MIC2171
100 kHz, 2.5A Switching Regulator
Features
General Description
•
•
•
•
•
•
•
•
•
The MIC2171 is a complete 100 kHz SMPS
current-mode controller with an internal 65V 2.5A
power switch.
2.5A, 65V Internal Switch Rating
3V to 40V Input Voltage Range
Current Mode Operation, 2.5A Peak
Internal Cycle-by-Cycle Current Limit
Twice the Frequency of the LM2577
Low External Electronic Components Count
Suitable for Most Switching Topologies
7 mA Quiescent Current (Operating)
Fits LT1171/LM2577 TO-220 and TO-263 Sockets
Applications
•
•
•
•
Laptop/Palmtop Computers
Battery Operated Equipment
Handheld Instruments
Off-Line Converter up to 50W (Requires External
Power Switch)
• Predriver for Higher Power Capability
Although primarily intended for voltage step-up
applications, the floating switch architecture of the
MIC2171 makes it practical for step-down, inverting,
and Cuk configurations, as well as isolated topologies.
Operating from 3V to 40V, the MIC2171 draws only
7 mA of quiescent current, making it attractive for
battery-operated applications.
The MIC2171 is available in a 5-pin TO-220 or TO-263
package that allows –40°C to +85°C ambient
temperature operation.
Package Types
5-Pin TO-220 (T)
5-Pin TO-263 (U)
5 IN
4 SW
3 GND
2 FB
1 COMP
Tab GND
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5 IN
4 SW
3 GND
2 FB
1 COMP
Tab GND
DS20006355A-page 1
MIC2171
Typical Application Circuits
5V to 12V Boost Converter
+5V
(4.75V min.)
C1*
47μF
L1
15μH
D1
VOUT
+12V, 0.25A
1N5822
R1
10.7Nȍ
1%
IN
SW
MIC2171
COMP
R3
1Nȍ
FB
R2
C2
1.24Nȍ
470μF 1%
GND
C3
1μF
* Locate near MIC2171 when supply leads > 2”
5V Flyback Converter
VIN
4V to 6V
VOUT
5V, 0.5A
T1
R4*
C1
47μF
D1*
IN
SW
COMP
D2
1N5818
R1
C4 3.74Nȍ
470μF 1%
1.8:1
LPRI = 12μH
MIC2171
R3
1Nȍ
C3*
FB
R2
1.24Nȍ
1%
GND
C2
1μF
* Optional voltage clipper (may be req’d if T1 leakage inductance too high)
Functional Block Diagram
IN
Reg.
D1
2.3V
SW
Anti-Sat.
100kHz
Osc.
Logic
Q1
Driver
Comparator
FB
Current
Amp.
Error
1.24V Amp.
Ref.
COMP
DS20006355A-page 2
GND
2022 Microchip Technology Inc. and its subsidiaries
MIC2171
1.0
ELECTRICAL CHARACTERISTICS
Absolute Maximum Ratings †
Supply Voltage (VIN) ...................................................................................................................................................40V
Switch Voltage (VSW) ..................................................................................................................................................65V
Feedback Voltage (VFB) (transient, 1 ms).................................................................................................................±15V
Storage Temperature (TS)...................................................................................................................... –65°C to +150°C
Lead Temperature (soldering 10 sec.) .................................................................................................................... 300°C
Operating Ratings ‡
Operating Ambient Temperature Range (TA) ........................................................................................... –40°C to +85°C
Thermal Resistance
TO-220-5 (JA) ......................................................................................................................................................45°C/W
TO-263-5 (JA) ......................................................................................................................................................45°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
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
—
0.6
—
—
310
750
—
—
1100
3.0
3.9
6.0
2.4
—
7.0
400
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
—
0.37
0.5
—
—
0.55
Units
Conditions
Reference
Feedback Voltage
VFB
Feedback Voltage Line
Regulation
∆VFB(LINE)
Feedback Bias Current
IFB
V
%/V
nA
VCOMP = 1.24V
3V ≤ VIN ≤ 40V, VCOMP = 1.24V
VFB = 1.24V
Error Amplifier
Transconductance
gm
Voltage Gain
AV
Output Current
Output Swing
Compensation Pin Threshold
ICOMP
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 = 2A, VFB = 0.8V
A
Duty Cycle = 50%, TJ < 25°C
Output Switch
ON Resistance
Current Limit
RSW(ON)
ICLIM
2022 Microchip Technology Inc. and its subsidiaries
2.5
3.6
5.0
2.5
4.0
5.5
2.5
3.0
5.0
Duty Cycle = 50%, TJ ≥ 25°C
Duty Cycle = 80%, Note 3
DS20006355A-page 3
MIC2171
ELECTRICAL CHARACTERISTICS (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.
Units
VBR
65
75
—
V
Breakdown Voltage
Oscillator
88
100
112
—
115
δmax
80
90
95
%
—
VIN(MIN)
—
2.7
3.0
V
—
IQ
—
7
9
mA
3V ≤ VIN ≤ 40V, VCOMP = 0.6V,
ISW = 0A
∆IIN
—
9
20
mA
∆ISW = 2A, VCOMP = 1.5V, during
tON
fO
Maximum Duty Cycle
Input Supply Voltage
Quiescent Current
Supply Current Increase
Note 1:
2:
3:
3V ≤ VIN ≤ 40V, ISW = 5 mA
85
Frequency
Minimum Operating Voltage
Conditions
—
kHz
—
Exceeding the absolute maximum rating may damage the device.
Devices are ESD sensitive. Handling precautions recommended.
For duty cycles (δ) between 50% and 95%, minimum guaranteed switch current is given by
ICLIM = 1.66 (2 – δ) Amp.
TEMPERATURE SPECIFICATIONS (Note 1)
Parameters
Symbol
Min.
Typ.
Max.
Units
Conditions
Operating Ambient Temperature
TA
–40
—
+85
°C
—
Operating Junction Temperature
TJ
–40
—
+125
°C
—
Maximum Junction Temperature
TJ(MAX)
—
—
+150
°C
—
TS
–65
—
+150
°C
—
TLEAD
—
—
+300
°C
Soldering, 10 sec.
Thermal Resistance 5-Pin TO-220-5
JA
—
45
—
Thermal Resistance 5-Pin TO-263-5
JA
—
45
—
Temperature Ranges
Storage Temperature
Lead Temperature
Package Thermal Resistances
Note 1:
2:
3:
°C/W
Note 2
Note 3
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.
Mounted vertically, no external heat sink, 1/4 inch leads soldered to PC board containing approximately 4
inch squared copper area surrounding leads.
All ground leads soldered to approximately 2 inches squared of horizontal PC board copper area.
DS20006355A-page 4
2022 Microchip Technology Inc. and its subsidiaries
MIC2171
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.
15
2.7
2.6
Switch Current = 2A
2.5
2.4
2.3
-100
FIGURE 2-1:
vs. Temperature.
-50
05
0
100
Temperature (°C)
13
12
D.C. = 90%
11
10
9
D.C. = 50%
Minimum Operating Voltage
Average Supply Current (mA)
500
400
300
200
100
5
4
3
-50
05
0
100
Temperature (°C)
Feedback Bias Current vs.
40
30
G = 90%
20
G = 50%
10
0
1
2
3
Switch Current (A)
FIGURE 2-5:
Current.
4
Supply Current vs. Switch
10
9
TJ = 125°C
1
0
T = 25°C
J
-1
-2
-3
T = -40°C
J
-4
FIGURE 2-3:
Regulation.
10
20
30
VIN Operating Voltage (V)
40
0
150
2
-5
0
FIGURE 2-4:
Supply Current vs.
Operating Voltage.
Supply Current (mA)
Feedback Bias Current (nA)
600
FIGURE 2-2:
Temperature.
D.C. = 0%
7
6
50
700
0
-100
8
5
150
800
Feedback Voltage Change (mV)
ISW = 0A
14
2.8
Supply Current (mA)
Minimum Operating Voltage (V)
2.9
0
10
20
30
VIN Operating (V)
VCO MP = 0.6V
8
7
6
5
4
3
2
1
40
Feedback Voltage Line
2022 Microchip Technology Inc. and its subsidiaries
0
-100
FIGURE 2-6:
Temperature.
-50
05
0
100
Temperature(°C)
150
Supply Current vs.
DS20006355A-page 5
MIC2171
1.6
5.0
Transconductance (μA/mV)
Switch ON Voltage (V)
1.4
T = 25°C
J
1.2
1.0
TJ = –40°C
0.8
0.6
TJ = 125°C
0.4
0.2
0
0
1
2
Switch Current (A)
Switch ON Voltage vs.
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
-100
FIGURE 2-10:
Temperature.
120
7000
110
6000
Transconductance (μS)
Frequency (kHz)
FIGURE 2-7:
Switch Current.
3
4.5
4.0
100
90
80
70
60
-50
05
0
100
Temperature(°C)
FIGURE 2-8:
Temperature.
150
Error Amplifier Gain vs.
5000
4000
3000
2000
1000
0
150
Oscillator Frequency vs.
-50
05
0
100
Temperature(°C)
11
FIGURE 2-11:
Frequency.
8
-30
6
30
0
100
1000
Frequency (kHz)
10000
Error Amplifier Gain vs.
25°C
–40°C
4
Phase Shift (°)
Switch Current (A)
0
125°C
2
60
90
120
150
180
0
210
02
04
FIGURE 2-9:
DS20006355A-page 6
06
08
0
Duty Cycle (%)
100
Current Limit vs. Duty Cycle.
11
FIGURE 2-12:
Frequency.
0
100
1000
Frequency (kHz)
10000
Error Amplifier Phase vs.
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MIC2171
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
1
COMP
2
FB
3
GND
4
SW
5
IN
Description
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.
Feedback: Inverting input of error amplifier. Connect to external resistive divider to set
switching regulator output voltage.
Ground: Connect directly to the input filter capacitor for proper operation (see
applications info).
Power Switch Collector: Collector of NPN switch. Connect to external inductor or input
voltage depending on circuit topology.
Supply Voltage: 3.0V to 40V
2022 Microchip Technology Inc. and its subsidiaries
DS20006355A-page 7
MIC2171
4.0
FUNCTIONAL DESCRIPTION
Refer to Functional Block Diagram section.
4.1
Internal Power
The MIC2171 operates when VIN is ≥ 2.6V. An internal
2.3V regulator supplies biasing to all internal circuitry,
including a precision 1.24V band gap reference.
4.2
PWM Operation
The 100 kHz 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
the output transistor Q1.
4.3
4.4
Anti-Saturation
The anti-saturation diode (D1) increases the usable
duty cycle range of the MIC2171 by eliminating the
base to collector stored charge which would delay Q1’s
turnoff.
4.5
Compensation
Loop stability compensation of the MIC21712 can be
accomplished by connecting an appropriate RC
network from either COMP to circuit ground (Typical
Application Circuits) or from 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 MIC2171 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.
DS20006355A-page 8
2022 Microchip Technology Inc. and its subsidiaries
MIC2171
5.0
APPLICATIONS INFORMATION
5.1
Soft-Start
A diode coupled capacitor from COMP to circuit ground
slows the output voltage rise at turn on (Figure 5-1).
VIN
IN
MIC2171
COMP
D1
R1
FIGURE 5-1:
C2
I IN
P BIAS + DRIVER = V IN I Q + V IN V CLIM -------------
I
SW
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.
5.2
Current Limit
VIN
SW
MIC2171
Q1
R1
C1
VOU T
FB
COMP
R3
IC LIM 0.6V/R2
and output
C2 Note: Input
returns not common
R2
FIGURE 5-2:
Current Limit.
The maximum current limit of the MIC2171 can be
reduced by adding a voltage clamp to the COMP output
(Figure 5-2). 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 lower than approximately 5 mA.
5.3
Where:
P(BIAS + DRIVER) =
Device operating losses
VIN =
Supply Voltage
IQ =
Quiescent supply current
ICLIM =
Power switch current limit
ΔIIN =
Maximum supply current increase
ΔISW =
Switch current increase
As a practical example, refer to Typical Application
Circuits
IN
GND
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 based on the supply voltage (VIN)
and device supply current (IQ). The MIC2171 supply
current is almost constant regardless of the supply
voltage (see Section 1.0, Electrical Characteristics).
The driver section losses (not including the switch) are
a function of supply voltage, power switch current, and
duty cycle.
EQUATION 5-1:
D2
C1
Firstly, the junction temperature is determined by
calculating the power dissipation of the device. For the
MIC2171, the total power dissipation is the sum of the
device operating losses and power switch losses.
Thermal Management
VIN =
5V
IQ =
0.007A
ICLIM =
2.21A
δ=
66.2% (0.662)
Then,
P(BIAS + DRIVER) =
5 x 0.007 + (5 x 2.21 x 0.02 x
0.662)/2
P(BIAS + DRIVER) =
0.108W
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-time conduction 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
Section 2.0, Typical Performance Curves). Power
switch losses are calculated by modeling the switch as
a resistor with the switch duty cycle modifying the
average power dissipation.
For the best reliability, MIC2171 should avoid
prolonged operation with junction temperatures near
the rated maximum.
2022 Microchip Technology Inc. and its subsidiaries
DS20006355A-page 9
MIC2171
5.4
EQUATION 5-2:
2
P SW = I SW R SW
Where:
δ=
Grounding
Refer to Figure 5-3. Heavy lines indicate high-current
ground paths.
VIN
Duty cycle
IN
SW
MIC2171
For boost converter,
V OUT + V F – V IN MIN
= ---------------------------------------------------------V OUT + V F
GND
FB
VC
Where:
VIN(MIN) =
VIN – VSW
VSW =
ICLIM x RSW
VOUT =
Output voltage
VF =
D1 forward voltage drop at IOUT
From the Typical performance Characteristics:
Single point ground
FIGURE 5-3:
Single Point Ground.
A single point ground is strongly recommended for
proper operation.
P(TOTAL) = 1.3W
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 tracks 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 MIC2171 or any switching
regulator from functioning properly.
The junction temperature for any semiconductor is
calculated using the following:
5.5
EQUATION 5-3:
Refer to the Typical Application Circuits for a typical
boost conversion application where a +5V logic supply
is available and a +12V at 0.25A output is required.
RSW = 0.37Ω
Then:
PSW = (2.21)2 × 0.37 × 0.662
PSW = 1.2W
P(TOTAL) = 1.2 + 0.1
Where:
T J = T A + P TOTAL JA
TJ =
Junction temperature
TA =
Ambient temperature (maximum)
P(TOTAL) =
θJA =
Total power dissipation
Junction to ambient thermal resistance
For the practical example:
TA = 70°C
θJA = 45°C/W (for TO-220)
Then:
TJ = 70 + (1.3 x 45)
TJ = 128.5°C
This junction temperature is below the rated maximum
of 150°C.
DS20006355A-page 10
Boost Conversion
The first step in designing a boost converter is
determining whether inductor L1 will cause the
converter to operate in either continuous or
discontinuous conduction mode. Discontinuous
conduction mode is preferred because the feedback
control of the converter is simpler.
When L1 discharges its current completely during the
MIC2171 off-time, it is operating in discontinuous
conduction mode.
L1 is operating in continuous conduction mode if it does
not discharge completely before the MIC2171 power
switch is turned on again.
5.5.1
DISCONTINUOUS CONDUCTION
MODE DESIGN
Given the maximum output current, solve Equation 5-4
to determine whether the device can operate in
discontinuous conduction mode without triggering the
internal device current limit.
2022 Microchip Technology Inc. and its subsidiaries
MIC2171
EQUATION 5-4:
EQUATION 5-7:
I CLIM
-------------- V
IN MIN
2
I OUT ---------------------------------------------------V OUT
2
4.178 0.662
4.178 0.662
------------------------------------- L1 ----------------------------------------5
5
2.235 1 10
2 3.0 1 10
V OUT + V F – V IN MIN
= ---------------------------------------------------------V OUT + V F
12.38H L1 19.26H
Equation 5-8 solves for L1’s maximum current value.
Where:
ICLIM =
Internal switch current limit
ICLIM =
1.25A when δ < 50%
ICLIM =
1.67 (2 – δ) when δ ≥ 50%
IOUT =
Maximum output current
VIN(MIN) =
Minimum input voltage = VIN – VSW
δ=
Duty cycle for boost converter in CRM
VOUT =
Required output voltage
VF =
Diode forward voltage drop
For the example in the Typical Application Circuits:
EQUATION 5-8:
V IN t ON
I L1 PEAK = -----------------------L1
Where:
tON =
–6
4.178 6.62 10
I L1 PEAK = ------------------------------------------------ = 1.096A
–6
15 10
ICL = 1.67 (2 – 0.662) = 2.24A
VIN(MIN) = 4.18V
δ = 0.662
I L1 PEAK = 1.84A
VOUT = 12.0V
VF = 0.36V (@ 0.26A, 70°C)
Use a 15 µH inductor with a peak current rating greater
than 2A.
Then:
EQUATION 5-5:
5.6
2.235
------------- 4.178
2
I OUT --------------------------------------12
I OUT 0.389A
This value is greater than the 0.25A output current
requirement, so one can proceed to find the inductance
value of L1 for discontinuous operation at POUT.
2
V IN
V IN
------------------------------ L1 --------------------------------------I CLIM f SW
2 P OUT f SW
Where:
12 x 0.25 = 3W
1.105 Hz (100 kHz)
For our practical example:
2022 Microchip Technology Inc. and its subsidiaries
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’s primary winding) current
can be continuous or discontinuous. In this particular
case, discontinuous operation is recommended.
The Typical Application Circuits shows a practical
flyback converter design using the MIC2171.
5.6.1
EQUATION 5-6:
fSW =
δ / fSW = 6.62 × 10-6 sec.
EQUATION 5-9:
IOUT = 0.25A
POUT =
(Use 15 µH)
SWITCH OPERATION
During Q1’s on time (Q1 is the internal NPN transistor,
see the Functional Block Diagram, 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 voltage ripple
making additional filter stages unnecessary.
C1’s value (input capacitor) may be reduced or it can
be eliminated if the MIC2171 is located near a low
impedance voltage source
DS20006355A-page 11
MIC2171
5.6.2
OUTPUT DIODE
The output diode allows T1 to store energy in its
primary inductance (D2 blocked/reverse biased) and
release energy into C4 (D2 forward biased); the low
forward voltage drop of a Schottky diode minimizes
power loss in D2.
5.6.3
The next step is to calculate the maximum transformer
turns ratio a, or NPRI/NSEC, that will guarantee a safe
operation of the MIC2171’s power switch.
EQUATION 5-11:
V CE F CE – V IN MAX
a -----------------------------------------------------------V SEC
FREQUENCY COMPENSATION
A simple frequency compensation network consisting
of R3 and C2 prevents output oscillations.
Where:
High impedance output stages (transconductance
type) in the MIC2171 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.
a=
Maximum transformer turn ratio
VCE =
Power switch collector to emitter
maximum voltage
FCE =
5.6.4
Safety derating factor (0.8 for most
commercial and industrial applications)
VIN(MAX) =
Maximum input voltage
VSEC =
Transformer secondary voltage
(VOUT + VF)
VOLTAGE CLIPPER
Extra care must be taken to minimize T1’s leakage
inductance, otherwise it may be necessary to add the
voltage clipper consisting of D1, R4, and C3 in order to
avoid second breakdown (failure) of the MIC2171’s
internal power switch.
5.6.5
DISCONTINUOUS CONDUCTION
MODE DESIGN
When designing a discontinuous conduction mode
flyback converter, first determine whether the device
can safely handle the peak primary current demand
drawn by the output power. Equation 5-10 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.
For the practical example:
VCE = 65V max. for the MIC2171
FCE = 0.8
VSEC = 5.6V
Then:
EQUATION 5-12:
65 0.8 – 6.0
a ------------------------------5.6
EQUATION 5-10:
2 P OUT
--------------------------------------------------------------I CLIM V IN MIN – V SW
Where:
POUT =
5.0V × 0.5A = 2.5W
VIN =
4.0V to 6.0V
ICLIM =
1.25A when δ < 50%
1.67 (2 – δ) when δ ≥ 50%
Then:
VIN(MIN) = VIN – (ICLIM × RSW)
VIN(MIN) = 4V – 0.78V
VIN(MIN) = 3.22V
δ ≥ 0.74 (76%), less than 0.8, so discontinuous is
permitted. A few iterations of Equation 5-10 may be
required if the duty cycle is found to be greater than
50% since the ICLIM is a function of duty cycle when δ
> 50%.
DS20006355A-page 12
a 8.2
(NPRI/NSEC)
Next, calculate the maximum primary inductance
required to store the needed output energy with the
power switch duty cycle of 76%.
EQUATION 5-13:
2
2
V IN MIN
0.5 f SW V IN MIN t ON
-------------------------------- L PRI --------------------------------------------------------------------------I CLIM f SW
P OUT
Where:
LPRI =
Maximum primary inductance
fSW =
Device switching frequency (100 kHz)
VIN(MIN) =
Minimum input voltage
tON =
Power switch on time
Then:
2022 Microchip Technology Inc. and its subsidiaries
MIC2171
EQUATION 5-14:
EQUATION 5-18:
5
-6 2
2
0.5 1 10 3.22 7.6 10
3.22 0.76
------------------------------- L PRI -----------------------------------------------------------------------------------------5
2.5
2.1 1 10
12
a ------- = 1.83
3.6
11.65H L PRI 12H
Use a 12 µH primary inductance to overcome circuit
inefficiencies.
To complete the design, the inductance value of the
secondary winding must be calculated, so it will ensure
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.
This ratio is less than the ratio calculated in
Equation 5-11. When selecting the transformer, it is
necessary to know the primary peak current which
must be handled without saturating the transformer
core.
EQUATION 5-19:
V IN MIN t ON
I PEAK PRI = --------------------------------------L PRI
EQUATION 5-15:
2
Where:
2
0.5 f SW V SEC t OFF
L SEC -------------------------------------------------------------------P OUT
LSEC =
Maximum secondary inductance
tOFF =
Power switch off time
So:
EQUATION 5-20:
–6
3.22 7.6 10
I PEAK PRI = -----------------------------------------–6
12 10
Then:
I PEAK PRI = 2.04A
EQUATION 5-16:
5
2
–6 2
0.5 1 10 5.6 2.4 10
L SEC ---------------------------------------------------------------------------------2.5
L SEC 3.6H
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 0.5A.
EQUATION 5-21:
Finally, recalculate the transformer turns ratio to ensure
that it is less than the value found earlier by using
Equation 5-11.
V IN MAX + V OUT a
V BR ------------------------------------------------------------F BR a
Where:
EQUATION 5-17:
L PRI
a -----------L SEC
Then:
2022 Microchip Technology Inc. and its subsidiaries
VBR =
Output rectifier maximum peak
reverse voltage rating
a=
Transformer turns ratio (1.8)
FBR =
Reverse voltage safety derating factor
(0.8)
Then:
DS20006355A-page 13
MIC2171
EQUATION 5-22:
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 depicted in Figure 5-4, the transformer
core is reset by the tertiary winding, discharging T1’s
peak magnetizing current through D2.
6.0 + 5.0 1.8
V BR ---------------------------------------0.8 1.8
V BR 10.4V
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 MIC2171 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.
A 1N5817 dioded will safely handle voltage and current
requirements provided in this example.
5.7
Forward Converters
The MIC2171 can be used in several circuit
configurations to generate an output voltage which is
lower than the input voltage (buck or step-down
topology). Figure 5-4 shows the MIC2171 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 MIC2171 is a
good choice.
To prevent core saturation, the application presented in
this section uses a duty cycle limiter consisting of Q1,
C4 and R3. Whenever the MIC2171 exceeds a duty
cycle of 50%, T1’s reset winding current turns Q1 on;
this action reduces the duty cycle of the MIC2171 until
T1 is able to reset during each cycle.
A 12V to 5V step-down converter using transformer
isolation (forward) is shown in Figure 5-4. Unlike the
isolated flyback converter which stores energy in the
primary inductance during the controller’s on-time and
T1
1:1:1
D3
1N5819
VIN
12V
R1*
C2*
L1 100μH
D4
1N5819
VOU T
5V, 1A
R4
C5
3.74k
470μF 1%
D1*
IN
SW
C1
22μF
MIC2171
GND
FB
COMP
R2
1k
C3
1μF
D2
1N5819
Q1†
R 3†
R5
1.24k
1%
C4†
* Voltage clipper
† Duty cycle limiter
FIGURE 5-4:
DS20006355A-page 14
12V to 5V Forward Converter.
2022 Microchip Technology Inc. and its subsidiaries
MIC2171
5.8
Output Voltage Setting
The MC2171 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 5-23.
EQUATION 5-23:
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 3kΩ to
15kΩ. 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 5-24.
EQUATION 5-24:
V REF R1
R2 = ---------------------------------V OUT – V REF
2022 Microchip Technology Inc. and its subsidiaries
DS20006355A-page 15
MIC2171
6.0
PACKAGING INFORMATION
6.1
Package Marking Information
5-Lead TO-220*
Example
XXX
XXXXXX
WNNNP
MIC
2171WT
1947P
5-Lead TO-263*
Example
XXX
XXXXXX
WNNNP
MIC
2171WU
2000P
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.
DS20006355A-page 16
2022 Microchip Technology Inc. and its subsidiaries
MIC2171
5-Lead TO-220 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.
2022 Microchip Technology Inc. and its subsidiaries
DS20006355A-page 17
MIC2171
5-Lead TO-263 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.
DS20006355A-page 18
2022 Microchip Technology Inc. and its subsidiaries
MIC2171
APPENDIX A:
REVISION HISTORY
Revision A (May 2022)
• Converted Micrel document MIC2171 to Microchip data sheet DS20006355A.
• Minor text changes throughout.
2022 Microchip Technology Inc. and its subsidiaries
DS20006355A-page 19
MIC2171
NOTES:
DS20006355A-page 20
2022 Microchip Technology Inc. and its subsidiaries
MIC2171
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
Ambient
Temperature
Range
Package
Option
Media Type
Device:
MIC2171: 100 kHz, 2.5A Switching Regulator
Ambient
Temperature Range:
W
=
–40°C to +85°C
Package:
T
U
=
=
5-Lead TO-220 (RoHS Compliant)
5-Lead TO-263 (RoHS Compliant)
Media Type:
= 50/Tube (TO-220 Package)
= 50/Tube (TO-263 Package)
TR
= 750/Reel
2022 Microchip Technology Inc. and its subsidiaries
Examples:
a) MIC2171WT:
100 kHz, 2.5A Switching Regulator,
–40°C to +85°C Ambient
Temperature Range, 5-Lead TO-220
Package, 50/Tube
b) MIC2171WU:
100 kHz, 2.5A Switching Regulator,
–40°C to +85°C Ambient
Temperature Range, 5-Lead TO-263
Package, 50/Tube
c) MIC2171WU-TR:
100 kHz, 2.5A Switching Regulator,
–40°C to +85°C Ambient
Temperature Range, 5-Lead TO-263
Package, 750/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.
DS20006355A-page 21
MIC2171
NOTES:
DS20006355A-page 22
2022 Microchip Technology Inc. and its subsidiaries
Note the following details of the code protection feature on Microchip products:
•
Microchip products meet the specifications contained in their particular Microchip Data Sheet.
•
Microchip believes that its family of products is secure when used in the intended manner, within operating specifications, and
under normal conditions.
•
Microchip values and aggressively protects its intellectual property rights. Attempts to breach the code protection features of
Microchip product is strictly prohibited and may violate the Digital Millennium Copyright Act.
•
Neither Microchip nor any other semiconductor manufacturer can guarantee the security of its code. Code protection does not
mean that we are guaranteeing the product is “unbreakable”. Code protection is constantly evolving. Microchip is committed to
continuously improving the code protection features of our products.
This publication and the information herein may be used only
with Microchip products, including to design, test, and integrate
Microchip products with your application. Use of this information in any other manner violates these terms. Information
regarding device applications 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. Contact your local Microchip sales office for
additional support or, obtain additional support at https://
www.microchip.com/en-us/support/design-help/client-supportservices.
THIS INFORMATION IS PROVIDED BY MICROCHIP "AS IS".
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 ANY IMPLIED WARRANTIES OF NONINFRINGEMENT, MERCHANTABILITY, AND FITNESS FOR A
PARTICULAR PURPOSE, OR WARRANTIES RELATED TO
ITS CONDITION, QUALITY, OR PERFORMANCE.
IN NO EVENT WILL MICROCHIP BE LIABLE FOR ANY INDIRECT, SPECIAL, PUNITIVE, INCIDENTAL, OR CONSEQUENTIAL LOSS, DAMAGE, COST, OR EXPENSE OF ANY
KIND WHATSOEVER RELATED TO THE INFORMATION OR
ITS USE, HOWEVER CAUSED, EVEN IF MICROCHIP HAS
BEEN ADVISED OF THE POSSIBILITY OR THE DAMAGES
ARE FORESEEABLE. TO THE FULLEST EXTENT
ALLOWED BY LAW, MICROCHIP'S TOTAL LIABILITY ON
ALL CLAIMS IN ANY WAY RELATED TO THE INFORMATION
OR ITS USE WILL NOT EXCEED THE AMOUNT OF FEES, IF
ANY, THAT YOU HAVE PAID DIRECTLY TO MICROCHIP
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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,
CryptoMemory, CryptoRF, dsPIC, flexPWR, HELDO, IGLOO,
JukeBlox, KeeLoq, Kleer, LANCheck, LinkMD, maXStylus,
maXTouch, MediaLB, megaAVR, Microsemi, Microsemi logo,
MOST, MOST logo, MPLAB, OptoLyzer, PIC, picoPower,
PICSTART, PIC32 logo, PolarFire, Prochip Designer, QTouch,
SAM-BA, SenGenuity, SpyNIC, SST, SST Logo, SuperFlash,
Symmetricom, SyncServer, Tachyon, TimeSource, tinyAVR, UNI/O,
Vectron, and XMEGA are registered trademarks of Microchip
Technology Incorporated in the U.S.A. and other countries.
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Company, EtherSynch, Flashtec, Hyper Speed Control, HyperLight
Load, IntelliMOS, Libero, motorBench, mTouch, Powermite 3,
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TimePictra, TimeProvider, TrueTime, WinPath, and ZL are
registered trademarks of Microchip Technology Incorporated in the
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SQTP is a service mark of Microchip Technology Incorporated in
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The Adaptec logo, Frequency on Demand, Silicon Storage
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GestIC is a registered trademark of Microchip Technology Germany
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All other trademarks mentioned herein are property of their
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© 2022, Microchip Technology Incorporated and its subsidiaries.
All Rights Reserved.
For information regarding Microchip’s Quality Management Systems,
please visit www.microchip.com/quality.
2022 Microchip Technology Inc. and its subsidiaries
ISBN: 978-1-6683-0387-0
DS20006355A-page 23
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