MIC2172/3172
100 kHz, 1.25A Switching Regulators
Features
General Description
•
•
•
•
•
•
•
•
The MIC2172 and MIC3172 are complete 100 kHz
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
Low External Parts Count
Operates in Most Switching Topologies
7 mA Quiescent Current (Operating)
2"
MIC3172 5V Flyback Converter
VIN
4V to 6V
R4*
C1
22μF
VSW
EN
C3*
D1*
VIN
Enable
Shutdown
MIC3172
R3
1k
VOUT
5V, 0.25A
T1
D2
1N5818
C4
470μF
R1
3.74k
1%
1:1.11
LPRI = 18μH
COMP GND
FB
P1 P2 S
R2
1.24k
1%
C2
1μF
* Optional voltage clipper (may be req’d if T1 leakage inductance too high)
Package Types
8-Pin DIP (N)
8-Pin DIP (M)
SGND 1
8
PGND1
SGND 1
8
PGND1
COMP 2
7
VSW
COMP 2
7
VSW
FB 3
*SYNC/†EN 4
DS20006208A-page 2
6 PGND2
5 VIN
FB 3
*SYNC/†EN 4
6 PGND2
5 VIN
2019 Microchip Technology Inc.
MIC2172/3172
Functional Block Diagrams
MIC2172 Functional Block Diagram
VSW
Pin 7
VI N
Pin 5
Reg.
D1
2.3V
Anti-Sat.
100kHz
Osc.
SYNC
Pin 4
Logic
Q1
Driver
Comparator
FB
Pin 3
1.24V
Ref.
Current
Amp.
Error
Amp.
COMP
Pin 2
SGND
Pin 1
PGND2 PGND1
Pin 6 Pin 8
MIC3172 Functional Block Diagram
VSW
Pin 7
VI N
Pin 5
Reg.
Anti-Sat.
100kHz
Osc.
EN
Pin 4
D1
2.3V
Logic
Driver
Q1
Comparator
FB
Pin 3
1.24V
Ref.
SGND
Pin 1
2019 Microchip Technology Inc.
Current
Amp.
Error
Amp.
COMP
Pin 2
PGND2 PGND1
Pin 6 Pin 8
DS20006208A-page 3
MIC2172/3172
1.0
ELECTRICAL CHARACTERISTICS
Absolute Maximum Ratings †
Input Voltage (VIN)...................................................................................................................................................... 40V
Switch Voltage (VSW) ................................................................................................................................................. 65V
Feedback Voltage (VFB) (Transient, 1 ms) ............................................................................................................... ±15V
Sync Current (ISYNC) .............................................................................................................................................. 50 mA
Junction Temperature (TJ)......................................................................................................................–55°C to +150°C
Storage Temperature (TS) ......................................................................................................................–65°C to +150°C
Lead Temperature (Soldering 10 sec.) ....................................................................................................................300°C
Operating Ratings ‡
Operating Junction Temperature (TJ) .....................................................................................................–40°C to +125°C
Operating Ambient Temperature Range (TA)
8-Pin PDIP................................................................................................................................................–40°C to +85°C
8-Pin SOIC ...............................................................................................................................................–40°C to +85°C
Thermal Resistance
8-Pin PDIP (JA) ................................................................................................................................................. 130°C/W
8-Pin SOIC (JA)................................................................................................................................................. 120°C/W
† Notice: Stresses above those listed under “Absolute Maximum Ratings” may cause permanent damage to the device.
This is a stress rating only and functional operation of the device at those or any other conditions above those indicated
in the operational sections of this specification is not intended. Exposure to maximum rating conditions for extended
periods may affect device reliability. Specifications are for packaged product only.
‡ Notice: The device is not guaranteed to function outside its operating ratings.
ELECTRICAL CHARACTERISTICS (MIC2172)
Electrical Characteristics: VIN = 5V; TA = 25°C, unless otherwise specified. Bold values indicate –40°C ≤ TA ≤
85°C. Note 1, Note 2
Parameter
Symbol
Min.
Typ.
Max.
1.220
1.240
1.264
1.214
—
1.274
—
—
0.03
—
310
750
—
—
1100
3.0
3.9
6.0
2.4
—
7.0
500
800
2000
125
175
350
100
—
400
Units
Conditions
Reference (Pin 2 tied to Pin 3)
Feedback Voltage
VFB
Feedback Voltage Line
Regulation
∆VFB(LINE)
Feedback Bias Current
IFB
V
%/V
nA
—
—
3V ≤ VIN ≤ 40V
—
—
Error Amplifier
Transconductance
∆ICOMP/∆VFB
Voltage Gain
∆VCOMP/∆VFB
Output Current
Output Swing
Compensation Pin Threshold
ICOMP
VCOMP(MAX)
1.8
2.1
2.3
VCOMP(MIN)
0.25
0.35
0.52
0.8
0.9
1.08
0.6
—
1.25
—
0.76
1
—
—
1.1
VCOMP_TH
μA/mV
∆ICOMP = ±25 μA
V/V
0.9V ≤ VCOMP ≤ 1.4V
μA
VCOMP = 1.5V
V
High Clamp, VFB = 1V
Low Clamp, VFB = 1.5V
V
Duty Cycle = 0
Ω
ISW = 1A, VFB = 0.8V
Output Switch
ON Resistance
DS20006208A-page 4
RSW(ON)
2019 Microchip Technology Inc.
MIC2172/3172
ELECTRICAL CHARACTERISTICS (MIC2172) (CONTINUED)
Electrical Characteristics: VIN = 5V; TA = 25°C, unless otherwise specified. Bold values indicate –40°C ≤ TA ≤
85°C. Note 1, Note 2
Parameter
Current Limit
Breakdown Voltage
Symbol
ICL
VBR
Min.
Typ.
Max.
1.25
—
3
1.25
—
3.5
1
—
2.5
65
75
—
88
100
Units
Conditions
Duty Cycle = 50%, TJ ≥ 25°C
A
Duty Cycle = 50%, TJ < 25°C
Duty Cycle = 80%, Note 3
V
3V ≤ VIN ≤ 40V, ISW = 5 mA
Oscillator
Frequency
fO
Maximum Duty Cycle
Sync Coupling Capacitor for
Frequency Lock
δmax
85
112
115
—
kHz
—
80
89
95
22
51
120
2.2
4.7
10
VIN(MIN)
—
2.7
3.0
V
IQ
—
7
9
mA
3V ≤ VIN ≤ 40V, VCOMP = 0.6V,
ISW = 0A
∆IIN
—
9
20
mA
∆ISW = 1A, VCOMP = 1.5V
CSYNC
%
—
VPP = 3.0V
pF
VPP = 40V
Input Supply Voltage
Minimum Operating Voltage
Quiescent Current
Supply Current Increase
Note 1:
2:
3:
—
Exceeding the absolute maximum rating may damage the device.
Devices are ESD sensitive. Handling precautions recommended. Human body model, 1.5 kΩ in series
with 100 pF.
For duty cycles (δ) between 50% and 95%, minimum guaranteed switch current is given by
ICL = 0.833 (2 - δ) for the MIC2172.
ELECTRICAL CHARACTERISTICS (MIC3172)
Electrical Characteristics: VIN = 5V; TA = 25°C, unless otherwise specified.
Bold values indicate –40°C ≤ TA ≤ 85°C. Note 1, Note 2
Parameter
Symbol
Min.
Typ.
Max.
1.224
1.240
1.264
1.214
—
1.274
—
0.07
—
—
310
750
—
—
1100
3.0
3.9
6.0
2.4
—
7.0
500
800
2000
125
175
350
100
—
400
VCOMP(MAX)
1.8
2.1
2.3
VCOMP(MIN)
0.25
0.35
0.52
0.8
0.9
1.08
0.6
—
1.25
Units
Conditions
Reference (Pin 2 tied to Pin 3)
Feedback Voltage
VFB
Feedback Voltage Line
Regulation
∆VFB(LINE)
Feedback Bias Current
IFB
V
%/V
nA
—
—
3V ≤ VIN ≤ 40V
—
—
Error Amplifier
Transconductance
∆ICOMP/∆VFB
Voltage Gain
∆VCOMP/∆VFB
Output Current
Output Swing
Compensation Pin Threshold
2019 Microchip Technology Inc.
ICOMP
VCOMP_TH
μA/mV
∆ICOMP = ±25 μA
V/V
0.9V ≤ VCOMP ≤ 1.4V
μA
VCOMP = 1.5V
V
V
High Clamp, VFB = 1V
Low Clamp, VFB = 1.5V
Duty Cycle = 0
DS20006208A-page 5
MIC2172/3172
ELECTRICAL CHARACTERISTICS (MIC3172) (CONTINUED)
Electrical Characteristics: VIN = 5V; TA = 25°C, unless otherwise specified.
Bold values indicate –40°C ≤ TA ≤ 85°C. Note 1, Note 2
Parameter
Symbol
Min.
Typ.
Max.
—
0.76
1
—
—
1.1
Units
Conditions
Output Switch
ON Resistance
RSW(ON)
1.25
—
3
Current Limit
ICL
1.25
—
3.5
1
—
2.5
Breakdown Voltage
VBR
65
75
—
88
100
112
Ω
ISW = 1A, VFB = 0.8V
Duty Cycle = 50%, TJ ≥ 25°C
A
Duty Cycle = 50%, TJ < 25°C
Duty Cycle = 80%, Note 3
V
3V ≤ VIN ≤ 40V, ISW = 5 mA
Oscillator
Frequency
fO
Maximum Duty Cycle
kHz
—
85
—
115
δmax
80
89
95
%
—
—
VIN(MIN)
—
2.7
3.0
V
—
IQ
—
7
9
mA
3V ≤ VIN ≤ 40V, VCOMP =
0.6V, ISW = 0
∆ISW = 1A, VCOMP = 1.5V
Input Supply Voltage
Minimum Operating Voltage
Quiescent Current
Supply Current Increase
∆IIN
—
9
20
mA
Enable Input Threshold
VEN_TH
0.4
1.2
2.4
V
–1
0
1
—
2
10
Enable Input Current
Note 1:
2:
3:
IEN
μA
—
VEN = 0V
VEN = 2.4V
Devices are ESD sensitive. Handling precautions required.
Specification for packaged product only.
For duty cycles (δ) between 50% and 95%, minimum guaranteed switch current is given by
ICL = 0.833 (2 - δ) for the MIC3172.
DS20006208A-page 6
2019 Microchip Technology Inc.
MIC2172/3172
TEMPERATURE SPECIFICATIONS (Note 1)
Parameters
Symbol
Min.
Typ.
Max.
Units
Conditions
TA
–40
—
+85
°C
—
Temperature Ranges
Operating Ambient Temperature Range
TJ
–40
—
+125
°C
—
TJ(ABSMAX)
—
—
+150
°C
—
Storage Temperature Range
TS
–65
—
+150
°C
—
Lead Temperature
—
—
—
+300
°C
Soldering, 10 sec.
Thermal Resistance 8-Lead PDIP
JA
—
130
—
Thermal Resistance 8-Lead SOIC
JA
—
120
—
Operating Junction Temperature Range
Maximum Junction Temperature
Package Thermal Resistances
Note 1:
°C/W
—
—
The maximum allowable power dissipation is a function of ambient temperature, the maximum allowable
junction temperature and the thermal resistance from junction to air (i.e., TA, TJ, JA). Exceeding the
maximum allowable power dissipation will cause the device operating junction temperature to exceed the
maximum +125°C rating. Sustained junction temperatures above +125°C can impact the device reliability.
2019 Microchip Technology Inc.
DS20006208A-page 7
MIC2172/3172
2.0
Note:
TYPICAL PERFORMANCE CURVES
The graphs and tables provided following this note are a statistical summary based on a limited number of
samples and are provided for informational purposes only. The performance characteristics listed herein
are not tested or guaranteed. In some graphs or tables, the data presented may be outside the specified
operating range (e.g., outside specified power supply range) and therefore outside the warranted range.
0A
FIGURE 2-1:
Minimum Operating Voltage
(MIC2172) vs. Temperature.
FIGURE 2-4:
Supply Current vs.
Operating Voltage.
FIGURE 2-2:
Temperature.
Feedback Bias Current vs.
FIGURE 2-5:
Supply Current (Shutdown
Mode) vs. Temperature.
FIGURE 2-3:
Regulation.
Feedback Voltage Line
FIGURE 2-6:
vs. Temperature.
DS20006208A-page 8
Enable Threshold Voltage
2019 Microchip Technology Inc.
MIC2172/3172
FIGURE 2-7:
Switch Current.
Average Supply Current vs.
FIGURE 2-10:
Temperature.
Supply Current vs.
FIGURE 2-8:
Switch Current.
Switch ON Voltage vs.
FIGURE 2-11:
Temperature.
Oscillator Frequency vs.
FIGURE 2-9:
Duty Cycle.
Switch Current Limit vs.
FIGURE 2-12:
Oscillator Frequency vs.
Adjusting Resistance.
2019 Microchip Technology Inc.
DS20006208A-page 9
MIC2172/3172
FIGURE 2-13:
Temperature.
Error Amplifier Gain vs.
FIGURE 2-14:
Frequency.
Error Amplifier Gain vs.
FIGURE 2-15:
Frequency.
Error Amplifier Phase vs.
DS20006208A-page 10
2019 Microchip Technology Inc.
MIC2172/3172
3.0
PIN DESCRIPTIONS
The descriptions of the pins are listed in Table 3-1.
TABLE 3-1:
PIN FUNCTION TABLE
Pin Number
Pin Name
Description
1
SGND
Signal Ground: Internal analog circuit ground. Connect directly to the input filter
capacitor for proper operation (see Section 5.0, Applications Information). 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
switching regulator output voltage.
Synchronization/Frequency Adjust: Capacitively coupled input signal greater than
device’s free running frequency (up to 135 kHz) will lock device’s oscillator on falling
edge. Oscillator frequency can be trimmed up to 135 kHz by adding a resistor to
ground. If unused, pin must float (no connection).
4 (MIC2172)
SYNC
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 ( 2"
This junction temperature is below the rated maximum
of 150°C.
FIGURE 5-12:
5.7
The first step in designing a boost converter is
determining whether inductor L1 will cause the
converter to operate in either continuous or
Grounding
Refer to Figure 5-11. Heavy lines indicate high-current
ground paths.
DS20006208A-page 16
5V to 12V Boost Converter.
2019 Microchip Technology Inc.
MIC2172/3172
discontinuous mode. Discontinuous mode is preferred
because the feedback control of the converter is
simpler.
Then:
EQUATION 5-7:
When L1 discharges its current completely during the
MIC2172/3172’s off-time, it is operating in
discontinuous mode.
I OUT
L1 is operating in continuous mode if it does not
discharge completely before the MIC2172/3172 power
switch is turned on again.
5.8.2
DISCONTINUOUS MODE DESIGN
Given the maximum output current, solve Equation 5-6
to determine whether the device can operate in
discontinuous mode without initiating the internal
device current limit.
I OUT 0.227 A
This value is greater than the 0.14A output current
requirement so we can proceed to find the inductance
value of L1.
EQUATION 5-8:
EQUATION 5-6:
I OUT
1.147
------------- 4.75
2
-----------------------------------12
2
I CL
------- V IN
2
------------------------------V OUT
V OUT + V F – V IN
= -------------------------------------------V OUT + V F
V IN
V IN
------------------------- L1 ---------------------------------------I CL f SW
2 P OUT f SW
Where:
POUT =
12 x 0.14 = 1.68W
fSW =
1.105 kHz (100 kHz)
For our practical example:
Where:
ICL =
Internal switch current limit
ICL =
1.25A when δ < 50%
ICL =
0.833 (2 - δ) when δ ≥ 50%
IOUT =
Maximum output current
VIN =
Minimum input voltage
δ=
Duty cycle for boost converter in CRM
VOUT =
Required output voltage
VF =
Diode forward voltage drop
For the example in Figure 5-12:
EQUATION 5-9:
2
4.75 0.623
4.75 0.623 - L1 -----------------------------------------------------------------------------5
5
1.147 1 10
2 1.68 1 10
25.80H L1 41.83H (Use 27 μH)
Equation 5-10 solves for L1’s maximum current value.
EQUATION 5-10:
IOUT = 0.14A
V IN t ON
I L1 PEAK = -----------------------L1
ICL = 1.147A
VIN = 4.75V (minimum)
δ = 0.623
VOUT = 12.0V
Where:
tON =
δ / fSW = 6.23×10-6 sec.
VF = 0.6V
EQUATION 5-11:
–6
4.75 6.23 10
I L1 PEAK = -------------------------------------------- = 1.096 A
–6
27 10
Use a 27 μH inductor with a peak current rating of at
least 1.4A.
2019 Microchip Technology Inc.
DS20006208A-page 17
MIC2172/3172
5.9
Flyback Conversion
5.9.6
DISCONTINUOUS MODE DESIGN
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.
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 5-12 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.
Figure 5-13 shows a practical flyback converter design
using the MIC3172.
EQUATION 5-12:
5.9.1
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
5.9.2
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.
5.9.3
Where:
POUT =
5.0V × 0.25A = 1.25W
VIN =
4.0V to 6.0V
ICL =
1.25A when δ < 50%
Then:
EQUATION 5-13:
2 1.25
------------------1.25 4
0.5 (Use 0.55)
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.
5.9.4
2 P OUT
-------------------------------------I CL V IN MIN
The slightly higher duty cycle value is used to
overcome circuit inefficiencies. A few iterations of
Equation 5-12 may 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.
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.
5.9.5
ENABLE/SHUTDOWN
The MIC3172 includes the enable/shutdown feature.
When the device is shutdown, total supply current is
less than 1 μA. 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.
DS20006208A-page 18
2019 Microchip Technology Inc.
MIC2172/3172
EQUATION 5-14:
Then:
EQUATION 5-17:
V CE F CE – V IN MAX
a ---------------------------------------------------------V SEC
5
a=
Maximum transformer turn ratio
VCE =
Power switch collector to emitter
maximum voltage
FCE =
Safety derating factor (0.8 for most
commercial and industrial applications)
VIN(MAX) =
maximum input voltage
VSEC =
transformer secondary voltage
(VOUT + VF)
–6 2
2
0.5 1 10 4.0 5.5 10
L PRI -------------------------------------------------------------------------------1.25
Where:
L PRI 19.36H
Use an 18 μH 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.
For the practical example:
EQUATION 5-18:
VCE = 65V max. for the MIC2172/3172
FCE = 0.8
2
2
0.5 f SW V SEC t OFF
L SEC ------------------------------------------------------------------P OUT
VSEC = 5.6V
Then:
Where:
EQUATION 5-15:
LSEC =
Maximum secondary inductance
tOFF =
Power switch off time
65 0.8 – 6.0
a -----------------------------5.6
Then:
a 8.2143
EQUATION 5-19:
Next, calculate the maximum primary inductance
required to store the needed output energy with the
power switch duty cycle of 55%.
5
2
–6 2
0.5 1 10 5.6 4.5 10
L SEC -------------------------------------------------------------------------------1.25
L SEC 25.4H
EQUATION 5-16:
2
2
0.5 f SW V IN MIN t ON
L PRI --------------------------------------------------------------------------P OUT
Where:
LPRI =
Maximum primary inductance
fSW =
Device switching frequency (100 kHz)
VIN(MIN) =
Minimum input voltage
tON =
Power switch on time
2019 Microchip Technology Inc.
DS20006208A-page 19
MIC2172/3172
VIN
4V to 6V
R4*
C1
22μF
VSW
EN
MIC3172
R3
1k
D2
1N5818
C3*
C4
470μF
D1*
VIN
Enable
Shutdown
VOUT
5V, 0.25A
T1
R1
3.74k
1%
1:1.11
LPRI = 18μH
COMP GND
FB
P1 P2 S
R2
1.24k
1%
C2
1μF
* Optional voltage clipper (may be req’d if T1 leakage inductance too high)
FIGURE 5-13:
MIC3172 5V 0.25A Flyback Converter.
Finally, recalculate the transformer turns ratio to ensure
that it is less than the value earlier found in
Equation 5-14.
So:
EQUATION 5-23:
EQUATION 5-20:
–6
a =
5.5 10 ------------------------------------I PEAK PRI = 4.0
–6
18 10
L PRI
-----------L SEC
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.
Then:
EQUATION 5-21:
–5
a =
EQUATION 5-24:
1.8 10
--------------------------–5
2.54 10
V IN MAX + V OUT a
V BR -----------------------------------------------------------F BR a
a = 0.89
Use 0.9 (same as 1:1.11)
Where:
This ratio is less than the ratio calculated in
Equation 5-14. When specifying the transformer it is
necessary to know the primary peak current which
must be withstood without saturating the transformer
core.
EQUATION 5-22:
V IN MIN t ON
I PEAK PRI = -------------------------------------L PRI
VBR =
Output rectifier maximum peak
reverse voltage rating
a=
Transformer turns ratio (0.9)
FBR =
Reverse voltage safety derating factor
(0.8)
Then:
EQUATION 5-25:
6.0 + 5.0 0.9
V BR ---------------------------------------0.8 0.9
V BR 14.58V
DS20006208A-page 20
2019 Microchip Technology Inc.
MIC2172/3172
A 1N5817 will safely handle voltage and current
requirements in this example.
5.10
Forward Converters
The 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 5-14 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 Microchip
(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 5-15. 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 ontime, 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 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.
5.11
absent, and the transistors’ emitters are grounded,
circuit operation is described in Section 5.11.1
“Oscillator Operation”
5.11.1
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.
5.11.2
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.
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 5-16 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
2019 Microchip Technology Inc.
DS20006208A-page 21
MIC2172/3172
5.11.3
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 1 μA while shutdown.
5.11.4
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.
5.12
Output Voltage Setting
The MIC2172/3172 requires a resistor divider
connected from the output to ground with the middle
point connected to the FB pin to set the desired output
voltage. The output voltage is set by Equation 5-26.
EQUATION 5-26:
R1
V OUT = V REF ------- + 1
R2
Where:
VREF =
1.24V internal reference voltage
R1 =
Upper feedback resistor
R2 =
Lower feedback resistor
A typical value of R1 can be in the range of 3 kΩ to
15 kΩ. If R1 is too large, it may allow noise to be
introduced into the voltage feedback loop. If R1 is too
small in value, it will decrease the efficiency of the
switching regulator, especially at light loads. Once R1
is selected, R2 can be calculated using Equation 5-27.
EQUATION 5-27:
V REF R1
R2 = ---------------------------------V OUT – V REF
DS20006208A-page 22
2019 Microchip Technology Inc.
MIC2172/3172
VIN
D1
1N4148
VIN
VSW
EN
C2
2.2μF
C1*
100μF
MIC3172
R3
470
D3
1N4148
R3†
COMP GND
FB
P1 P2 S
3.7k
R2†
1.2k
C3
1μF
R4
10
C4
1μF
L1
100μH
C5
330μF
D2
5V, 0.1A to 1A
(ILOAD > 100mA)
* Locate near MIC2172/3172 when supply leads > 2"
† R3/R2 sets output voltage
FIGURE 5-14:
Step-Down or Buck Regulator.
T1
1:1:1
D3
1N5819
L1 100μH
VOUT
5V, 1A
R4
C5
3.74k
470μF 1%
VIN
12V
R1*
D1*
VIN
Enable
Shutdown
EN
C1
22μF
D4
1N5819
C2*
VSW
MIC3172
GND
P1 P2 S
FB
COMP
D2
1N5819
R2
1k
C3
1μF
Q1†
R5
1.24k
1%
R3 †
C4 †
* Voltage clipper
† Duty cycle limiter
FIGURE 5-15:
12V to 5V Forward Converter.
Cold Cathode
Fluorescent
Lamp
FB
T1
EN
GND
P1 P2 S
Q2
FB
COMP
C1
FIGURE 5-16:
C3
300μH
R1
D2
1N4148
D3
1N4148
L1
VSW
MIC3172
C2
Sec
D1
VIN
Pri 1
Q1
Pri 2
Enable (On)
Shutdown (Off)
C4
R2
VIN
4.5V to 20V
R3
C5
L1:
T1:
C2:
C4:
R4
R5
Intensity
Control
Coiltronics CTX300-4P
Coiltronics CTX110602
Polyfilm, WIMA FKP2 0.1μF to 0.68μF
15pF to 30pF, 3kV min.
LCD Backlight Fluorescent Lamp Supply.
2019 Microchip Technology Inc.
DS20006208A-page 23
MIC2172/3172
6.0
PACKAGING INFORMATION
6.1
Package Marking Information
8-Lead SOIC*
Example
XXX
XXXXXX
WNNN
MIC
2172YM
1947
8-Lead PDIP*
Example
XXX
XXXXXX
WNNN
MIC
3172YN
2000
Legend: XX...X
Y
YY
WW
NNN
e3
*
Product code or customer-specific information
Year code (last digit of calendar year)
Year code (last 2 digits of calendar year)
Week code (week of January 1 is week ‘01’)
Alphanumeric traceability code
Pb-free JEDEC® designator for Matte Tin (Sn)
This package is Pb-free. The Pb-free JEDEC designator ( e3 )
can be found on the outer packaging for this package.
●, ▲, ▼ Pin one index is identified by a dot, delta up, or delta down (triangle
mark).
Note:
In the event the full Microchip part number cannot be marked on one line, it will
be carried over to the next line, thus limiting the number of available
characters for customer-specific information. Package may or may not include
the corporate logo.
Underbar (_) and/or Overbar (‾) symbol may not be to scale.
DS20006208A-page 24
2019 Microchip Technology Inc.
MIC2172/3172
8-Lead PDIP Package Outline and Recommended Land Pattern
Note:
For the most current package drawings, please see the Microchip Packaging Specification located at
http://www.microchip.com/packaging.
2019 Microchip Technology Inc.
DS20006208A-page 25
MIC2172/3172
8-Lead SOIC Package Outline and Recommended Land Pattern
Note:
For the most current package drawings, please see the Microchip Packaging Specification located at
http://www.microchip.com/packaging.
DS20006208A-page 26
2019 Microchip Technology Inc.
MIC2172/3172
APPENDIX A:
REVISION HISTORY
Revision A (July 2019)
• Converted Micrel document MIC2172/3172 to
Microchip data sheet DS20006208A.
• Minor text changes throughout.
• Updated Section 1.0 “Electrical Characteristics” tables for both MIC2172/3172.
• Updated Equation 5-1 through Equation 5-25 and
also added a new Section 5.12 “Output Voltage
Setting” in the Section 5.0 “Applications Information”.
2018 Microchip Technology Inc.
DS20006208A-page 27
MIC2172/3172
NOTES:
DS20006208A-page 28
2018 Microchip Technology Inc.
MIC2172/3172
PRODUCT IDENTIFICATION SYSTEM
To order or obtain information, e.g., on pricing or delivery, contact your local Microchip representative or sales office.
PART NO.
X
XX
–XX
Device
Junction
Temperature
Range
Package
Option
Media Type
Device:
MIC2172: 100 kHz, 1.25A Switching Regulator
(External frequency synchronization or
frequency adjustment)
MIC3172: 100 kHz, 1.25A Switching Regulator
(Enable/shutdown control input)
Junction
Temperature Range:
Y
=
–40°C to +125°C
Package:
M
N
=
=
8-Lead SOIC (Pb-Free)
8-Lead PDIP (Pb-Free)
Media Type:
Blank
Blank
TR
= 95/Tube (SOIC Package)
= 50/Tube (PDIP Package)
= 2,500/Reel
2019 Microchip Technology Inc.
Examples:
a) MIC2172YM:
100 kHz, 1.25A Switching Regulator,
–40°C to +125°C Junction
Temperature Range, Pb-Free, 8Lead SOIC Package, 95/Tube
b) MIC3172YN:
100 kHz, 1.25A Switching Regulator,
–40°C to +125°C Junction
Temperature Range, Pb-Free, 8Lead PDIP Package, 50/Tube
c) MIC2172YM-TR:
100 kHz, 1.25A Switching Regulator,
–40°C to +125°C Junction
Temperature Range, Pb-Free, 8Lead SOIC Package, 2,500/Reel
d) MIC3172YM-TR:
100 kHz, 1.25A Switching Regulator,
–40°C to +125°C Junction
Temperature Range, Pb-Free, 8Lead SOIC Package, 2,500/Reel
Note 1:
Tape and Reel identifier only appears in the
catalog part number description. This identifier is
used for ordering purposes and is not printed on
the device package. Check with your Microchip
Sales Office for package availability with the
Tape and Reel option.
DS20006208A-page 29
MIC2172/3172
NOTES:
DS20006208A-page 30
2019 Microchip Technology Inc.
Note the following details of the code protection feature on Microchip devices:
•
Microchip products meet the specification contained in their particular Microchip Data Sheet.
•
Microchip believes that its family of products is one of the most secure families of its kind on the market today, when used in the
intended manner and under normal conditions.
•
There are dishonest and possibly illegal methods used to breach the code protection feature. All of these methods, to our
knowledge, require using the Microchip products in a manner outside the operating specifications contained in Microchip’s Data
Sheets. Most likely, the person doing so is engaged in theft of intellectual property.
•
Microchip is willing to work with the customer who is concerned about the integrity of their code.
•
Neither Microchip nor any other semiconductor manufacturer can guarantee the security of their code. Code protection does not
mean that we are guaranteeing the product as “unbreakable.”
Code protection is constantly evolving. We at Microchip are committed to continuously improving the code protection features of our
products. Attempts to break Microchip’s code protection feature may be a violation of the Digital Millennium Copyright Act. If such acts
allow unauthorized access to your software or other copyrighted work, you may have a right to sue for relief under that Act.
Information contained in this publication regarding device
applications and the like is provided only for your convenience
and may be superseded by updates. It is your responsibility to
ensure that your application meets with your specifications.
MICROCHIP MAKES NO REPRESENTATIONS OR
WARRANTIES OF ANY KIND WHETHER EXPRESS OR
IMPLIED, WRITTEN OR ORAL, STATUTORY OR
OTHERWISE, RELATED TO THE INFORMATION,
INCLUDING BUT NOT LIMITED TO ITS CONDITION,
QUALITY, PERFORMANCE, MERCHANTABILITY OR
FITNESS FOR PURPOSE. Microchip disclaims all liability
arising from this information and its use. Use of Microchip
devices in life support and/or safety applications is entirely at
the buyer’s risk, and the buyer agrees to defend, indemnify and
hold harmless Microchip from any and all damages, claims,
suits, or expenses resulting from such use. No licenses are
conveyed, implicitly or otherwise, under any Microchip
intellectual property rights unless otherwise stated.
Trademarks
The Microchip name and logo, the Microchip logo, Adaptec,
AnyRate, AVR, AVR logo, AVR Freaks, BesTime, BitCloud, chipKIT,
chipKIT logo, CryptoMemory, CryptoRF, dsPIC, FlashFlex,
flexPWR, HELDO, IGLOO, JukeBlox, KeeLoq, Kleer, LANCheck,
LinkMD, maXStylus, maXTouch, MediaLB, megaAVR, Microsemi,
Microsemi logo, MOST, MOST logo, MPLAB, OptoLyzer,
PackeTime, PIC, picoPower, PICSTART, PIC32 logo, PolarFire,
Prochip Designer, QTouch, SAM-BA, SenGenuity, SpyNIC, SST,
SST Logo, SuperFlash, Symmetricom, SyncServer, Tachyon,
TempTrackr, TimeSource, tinyAVR, UNI/O, Vectron, and XMEGA
are registered trademarks of Microchip Technology Incorporated in
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APT, ClockWorks, The Embedded Control Solutions Company,
EtherSynch, FlashTec, Hyper Speed Control, HyperLight Load,
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Edge, ProASIC, ProASIC Plus, ProASIC Plus logo, Quiet-Wire,
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TimePictra, TimeProvider, Vite, WinPath, and ZL are registered
trademarks of Microchip Technology Incorporated in the U.S.A.
Adjacent Key Suppression, AKS, Analog-for-the-Digital Age, Any
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ZENA are trademarks of Microchip Technology Incorporated in the
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SQTP is a service mark of Microchip Technology Incorporated in
the U.S.A.
The Adaptec logo, Frequency on Demand, Silicon Storage
Technology, and Symmcom are registered trademarks of Microchip
Technology Inc. in other countries.
GestIC is a registered trademark of Microchip Technology Germany
II GmbH & Co. KG, a subsidiary of Microchip Technology Inc., in
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All other trademarks mentioned herein are property of their
respective companies.
© 2019, Microchip Technology Incorporated, All Rights Reserved.
For information regarding Microchip’s Quality Management Systems,
please visit www.microchip.com/quality.
2019 Microchip Technology Inc.
ISBN: 978-1-5224-4723-8
DS20006208A-page 31
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