MIC3172YM [MICROCHIP]
3.5A SWITCHING REGULATOR, 115kHz SWITCHING FREQ-MAX, PDSO8;
| 型号： | MIC3172YM |
| 厂家： | 
MICROCHIP |
| 描述： | 3.5A SWITCHING REGULATOR, 115kHz SWITCHING FREQ-MAX, PDSO8 开关 光电二极管 |
| 文件： | 总20页 (文件大小：909K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
MIC2172/3172
100kHz 1.25A Switching Regulators
General Description
Features
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.
• 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)
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.
Applications
• Laptop/palmtop computers
• Toys
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.
• Hand-held instruments
• Off-line converter up to 50W (requires external power
switch)
• Predriver for higher power capability
• Master/slave configurations (MIC2172)
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.
___________________________________________________________________________________________________________
Micrel Inc. • 2180 Fortune Drive • San Jose, CA 95131 • USA • tel +1 (408) 944-0800 • fax + 1 (408) 474-1000 • http://www.micrel.com
M9999-041806
(408) 955-1690
April 2006
Micrel
MIC2172/3172
Typical Applications
Figure 1. MIC2172 5V to 12V Boost Converter
Figure 2. MIC3172 Flyback Converter
Ordering Information
Part Number
Junction Temp. Range
Package
Standard
MIC2172BN
MIC2172BM
MIC3172BN
MIC3172BM
Pb-Free
MIC2172YN
MIC2172YM
MIC3172YN
MIC3172YM
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
8-pin plastic DIP
8-pin SOIC
8-pin plastic DIP
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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MIC2172/3172
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 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
6
VIN
Supply Voltage: 3.0V to 40V
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
8
VSW
Power Switch Collector: Collector of NPN switch. Connect to external inductor or
input voltage depending on circuit topology.
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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MIC2172/3172
Absolute Maximum Ratings MIC2172
Input Voltage.................................................................. 40V
Switch Voltage............................................................... 65V
Sync Current............................................................... 50mA
Feedback Voltage (Transient, 1ms) ............................ 15V
Operating Temperature Range
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
8-pin PDIP.................................................–40 to +85°C
8-pin SOIC ................................................–40 to +85°C
Electrical Characteristics MIC2172
Note 1, 3. Unless otherwise specified, VIN = 5V.
Parameter
Condition
Min
Typ
Max
Units
Reference Section
Feedback Voltage (VFB)
Pin 2 tied to pin 3
1.220 1.240
1.214
1.264
1.274
V
V
0.03
Feedback Voltage Line
Regulation
3V ≤ VIN ≤ 40V
%/V
Feedback Bias Current
(IFB)
310
750
1100
nA
nA
Error Amplifier Section
Transconductance
(∆ICOMP/∆VFB)
∆ICOMP = ±25µA
0.9V ≤ VCOMP ≤ 1.4V
VCOMP = 1.5V
3.0
2.4
3.9
800
175
6.0
7.0
µA/mV
µA/mV
Voltage Gain
(∆VCOMP/∆VFB)
500
2000
V/V
Output Current
125
100
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
V
Compensation Pin
Threshold
Duty Cycle = 0
0.8
0.6
0.9
1.08
1.25
V
V
Output Switch Section
Ω
Ω
ON Resistance
Current Limit
ISW = 1A, VFB = 0.8V
0.76
1
1.1
1.25
1.25
1
3
3.5
2.5
Duty Cycle = 50%, TJ ≥ 25°C
Duty Cycle = 50%, TJ < 25°C
Duty Cycle = 80%, Note 2
A
A
A
65
Breakdown Voltage (BV) 3V ≤ VIN ≤ 40V
75
V
ISW = 5mA
Notes:
1. Exceeding the absolute maximum rating may damage the device.
2. The device is not guaranteed to function outside its operating rating.
3. Devices are ESD sensitive. Handling precautions recommended. Human body model, 1.5k in series with 100pF.
4. Specification for packaged product only.
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MIC2172/3172
Typical Characteristics MIC2172 (cont)
Parameter
Condition
Min
Typ
Max
Units
Oscillator Section
Frequency (fO)
88
85
100
89
112
115
kHz
kHz
95
Duty Cycle [δ(max)]
80
%
Sync Coupling Capacitor VPP = 3.0V
22
2.2
51
4.7
120
10
pF
pF
Required for Frequency
Lock
VPP = 40V
Input Supply Voltage Section
3.0
Minimum Operating
Voltage
2.7
V
9
Quiescent Current (IQ)
3V ≤ VIN ≤ 40V, VCOMP = 0.6V, ISW = 0
∆ISW = 1A, VCOMP = 1.5V
7
9
mA
mA
Supply Current Increase
20
(∆IIN)
Electrical Characteristics MIC3172
Note 1, 3. Unless otherwise specified, VIN = 5V.
Parameter
Condition
Min
Typ
Max
Units
Reference Section
Feedback Voltage (VFB)
Pin 2 tied to pin 3
1.224 1.240
1.214
1.264
1.274
V
V
Feedback Voltage Line
Regulation
3V ≤ VIN ≤ 40V
0.07
310
%/V
Feedback Bias Current
(IFB)
750
1100
nA
nA
Error Amplifier Section
Transconductance
(∆ICOMP/∆VFB)
∆ICOMP = ±25µA
0.9V ≤ VCOMP ≤ 1.4V
VCOMP = 1.5V
3.0
2.4
3.9
800
175
6.0
7.0
µA/mV
µA/mV
Voltage Gain
(∆VCOMP/∆VFB)
500
2000
V/V
Output Current
Output Swing
125
100
350
400
µA
µA
High Clamp, VFB = 1V
Low Clamp, VFB = 1.5V
1.8
0.25
2.1
0.35
2.3
0.52
V
V
Compensation Pin
Threshold
Duty Cycle = 0
0.8
0.6
0.9
1.08
1.25
V
V
Output Switch Section
Ω
Ω
ON Resistance
ISW = 1A, VFB = 0.8V
0.76
1
1.1
1.25
1.25
1
3
3.5
2.5
Current Limit
Duty Cycle = 50%, TJ ≥ 25°C
Duty Cycle = 50%, TJ < 25°C
Duty Cycle = 80%, Note 2
A
A
A
65
Breakdown Voltage (BV) 3V ≤ VIN ≤ 40V
75
V
ISW = 5mA
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MIC2172/3172
Typical Characteristics MIC3172 (cont)
Parameter
Condition
Min
Typ
Max
Units
Oscillator Section
Frequency (fO)
88
85
100
89
112
115
kHz
kHz
95
Duty Cycle [δ(max)]
80
%
Sync Coupling Capacitor VPP = 3.0V
22
2.2
51
4.7
120
10
pF
pF
Required for Frequency
Lock
VPP = 40V
Input Supply Voltage Section and Enable Section
3.0
Minimum Operating
Voltage
2.7
V
9
Quiescent Current (IQ)
3V ≤ VIN ≤ 40V, VCOMP = 0.6V, ISW = 0
7
9
mA
mA
Supply Current Increase
∆ISW = 1A, VCOMP = 1.5V
20
(∆IIN)
0.4
2.4
Enable Input Threshold
Enable Input Current
1.2
V
1
10
VEN = 0V
VEN = 2.4V
–1
0
2
µA
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 I = 0.833 (2- δ) for the MIC3172.
CL
Note 3. Specification for packaged product only.
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April 2006
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MIC2172/3172
Typical Characteristics
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MIC2172/3172
Typical Characteristics (cont.)
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MIC2172/3172
Functional Characteristics
MIC2172 Block Diagram
MIC3172 Block Diagram
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MIC2172/3172
Functional Description
Refer to “Block Diagram MIC2172” and “Block Diagram
MIC3172.”
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.
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.
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.
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.
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.
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
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MIC2172/3172
By using the MIC3172, U1 and Q1 shown in figure 5 can
be eliminated, reducing the total components count.
Application Information
Using the MIC3172 Enable Control (New Designs)
Synchronizing the MIC2172
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.
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.
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
VIN to continuously enable the MIC3172 or use an
MIC2172 with SYNC open (figure 4).
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.
Figure 4. MIC2172/3172 Always Enabled
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.
Figure 7. External Synchronization
Care must be exercised to insure that the master
MIC2172 is always operating in continuous mode.
Figure 5. Adapting to the LT1172 Socket
Figure 7 shows how one or more MIC2172s can be
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MIC2172/3172
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 8b. Without Soft Start
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.
Another soft start circuit is shown in figure 8A. The
circuit uses capacitor C1 to control the output risetime by
providing feedback from the output to the FB pin. The
output voltage starts to rise when the MIC3172 regulator
starts switching. This is the dv/dt of the output will force
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 duty cycle at the VSW pin, limiting the turn-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 supply is turned off.
Figure 8c. With Soft Start
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.
Figure 8a. Additional Soft Start Circuit
This circuit only limits the dv/dt of the output when the
boost converter is running. It will not decrease the dv/dt
or the initial inrush caused by applying the input voltage.
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 9. Current Limit
Alternatively, the maximum current limit of the
MIC2172/3172 can be reduced by adding a voltage
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MIC2172/3172
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.
⎡
⎤
0.004 + 0.6
⎛
⎜
⎞
⎟
P(bias+driver
=
(
5× 0.006
)
+ 5 0.625
)
⎢
⎥
⎦
50
⎝
⎠
⎣
P(bias+driver = 0.068W
)
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
Thermal Management
Although the MIC2172/3172 family contains thermal
protection circuitry, for best reliability, avoid prolonged
operation with junction temperatures near the rated
maximum.
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.
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.
PSW = (ISW)2 RSW
δ
From the Typical performance Characteristics:
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.
RSW = 1Ω
Then:
P
SW = (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
⎡
⎤
0.004 + δ
⎛
⎜
⎞
⎟
P(bias+driver
where:
=
(
VIN Q
I
)
+ VIN ⎢ SW
I
)
⎥
⎦
50
⎝
⎠
⎣
Where:
TJ = junction temperature
TA = ambient temperature (maximum)
P(total) = total power dissipation
P(bias+driver) = device operating losses
VIN = supply voltage
IQ = quiescent supply current
θ
JA = junction to ambient thermal resistance
ISW = power switch current
For the practical example:
(see “Design Hints: Switch Current Calculations”)
TA = 70°C
θ
δ = duty cycle
JA = 130°C/W (for plastic DIP)
Then:
TJ = 70 + 0.30 ⋅ 130
VOUT + VF VIN
δ =
VOUT + VF
V
OUT = output voltage
VF = D1 forward voltage drop
As a practical example refer to figure 1.
TJ = 109°C
This junction temperature is below the rated maximum of
150°C.
VIN = 5.0V
Grounding
IQ = 0.006A
Refer to figure 10. Heavy lines indicate high current
paths.
I
SW = 0.625A
δ = 60% (0.6)
Then:
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MIC2172/3172
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
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
⎛
⎜
⎞
⎟
CL
VIN δ
2
⎝
⎠
IOUT
≤
(1)
VOUT
Figure 10. Single Point Ground
A single point ground is strongly recommended for
proper operation.
VOUT + VF
V
IN
δ =
(1a)
VOUT + VF
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.
Where:
CL = internal switch current limit
ICL = 1.25A when δ < 50%
CL = 0.833 (2 – δ) when δ ≥ 50%
I
I
(Refer to Electrical Characteristics.)
IOUT = maximum output current
VIN = minimum input voltage
δ = duty cycle
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.
VOUT = required output voltage
VF = D1 forward voltage drop
For the example in figure 11.
I
OUT = 0.14A
Boost Conversion
ICL = 1.147A
Refer to figure 11 for a typical boost conversion
application where a +5V logic supply is available but
+12V at 0.14A is required.
VIN = 4.75V (minimum)
δ = 0.623
VOUT = 12.0V
VF = 0.6V
Then:
1.147
2
⎛
⎜
⎞
⎟
× 4.75×0.623
⎝
⎠
IOUT
≤
12
IOUT ≤ 0.141A
This value is greater than the 0.14A output current
requirement so we can proceed to find the inductance
value of 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.
2
(
V
IN δ
)
L1≤
(2)
2POUT SW
f
Where:
OUT = 12 ⋅ 0.14 = 1.68W
SW = 1⋅105kHz (100kHz)
For our practical example:
P
When L1 discharges its current completely during the
MIC2172/3172’s off-time, it is operating in discontinuous
mode.
f
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MIC2172/3172
2
inverting input is also possible.
(
4.75 ×0.623
)
L1≤
2×1.68×1×105
L1 ≤ 26.062µH (use 27µH)
Equation (3) solves for L1’s maximum current value.
IN TON
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.
I
V
IL1(peak)
=
(3)
L1
Where:
Enable/Shutdown
TON = δ / fSW = 6.23×10-6 sec
4.75× 6.23×10−6
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.
IL1(peak)
=
27×10−6
L1(peak) = 1.096A
I
Use a 27µH inductor with a peak current rating of at
least 1.4A.
Discontinuous Mode Design
Flyback Conversion
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.
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.
2POUT
δ ≥
(8)
Figure 12 shows a practical flyback converter design
using the MIC3172.
ICL
V
IN(min)
For a practical example let:
POUT = 5.0V × 0.25A = 1.25W
VIN = 4.0V to 6.0V
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.
I
CL = 1.25A when δ < 50%
Then:
δ ≥
2×1.25
1.25× 4
C1 (input capacitor) may be reduced or eliminated if the
MIC3172 is located near a low impedance voltage
source.
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%.
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.
Calculate the maximum transformer turns ratio a, or
NPRI/NSEC, that will guarantee safe operation of the
MIC2172/3172 power switch.
Frequency Compensation
VCE FCE
V
IN(max)
a ≤
(9)
A simple frequency compensation network consisting of
R3 and C2 prevents output oscillations.
VSEC
Where:
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
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
MIC2172/3172
Then:
LPRI
commercial and industrial applications)
2
0.5×1×105 × 4.02
(
5.5×10−6
)
VIN(max) = maximum input voltage
≤
1.25
VSEC = transformer secondary voltage (VOUT + VF)
For the practical example:
LPRI ≤ 19.23µH
Use an 18µH primary inductance to overcome circuit
inefficiencies.
VCE = 65V max. for the MIC2172/3172
FCE = 0.8
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.
VSEC = 5.6V
Then:
65×0.8 6.0
5.6
a ≤
a ≤ 8.2143
2
2
0.5 fSW VSEC TOFF
LSEC
≤
(11)
Next, calculate the maximum primary inductance
required to store the needed output energy with a power
switch duty cycle of 55%.
POUT
Where:
SEC = maximum secondary inductance
TOFF = power switch off time
Then:
2
0.5 fSW
V
2 TON
L
IN(min)
LPRI
≤
(10)
POUT
Where:
PRI = maximum primary inductance
2
0.5×1×105 ×5.62 ×
(
4.5×10−6
)
L
LSEC
≤
1.25
fSW = device switching frequency (100kHz)
VIN(min) = minimum input voltage
TON = power switch on time
L
SEC ≤ 25.4µH
Figure 12. MIC3172 5V 0.25A Flyback Converter
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MIC2172/3172
Finally, recalculate the transformer turns ratio to insure
that it is less than the value earlier found in equation (9).
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.
LPRI
a ≤
Then:
a ≤
(12)
LSEC
−5
1.8×10
−5
2.54×10
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.
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 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.
V
T
IN(min) ON
I
=
PEAK(pri)
L
PRI
So:
−6
4.0×5.5×10
18µ8
I
=
(13)
PEAK(pri)
I
PEAK(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.
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.
V
+
(
V
a
)
IN(max)
OUT
V
≥
(14)
BR
F
a
BR
Where:
BR = output rectifier maximum peak reverse
Fluorescent Lamp Supply
V
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.
voltage rating
a = transformer turns ratio (0.8)
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.”
F
BR = reverse voltage safety derating factor (0.8)
Then:
6.0 +
(
5.0×0.8
0.8×0.8
VBR ≥ 15.625V
)
VBR
≥
A 1N5817 will safely handle voltage and current
requirements in this example.
Oscillator Operation
Forward Converters
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
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
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MIC2172/3172
Figure 13. Step-Down or Buck Regulator
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.
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.
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.
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
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:
(407) 241-7876.
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Micrel
MIC2172/3172
Figure 14. 12V to 5V Forward Converter
Figure 15. LCD Backlight Fluorescent Lamp Supply
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MIC2172/3172
Package Information
8-Pin Plastic DIP (N)
8-Pin SOIC (M)
MICREL, INC. 2180 FORTUNE DRIVE SAN JOSE, CA 95131 USA
TEL +1 (408) 944-0800 FAX +1 (408) 474-1000 WEB http:/www.micrel.com
The information furnished by Micrel in this data sheet is believed to be accurate and reliable. However, no responsibility is assumed by Micrel for its
use. Micrel reserves the right to change circuitry and specifications at any time without notification to the customer.
Micrel Products are not designed or authorized for use as components in life support appliances, devices or systems where malfunction of a product
can reasonably be expected to result in personal injury. Life support devices or systems are devices or systems that (a) are intended for surgical implant
into the body or (b) support or sustain life, and whose failure to perform can be reasonably expected to result in a significant injury to the user. A
Purchaser’s use or sale of Micrel Products for use in life support appliances, devices or systems is a Purchaser’s own risk and Purchaser agrees to fully
indemnify Micrel for any damages resulting from such use or sale.
© 2004 Micrel, Incorporated.
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