Datasheet MIC3172BM, MIC3172BN, MIC2172BM, MIC2172BN Datasheet (MICREL)

MIC2172/3172 Micrel

MIC2172/3172

100kHz 1.25A Switching Regulators

Preliminary Information

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). When disabled, the
MIC3172 draws only leakage current (typically less than
1µA). EN must be high for normal operation. For applications
not requiring control, EN must be tied to VIN or TTL high.

The MIC2172 is for applications requiring two or more SMPS
regulators that operate from the same input supply. 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 independent SMPS
regulators.

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)

4

T ypical Applications

+5V

(4.75V min.)

V

IN

SYNC

N/C

MIC2172

COMP

C3
1µF

GND

P1 P2 S

R3

1k

* Locate near MIC2172 when supply leads > 2

V

FB

27µH

SW

L1

D1

1N5822

C1*
22µF

C2
470µF

V

OUT

+12V, 0.14A

R1

10.7k
1%

R2

1.24k
1%

"

Figure 1.

MIC2172 5V to 12V Boost Converter

1997 4-13

4V to 6V

Enable

Shutdown

R3

1k

* Optional voltage clipper (may be req’d if T1 leakage inductance too high)

V

R1

3.74k
1%

R2

1.24k
1%

OUT

5V, 0.25A

V

IN

C1

22µF

COMP

C2
1µF

EN

MIC3172

P1 P2 S

V

GND

R4*

IN

V

SW

L

FB

C3*

D1*

1:1.25

= 100µH

PRI

T1

D2

1N5818

C4

470µF

Figure 2.

MIC3172 5V Flyback Converter

MIC2172/3172 Micrel

Ordering Information

Part Number Temperature Range Package

MIC2172BN –40°C to +85°C 8-pin plastic DIP
MIC2172BM –40°C to +85°C 8-lead SOIC
MIC3172BN –40°C to +85°C 8-pin plastic DIP
MIC3172BM –40°C to +85°C 8-lead SOIC

Pin Configuration

S GND

*SYNC/

COMP

FB

†

EN

MIC2172*/3172

1
2

3
4

†

8
7

6
5

P GND 1
V

SW

P GND 2
V

IN

S GND

COMP

*SYNC/

MIC2172*/3172

1
2
3

FB

†

4

EN

†

8
7

6
5

P GND 1
V

SW

P GND 2
V

IN

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

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 current limit tailoring.

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 V

TTL low or connect to ground to disable the regulator. Device draws only
leakage current (<1µA) when disabled.

5V

IN

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.

7V

SW

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.

to enable the regulator. Apply

IN

4-14 1997

MIC2172/3172 Micrel

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. Unless otherwise specified, V

Parameter Conditions Min Typ Max Units
Reference Section Pin 2 tied to pin 3

Feedback Voltage (V

Feedback Voltage 3V ≤ V
Line Regulation

Feedback Bias Current (I

Error Amplifier Section

Transconductance (∆I

Voltage Gain (∆V
Output Current V

Output Swing High Clamp, V

Compensation Pin Duty Cycle = 0 0.8 0.9 1.08 V
Threshold 0.6 1.25 V

Output Switch Section

) 1.220 1.240 1.264 V

FB

≤ 40V 0.03 %/V

IN

) 310 750 nA

FB

/∆VFB) ∆I

COMP

/∆VFB) 0.9V ≤ V

COMP

= ±25µA 3.0 3.9 6.0 µA/mV

COMP

≤ 1.4V 500 800 2000 V/V

COMP

= 1.5V 125 175 350 µA

COMP

= 1V 1.8 2.1 2.3 V

Low Clamp, VFB = 1.5V 0.25 0.35 0.52 V

FB

= 5V.

IN

1.214 1.274 V

1100 nA

2.4 7.0 µA/mV

100 400 µA

4

ON Resistance I

Current Limit Duty Cycle = 50%, T

Breakdown Voltage (BV) 3V ≤ VIN ≤ 40V 65 75 V

= 1A, VFB = 0.8V 0.76 1 Ω

SW

≥ 25°C 1.25 3 A
Duty Cycle = 50%, T
Duty Cycle = 80% Note 2 1 2.5 A

ISW = 5mA

J

< 25°C 1.25 3.5 A

J

1.1 Ω

1997 4-15

MIC2172/3172 Micrel

Parameter Conditions Min Typ Max Units
Oscillator Section

Frequency (fO) 88 100 112 kHz

85 115 kHz

Duty Cycle [δ(max)] 80 89 95 %

Sync Coupling Capacitor VPP = 3.0V 22 51 120 pF
Required for Frequency Lock VPP = 40V 2.2 4.7 10 pF

Peak-to-Peak Voltage C
Required for Frequency Lock

Input Supply Voltage Section

Minimum Operating Voltage 2.7 3.0 V
Quiescent Current (IQ) 3V ≤ VIN ≤ 40V, V
Supply Current Increase (∆IIN) ∆ISW = 1A, V

Bold type denotes specifications applicable to the full operating temperature range.
Note 1 Devices are ESD sensitive. Handling 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.

COUPLING

= 12pF 2.2 12 30 V

= 0.6V, ISW = 0 7 9 mA

COMP

= 1.5V 9 20 mA

COMP

Absolute Maximum Ratings MIC3172

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

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

Enable Voltage..............................................................40V

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

Operating Temperature Range

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

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

8-pin CerDIP ...........................................–55 to +125°C

Electrical Characteristics MIC3172 Note 1. Unless otherwise specified, V

Parameter Conditions Min Typ Max Units
Reference Section Pin 2 tied to pin 3

Feedback Voltage (V

Feedback Voltage 3V ≤ V
Line Regulation

) 1.224 1.240 1.264 V

FB

≤ 40V 0.07 %/V

IN

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

Thermal Resistance

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

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

θJA 8-pin CerDIP ..............................................100°C/W

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

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

= 5V.

IN

1.214 1.274 V

Feedback Bias Current (I

) 310 750 nA

FB

1100 nA

4-16 1997

MIC2172/3172 Micrel

Parameter Conditions Min Typ Max Units
Error Amplifier Section

Transconductance (∆I

COMP

/∆VFB) ∆I

= ±25µA 3.0 3.9 6.0 µA/mV

COMP

2.4 7.0 µA/mV
Voltage Gain (∆V
Output Current V

/∆VFB) 0.9V ≤ V

COMP

≤ 1.4V 500 800 2000 V/V

COMP

= 1.5V 125 175 350 µA

COMP

100 400 µA

Output Swing High Clamp, V

= 1V 1.8 2.1 2.3 V

FB

Low Clamp, VFB = 1.5V 0.25 0.35 0.52 V

Compensation Pin Duty Cycle = 0 0.8 0.9 1.08 V
Threshold 0.6 1.25 V

Output Switch Section

ON Resistance I

= 1A, VFB = 0.8V 0.76 1 Ω

SW

1.1 Ω

Current Limit Duty Cycle = 50%, T

Duty Cycle = 50%, T

≥ 25°C 1.25 3 A

J

< 25°C 1.25 3.5 A

J

Duty Cycle = 80% Note 2 1 2.5 A

Breakdown Voltage (BV) 3V ≤ VIN ≤ 40V 65 75 V

ISW = 5mA

Oscillator Section

Frequency (f

) 88 100 112 kHz

O

85 115 kHz

Duty Cycle [δ(max)] 80 89 95 %

4

Input Supply Voltage Section and Enable Section

Minimum Operating Voltage 2.7 3.0 V
Quiescent Current (IQ) 3V ≤ VIN ≤ 40V, V

= 0.6V, ISW = 0 7 9 mA

COMP

Shutdown, VEN = 0V 0.1 5 µA

Quiescent Current Increase (∆I

) ∆ISW = 1A, V

IN

= 1.5V 9 20 mA

COMP

Enable Input Threshold 0.4 1.2 2.4 V
Enable Input Current VEN = 0V –1 0 1 µA

VEN = 2.4V 2 10 µA

Bold type denotes specifications applicable to the full operating temperature range.
Note 1 Devices are ESD sensitive. Handling 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.

1997 4-17

MIC2172/3172 Micrel

Typical Performance Characteristics

MIC2172 Minimum
Operating Voltage

2.9

2.8

2.7

2.6
Switch Current = 1A

2.5

2.4

Minimum Operating Voltage (V)

2.3

-100 -50 0 50 100 150

15
14
13
12
11
10

Supply Current (mA)

Temperature (°C)

Supply Current

ISW = 0

D.C. = 90%

D.C. = 50%

9
8

D.C. = 0%

7
6
5

0 10203040

VIN Operating Voltage (V)

Feedback Bias Current

800
700
600
500
400
300
200
100

Feedback Bias Current (nA)

0

-100 -50 0 50 100 150
Temperature (°C)

Supply Current

(Shutdown Mode)

8

MIC3172

7

V

= 40V

IN

6
5
4
3
2

Supply Current (µA)

1
0

-100 -50 0 50 100 150
Temperature (°C)

Feedback Voltage

Line Regulation

5
4
3
2
1
0

-1

-2

-3

-4

Feedback Voltage Change (mV)

-5
0 10203040

Enable Thresholds

1.4

MIC3172

1.3

1.2

1.1

1

0.9

Enable Pin Voltage (mV)

0.8

-100 -50 0 50 100 150

TJ = 125°C

TJ = 25°C

TJ = -40°C

VIN Operating (V)

ON

OFF

Temperature (°C)

50

40

30

20

10

Average Supply Current (mA)

10

Supply Current (mA)

Supply Current

δ = 90%

δ = 50%

0

0.0 0.5 1.0 1.5 2.0
Switch Current (A)

Supply Current

V

9
8
7
6
5
4
3
2
1
0

-100 -50 0 50 100 150

= 0.6V

COMP

Temperature (°C)

Switch ON Voltage

1.6

1.4

1.2

1.0

TJ = 25°C

0.8

0.6

0.4

Switch ON Voltage (V)

0.2

0.0

0.0 0.5 1.0 1.5

Oscillator Frequency

120

110

100

90

80

Frequency (kHz)

70

60

-50 0 50 100 150

TJ = –40°C

TJ = 125°C

Switch Current (A)

Temperature (°C)

4

3

2

1

Switch Current (A)

0

140

130

120

(kHz)

110

OSC

f

100

90

1 10 100 1000

Current Limit

–40°C

25°C

125°C

0 20406080100

Duty Cycle (%)

Oscillator Frequency

MIC2172

R

(kΩ)

ADJ

4-18 1997

MIC2172/3172 Micrel

Typical Performance Characteristics

5.0

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

Transconductance (µA/mV)

0.5

0.0

-100 -50 0 50 100 150
Temperature (°C)

Block Diagram MIC2172

7000
6000
5000
4000
3000
2000

Transconductance (µS)

1000

Error Amplifier GainError Amplifier Gain

0

1 10 100 1000 10000

Frequency (kHz)

Error Amplifier Phase

-30
0

30
60
90

120

Phase Shift (°)

150
180
210

1 10 100 1000 10000

Frequency (kHz)

4

V

Pin 5

SYNC

Pin 4

FB

Pin 3

V

SW

Pin 7

D1

P

GND

2

Pin 6

Q1

P

GND

1

Pin 8

IN

1.24V
Ref.

S

GND

Pin 1

Reg.

2.3V

Error
Amp.

100kHz

Osc.

COMP

Pin 2

Logic

Com-

parator

Anti-Sat.

Driver

Current

Amp.

1997 4-19

MIC2172/3172 Micrel

Block Diagram MIC3172

V

SW

Pin 7

V

Pin 5

EN

Pin 4

FB

Pin 3

IN

1.24V
Ref.

S
GND
Pin 1

Reg.

2.3V

Error
Amp.

100kHz

Osc.

COMP

Pin 2

Logic

Com-

parator

Anti-Sat.

Driver

Current

Amp.

D1

P

GND

2

Pin 6

Q1

P

GND

1

Pin 8

Functional Description

Refer to “Block Diagram MIC2172” and “Block Diagram
MIC3172.”

Internal Power

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 comparator output is
used to reduce the duty cycle when 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 operates in current 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. Finally, current-mode op­eration provides automatic input voltage feed forward which
prevents instantaneous input voltage changes from disturb­ing the output voltage setting.

Anti-Saturation

The anti-saturation diode (D1) increases the usable duty
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
transconductance type, the output impedance is relatively
high which means the output voltage can be easily clamped
or adjusted externally.

4-20 1997

MIC2172/3172 Micrel

Applications 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.

U1

Enable

Shutdown

Logic
Gate

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) below 0.15V, the MIC3172 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 shutdown feature is not being used, connect EN to V
to continuously enable the MIC3172 or use an MIC2172 with
SYNC open (figure 4).

V

IN

V

N/C

4

SYNC

IN

MIC2172

Figure 4. MIC2172/3172 Always Enabled

4

EN

MIC3172

V

IN

V

4

EN

IN

MIC3172

By using the MIC3172, U1 and Q1 shown in figure 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 1.20 times the
slave’s 100kHz nominal frequency to guarantee synchroni­zation.

U2

45

U1

45

SYNC

0kΩ

IN

MIC2172

Master

V

SW

Additional

Slaves

SYNC

MIC2172

Slave

U3

45

SYNC

MIC2172

Slave

Figure 6. Master/Slave Synchronization

Figure 6 shows a typical application where several MIC2172s
operate from the same supply voltage. U1’s oscillator fre­quency is increased above U2’s and U3’s by 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.

V

SW

V

SW

4

Circuits with Shutdown

If shutdown was used in the original LT1172 application,
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 performing
the functions of U1 and Q1 may vary according to the original
application.

4

EN

MIC3172

Existing
Q1
VN2222
or equiv.

COMP

R1

C1

Enable

Shutdown

add
connection

U1

Existing

Logic

Gate

Figure 5. Adapting to the LT1172 Socket

1997 4-21

U1

45

xternal

Signal

Additional

Slaves

SYNC

MIC2172

45

SYNC

MIC2172

Slave

U2

Slave

V

SW

V

SW

Figure 7. External Synchronization

Care must be exercised to insure that the master MIC2172 is
always operating in continuous mode.

MIC2172/3172 Micrel

Figure 7 shows how one or more MIC2172s can be 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).

V

IN

V

IN

MIC2172/3172

COMP

D1

D2

C1

R1

C2

Figure 8. Soft Start

The additional time it takes for the error amplifier to charge the
capacitor corresponds to the time it takes the output to reach
regulation. Diode D1 discharges C1 when VIN is removed.

Current Limit

For designs demanding less output current than the MIC2172/
3172 is capable of delivering, P GND 1 can be left open
reducing the current capability of Q1 by one-half.

V

IN

Q1

V

IN

MIC2172/3172

GND

P1 P2 S

R1

C1

R2

V

COMP

SW

FB

≈ 0.6V/R2

I

R3

CL

Note: Input and output

C2

returns not common.

V

OUT

Figure 9. Current Limit

Alternatively, the maximum current limit of the MIC2172/3172
can be reduced by adding a voltage 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-back limiting. This use of the COMP
output does 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 losses (not including the switch) are a
function of supply voltage, power switch current, and duty
cycle.

P

(bias+ driver )

=VIN I

()

Q



+V

ISW



IN



0.004+






50



δ










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

V

OUT

δ

=

V

OUT

V

= output voltage

OUT

+ VF– V

+V

IN

F

VF = 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:

P

(bias+driver)

P

(bias+driver)

=5×0.006

()



+5 0.625




= 0.068W

0.004+0.6






50












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 due to
on-losses. To find these losses, assign 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

4-22 1997

MIC2172/3172 Micrel

Then:

PSW = (0.625)2 × 1 × 0.6

P

= 0.234W

(SW)

P

= 0.068 + 0.234

(total)

P

= 0.302W

(total)

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 power dissipation

(total)

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.

V

IN

V

IN

*

EN

MIC2172/3172

GND

P1 P2 S

V

SW

FB

V

C

Applications and Design Hints

Access to both the collector and emitter(s) of the NPN power
switch makes the MIC2172/3172 extremely versatile and
suitable for use in most PWM power supply 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.

+5V

(4.75V min.)

V

IN

SYNC

N/C

MIC2172

COMP

C3
1µF

GND

P1 P2 S

R3

1k

* Locate near MIC2172 when supply leads > 2

V

FB

27µH

SW

L1

Figure 11. 5V to 12V Boost Converter

The first step in designing a boost converter is determining
whether inductor L1 will cause the converter 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.

Discontinuous Mode Design

D1

1N5822

C1*
22µF

C2
470µF

V

OUT

+12V, 0.14A

R1

10.7k
1%

R2

1.24k
1%

"

4

* MIC3172 only

Single point ground

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.

1997 4-23

Given the maximum output current, solve equation (1) to
determine whether the device can operate in discontinuous
mode without initiating the internal device current limit.

I





(1)

(1a)

I

OUT

δ

CL





2

≤

V

=

+ VF– V

OUT

V

OUT

V

V





OUT

+V

δ

IN

IN

F

Where:

ICL = internal switch current limit

ICL = 1.25A when δ < 50%
ICL = 0.833 (2 – δ) when δ ≥ 50%
(Refer to Electrical Characteristics.)

I

= maximum output current

OUT

VIN = minimum input voltage
δ = duty cycle

MIC2172/3172 Micrel

V

= required output voltage

OUT

VF = D1 forward voltage drop

For the example in figure 11.

I

= 0.14A

OUT

ICL = 1.147A
VIN = 4.75V (minimum)
δ = 0.623
V

= 12.0V

OUT

VF = 0.6V

Then:

1.147





× 4. 75 × 0.623





2

12

I

OUT

I

OUT





≤

≤ 0.141A

This value is greater than the 0.14A output current require­ment so we can proceed to find the inductance value of L1.

2

V

δ

()

(2)

L1 ≤

2 P

IN

OUT fSW

Where:

Switch Operation

During Q1’s on time (Q1 is the internal NPN 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) may be reduced or eliminated if the
MIC3172 is located near a low impedance voltage source.

Output Diode

The output diode allows T1 to store energy in its primary
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 permit simplified loop-stability solu­tions to be connected to circuit ground, although a more
conventional technique of connecting the components from
the error amplifier output to its inverting input is also possible.

P

= 12 × 0.14 = 1.68W

OUT

fSW = 1×105Hz (100kHz)

For our practical example:

4.75 × 0. 623

L1 ≤

()

2 × 1.68 × 1×10

2

5

IL1 ≤ 26.062µH (use 27µH)

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

V

T

IN

(3)

I

L1(peak)

=

ON

L1

Where:

TON = δ / fSW = 6.23×10-6 sec

I

L1(peak)

I

L1(peak)

4.75 × 6.2 3 × 10

=

27×10

= 1.096A

-6

-6

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 MIC3172.

Voltage Clipper

Care must be taken to minimize T1’s leakage inductance,
otherwise it may be necessary to incorporate the voltage
clipper consisting of D1, R4, and C3 to avoid second break­down (failure) of the MIC3172’s power NPN Q1.

Enable/Shutdown

The MIC3172 includes the enable/shutdown feature. When
the device is shutdown, total supply current is less 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.

Discontinuous Mode Design

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.

2 P

ICL V

OUT

IN(min)

(8)

δ

≥

For a practical example let:

P

= 5.0V × 0.25A = 1.25W

OUT

VIN = 4.0V to 6.0V
ICL = 1.25A when δ < 50%

0.833 (2 – δ) when δ ≥ 50%

4-24 1997

MIC2172/3172 Micrel

Then:

2 × 1. 2 5

δ

≥

1. 25 × 4

δ ≥ 0.5 (50%) Use 0.55.

The slightly higher duty cycle value is used to overcome
circuit inefficiencies. A few iterations of equation (8) may be
required if the duty cycle is found to be greater than 50%.

Calculate the maximum transformer turns ratio a, or
N

PRI/NSEC

, that will guarantee safe operation of the MIC2172/

3172 power switch.

(9)

a ≤

V

CE FCE

– V

V

SEC

IN(max)

Where:

a = transformer maximum turns ratio
VCE = power switch collector to emitter

maximum voltage

FCE = safety derating factor (0.8 for most

commercial and industrial applications)
V
V

= maximum input voltage

IN(max)

= transformer secondary voltage (V

SEC

OUT

+ VF)

For the practical example:

VCE = 65V max. for the MIC2172/3172
FCE = 0.8
V

= 5.6V

SEC

Then:

65 × 0.8 – 6.0

a ≤

5.6

a ≤ 8.2143

Next, calculate the maximum primary inductance required to
store the needed output energy with a power switch duty
cycle of 55%.

2

(10)

SW

V

P

IN(min)

OUT

L

0.5 f

≤

PRI

T

ON

2

Where:

L

= maximum primary inductance

PRI

fSW = device switching frequency (100kHz)
V

= minimum input voltage

IN(min)

TON = power switch on time

Then:

2

-6

L

≤

PRI

L

≤ 19.23µH

PRI

0.5 × 1×10

5

× 4. 02 5.5 × 10

()

1. 2 5

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.

(11)

L

SEC

≤

0.5 f

SW VSEC

P

OUT

2

T

OFF

2

Where:

L

= maximum secondary inductance

SEC

T

= power switch off time

OFF

Then:

2

-6

L

L

SEC

SEC

0.5 × 1×10

≤

≤ 25.4µH

5

× 5. 62 × 4.5×10

()

1. 2 5

4

V

IN

4V to 6V

C1

22µF

Enable

Shutdown

R3

1k

C2
1µF

* Optional voltage clipper (may be req’d if T1 leakage inductance too high)

EN

COMP

P1 P2 S

V

MIC3172

GND

R4*

IN

V

SW

L

FB

Figure 12. MIC3172 5V 0.25A Flyback Converter

1997 4-25

C3*

D1*

1:1.25

= 100µH

PRI

T1

D2

1N5818

C4

470µF

R1

3.74k
1%

R2

1.24k
1%

V

OUT

5V, 0.25A

MIC2172/3172 Micrel

Finally, recalculate the transformer turns ratio to insure that
it is less than the value earlier found in equation (9).

L

(12)

a ≤

L

PRI

SEC

Then:

a ≤

1. 8 × 10

2.54 ×10

-5

-5

a ≤ 0.84 Use 0.8 (same as 1:1.25).

This ratio is less than the ratio calculated in equation (9).
When specifying the transformer it is necessary to know the
primary peak current which must be withstood without satu­rating the transformer core.

(13)

I

PEAK(pri)

V

=

IN(min)

L

PRI

T

ON

So:

I

PEAK(pri)

I

PEAK(pri)

4.0 × 5.5 ×10

=

= 1.22A

18µH

-6

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.

(14)

VBR≥

V

IN(max)

+ V

F

a

BR

a

()

OUT

Where:

VBR = output rectifier maximum peak

reverse voltage rating

a = transformer turns ratio (0.8)
FBR = reverse voltage safety derating factor (0.8)

Then:

VBR≥

6.0 + 5. 0 × 0.8

()

0. 8 × 0.8

VBR ≥ 15.625V

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 input voltage (buck or step-down topology). Figure 13
shows the MIC3172 in a voltage step-down application.
Because of the internal architecture of these devices, more
external components are required to implement a step-down
regulator than with other devices offered by Micrel (refer to
the LM257x or LM457x family of buck switchers). However,
for step-down conversion requiring a transformer (forward),
the MIC2172/3172 is 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 transformer flux 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

V

IN

EN

C2

2.2µF

470

R3

COMP

C3
1µF

C1*

100µF

* Locate near MIC2172/3172 when supply leads > 2

†

R3/R2 sets output voltage

D1
1N4148

V

IN

MIC3172

GND

P1 P2 S

V

SW

FB

Figure 13. Step-Down or Buck Converter

D3

1N4148

†

R3

3.7k

R2

1.2k

†

C4
1µF

100µH

D2

"

330µF

R4
10Ω

L1

C5

5V, 0.1A to 1A

(I

> 100mA)

LOAD

4-26 1997

MIC2172/3172 Micrel

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 the duty cycle of the
MIC3172 until T1 is able to 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 determined by the
series L/C circuit comprised of T1 and C2. Assuming 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 Q1 forcing it well

into saturation for 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 series with Q1, this drop in primary
current causes the flux in T1 to change and because of the
mutual coupling to the feedback winding further reduces
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 oscillator and
maintains the lamp current. During the negative half cycle,
lamp current is rectified by D3. During the positive half cycle,
lamp current is rectified by D2 through R4 and R5. R3 and C5
filter the voltage dropped across R4 and R5 to the MIC3172’s
feedback pin. The MIC3172 maintains a constant lamp
current by adjusting its duty cycle to keep the feedback
voltage at 1.24V. The intensity of the lamp 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.

4

T1

1:1:1

V

IN

12V

C2*

D1*

D2

1N5819

†

Q1

C4

* Voltage clipper

†

Duty cycle limiter

Enable

Shutdown

C1
22µF

EN

MIC3172

GND

P1 P2 S

V

IN

C3

1µF

COMP

R2

1k

R1*

V

FB

SW

Figure 14. 12V to 5V Forward Converter

1997 4-27

R3

D3

1N5819

†

†

L1 100µH

D4

1N5819

C5

470µF

R4

3.74k
1%

R5

1.24k
1%

V

OUT

5V, 1A

MIC2172/3172 Micrel
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.

R2

D1

L1

300µH

L1: Coiltronics CTX300-4P
T1: Coiltronics CTX110602
C2: Polyfilm, WIMA FKP2 0.1µF to 0.68µF
C4: 15pF to 30pF, 3kV min.

4.5V to 20V

Enable (On)

Shutdown (Off)

V

IN

V

MIC3172

GND

P1 P2 S

IN

V

COMP

SW

FB

R1

C1

EN

Efficiency

To obtain maximum circuit efficiency careful selection of Q1
and Q2 for low collector to emitter saturation voltage is a
must. Inductor L1 should be chosen for minimal core 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: [phone].

Cold Cathode

T1

Fluorescent

C4

Sec

Lamp

D2

1N4148

R3 R4

C5

D3
1N4148

R5
Intensity
Control

Q1

Q2

C2

C3

FB

Pri 1

Pri 2

Figure 15. LCD Backlight Fluorescent Lamp Supply

4-28 1997