MIC2172YN - Micrel Semiconductor, Inc.

MIC2172/3172

100kHz 1.25A Switching Regulators

Micrel Inc. • 2180 Fort une Drive • San Jose, CA 95131 • USA • tel +1 (408) 944-0800 • fax + 1 (408) 474-1000 • http://www.micrel .com

April 2006 M9999-041806

(408) 955-1690

General Description

The MIC2172 and MIC3172 are complete 100kHz SMPS

current-mode controllers with internal 65V 1.25A power

switches. The MIC2172 features external frequency

synchronization or frequency adjustment, while the

MIC3172 features an enable/shutdown control input.

Although primarily intended for voltage step-up

applications, the floating switch architecture of the

MIC2172/3172 makes it practical for step-down, inverting,

and Cuk configurations as well as isolated topologies.

Operating from 3V to 40V, the MIC2172/3172 draws only

7mA of quiescent current making it attractive for battery

operated supplies.

The MIC3172 is for applications that require on/off control

of the regulator. The MIC3172 is externally shutdown by

applying a TTL low signal to EN (enable). W hen disabled,

the MIC3172 draws only leakage current (typically less

than 1µA). EN must be high for normal operation. For

applicat ions n ot r equ ir in g contr ol , EN must be tied to VIN or

TTL high.

The MIC2172 is for applications requiring two or more

SMPS re gulators that op erate from the s ame input su pply.

The MIC2172 features a SYNC input which allows locking

of its internal oscillator to an external reference. This

makes it possible to avoid the audible beat frequencies

that result from the unequal oscillator frequencies of

indepe nde nt SM PS regulator s .

A reference signal can be supplied by one MIC2172

designated as a master. To insure locking of the slave’s

oscillators, the reference oscillator frequency must be

higher than the slave’s. The master MIC2172’s oscillator

frequency is increased up to 135kHz by connecting a

resistor from SYNC to ground (see applications

information).

The MIC2172/3172 is available in an 8-pin plastic DIP or

SOIC for –40°C to +85°C operation.

Features

• 1.25A, 65V internal switch rating

• 3V to 40V input voltage range

• Current-mode operation

• Internal cycle-by-cycle current limit

• Thermal shutdown

• Low external parts count

• Operates in most switching topologies

• 7mA quiescent current (operating)

• <1µA quiescent current, shutdown mode (MIC3172)

• TTL shutdown compatibility (MIC3172)

• External frequency synchronization (MIC2172)

• External frequency trim (MIC2172)

• Fits most LT1172 sockets (see applications info)

Applications

• Laptop/palmtop computers

• Toys

• Hand-held instruments

• Off-line converter up to 50W (requires external power

switch)

• Predriver for higher power capability

• Master/slave configurations (MIC2172)

___________________________________________________________________________________________________________

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Typical Applications

Figure 1. MIC2172 5V to 12V Boost Converter

Figure 2. MIC3172 Flyback Converter

Ordering Information

Part Number

Standard Pb-Free

Junction Temp. Range Package

MIC 2172BN MIC 2172Y N –40°C to +85°C 8-pin plastic DIP

MIC2172BM MIC2172YM –40°C to +85°C 8-pin SOIC

MIC 3172BN MIC 3172Y N –40°C to +85°C 8-pin plastic DIP

MIC3172BM MIC3172YM –40°C to +85°C 8-pin SOIC

Note:

1. Other Voltage available. Contact Micrel for details.

Pin Configuration

8-pin DIP (N) 8-pin SOIC (M)

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Pin Description

Pin Number Pin Name Pin Function

1 S GND Signal Ground: Internal analog circuit ground. Connect directly to the input filter

capacitor for proper operation (see applications info). Keep separate from power

grounds.

2 COMP Frequency Compensation: Output of transconductance type error amplifier.

Primary function is for loop stabilization. Can also be used for output voltage

soft-start and curr ent lim it tailo ring.

3 FB Feedback: Inverting input of error amplifier. Connect to external resistive divider

to set power supply output voltage.

4 (MIC2172) SYNC Synchronization/Frequency Adjust: Capacitively coupled input signal greater

than device’s free running frequency (up to 135kHz) will lock device’s oscillator

on falling edge. Oscillator frequency can be trimmed up to 135kHz by adding a

resistor to ground. If unused, pin must float (no connection).

4 (MIC3172) EN Enable: Apply TTL high or connect to VIN to enable the regulator. Apply TTL low

or connect to ground to disable the regulator. Device draws only leakage current

(<1µA) when disabled.

5 VIN Supply Voltage: 3.0V to 40V

6 P GND 2 Power Ground #2: One of two NPN power switch emitters with 0.3Ω current

sense resistor in series. Required. Connect to external inductor or input voltage

ground depending on circuit topology.

7 VSW Power Switch Collector: Collector of NPN switch. Connect to external inductor or

input voltage depending on circuit topology.

8 P GND 1 Power Ground #1: One of two NPN power switch emitters with 0.3Ω current

sense resistor in series. Optional. For maximum power capability connect to P

GND 2. Floating pin reduces current limit by a factor of two.

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Absolute Maximum Ratings MIC2172

Input Voltage.................................................................. 40V

Switch Voltage............................................................... 65V

Sync Current............................................................... 50mA

Feedback Voltage (Transient, 1ms) ............................ ±15V

Operating Temperature Range

8-pin PDIP.................................................–40 to +85°C

8-pin SOIC ................................................–40 to +85°C

Junction Temperature................................–55°C to +150°C

Thermal Resistance

θJA 8-pin PDIP................................................. 130°C/W

θJA 8-pin SOIC.................................................120°C/W

Storage Temperature ................................–65°C to +150°C

Soldering (10 sec.) ...................................................+300°C

Electrical Characteristics MIC2172

Note 1, 3. Unless otherwise specified, VIN = 5V.

Parameter Condition Min Typ Max Units

Reference Section Pin 2 tied to pin 3

Feedback Voltage (VFB) 1.220

1.214 1.240 1.264

1.274 V

Feedback Voltage Line

Regulation 3V ≤ VIN ≤ 40V 0.03 %/V

Feedback Bias Current

(IFB) 310 750

1100 nA

Error Amplifier Section

Transconductance

(∆ICOMP/∆VFB) ∆ICOMP = ±25µA 3.0

2.4 3.9 6.0

7.0 µA/mV

µA/mV

Voltage Gain

(∆VCOMP/∆VFB) 0.9V ≤ VCOMP ≤ 1.4V 500 800 2000 V/V

Output Current VCOMP = 1.5V 125

100 175 350

400 µA

µA

Output Swing High Clamp, VFB = 1V

Low Clamp, VFB = 1.5V 1.8

0.25 2.1

0.35 2.3

0.52 V

Compensation Pin

Threshold Duty Cycle = 0 0.8

0.6 0.9 1.08

1.25 V

Output Switch Section

ON Resistance ISW = 1A, VFB = 0.8V 0.76 1

1.1

Ω

Current Limit Duty Cycle = 50%, TJ ≥ 25°C

Duty Cycle = 50%, TJ < 25°C

Duty Cycle = 80%, Note 2

1.25

3.5

2.5

Breakdown Voltage (BV) 3V ≤ VIN ≤ 40V

ISW = 5mA 65 75 V

Notes:

1. Exceeding the absolute maximum rat i ng may damage the device.

2. The device is not guaranteed to function outside its operat i ng rating.

3. Devices are ESD sensit i ve. Handling precautions recommended. Human body model, 1.5k in series with 100pF.

4. Specification f or packaged product only.

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Typical Characteris tics MI C2172 (cont)

Parameter Condition Min Typ Max Units

Oscillator Section

Frequency (fO) 88

85 100 112

115 kHz

kHz

Duty Cycle [δ(max)]

80 89

95 %

Sync Coupling Capacitor

Required for Frequency

Lock

VPP = 3.0V

VPP = 40V 22

2.2 51

4.7 120

10 pF

Input Supply Voltage Section

Minimum Operating

Voltage 2.7

3.0 V

Quiescent Current (IQ) 3V ≤ VIN ≤ 40V, VCOMP = 0.6V, ISW = 0 7 9 mA

Supply Current Increase

(∆IIN) ∆ISW = 1A, VCOMP = 1.5V 9 20 mA

Electrical Characteristics MIC3172

Note 1, 3. Unless otherwise specified, VIN = 5V.

Parameter Condition Min Typ Max Units

Reference Section Pin 2 tied to pin 3

Feedback Voltage (VFB) 1.224

1.214 1.240 1.264

1.274 V

Feedback Voltage Line

Regulation 3V ≤ VIN ≤ 40V 0.07 %/V

Feedback Bias Current

(IFB) 310 750

1100 nA

Error Amplifier Section

Transconductance

(∆ICOMP/∆VFB) ∆ICOMP = ±25µA 3.0

2.4 3.9 6.0

7.0 µA/mV

µA/mV

Voltage Gain

(∆VCOMP/∆VFB) 0.9V ≤ VCOMP ≤ 1.4V 500 800 2000 V/V

Output Current VCOMP = 1.5V 125

100 175 350

400 µA

µA

Output Swing High Clamp, VFB = 1V

Low Clamp, VFB = 1.5V 1.8

0.25 2.1

0.35 2.3

0.52 V

Compensation Pin

Threshold Duty Cycle = 0 0.8

0.6 0.9 1.08

1.25 V

Output Switch Section

ON Resistance ISW = 1A, VFB = 0.8V 0.76 1

1.1

Ω

Current Limit Duty Cycle = 50%, TJ ≥ 25°C

Duty Cycle = 50%, TJ < 25°C

Duty Cycle = 80%, Note 2

1.25

3.5

2.5

Breakdown Voltage (BV) 3V ≤ VIN ≤ 40V

ISW = 5mA 65 75 V

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Typical Characteris tics MI C3172 (cont)

Parameter Condition Min Typ Max Units

Oscillator Section

Frequency (fO) 88

85 100 112

115 kHz

kHz

Duty Cycle [δ(max)]

80 89

95 %

Sync Coupling Capacitor

Required for Frequency

Lock

VPP = 3.0V

VPP = 40V 22

2.2 51

4.7 120

10 pF

Input Supply Voltage Section and Enable Section

Minimum Operating

Voltage 2.7

3.0 V

Quiescent Current (IQ) 3V ≤ VIN ≤ 40V, VCOMP = 0.6V, ISW = 0 7 9 mA

Supply Current Increase

(∆IIN) ∆ISW = 1A, VCOMP = 1.5V 9 20 mA

Enable Input Threshold 0.4 1.2 2.4 V

Enable Input Current VEN = 0V

VEN = 2.4V –1 0

2 1

10 µA

µA

Bold type denotes specifications applicabl e to the full operat i ng temperature range.

Note 1. Devices are ESD sens iti ve. Handli ng precautions required.

Note 2. For duty cycles (δ) between 50% and 95%, minimum guaranteed switch current is given by ICL = 0.833 (2- δ) for the MIC3172.

Note 3. Specification for packaged product only.

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Typical Characteris t ics

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Typical Characteris tics (cont.)

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Functional Characteristics

MIC2172 Block Diagram

MIC3172 Block Diagram

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Functional Description

Refer to “Block Diagram MIC2172” and “Block Diagram

MIC3172.”

Internal Po w er

The MIC2172/3172 operates when VIN is ≥ 2.6V (and

VEN ≥ 2.0V for the MIC3172). An internal 2.3V regulator

supplies biasing to all internal circuitry including a

precision 1.24V band gap reference.

The enable control (MIC3172 only) enables or disables

the internal regulator which supplies power to all other

internal circuitry.

PWM Operation

The 100kHz oscillator generates a signal with a duty

cycle of approximately 90%. The current-mode

compar ator o utpu t is us ed t o reduce th e d uty cycle wh en

the current amplifier output voltage exceeds the error

amplifier output voltage. The resulting PWM signal

controls a driver which supplies base current to output

transistor Q1.

Current Mode Advantages

The MIC2172 /3172 op erates in curr ent mode rather than

voltage mode. There are three distinct advantages to

this technique. Feedback loop compensation is greatly

simplified because inductor current sensing removes a

pole from the closed loop response. Inherent cycle-by-

cycle current limiting greatly improves the power switch

reliability and provides automatic output current limiting.

Finall y, curre nt-mode oper ation pro vides autom atic input

voltag e feed f orward whic h pre vents inst antaneous input

voltage changes from disturbing the output voltage

setting.

Anti-Saturation

The anti-s aturation diode ( D1) increases the us able dut y

cycle range of the MIC2172/3172 by eliminating the

base to collector stored charge which would delay Q1’s

turnoff.

Compensation

Loop stability compensation of the MIC2172/3172 can

be accomplished by connecting an appropriate network

from either COMP to circuit ground (Typical

Applications) or COMP to FB.

The error amplifier output (COMP) is also useful for soft

start and current limiting. Because the error amplifier

output is a transconductanc e type, the o utpu t impedanc e

is rel atively high which m eans the outp ut voltage c an be

easily clamped or adjusted externally.

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Application Information

Using the MIC3172 Enable Control (New Designs)

For new designs requiring enable/shutdown control,

connect EN to a TTL or CMOS control signal (figure 3).

The very low driver current requirement ensures

compatibility regardless of the driver or gate used.

Figure 3. MIC3172 TTL Enable/Shutdown

Using the MIC3172 in LT1172 Applications

The MIC3172 can be used in most original LT1172

applications by adapting the MIC3172’s

enable/shutdown feature to the existing LT1172 circuit.

Unlike the LT1172 which can be shutdown by reducing

the voltage on pin 2 (VC) belo w 0.15V, the MIC 3172 has

a dedicated enable/shutdown pin. To replace the

LT1172 with the MIC3172, determine if the LT1172’s

shutdown feature is used.

Circuits without Shutdown

If the shutd own feat ure is not be ing used, connect EN to

VIN to continuously enable the MIC3172 or use an

MIC2172 with SYNC open (figure 4).

Figure 4. MIC2172/3172 Always Enabled

Circuits with Shutdown

If shutdo wn was used in the or iginal LT1172 a pplicati on,

connect EN to a logic gate that produces a TTL logic-

level output signal that matches the shutdown signal.

The MIC3172 will be enabled by a logic-high input and

shutdown with a logic-low input (figure 5). The actual

components perform ing the functions of U1 and Q1 may

vary according to the original application.

Figure 5. Adapting to the LT1172 Socket

By using the MIC3172, U1 and Q1 sho wn in f igure 5 can

be eliminated, reducing the total components count.

Synchronizing the MIC2172

Using several unsynchronized switching regulators in the

same circuit will cause beat frequencies to appear on the

inputs and outputs. These beat frequencies can be very

low making them difficult to filter.

Micrel’s MIC2172 can be synchronized to a single

master frequency avoiding the possibility of undesirable

beat frequencies in multiple regulator circuits. The

master frequency can be an external oscillator or a

designated master MIC2172. The master frequency

should be 1 .05 to 1.2 0 tim es the slav e’s 1 00kHz nom inal

frequency to guarantee synchronization.

Figure 6. Master/Slave Synchronization

Figure 6 shows a typical application where several

MIC2172s operate from the same supply voltage. U1’s

oscill ator fr equenc y is incre ased above U2 ’s and U3 ’s b y

connecting a resistor from SYNC to ground. U2-SYNC

and U3-SYNC are capacitively coupled to the master’s

output (VSW). The slaves lock to the negative (falling

edge) of U1’s output waveform.

Figure 7. External Synchronization

Care must be exercised to insure that the master

MIC2172 is always operating in continuous mode.

Figure 7 shows how one or more MIC2172s can be

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locked to an external reference frequency. The slaves

lock to the negative (falling edge) of the external

reference waveform.

Soft Start

A diode-coupled capacitor from COMP to circuit ground

slows the output voltage rise at turn on (figure 8).

Figure 8. Soft Start

The additional time it takes for the error amplifier to

charge th e capacitor c orresponds to the tim e it takes the

output to reach regulation. Diode D1 discharges C1

when VIN is removed.

Another soft start circuit is shown in figure 8A. The

circuit us es c apac it or C1 to contr ol th e out put r ise time b y

providing feedback from the output to the FB pin. The

output vol tage starts to rise when the MI C3172 re gulator

starts switching. T his is the dv/dt of the output will forc e

a current through capacitor C1, which flows through the

lower feedback resistor, R2, increasing the voltage on

the FB pin. This increased voltage on the FB pin

reduces the d uty cycle at th e VSW pin, lim iting the tur n- on

time of the output. Increasing the value of C1 causes

the output voltage to rise more slowly. Diode D1 is

reverse biased in normal operation and prevents C1

from appearing in parallel with the upper voltage divider

resistor, which would affect stability and transient

response. Zener diode D2 clamps the voltage seen by

the feedback pin and provides a discharge path for C1

when the power supp ly is turned of f.

Figure 8a. Additional Soft Start Circuit

This circuit only limits the dv/dt of the output when the

boost con verter is runn ing. It will n ot decrease th e dv/dt

or the i nitial inrush c aused by app lying the input voltag e.

Figure 8B shows the turn-on without a soft start circuit

and Figure 8C shows how the soft start circuit reduces

inrush and prevents output voltage overshoot.

Figure 8b. Without Soft Start

Figure 8c. With Soft Start

Current Limit

For designs demanding less output current than the

MIC2172/ 3172 is c apable of deliv ering, P G ND 1 can be

left open reducing the current capability of Q1 by one-

half.

Figure 9. Current Limit

Alternatively, the maximum current limit of the

MIC2172/3172 can be reduced by adding a voltage

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clamp to the COMP output (figure 9). This feature can be

useful in applications requiring either a complete

shutdown of Q1’s switching action or a form of current

fold-bac k lim iting. This use of the COMP o utput do es not

disable the oscillator, amplifiers or other circuitry,

therefore the supply current is never less than

approximately 5mA.

Thermal Management

Although the MIC2172/3172 family contains thermal

protection circuitry, for best reliability, avoid prolonged

operation with junction temperatures near the rated

maximum.

The junction temperature is determined by first

calculating the power dissipation of the device. For the

MIC2172/3172, the total power dissipation is the sum of

the device operating losses and power switch losses.

The device operating losses are the dc losses

associated with biasing all of the internal functions plus

the losses of the power switch driver circuitry. The dc

losses are calculated from the supply voltage (VIN) and

device supply current (IQ). The MIC2172/3172 supply

current is almost constant regardless of the supply

voltage (see “Electrical Characteristics”). The driver

section l osses (not inc ludin g the s witch) ar e a func tion of

supply voltage, power switch current, and duty cycle.

()

⎥

⎦

⎤

⎢

⎣

⎡⎟

⎠

⎞

⎜

⎝

⎛+

+50

δ0.004

IVI VP SWINQINdriverbias

where:

P(bias+driver) = device operating losses

VIN = supply voltage

IQ = quiescent supply current

ISW = power switch current

(see “Design Hints: Switch Current Calculations”)

δ = duty cycle

FOUT

INFOUT VV VVV

±+

OUT = output vo lta ge

F = D1 forward voltage drop

As a practical example refer to figure 1.

VIN = 5.0V

IQ = 0.006A

ISW = 0.625A

δ = 60% (0 .6)

Then:

()

0.068WP50 0.60.004

0.62550.0065P

driverbias

⎥

⎦

⎤

⎢

⎣

⎡⎟

⎠

⎞

⎜

⎝

⎛+

+×=

Power switch dissipation calculations are greatly

simplified by making two assumptions which are usually

fairly accurate. First, the majority of losses in the power

switch are du e to o n-losses . T o find thes e loss es, as sign

a resistance value to the collector/emitter terminals of

the device using the saturation voltage versus collector

current curves (see Typical Performance

Characteristics). Power switch losses are calculated by

modeling the switch as a resistor with the switch duty

cycle modifying the average power dissipation.

PSW = (ISW)2 RSW δ

From the Typical performance Characteristics:

RSW = 1Ω

Then:

PSW = (0.625)2 × 1 × 0.6

PSW = 0.234W

P(total) = 0.068 + 0.234

P(total) = 0.302W

The junction temperature for any semiconductor is

calculated using the following:

TJ = TA + P(total) θJA

Where:

TJ = junction temperature

TA = ambient temperature (maximum)

P(total) = total power dissipation

θJA = junction to ambient thermal resistance

For the practical example:

TA = 70°C

θJA = 130°C/W (for plastic DIP)

Then:

TJ = 70 + 0.30 ⋅ 130

TJ = 109°C

This junction temperature is below the rated maximum of

150°C.

Grounding

Refer to figure 10. Heavy lines indicate high current

paths.

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Figure 10. Single Point Ground

A single point ground is strongly recommended for

proper operation.

The signal ground, compensation network ground, and

feedback network connections are sensitive to minor

voltage variations. The input and output capacitor

grounds and power ground conductors will exhibit

voltage drop when carrying large currents. Keep the

sensitive circuit ground traces separate from the power

ground traces. Small voltage variations applied to the

sensitive circuits can prevent the MIC2172/3172 or any

switching regulator from functioning properly.

Applications and Design Hints

Access to both the collector and emitter(s) of the NPN

power switch makes the MIC2172/3172 extremely

versati le and suitable f or us e in most PW M power s up p l y

topologies.

Boost Conversion

Refer to figure 11 for a typical boost conversion

application where a +5V logic supply is available but

+12V at 0.14A is required.

Figure 11. 5V to 12V Boost Converter

The first step in designing a boost converter is

determ ining whether induct or L1 will caus e the con verter

to operate in either continuous or discontinuous mode.

Discontinuous mode is preferred because the feedback

control of the converter is simpler.

When L1 discharges its current completely during the

MIC2172/3172’s off-time, it is operating in discontinuous

mode.

L1 is operating in continuous mode if it does not

discharge completely before the MIC2172/3172 power

switch is turned on again.

Discontinuo us Mode Des ig n

Given the m aximum output current, s olve equati on (1) to

determine whether the device can operate in

discontinuous mode without initiating the internal device

current limit.

OUT

OUT V

δ V

⎟

⎠

⎞

⎜

⎝

⎛

≤ (1)

FOUT

INFOUT VV VVV

±+

= (1a)

Where:

ICL = internal switch current limit

ICL = 1.25A when δ < 50%

ICL = 0.833 (2 – δ) when δ ≥ 50%

(Refer to Electrical Characteristics.)

IOUT = maximum output current

VIN = minimum input voltage

δ = duty cycle

VOUT = required outp u t vo lta ge

VF = D1 forward voltage drop

For the example in figure 11.

IOUT = 0.14A

ICL = 1.147A

VIN = 4.75V (minimum)

δ = 0.623

VOUT = 12.0V

VF = 0.6V

Then:

0.141AI12

0.6234.75

1.147

OUT

≤

××

⎟

⎠

⎞

⎜

⎝

⎛

≤

This value is greater than the 0.14A output current

requirement so we can proceed to find the inductance

value of L1.

()

SWOUT

IN f P 2

δ V

L1≤ (2)

Where:

POUT = 12 ⋅ 0.14 = 1.68W

fSW = 1⋅105kHz (100kHz)

For our practical example:

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()

1011.6820.6234.75

L1 ×××

≤

IL1 ≤ 26.062µH (use 27µH)

Equation (3) solves for L1’s maximum current value.

T V

IONIN

L1(peak) = (3)

Where:

TON = δ / fSW = 6.23×10-6 sec

L1(peak) 1027 106.234.75

I−

−

××

IL1(peak) = 1.096A

Use a 27µH inductor with a peak current rating of at

least 1.4A.

Flyback Conversion

Flyback converter topology may be used in low power

applications where voltage isolation is required or

whenever the input voltage can be less than or greater

than the output voltage. As with the step-up converter

the inductor (transformer primary) current can be

continuous or discontinuous. Discontinuous operation is

recommended.

Figure 12 shows a practical flyback converter design

using the MIC317 2.

Switch Operation

During Q1’s on time ( Q1 is the internal N PN transistor —

see block diagrams), energy is stored in T1’s primary

inductance. During Q1’s off time, stored energy is

partially discharged into C4 (output filter capacitor).

Careful selection of a low ESR capacitor for C4 may

provide satisfactory output ripple voltage making

additional filter stages unnecessary.

C1 (input capacitor) m ay be reduced or eliminat ed if the

MIC3172 is located near a low impedance voltage

source.

Output Diode

The output di ode allo ws T1 to stor e energy in its prim ar y

inductance (D2 nonconducting) and release energy into

C4 (D2 conducting). The low forward voltage drop of a

Schottky diode minimizes power loss in D2.

Frequency Compensation

A simple frequency compensation network consisting of

R3 and C2 prevents output oscillations.

High impedance output stages (transconductance type)

in the MIC2172/ 3172 often perm it simp lified lo op-st abil ity

solutions to be connected to circuit ground, although a

more conventional technique of connecting the

components from the error amplifier output to its

inverti ng inp ut is also pos s i ble.

Voltage Clipper

Care must be taken to minimize T1’s leakage

inductance, otherwise it may be necessary to

incorpora te the voltage c lipper cons isting of D1, R4, a nd

C3 to avoid second breakdown (failure) of the

MIC3172’s power NPN Q1.

Enable/Shutdown

The MIC3172 includes the enable/shutdown feature.

W hen the de vic e is s h utdo wn, total s up ply curren t is les s

than 1µA. This is ideal for battery applications where

portions of a system are powered only when needed. If

this feature is not required, simply connect EN to VIN or

to a TTL high voltage.

Discontinuo us Mode Des ig n

When designing a discontinuous flyback converter, first

determine whether the device can safely handle the

peak primary current demand placed on it by the output

power. Equation (8) finds the maximum duty cycle

required for a given input voltage and output power. If

the duty cycle is greater than 0.8, discontinuous

operation cannot be used.

IN(min)CL

OUT

VIP 2

δ≥ (8)

For a practical example let:

POUT = 5.0V × 0.25A = 1.25W

VIN = 4.0V to 6.0V

ICL = 1.25A when δ < 50%

Then:

41.25

1.252

≥

δ ≥ 0.5 (50%) Use 0.55.

The s lightly hig her dut y c ycle valu e is used to overco me

circuit i neff iciencies. A few iter ations of equati on (8) m ay

be required if the duty cycle is found to be greater than

50%.

Calculate the maximum transformer turns ratio a, or

NPRI/NSEC, that will guarantee safe operation of the

MIC2172/3172 power switch.

SEC

IN(max)CECE VVF V

a±

≤ (9)

Where:

a = transformer maximum turns ratio

VCE = power switch collector to emitter maximum

voltage

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FCE = safety derating factor (0.8 for most

commercial and industrial applications)

VIN(max) = maximum input voltage

VSEC = transformer secondary voltage (VOUT + VF)

For the practical example:

VCE = 65V max. for the MIC2172/3172

FCE = 0.8

VSEC = 5.6V

Then:

5.6 6.00.865

a±×

≤

a ≤ 8.2143

Next, calculate the maximum primary inductance

required t o store the need ed output energ y with a power

switch duty cycle of 55%.

OUT

IN(min)SW

PRI PT Vf 0.5

L≤ (10)

Where:

LPRI = maximum primary inductance

fSW = device switching frequency (100kHz)

VIN(min) = minimum input voltage

TON = power switch on time

Then:

()

1.25 105.54.01010.5

625

PRI

−

××××

≤

LPRI ≤ 19.23µH

Use an 18µH primary inductance to overcome circuit

inefficiencies.

To complete the design the inductance value of the

secondary is found which will guarantee that the energy

stored in the transformer during the power switch on

time will be completed discharged into the output during

the off-time. This is necessary when operating in

discontinuous-mode.

OUT

OFF

SECSW

SEC PT Vf 0.5

L≤ (11)

Where:

LSEC = maximum secondary inductance

TOFF = power switch off time

Then:

()

1.25 104.55.61010.5

625

SEC

−

×××××

≤

LSEC ≤ 25.4µH

Figure 12. MIC3172 5V 0.25A Flyback Converter

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Finally, recalculate the transformer turns ratio to insure

that it is less than the value earlier found in equation (9).

SEC

PRI

a≤ (12)

Then:

102.54 101.8

a−

−

≤

a ≤ 0.84 Use 0.8 (same as 1:1.25).

This ratio is les s than th e ratio ca lcula ted in eq uat ion ( 9).

When specifying the transf ormer it is necessar y to k now

the primary peak current which must be withstood

without satur at ing the transf ormer core.

PRI

ONIN(min)

PEAK(pri) LT V

So:

18µ8105.54.0

PEAK(pri)

−

××

= (13)

IPEAK(pri) = 1.22A

Now find the minimum reverse voltage requirement for

the output rectifier. This rectifier must have an average

current rating greater than the maximum output current

of 0.25A.

()

OUTIN(max)

BR FVV

V+

≥ (14)

Where:

VBR = output rectifier maximum peak reverse

voltage rating

a = transformer turns ratio (0.8)

FBR = reverse voltage safety derating factor (0.8)

Then:

()

15.625VV0.80.8 0.85.06.0

≥

×+

≥

A 1N5817 will safely handle voltage and current

requirements in this example.

Forward Converters

Micrel’s MIC2172/3172 can be used in several circuit

configurations to generate an output voltage which is

less than the i nput vo ltage (buc k or step-down topo logy) .

Figure 13 shows the MIC3172 in a voltage step-down

applicat ion. Becaus e of the internal arc hitecture of these

devices, more external components are required to

implem ent a s tep-down r egulat or than with other devic es

offered by Micrel (refer to the LM257x or LM457x family

of buck switchers). However, for step-down conversion

requiring a tr ansf ormer (f orward), th e MIC2 172/31 72 i s a

good choice.

A 12V to 5V step-down converter using transformer

isolation (forward) is shown in figure 14. Unlike the

isolated flyback converter which stores energy in the

primary inductance during the controller’s on-time and

releases it to the load during the off-time, the forward

converter transfers energy to the output during the on-

time, using the off-time to reset the transformer core. In

the application shown, the transformer core is reset by

the tertiary winding discharging T1’s peak magnetizing

current through D2.

For most forward converters the duty cycle is limited to

50%, allowing the transform er f lux to reset with only two

times the input voltage appearing across the power

switch. Although during normal operation this circuit’s

duty cycle is well below 50%, the MIC2172 (and

MIC3172) has a maximum duty cycle capability of 90%.

If 90% was required during operation (start-up and high

load currents), a complete reset of the transformer

during the off-time would require the voltage across the

power switch to be ten times the input voltage. This

would limit the input voltage to 6V or less for forward

converter applications.

To prevent core saturation, the application given here

uses a duty cycle limiter consisting of Q1, C4 and R3.

Whenever the MIC3172 exceeds a duty cycle of 50%,

T1’s reset winding current turns Q1 on. This action

reduces th e duty cyc le of the MIC3 172 un til T 1 is able t o

reset during each cycle.

Fluorescent Lamp Supply

An extremely useful application of the MIC3172 is

generating an ac voltage for fluorescent lamps used as

liquid crystal display back lighting in portable computers.

Figure 15 shows a complete power supply for lighting a

fluorescent lamp. Transistors Q1 and Q2 together with

capacitor C2 form a Royer oscillator. The Royer

oscillator generates a sine wave whose frequency is

determ ined b y th e ser i es L/ C c irc uit c omprised of T1 and

C2. Assum ing that the MIC3172 and L1 are absent, and

the transistors’ emitters are grounded, circuit operation is

described in “Oscillator Operation.”

Oscillator Operation

Resistor R2 provides initial base current that turns

transistor Q1 on and impresses the input voltage across

one half of T1’s primary winding (Pri 1). T1’s feedback

winding provides additional base drive (positive

feedback) to Q 1 f or c ing it well in to s aturat ion f or a period

determined by the Pri 1/C2 time constant. Once the

voltage across C2 has reached its maximum circuit

value, Q1’s collector current will no longer increase.

Since T1 is in s er ies with Q1 , th is drop in pr imar y curr ent

causes the flux in T1 to change and because of the

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mutual coupling to t he feedback winding further reduc es

primary current eventually turning Q1 off. The primary

windings now change state with the feedback winding

forcing Q2 on repeating the alternate half cycle exactly

as with Q1. This action produces a sinusoidal voltage

wave form; whose amplitude is proportional to the input

voltage, across T1’s primary winding which is stepped

up and capacitively coupled to the lamp.

Lamp Current Regulation

Initial ionization (lighting) of the fluorescent lamp

requires several times the ac voltage across it than is

required to sustain current through the device. The

current through the lamp is sampled and regulated by

the MIC3172 to achieve a given intensity. The MIC3172

uses L1 to maintain a constant average current through

the transistor emitters. This current controls the voltage

amplitude of the Royer osc illator a nd mainta ins the lam p

current. During the negative half cycle, lamp current is

rectified by D3. During the positive half cycle, lamp

current is rectif ied by D2 th rough R4 and R 5. R3 and C5

filter the voltage dropped across R4 and R5 to the

MIC3172’s feedback pin. The MIC3172 maintains a

constant lamp curr ent by adjusting its duty cycle to k eep

the feedback voltage at 1.2 4V. The intensity of the la mp

is adjusted using potentiometer R5. The MIC3172

adjusts its duty cycle accordingly to bring the average

voltage across R4 and R5 back to 1.24V.

On/Off Control

Especially important for battery powered applications,

the lamp can be remotely or automatically turned off

using the MIC3172’s EN pin. The entire circuit draws

less than 1µA while shutdown.

Efficiency

To obtain maximum circuit efficiency careful selection of

Q1 and Q2 for low c ollecto r to em itter saturation voltage

is a m us t. Induc tor L1 sh ou ld b e c hos e n f or minim al c or e

and copper losses at the switching frequency of the

MIC3172, and T1 should be carefully constructed from

magnetic materials optimized for the output power

required at the Royer oscillator frequency. Suitable

inductors may be obtained from Coiltronics, Inc., tel:

(407) 241-7876.

Figure 13. Step-Down or Buck Regulator

Micrel MIC2172/3172

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Figure 14. 12V to 5V Forward Converter

Figure 15. LCD Backlight Fluorescent Lamp Supply

Micrel MIC2172/3172

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Package Information

8-Pin Plastic DIP (N)

8-Pin SOIC (M)

MICREL, INC. 2180 FORTUNE DRIVE SAN JOSE, CA 95131 US

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 t ime without notification to the customer.

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

can reasonably be expected to result i n personal injury. Lif e support devices or systems are devices or syst ems that (a) are int ended for surgical implan

into the body or (b) support or sustain li f e, and whose failure to perform can be reasonably expected to result in a significan t injury to the user. A

Purchas er’s use or sal e of Micrel Products for use in life support appliances, devices or syst ems is a Purchaser’s own risk and Purchaser agrees to fully

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