FBGND

SNKIN

V 1.245VR2R1

1TRIP = +

Figure 6. Off-Line 5V/2A Power Supply

Figure 6b. 5V/2A Output Bode Plot(θ margin = 61°)

Figure 6a. 5V/1A Output Bode Plot(θ margin = 67°)

November 2000 11 MIC4043

MIC4043 Micrel

Package Information

0.150 (0.0059)0.089 (0.0035)8°

0°

0.400 (0.016) TYP 3 PLACES

2.50 (0.098)2.10 (0.083)

1.40 (0.055)1.20 (0.047)

0.950 (0.0374) TYP

3.05 (0.120)2.67 (0.105)

0.800 (0.031) TYP

1.12 (0.044)0.81 (0.032)

0.10 (0.004)0.013 (0.0005)

DIMENSIONS:MM (INCH)

0.41 (0.016)0.13 (0.005)

CL

CL

SOT-143 (M4)

MIC4043 Micrel

MIC4043 12 November 2000

MICREL INC. 1849 FORTUNE DRIVE SAN JOSE, CA 95131 USATEL + 1 (408) 944-0800 FAX + 1 (408) 944-0970 WEB http://www.micrel.com

This information is believed to be accurate and reliable, however no responsibility is assumed by Micrel for its use nor for any infringement of patents orother rights of third parties resulting from its use. No license is granted by implication or otherwise under any patent or patent right of Micrel Inc.

© 2000 Micrel Incorporated