MIC2172BN [MICREL]
100kHz 1.25A Switching Regulators Preliminary Information; 100kHz的1.25A开关稳压器的初步信息
| 型号： | MIC2172BN |
| 厂家： | 
MICREL SEMICONDUCTOR |
| 描述： | 100kHz 1.25A Switching Regulators Preliminary Information |
| 文件： | 总16页 (文件大小：133K) |
| 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
Preliminary Information
slave’s. The master MIC2172’s oscillator frequency is in-
creased up to 135kHz by connecting a resistor from SYNC to
ground (see applications information).
General Description
The MIC2172 and MIC3172 are complete 100kHz SMPS
current-mode controllers with internal 65V 1.25A power
switches. The MIC2172 features external frequency syn-
chronization or frequency adjustment, while the MIC3172
features an enable/shutdown control input.
The MIC2172/3172 is available in an 8-pin plastic DIP or
SOIC for –40°C to +85°C operation.
Features
• 1.25A, 65V internal switch rating
• 3V to 40V input voltage range
• Current-mode operation
• Internal cycle-by-cycle current limit
• Thermal shutdown
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.
• 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)
The MIC3172 is for applications that require on/off control of
the regulator. The MIC3172 is externally shutdown by
applyingaTTLlowsignaltoEN(enable). Whendisabled, the
MIC3172 draws only leakage current (typically less than
1µA). ENmustbehighfornormaloperation. Forapplications
4
not requiring control, EN must be tied to V or TTL high.
IN
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
possibletoavoidtheaudiblebeatfrequenciesthatresultfrom
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 desig-
natedasamaster. Toinsurelockingoftheslave’soscillators,
the reference oscillator frequency must be higher than the
Typical Applications
VOUT
5V, 0.25A
VIN
4V to 6V
+5V
T1
(4.75V min.)
D2
1N5818
C1
R4*
C3*
D1*
C1*
22µF
L1
27µH
22µF
R1
C4
470µF
3.74k
1%
VOUT
+12V, 0.14A
VIN
D1
VIN
Enable
VSW
N/C
SYNC
VSW
EN
Shutdown
R1
10.7k
1%
1:1.25
PRI = 100µH
1N5822
MIC2172
L
MIC3172
COMP
FB
COMP
FB
GND
R2
GND
R3
1k
R2
1.24k
1%
C2
470µF
P1 P2
S
R3
1k
1.24k
P1 P2
S
1%
C3
1µF
C2
1µF
* Locate near MIC2172 when supply leads > 2"
* Optional voltage clipper (may be req’d if T1 leakage inductance too high)
Figure 1.
Figure 2.
MIC2172 5V to 12V Boost Converter
MIC3172 5V Flyback Converter
1997
4-13
MIC2172/3172
Micrel
Ordering Information
Part Number
MIC2172BN
MIC2172BM
MIC3172BN
MIC3172BM
Temperature Range
Package
–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-lead SOIC
8-pin plastic DIP
8-lead SOIC
Pin Configuration
†
†
MIC2172*/3172
MIC2172*/3172
S GND
1
8
P GND 1
S GND
COMP
1
2
3
4
8
P GND 1
VSW
7
6
5
COMP
FB
2
3
4
7
6
5
VSW
P GND 2
VI N
FB
P GND 2
VI N
*SYNC/†EN
*SYNC/†EN
8-lead DIP (N)
8-lead SOIC (M)
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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1997
MIC2172/3172
Micrel
Absolute Maximum Ratings MIC2172
Input Voltage .................................................................40V
Switch Voltage ..............................................................65V
Sync Current ..............................................................50mA
Feedback Voltage (Transient, 1ms) ........................... ±15V
Operating Temperature Range
Junction Temperature .............................. –55°C to +150°C
Thermal Resistance
θ
θ
8-pin PDIP .................................................130°C/W
8-pin SOIC .................................................120°C/W
JA
JA
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. Unless otherwise specified, VIN = 5V.
Parameter
Conditions
Min
Typ
Max
Units
Reference Section
Feedback Voltage (VFB
Pin 2 tied to pin 3
)
1.220 1.240 1.264
V
V
1.214
1.274
Feedback Voltage
Line Regulation
3V ≤ VIN ≤ 40V
0.03
%/V
Feedback Bias Current (IFB
)
310
3.9
750
1100
nA
nA
4
Error Amplifier Section
Transconductance (∆ICOMP/∆VFB
)
∆ICOMP = ±25µA
3.0
2.4
6.0
7.0
µA/mV
µA/mV
Voltage Gain (∆VCOMP/∆VFB
)
0.9V ≤ VCOMP ≤ 1.4V
500
800
175
2000
V/V
Output Current
VCOMP = 1.5V
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
ON Resistance
I
SW = 1A, VFB = 0.8V
0.76
1
1.1
Ω
Ω
Current Limit
Duty Cycle = 50%, TJ ≥ 25°C
Duty Cycle = 50%, TJ < 25°C
Duty Cycle = 80% Note 2
1.25
1.25
1
3
3.5
2.5
A
A
A
Breakdown Voltage (BV)
3V ≤ VIN ≤ 40V
65
75
V
ISW = 5mA
1997
4-15
MIC2172/3172
Micrel
Parameter
Conditions
Min
Typ
Max
Units
Oscillator Section
Frequency (fO)
88
85
100
89
112
115
kHz
kHz
Duty Cycle [δ(max)]
80
95
%
Sync Coupling Capacitor
Required for Frequency Lock
VPP = 3.0V
VPP = 40V
22
2.2
51
4.7
120
10
pF
pF
Peak-to-Peak Voltage
CCOUPLING = 12pF
2.2
12
30
V
Required for Frequency Lock
Input Supply Voltage Section
Minimum Operating Voltage
Quiescent Current (IQ)
2.7
7
3.0
9
V
3V ≤ VIN ≤ 40V, VCOMP = 0.6V, ISW = 0
∆ISW = 1A, VCOMP = 1.5V
mA
mA
Supply Current Increase (∆IIN)
9
20
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
Absolute Maximum Ratings MIC3172
Input Voltage .................................................................40V
Switch Voltage ..............................................................65V
Enable Voltage ..............................................................40V
Feedback Voltage (Transient, 1ms) ........................... ±15V
Operating Temperature Range
Junction Temperature ................................ –55°C to 150°C
Thermal Resistance
θ
θ
θ
8-pin PDIP .................................................130°C/W
8-pin SOIC .................................................120°C/W
8-pin CerDIP ..............................................100°C/W
JA
JA
JA
8-pin PDIP................................................. –40 to +85°C
8-pin SOIC ................................................ –40 to +85°C
8-pin CerDIP ........................................... –55 to +125°C
Storage Temperature ................................. –65°C to 150°C
Soldering (10 sec.) .................................................... 300°C
Electrical Characteristics MIC3172 Note 1. Unless otherwise specified, VIN = 5V.
Parameter
Conditions
Min
Typ
Max
Units
Reference Section
Feedback Voltage (VFB
Pin 2 tied to pin 3
)
1.224 1.240 1.264
V
V
1.214
1.274
Feedback Voltage
Line Regulation
3V ≤ VIN ≤ 40V
0.07
310
%/V
Feedback Bias Current (IFB
)
750
1100
nA
nA
4-16
1997
MIC2172/3172
Micrel
Parameter
Conditions
Min
Typ
Max
Units
Error Amplifier Section
Transconductance (∆ICOMP/∆VFB
)
∆ICOMP = ±25µA
3.0
2.4
3.9
6.0
7.0
µA/mV
µA/mV
Voltage Gain (∆VCOMP/∆VFB
)
0.9V ≤ VCOMP ≤ 1.4V
500
800
175
2000
V/V
Output Current
VCOMP = 1.5V
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
ON Resistance
I
SW = 1A, VFB = 0.8V
0.76
1
1.1
Ω
Ω
Current Limit
Duty Cycle = 50%, TJ ≥ 25°C
Duty Cycle = 50%, TJ < 25°C
Duty Cycle = 80% Note 2
1.25
1.25
1
3
3.5
2.5
A
A
A
Breakdown Voltage (BV)
3V ≤ VIN ≤ 40V
65
75
V
ISW = 5mA
Oscillator Section
4
Frequency (fO)
88
85
100
89
112
115
kHz
kHz
Duty Cycle [δ(max)]
80
95
%
Input Supply Voltage Section and Enable Section
Minimum Operating Voltage
2.7
3.0
V
Quiescent Current (IQ)
3V ≤ VIN ≤ 40V, VCOMP = 0.6V, ISW = 0
Shutdown, VEN = 0V
7
0.1
9
5
mA
µA
Quiescent Current Increase (∆IIN) ∆ISW = 1A, VCOMP = 1.5V
9
20
mA
Enable Input Threshold
0.4
1.2
2.4
V
Enable Input Current
VEN = 0V
VEN = 2.4V
–1
0
2
1
10
µA
µA
Bold type denotes specifications 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
1997
4-17
MIC2172/3172
Micrel
Typical Performance Characteristics
Feedback Voltage
Line Regulation
MIC2172 Minimum
Operating Voltage
Feedback Bias Current
5
4
2.9
2.8
2.7
2.6
2.5
2.4
2.3
800
700
600
500
400
300
200
100
0
T
= 125°C
3
J
2
1
0
T
= 25°C
J
Switch Current = 1A
-1
-2
-3
-4
-5
T
= -40°C
J
0
10
V
20
30
40
-100 -50
0
50
100 150
-100 -50
0
50
100 150
Operating (V)
Temperature (°C)
Temperature (°C)
IN
Supply Current
(Shutdown Mode)
Supply Current
Enable Thresholds
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
1.4
1.3
1.2
1.1
1
MIC3172
IN = 40V
I
= 0
MIC3172
SW
V
ON
D.C. = 90%
D.C. = 50%
D.C. = 0%
OFF
8
7
0.9
0.8
6
5
0
10
20
30
40
-100 -50
0
50
100 150
-100 -50
0
50
100 150
Temperature (°C)
V
Operating Voltage (V)
Temperature (°C)
IN
Supply Current
Current Limit
Switch ON Voltage
50
40
30
20
10
0
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
4
3
2
1
0
T
= –40°C
J
–40°C
125°C
T
= 25°C
J
25°C
δ = 90%
T
= 125°C
J
δ = 50%
0.0
0.5
1.0
1.5
2.0
0.0
0.5
1.0
1.5
0
20
40
60
80
100
Switch Current (A)
Switch Current (A)
Duty Cycle (%)
Supply Current
Oscillator Frequency
Oscillator Frequency
MIC2172
10
120
110
100
90
140
130
120
110
100
90
VCOMP = 0.6V
9
8
7
6
5
4
3
2
1
0
80
70
60
-100 -50
0
50
100 150
-50
0
50
100
150
1
10
100
(kΩ)
1000
Temperature (°C)
Temperature (°C)
R
ADJ
4-18
1997
MIC2172/3172
Micrel
Typical Performance Characteristics
Error Amplifier Gain
Error Amplifier Gain
Error Amplifier Phase
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
7000
6000
5000
4000
3000
2000
1000
0
-30
0
30
60
90
120
150
180
210
-100 -50
0
50
100 150
1
10
100
1000 10000
1
10
100
1000 10000
Temperature (°C)
Frequency (kHz)
Frequency (kHz)
4
Block Diagram MIC2172
VSW
Pin 7
D1
2.3V
VI N
Pin 5
Reg.
Anti-Sat.
Driver
100kHz
Osc.
Logic
SYNC
Pin 4
Q1
Com-
parator
FB
Pin 3
Current
Amp.
Error
Amp.
1.24V
Ref.
S
GND
Pin 1
COMP
Pin 2
P
P
GND GND
2
1
Pin 6 Pin 8
1997
4-19
MIC2172/3172
Micrel
Block Diagram MIC3172
VSW
Pin 7
D1
2.3V
VI N
Pin 5
Reg.
Anti-Sat.
Driver
100kHz
Osc.
Logic
EN
Pin 4
Q1
Com-
parator
FB
Pin 3
Current
Amp.
Error
Amp.
1.24V
Ref.
S
GND
Pin 1
COMP
Pin 2
P
P
GND GND
2
1
Pin 6 Pin 8
technique. Feedbackloopcompensationisgreatlysimplified
because inductor current sensing removes a pole from the
closed loop response. Inherent cycle-by-cycle current limit-
ing greatly improves the power switch reliability and provides
automatic output current limiting. Finally, current-mode op-
eration provides automatic input voltage feed forward which
prevents instantaneous input voltage changes from disturb-
ing the output voltage setting.
Functional Description
Refer to “Block Diagram MIC2172” and “Block Diagram
MIC3172.”
Internal Power
The MIC2172/3172 operates when V is ≥ 2.6V (and V
≥
IN
EN
2.0V for the MIC3172). An internal 2.3V regulator supplies
biasing to all internal circuitry including a precision 1.24V
band gap reference.
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.
The enable control (MIC3172 only) enables or disables the
internal regulator which supplies power to all other internal
circuitry.
Compensation
PWM Operation
Loop stability compensation of the MIC2172/3172 can be
accomplished by connecting an appropriate network from
eitherCOMPtocircuitground(TypicalApplications)orCOMP
to FB.
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.
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
4-20
1997
MIC2172/3172
Micrel
By using the MIC3172, U1 and Q1 shown in figure 5 can be
eliminated, reducing the total components count.
Applications 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.
U1
Micrel’s MIC2172 can be synchronized to a single master
frequency avoiding the possibility of undesirable beat fre-
quencies in multiple regulator circuits. The master frequency
canbeanexternaloscillatororadesignatedmasterMIC2172.
The master frequency should be 1.05 to 1.20 times the
slave’s 100kHz nominal frequency to guarantee synchroni-
zation.
Enable
Shutdown
4
EN
Logic
Gate
MIC3172
Figure 3. MIC3172 TTL Enable/Shutdown
Using the MIC3172 in LT1172 Applications
U2
The MIC3172 can be used in most original LT1172 applica-
tions by adapting the MIC3172’s enable/shutdown feature to
the existing LT1172 circuit.
4
5
SYNC
MIC2172
VSW
U1
4
5
Unlike the LT1172 which can be shutdown by reducing the
SYNC
VSW
voltage on pin 2 (V ) below 0.15V, the MIC3172 has a
C
10kΩ
MIC2172
dedicated enable/shutdown pin. To replace the LT1172 with
the MIC3172, determine if the LT1172’s shutdown feature is
used.
Slave
U3
Master
4
5
4
SYNC
VSW
Circuits without Shutdown
MIC2172
If the shutdown feature is not being used, connect EN to V
to continuously enable the MIC3172 or use an MIC2172 with
SYNC open (figure 4).
IN
Additional
Slaves
Slave
Figure 6. Master/Slave Synchronization
VIN
VIN
Figure6showsatypicalapplicationwhereseveralMIC2172s
operate from the same supply voltage. U1’s oscillator fre-
quency is increased above U2’s and U3’s by connecting a
resistor from SYNC to ground. U2-SYNC and U3-SYNC are
VIN
VIN
4
4
N/C
SYNC
EN
MIC2172
MIC3172
capacitively coupled to the master’s output (V ). The
SW
slaves lock to the negative (falling edge) of U1’s output
waveform.
Figure 4. MIC2172/3172 Always Enabled
Circuits with Shutdown
U1
4
5
If shutdown was used in the original LT1172 application,
connect EN to a logic gate that produces a TTL logic-level
outputsignalthatmatchestheshutdownsignal. TheMIC3172
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.
SYNC
MIC2172
VSW
External
Signal
Slave
U2
4
5
SYNC
VSW
4
EN
MIC2172
MIC3172
Additional
Slaves
add
connection
COMP
Slave
Figure 7. External Synchronization
U1
Existing
Enable
Shutdown
R1
C1
Q1
Care must be exercised to insure that the master MIC2172 is
always operating in continuous mode.
VN2222
or equiv.
Existing
Logic
Gate
Figure 5. Adapting to the LT1172 Socket
1997
4-21
MIC2172/3172
Micrel
Figure 7 shows how one or more MIC2172s can be locked to
an external reference frequency. The slaves lock to the
negative (falling edge) of the external reference waveform.
the total power dissipation is the sum of the device operating
losses and power switch losses.
The device operating losses are the dc losses associated
with biasing all of the internal functions plus the losses of the
power switch driver circuitry. The dc losses are calculated
from the supply voltage (V ) and device supply current (I ).
Soft Start
A diode-coupled capacitor from COMP to circuit ground
slows the output voltage rise at turn on (figure 8).
IN
Q
TheMIC2172/3172supplycurrentisalmostconstantregard-
less 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.
VIN
VIN
MIC2172/3172
0.004+δ
COMP
P
= V
(
I
+ V I
IN SW
)
(bias+driver)
IN Q
50
D1
D2
C1
R1
C2
where:
P
V
= device operating losses
(bias+driver)
= supply voltage
IN
Figure 8. Soft Start
I = quiescent supply current
Q
Theadditionaltimeittakesfortheerroramplifiertochargethe
capacitor corresponds to the time it takes the output to reach
regulation. Diode D1 discharges C1 when V is removed.
I
= power switch current
(see “ Design Hints: Switch Current
Calculations”)
SW
IN
Current Limit
δ = duty cycle
FordesignsdemandinglessoutputcurrentthantheMIC2172/
3172 is capable of delivering, P GND 1 can be left open
reducing the current capability of Q1 by one-half.
V
+ V – V
OUT
F
IN
δ =
V
+ V
OUT
F
V
= output voltage
OUT
VIN
V = D1 forward voltage drop
F
VIN
VSW
As a practical example refer to figure 1.
MIC2172/3172
V
= 5.0V
IN
VOUT
FB
P1 P2 S COMP
GND
I = 0.006A
Q
I
= 0.625A
SW
δ = 60% (0.6)
R1
ICL ≈ 0.6V/R2
R3
C2
Q1
Then:
C1
R2
Note: Input and output
returns not common.
0.004+0.6
50
P(bias+driver) = 5 × 0.006 + 5 0.625
(
)
Figure 9. Current Limit
P
= 0.068W
(bias+driver)
Alternatively,themaximumcurrentlimitoftheMIC2172/3172
can be reduced by adding a voltage clamp to the COMP
output (figure 9). This feature can be useful in applications
requiringeitheracompleteshutdownofQ1’sswitchingaction
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.
Power switch dissipation calculations are greatly simplified
bymakingtwoassumptionswhichareusuallyfairlyaccurate.
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 satura-
tion voltage versus collector current curves (see Typical
Performance Characteristics). Power switch losses are
calculatedbymodelingtheswitchasaresistorwiththeswitch
duty cycle modifying the average power dissipation.
Thermal Management
Although the MIC2172/3172 family contains thermal protec-
tion circuitry, for best reliability, avoid prolonged operation
with junction temperatures near the rated maximum.
2
P
= (I ) R
δ
SW
SW
SW
From the Typical performance Characteristics:
The junction temperature is determined by first calculating
the power dissipation of the device. For the MIC2172/3172,
R
= 1Ω
SW
4-22
1997
MIC2172/3172
Then:
Micrel
Applications and Design Hints
2
P
P
P
P
= (0.625) × 1 × 0.6
SW
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.
= 0.234W
(SW)
(total)
(total)
= 0.068 + 0.234
= 0.302W
Boost Conversion
Thejunctiontemperatureforanysemiconductoriscalculated
using the following:
Refer to figure 11 for a typical boost conversion application
where a +5V logic supply is available but +12V at 0.14A is
required.
T = T + P θ
(total) JA
J
A
Where:
+5V
(4.75V min.)
C1*
22µF
L1
27µH
T = junction temperature
J
VOUT
+12V, 0.14A
T = ambient temperature (maximum)
A
VIN
D1
VSW
N/C
SYNC
P
= total power dissipation
(total)
R1
10.7k
1%
1N5822
MIC2172
θ
= junction to ambient thermal resistance
JA
COMP
FB
For the practical example:
GND
R2
R3
1k
C2
470µF
P1 P2
S
1.24k
1%
C3
1µF
T = 70°C
A
θ
= 130°C/W (for plastic DIP)
JA
* Locate near MIC2172 when supply leads > 2"
Then:
T = 70 + 0.30 × 130
Figure 11. 5V to 12V Boost Converter
J
T = 109°C
J
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.
This junction temperature is below the rated maximum of
150°C.
4
Grounding
Refer to figure 10. Heavy lines indicate high current paths.
When L1 discharges its current completely during the
MIC2172/3172’s off-time, it is operating in discontinuous
mode.
VIN
VIN
EN*
VSW
MIC2172/3172
GND FB
L1 is operating in continuous mode if it does not discharge
completely before the MIC2172/3172 power switch is turned
on again.
P1 P2 S
VC
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.
Single point ground
* MIC3172 only
ICL
VIN
δ
Figure 10. Single Point Ground
2
IOUT
≤
(1)
VOUT
A single point ground is strongly recommended for proper
operation.
V
+ V – V
OUT
F
IN
The signal ground, compensation network ground, and feed-
back 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.
δ =
(1a)
V
+ V
OUT
F
Where:
I
= internal switch current limit
I
I
CL
= 1.25A when δ < 50%
= 0.833 (2 – δ) when δ ≥ 50%
CL
CL
(Refer to Electrical Characteristics.)
I
= maximum output current
OUT
V
= minimum input voltage
IN
δ = duty cycle
1997
4-23
MIC2172/3172
= required output voltage
Micrel
V
Switch Operation
OUT
V = D1 forward voltage drop
During Q1’s on time (Q1 is the internal NPN transistor—see
block diagrams), energy is stored in T1’s primary inductance.
DuringQ1’sofftime,storedenergyispartiallydischargedinto
C4 (output filter capacitor). Careful selection of a low ESR
capacitor for C4 may provide satisfactory output ripple volt-
age making additional filter stages unnecessary.
F
For the example in figure 11.
I
I
= 0.14A
OUT
= 1.147A
CL
V
= 4.75V (minimum)
IN
δ = 0.623
= 12.0V
C1 (input capacitor) may be reduced or eliminated if the
MIC3172 is located near a low impedance voltage source.
V
OUT
V = 0.6V
Output Diode
F
Then:
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.
1.147
2
× 4.75 × 0.623
I
≤
OUT
12
Frequency Compensation
I
≤ 0.141A
OUT
A simple frequency compensation network consisting of R3
and C2 prevents output oscillations.
This value is greater than the 0.14A output current require-
ment so we can proceed to find the inductance value of L1.
High impedance output stages (transconductance type) in
the MIC2172/3172 often permit simplified loop-stability solu-
tions 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.
2
V
δ
(
)
IN
L1 ≤
(2)
2 POUT fSW
Where:
P
= 12 × 0.14 = 1.68W
Voltage Clipper
OUT
5
f
= 1×10 Hz (100kHz)
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 break-
down (failure) of the MIC3172’s power NPN Q1.
SW
For our practical example:
2
4.75 × 0.623
(
)
L1 ≤
2 × 1.68 × 1×105
Enable/Shutdown
I
≤ 26.062µH (use 27µH)
L1
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
Equation (3) solves for L1’s maximum current value.
V
T
IN ON
I
=
(3)
L1(peak)
L1
required, simply connect EN to V or to a TTL high voltage.
IN
Where:
Discontinuous Mode Design
-6
T
= δ / f
= 6.23×10 sec
SW
When designing a discontinuous flyback converter, first de-
termine 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.
ON
-6
4.75 × 6.23 × 10
I
=
L1(peak)
-6
27 ×10
I
= 1.096A
L1(peak)
Use a 27µH inductor with a peak current rating of at least
1.4A.
2 P
OUT
(8)
Flyback Conversion
δ ≥
I
V
CL IN(min)
Flyback converter topology may be used in low power appli-
cations 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 (trans-
former primary) current can be continuous or discontinuous.
Discontinuous operation is recommended.
For a practical example let:
P
= 5.0V × 0.25A = 1.25W
OUT
V
I
= 4.0V to 6.0V
= 1.25A when δ < 50%
IN
CL
0.833 (2 – δ) when δ ≥ 50%
Figure 12 shows a practical flyback converter design using
the MIC3172.
4-24
1997
MIC2172/3172
Then:
Micrel
2
2
0.5 f
V
T
SW IN(min)
ON
L
≤
(10)
PRI
2 × 1. 2 5
P
OUT
δ ≥
1.25 × 4
Where:
δ ≥ 0.5 (50%) Use 0.55.
L
= maximum primary inductance
PRI
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%.
f
= device switching frequency (100kHz)
= minimum input voltage
SW
V
T
IN(min)
= power switch on time
Calculate the maximum transformer turns ratio a, or
ON
N
/N
,thatwillguaranteesafeoperationoftheMIC2172/
Then:
PRI SEC
3172 power switch.
2
5
2
-6
0.5 × 1×10 × 4.0 5.5 × 10
(
)
V
F
– V
L
≤
CE CE
IN(max)
PRI
(9)
1. 2 5
a ≤
V
SEC
L
≤ 19.23µH
PRI
Where:
a = transformer maximum turns ratio
Use an 18µH primary inductance to overcome circuit ineffi-
ciencies.
V
= power switch collector to emitter
maximum voltage
CE
To complete the design the inductance value of the second-
ary is found which will guarantee that the energy stored in the
transformer during the power switch on time will be com-
pleted discharged into the output during the off-time. This is
necessary when operating in discontinuous-mode.
F
= safety derating factor (0.8 for most
commercial and industrial applications)
CE
V
= maximum input voltage
IN(max)
V
= transformer secondary voltage (V
+ V )
OUT F
SEC
2
2
4
0.5 fSW VSEC TOFF
For the practical example:
LSEC
Where:
≤
(11)
POUT
V
F
= 65V max. for the MIC2172/3172
CE
= 0.8
CE
V
= 5.6V
L
= maximum secondary inductance
= power switch off time
SEC
SEC
Then:
T
OFF
Then:
65 × 0.8 – 6.0
a ≤
5.6
2
0.5 × 1×105 × 5.62 × 4.5 × 10-6
(
)
a ≤ 8.2143
LSEC
≤
1. 2 5
Next, calculate the maximum primary inductance required to
store the needed output energy with a power switch duty
cycle of 55%.
L
≤ 25.4µH
SEC
VOUT
5V, 0.25A
VIN
4V to 6V
T1
D2
1N5818
C1
22µF
R4*
C3*
R1
C4
470µF
3.74k
1%
D1*
VIN
Enable
VSW
EN
Shutdown
1:1.25
PRI = 100µH
L
MIC3172
COMP
FB
GND
R2
1.24k
1%
R3
1k
P1 P2
S
C2
1µF
* Optional voltage clipper (may be req’d if T1 leakage inductance too high)
Figure 12. MIC3172 5V 0.25A Flyback Converter
1997
4-25
MIC2172/3172
Micrel
Finally, recalculate the transformer turns ratio to insure that
it is less than the value earlier found in equation (9).
a = transformer turns ratio (0.8)
F
= reverse voltage safety derating factor (0.8)
BR
Then:
L
PRI
a ≤
(12)
L
6.0 + 5.0 × 0.8
(
)
SEC
V
≥
BR
0.8 × 0.8
Then:
V
≥ 15.625V
BR
1. 8 × 10-5
2.54 × 10-5
A 1N5817 will safely handle voltage and current require-
ments in this example.
a ≤
Forward Converters
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 satu-
rating the transformer core.
Micrel’s MIC2172/3172 can be used in several circuit con-
figurations to generate an output voltage which is less than
the input voltage (buck or step-down topology). Figure 13
shows the MIC3172 in a voltage step-down application.
Because of the internal architecture of these devices, more
external components are required to implement a step-down
regulator than with other devices offered by Micrel (refer to
the LM257x or LM457x family of buck switchers). However,
for step-down conversion requiring a transformer (forward),
the MIC2172/3172 is a good choice.
V
T
IN(min) ON
I
=
(13)
So:
PEAK(pri)
L
PRI
-6
4.0 × 5.5 × 10
18µH
I
=
PEAK(pri)
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.
I
= 1.22A
PEAK(pri)
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.
VIN(max) + V
a
(
)
OUT
VBR
Where:
≥
(14)
FBR
a
For most forward converters the duty cycle is limited to 50%,
allowing the transformer flux to reset with only two times the
input voltage appearing across the power switch. Although
during normal operation this circuit’s duty cycle is well below
V
= output rectifier maximum peak
reverse voltage rating
BR
VIN
D1
1N4148
VIN
VSW
EN
C2
D3
MIC3172
2.2µF
1N4148
R3†
COMP
FB
GND
P1 P2
C1*
100µF
3.7k
R3
470
S
R2†
1.2k
C4
1µF
R4
10Ω
C3
1µF
L1
100µH
C5
5V, 0.1A to 1A
(ILOAD > 100mA)
D2
330µF
* Locate near MIC2172/3172 when supply leads > 2"
†
R3/R2 sets output voltage
Figure 13. Step-Down or Buck Converter
4-26
1997
MIC2172/3172
Micrel
50%,theMIC2172(andMIC3172)hasamaximumdutycycle
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.
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 wind-
ings 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
primarywindingwhichissteppedupandcapacitivelycoupled
to the lamp.
To prevent core saturation, the application given here uses a
duty cycle limiter consisting of Q1, C4 and R3. Whenever the
MIC3172 exceeds a duty cycle of 50%, T1’s reset winding
current turns Q1 on. This action reduces the duty cycle of the
MIC3172 until T1 is able to reset during each cycle.
Fluorescent Lamp Supply
Lamp Current Regulation
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.
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,
lampcurrentisrectifiedbyD2throughR4andR5. R3andC5
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.
Figure 15 shows a complete power supply for lighting a
fluorescent lamp. Transistors Q1 and Q2 together with ca-
pacitor C2 form a Royer oscillator. The Royer oscillator
generatesasinewavewhosefrequencyisdeterminedbythe
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 Opera-
tion.”
4
Oscillator Operation
Resistor R2 provides initial base current that turns transistor
Q1onandimpressestheinputvoltageacrossonehalfofT1’s
primary winding (Pri 1). T1’s feedback winding provides
additional base drive (positive feedback) to Q1 forcing it well
T1
1:1:1
D3
1N5819
L1 100µH
VOUT
VIN
12V
5V, 1A
R4
D4
1N5819
C5
470µF
3.74k
1%
R1*
C2*
D1*
VIN
Enable
Shutdown
EN
VSW
FB
MIC3172
C1
22µF
GND
D2
1N5819
P1 P2 S COMP
R5
1.24k
1%
R2
1k
Q1†
R3†
C3
1µF
C4†
* Voltage clipper
† Duty cycle limiter
Figure 14. 12V to 5V Forward Converter
1997
4-27
MIC2172/3172
On/Off Control
Micrel
Efficiency
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.
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
copperlossesattheswitchingfrequencyoftheMIC3172,and
T1 should be carefully constructed from magnetic materials
optimizedfortheoutputpowerrequiredattheRoyeroscillator
frequency. Suitable inductors may be obtained from
Coiltronics, Inc., tel: (407) 241-7876.
Cold Cathode
T1
Fluorescent
Lamp
BF
C4
R2
VIN
4.5V to 20V
Q1
Q2
D2
D3
eSc
rPi1
D1
1N4148
1N4148
C2
C3
VIN
L1
Enable (On)
EN
VSW
FB
Shutdown (Off)
rPi2
MIC3172
300µH
R3
C5
R4
GND
R5
Intensity
Control
P1 P2 S COMP
R1
C1
L1: Coiltronics CTX300-4P
T1: Coiltronics CTX110602
C2: Polyfilm, WIMA FKP2 0.1µF to 0.68µF
C4: 15pF to 30pF, 3kV min.
Figure 15. LCD Backlight Fluorescent Lamp Supply
4-28
1997
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