Page 1
General Description
MIC4723
3A 2MHz Integrated Switch
Features
Buck Regulator
The Micrel MIC4723 is a high efficiency PWM buck (stepdown) regulator that provides up to 3A of output current.
The MIC4723 operates at 2.0MHz and has proprietary
internal compensation that allows a closed loop bandwidth
of over 200KHz.
The low on-resistance internal p-channel MOSFET of the
MIC4723 allows efficiencies over 92%, reduces external
components count and eliminates the need for an
expensive current sense resistor.
The MIC4723 operates from 2.7V to 5.5V input and the
output can be adjusted down to 1V. The devices can
operate with a maximum duty cycle of 100% for use in lowdropout conditions.
The MIC4723 is available in the exposed pad 12-pin
3mm x 3mm MLF
®
and 10-pin ePAD MSOP packages with
a junction operating range from –40°C to +125°C.
Data sheets and support documentation can be found on
Micrel’s web site at: www.micrel.com.
• 2.7/3.0V to 5.5V supply voltage
• 2.0MHz PWM mode
• Output current to 3A
• Up to 94% efficiency
• 100% maximum duty cycle
• Adjustable output voltage option down to 1V
• Ultra-fast transient response
• Ultra-small external components
Stable with a 1µH inductor and a 4.7µF output
capacitor
• Fully integrated 3A MOSFET switch
• Micropower shutdown
• Thermal shutdown and current limit protection
• Pb-free 12-pin 3mm x 3mm MLF
®
package
• Pb-free 10-pin ePAD MSOP package
• –40°C to +125°C junction temperature range
Applications
• FPGA/DSP/ASIC applications
• General point of load
• Broadband communications
• DVD/TV recorders
• Point of sale
• Printers/Scanners
• Set top boxes
• Computing peripherals
• Video cards
___________________________________________________________________________________________________________
Typical Application
MIC4723
Efficiency
3.3V
OUT
4.5V
IN
5V
IN
5.5V
IN
0
0.5 1 1.5 2 2.5 3
OUTPUT CURRENT (A)
3A 2MHz Buck Regulator
96
94
92
90
88
86
84
82
80
78
76
MLF and MicroLeadFrame are registered trademarks of Amkor Technology, Inc.
Micrel Inc. • 2180 Fortune Drive • San Jose, CA 95131 • USA • tel +1 (
June 2008
408) 944-0800 • fax + 1 (408) 474-1000 • http://www.micrel.com
M9999-060308-E
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Micrel, Inc. MIC4723
Ordering Information
Part Number Voltage Temperature Range Package Lead Finish
MIC4723YML Adj. –40° to +125°C 12-Pin 3x3 MLF® Pb-Free
MIC4723YMME Adj. –40° to +125°C 10-Pin ePAD MSOP Pb-Free
Note
MLF
®
is a GREEN RoHS compliant package. Lead finish is NiPdAu. Mold compound is Halogen Free.
Pin Configuration
SW
1
VIN
2
PGND
SGND
3
4
BIAS EN
5
67
FB NC
12-Pin 3mm x 3mm MLF (ML) 10-Pin ePAD MSOP (MME)
EP
12
11
10
9
8
SW
VIN
NC
PGOOD
1SW
2
VIN
FB EN65
3
4
EP
SGND
BIAS
Pin Description
Pin Number
MLF-12
1, 12 1, 10 SW Switch (Output): Internal power P-Channel MOSFET output switch.
2, 11 2, 9 VIN
3 8 PGND Power Ground. Provides the ground return path for the high-side drive current.
4 3 SGND
5 4 BIAS
6 5 FB
7, 10
8 6 EN
9 7 PGOOD
EP EP GND Connect to ground.
Pin Number
MSOP-10
—
Pin Name Pin Function
Supply Voltage (Input): Supply voltage for the source of the internal P-channel
MOSFET and driver. Requires bypass capacitor to GND.
Signal (Analog) Ground. Provides return path for control circuitry and internal
reference.
Internal circuit bias supply. Must be bypassed with a 0.1µF ceramic capacitor to
SGND.
Feedback. Input to the error amplifier, connect to the external resistor divider network
to set the output voltage.
NC
No Connect. Not internally connected to die. This pin can be tied to any other pin if
desired.
Enable (Input). Logic level low, will shutdown the device, reducing the current draw to
less than 5µA.
Power Good. Open drain output that is pulled to ground when the output voltage is
within ±7.5% of the set regulation voltage.
10 SW
9
VIN
8
PGND
7
PGOOD
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Micrel, Inc. MIC4723
Absolute Maximum Ratings
(1)
Operating Ratings
(2)
Supply Voltage (VIN).......................................................+6V
Output Switch Voltage (V
Output Switch Current (I
Logic Input Voltage (V
Storage Temperature (T
)..........................................+6V
SW
)............................................11A
SW
) ..................................–0.3V to VIN
EN
).........................–60°C to +150°C
s
Supply Voltage (VIN).....................................+2.7V to +5.5V
Logic Input Voltage (V
Junction Temperature (T
).......................................0V to VIN
EN
) ........................–40°C to +125°C
J
Junction Thermal Resistance
3mm x 3mm MLF-12 (θ
3mm x 3mm MLF-12 (θ
ePAD MSOP-10 (θ
ePAD MSOP-10 (θ
).................................60°C/W
JA
) .................................28°C/W
Jc
)........................................76°C/W
JA
) ........................................28°C/W
Jc
Electrical Characteristics
= VEN = 3.6V; L = 1µH; C
V
IN
Parameter Condition Min Typ Max Units
Supply Voltage Range
Under-Voltage Lockout
Threshold
UVLO Hysteresis 100 mV
Quiescent Current VFB = 0.9 * V
Shutdown Current VEN = 0V 2
[Adjustable] Feedback
Voltage
FB pin input current 1 nA
Current Limit in PWM Mode VFB = 0.9 * V
Output Voltage Line
Regulation
Output Voltage Load
Regulation
Maximum Duty Cycle
PWM Switch ON-
Resistance
Oscillator Frequency
Enable Threshold
Enable Hysteresis 50 mV
Enable Input Current 0.1
Power Good Range ±7 ±10 %
Power Good Resistance I
Over-Temperature
Shutdown
Over-Temperature
Hysteresis
Notes:
1. Exceeding the absolute maximum rating may damage the device.
2. The device is not guaranteed to function outside its operating rating.
3. Specification for packaged product only.
(4)
= 4.7µF; TA = 25°C, unless noted. Bold values indicate –40°C< TJ < +125°C.
OUT
MIC4723YML
MIC4723YMME
(turn-on)
(not switching) 570
NOM
± 2% (over temperature) I
3.5 5 A
NOM
V
> 2V; VIN = V
OUT
< 2V; VIN = 2.7V to 5.5V; I
V
OUT
20mA < I
V
≤ 0.4V
FB
I
= 50mA; VFB = GND (High Side Switch) 95
SW
< 3A 0.2 %
LOAD
+500mV to 5.5V; I
OUT
LOAD
= 100mA
= 100mA
LOAD
LOAD
= 100mA
2.7
3.0
2.45
0.98
0.07
100
1.8
0.5
= 500µA 150
PGOOD
2.55
%
2
0.85
160
25
5.5
5.5
2.65
900
10
1.02
200
300
2.2
1.3
2.3
250
V
V
V
µA
µA
V
%
%
mΩ
mΩ
MHz
V
µA
Ω
°C
°C
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Micrel, Inc. MIC4723
Typical Characteristics
MIC4723
5.5V
Efficiency
OUT
5V
IN
IN
3.3V
96
4.5V
IN
94
92
90
88
86
84
82
80
78
76
0.5 1 1.5 2 2.5 3
0
OUTPUT CURRENT (A)
MIC4723
IN
Efficiency
OUT
5V
IN
IN
1.8V
90
4.5V
88
86
84
82
80
5.5V
78
76
74
72
70
68
66
64
62
60
0.5 1 1.5 2 2.5 3
0
OUTPUT CURRENT (A)
MIC4723
OUT
3.3V
IN
Efficiency
IN
1.5V
92
90
3V
IN
88
86
84
82
3.6V
80
78
76
74
72
70
68
66
64
0 0.5 1 1.5 2 2.5 3
OUTPUT CURRENT (A)
MIC4723
Efficiency
1V
78
76
74
72
70
68
66
64
62
60
0
OUT
4.5V
IN
5V
IN
5.5V
IN
0.5 1 1.5 2 2.5 3
OUTPUT CURRENT (A)
MIC4723
5.5V
IN
OUT
5V
IN
Efficiency
IN
100
98
96
94
92
90
88
86
84
82
80
0
2.5V
94
92
4.5V
90
88
86
84
82
80
0.5 1 1.5 2 2.5 3
0
OUTPUT CURRENT (A)
MIC4723
OUT
3.3V
IN
Efficiency
IN
86
84
82
80
78
76
74
72
70
68
66
64
0
1.8V
96
94
3V
IN
92
90
88
86
84
3.6V
82
80
78
76
74
72
70
68
66
0 0.5 1 1.5 2 2.5 3
OUTPUT CURRENT (A)
MIC4723
IN
OUT
5V
IN
Efficiency
IN
90
88
86
3V
84
82
80
78
76
74
72
70
68
66
64
62
60
0
1.2V
90
88
86
84
82
4.5V
80
78
76
74
72
5.5V
70
68
66
64
62
60
0.5 1 1.5 2 2.5 3
0
OUTPUT CURRENT (A)
MIC4723
2.5V
3V
IN
3.6V
0.5 1 1.5 2 2.5 3
OUTPUT CURRENT (A)
OUT
3.3V
IN
Efficiency
IN
MIC4723
IN
5.5V
Efficiency
OUT
5V
IN
IN
1.5V
4.5V
0.5 1 1.5 2 2.5 3
OUTPUT CURRENT (A)
MIC4723
OUT
3.3V
IN
Efficiency
IN
1.2V
IN
3.6V
0.5 1 1.5 2 2.5 3
OUTPUT CURRENT (A)
MIC4723
Efficiency
1V
84
82
80
78
76
74
72
70
68
66
64
0 0.5 1 1.5 2 2.5 3
OUT
3V
IN
3.3V
IN
3.6V
IN
OUTPUT CURRENT (A)
1.010
1.005
1.000
0.995
OUTPUT VOLTAGE (V)
0.990
Load Regulation
3.3V
VIN = 3.3V
0 0.5 1 1.5 2 2.5 3
OUTPUT CURRENT (A)
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Micrel, Inc. MIC4723
Typical Characteristics (continue)
1.0010
1.0008
1.0006
1.0004
1.0002
1.0000
0.9998
0.9996
0.9994
0.9992
0.9990
Line Regulation
2.7 3.2 3.7 4.2 4.7 5.2
SUPPLY VOLTAGE (V)
Feedback Voltage
vs. Supply Voltage
1.2
1.0
0.8
0.6
0.4
0.2
VEN = V
0
012345
160
140
120
100
80
60
40
20
0
IN
SUPPLY VOLTAGE (V)
R
DSON
vs. Temperature
VIN = 3.3V
20 40 60 80
TEMPERATURE (°C)
Feedback Voltage
1.010
1.008
1.006
1.004
1.002
1.000
0.998
0.996
0.994
0.992
0.990
vs. Temperature
VIN = 3.3V
20 40 60 80
TEMPERATURE (°C)
2.5
2.4
2.3
2.2
2.1
2.0
1.9
1.8
1.7
1.6
1.5
Quiescent Current
vs. Supply Voltage
800
700
600
500
400
300
200
100
0
0123456
SUPPLY VOLTAGE (V)
VEN= V
IN
120
115
110
105
100
95
90
85
80
75
70
2.7 3.2 3.7 4.2 4.7 5.2
Enable Threshold
vs. Supply Voltage
1.2
1.0
0.8
0.6
0.4
0.2
0
2.7
SUPPLY VOLTAGE (V)
3.73.2 4.2 4.7
1.2
1.0
0.8
0.6
0.4
0.2
0
Frequency
vs. Temperature
VIN = 3.3V
20 40 60 80
TEMPERATURE (°C)
R
DSON
vs. Supply Voltage
SUPPLY VOLTAGE (V)
Enable Threshold
vs. Temperature
VIN = 3.3V
20 40 60 80
TEMPERATURE (°C)
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Functional Characteristics
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Micrel, Inc. MIC4723
Functional Diagram
VIN
VIN
P-Channel
Current Limit
BIAS
HSD
PWM
Control
SW
SW
Bias,
EN
Enable and
Control Logic
UVLO,
Thermal
Shutdown
Soft
Start
EA
FB
1.0V
PGOOD
1.0V
SGND
PGND
MIC4723 Block Diagram
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Micrel, Inc. MIC4723
Pin Description
VIN
Two pins for VIN provide power to the source of the
internal P-channel MOSFET along with the current
limiting sensing. The VIN operating voltage range is from
2.7V to 5.5V for the MIC4723YML or 3.0V to 5.5V for the
MIC4723YMME. Due to the high switching speeds, a
10µF capacitor is recommended close to VIN and the
power ground (PGND) for each pin for bypassing.
Please refer to layout recommendations.
BIAS
The bias (BIAS) provides power to the internal reference
and control sections of the MIC4723. A 10Ω resistor
from VIN to BIAS and a 0.1µF from BIAS to SGND is
required for clean operation.
EN
The enable pin provides a logic level control of the
output. In the off state, supply current of the device is
greatly reduced (typically <1µA). Do not drive the enable
pin above the supply voltage.
FB
The feedback pin (FB) provides the control path to
control the output. For adjustable versions, a resistor
divider connecting the feedback to the output is used to
adjust the desired output voltage. The output voltage is
calculated as follows:
R1
R2
+×= 1
⎞
⎟
⎠
where V
VV
is equal to 1.0V.
REF
⎛
⎜
REFOUT
⎝
A feedforward capacitor is recommended for most
designs using the adjustable output voltage option. To
reduce current draw, a 10K feedback resistor is
recommended from the output to the FB pin (R1). Also, a
feedforward capacitor should be connected between the
output and feedback (across R1). The large resistor
value and the parasitic capacitance of the FB pin can
cause a high frequency pole that can reduce the overall
system phase margin. By placing a feedforward
capacitor, these effects can be significantly reduced.
Feedforward capacitance (C
) can be calculated as
FF
follows:
=
C
FF
π
1
200kHzR12
××
SW
The switch (SW) pin connects directly to the inductor
and provides the switching current necessary to operate
in PWM mode. Due to the high speed switching on this
pin, the switch node should be routed away from
sensitive nodes. This pin also connects to the cathode of
the free-wheeling diode.
PGOOD
Power good is an open drain pull down that indicates
when the output voltage has reached regulation. When
power good is low, then the output voltage is within
±10% of the set regulation voltage. For output voltages
greater or less than 10%, the PGOOD pin is high. This
should be connected to the input supply through a pull
up resistor. A delay can be added by placing a capacitor
from PGOOD to ground.
PGND
Power ground (PGND) is the ground path for the
MOSFET drive current. The current loop for the power
ground should be as small as possible and separate
from the Signal ground (SGND) loop. Refer to the layout
considerations for more details.
SGND
Signal ground (SGND) is the ground path for the biasing
and control circuitry. The current loop for the signal
ground should be separate from the power ground
(PGND) loop. Refer to the layout considerations for more
details.
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Micrel, Inc. MIC4723
(
−
Application Information
switch is turned on, current flows from the input supply
through the inductor and to the output. The inductor
The MIC4723 is a 3A PWM non-synchronous buck
current is:
regulator. By switching an input voltage supply, and
filtering the switched voltage through an Inductor and
capacitor, a regulated DC voltage is obtained. Figure 1
shows a simplified example of a non-synchronous buck
converter.
Figure 1. Example of non-synchronous buck converter
For a non-synchronous buck converter, there are two
modes of operation; continuous and discontinuous.
Continuous or discontinuous refer to the inductor
current. If current is continuously flowing through the
inductor throughout the switching cycle, it is in
continuous operation. If the inductor current drops to
zero during the off time, it is in discontinuous operation.
Critically continuous is the point where any decrease in
output current will cause it to enter discontinuous
operation. The critically continuous load current can be
calculated as follows;
2
⎡
⎢
V
⎢
I
OUT
⎣
=
OUT
V
−
OUT
V
IN
⎤
⎥
⎥
⎦
L22.0MHz
××
Continuous or discontinuous operation determines how
we calculate peak inductor current.
Continuous Operation
Figure 2 illustrates the switch voltage and inductor
current during continuous operation.
Figure 3. On-Time
charged at the rate;
)
VV
OUTIN
L
To determine the total on-time, or time at which the
inductor charges, the duty cycle needs to be calculated.
The duty cycle can be calculated as;
V
D =
OUT
V
IN
and the On time is;
T
ON
D
=
2.0MHz
Therefore, peak to peak ripple current is;
V
()
VV
OUTIN
I
=
pkpk
−
OUT
×−
V
IN
L2.0MHz
×
Since the average peak to peak current is equal to the
load current. The actual peak (or highest current the
inductor will see in a steady-state condition) is equal to
the output current plus ½ the peak-to-peak current.
V
OUT
×−
V
IN
L2.0MHz2
××
Figure 2. Continuous Operation
()
VV
OUTIN
II
+=
OUTpk
Figure 4 demonstrates the off-time. During the off-time,
the high-side internal P-channel MOSFET turns off.
The output voltage is regulated by pulse width
modulating (PWM) the switch voltage to the average
required output voltage. The switching can be broken up
into two cycles; On and Off.
Since the current in the inductor has to discharge, the
current flows through the free-wheeling Schottky diode
to the output. In this case, the inductor discharge rate is
(where V
is the diode forward voltage);
D
During the on-time, Figure 3 illustrates the high side
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Micrel, Inc. MIC4723
×
=
×
=
×
=
()
−
+
VV
DOUT
L
The total off time can be calculated as;
−
=
T
OFF
D1
2.0MHz
When the inductor current (IL) has completely
discharged, the voltage on the switch node rings at the
frequency determined by the parasitic capacitance and
the inductor value. In Figure 5, it is drawn as a DC
voltage, but to see actual operation (with ringing) refer to
the functional characteristics.
Discontinuous mode of operation has the advantage
over full PWM in that at light loads, the MIC4723 will skip
pulses as nessasary, reducing gate drive losses,
drastically improving light load efficiency.
Efficiency Considerations
Calculating the efficiency is as simple as measuring
power out and dividing it by the power in;
P
OUT
100
×=
P
IN
) is;
IN
IVP
INININ
) is calculated as;
OUT
IVP
OUTOUTOUT
Figure 4. Off-Time
Efficiency
Where input power (P
and output power (P
The Efficiency of the MIC4723 is determined by several
factors.
• Rdson (Internal P-channel Resistance)
Discontinuous Operation
Discontinuous operation is when the inductor current
discharges to zero during the off cycle. Figure 5
demonstrates the switch voltage and inductor currents
during discontinuous operation.
• Diode conduction losses
• Inductor Conduction losses
• Switching losses
Rdson losses are caused by the current flowing through
the high side P-channel MOSFET. The amount of power
loss can be approximated by;
2
DIRP
××=
OUTDSONSW
Where D is the duty cycle.
Since the MIC4723 uses an internal P-channel
MOSFET, Rdson losses are inversely proportional to
supply voltage. Higher supply voltage yields a higher
gate to source voltage, reducing the Rdson, reducing the
MOSFET conduction losses. A graph showing typical
Rdson vs input supply voltage can be found in the typical
characteristics section of this datasheet.
Diode conduction losses occur due to the forward
voltage drop (V
) and the output current. Diode power
F
losses can be approximated as follows;
()
OUTFD
D1IVP
−×
Figure 5. Discontinuous Operation
For this reason, the Schottky diode is the rectifier of
choice. Using the lowest forward voltage drop will help
reduce diode conduction losses, and improve efficiency.
Duty cycle, or the ratio of output voltage to input voltage,
determines whether the dominant factor in conduction
losses will be the internal MOSFET or the Schottky
diode. Higher duty cycles place the power losses on the
high side switch, and lower duty cycles place the power
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Micrel, Inc. MIC4723
losses on the Schottky diode.
Inductor conduction losses (P
) can be calculated by
L
multiplying the DC resistance (DCR) times the square of
the output current;
2
IDCRP ×=
OUTL
Also, be aware that there are additional core losses
associated with switching current in an inductor. Since
most inductor manufacturers do not give data on the
Figure 6. Switching Transition Losses
type of material used, approximating core losses
becomes very difficult, so verify inductor temperature
rise.
Switching losses occur twice each cycle, when the
switch turns on and when the switch turns off. This is
caused by a non-ideal world where switching transitions
are not instantaneous, and neither are currents. Figure 6
demonstrates how switching losses due to the
transitions dissipate power in the switch.
Normally, when the switch is on, the voltage across the
switch is low (virtually zero) and the current through the
switch is high. This equates to low power dissipation.
When the switch is off, voltage across the switch is high
and the current is zero, again with power dissipation
being low. During the transitions, the voltage across the
switch (V
) and the current through the switch (I
S-D
S-D
at middle, causing the transition to be the highest
instantaneous power point. During continuous mode,
these losses are the highest. Also, with higher load
currents, these losses are higher. For discontinuous
operation, the transition losses only occur during the “off”
transition since the “on” transitions there is no current
flow through the inductor.
) are
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Micrel, Inc. MIC4723
Component Selection
Input Capacitor
A 10µF ceramic is recommended on each VIN pin for
bypassing. X5R or X7R dielectrics are recommended for
the input capacitor. Y5V dielectrics lose most of their
capacitance over temperature and are therefore, not
recommended. Also, tantalum and electrolytic capacitors
alone are not recommended due their reduced RMS
current handling, reliability, and ESR increases.
An additional 0.1µF is recommended close to the VIN
and PGND pins for high frequency filtering. Smaller case
size capacitors are recommended due to their lower
ESR and ESL. Please refer to layout recommendations
for proper layout of the input capacitor.
Output Capacitor
The MIC4723 is designed for a 4.7µF output capacitor.
X5R or X7R dielectrics are recommended for the output
capacitor. Y5V dielectrics lose most of their capacitance
over temperature and are therefore not recommended.
In addition to a 4.7µF, a small 0.1µF is recommended
close to the load for high frequency filtering. Smaller
case size capacitors are recommended due to there
lower equivalent series ESR and ESL.
The MIC4723 utilizes type III voltage mode internal
compensation and utilizes an internal zero to
compensate for the double pole roll off of the LC filter.
For this reason, larger output capacitors can create
instabilities. In cases where a 4.7µF output capacitor is
not sufficient, other values of capacitance can be used
but the original LC filter pole frequency determined by
CO = 4.7µF + L = 1µH (which is approximately 73.4KHz)
must remain fixed. Increasing COUT forces L to
decrease and vice versa.
Inductor Selection
The MIC4723 is designed for use with a 1µH inductor.
Proper selection should ensure the inductor can handle
the maximum average and peak currents required by the
load. Maximum current ratings of the inductor are
generally given in two methods; permissible DC current
and saturation current. Permissible DC current can be
rated either for a 40°C temperature rise or a 10% to 20%
loss in inductance. Ensure the inductor selected can
handle the maximum operating current. When saturation
current is specified, make sure that there is enough
margin that the peak current will not saturate the
inductor.
Diode Selection
Since the MIC4723 is non-synchronous, a free-wheeling
diode is required for proper operation. A Schottky diode
is recommended due to the low forward voltage drop
and their fast reverse recovery time. The diode should
be rated to be able to handle the average output current.
Also, the reverse voltage rating of the diode should
exceed the maximum input voltage. The lower the
forward voltage drop of the diode the better the
efficiency. Please refer to the layout recommendations to
minimize switching noise.
Feedback Resistors
The feedback resistor set the output voltage by dividing
down the output and sending it to the feedback pin. The
feedback voltage is 1.0V. Calculating the set output
voltage is as follows;
R1
R2
+= 1
⎞
⎟
⎠
⎛
VV
⎜
FBOUT
⎝
Where R1 is the resistor from VOUT to FB and R2 is the
resistor from FB to GND. The recommended feedback
resistor values for common output voltages are available
in the bill of materials on page 19. Although the range of
resistance for the FB resistors is very wide, R1 is
recommended to be 10K. This minimizes the effect the
parasitic capacitance of the FB node.
Feedforward Capacitor (CFF)
A capacitor across the resistor from the output to the
feedback pin (R1) is recommended for most designs.
This capacitor can give a boost to phase margin and
increase the bandwidth for transient response. Also,
large values of feedforward capacitance can slow down
the turn-on characteristics, reducing inrush current. For
maximum phase boost, C
Bias filter
C
=
FF
π
can be calculated as follows;
FF
1
R1200kHz2
××
A small 10Ω resistor is recommended from the input
supply to the bias pin along with a small 0.1µF ceramic
capacitor from bias to ground. This will bypass the high
frequency noise generated by the violent switching of
high currents from reaching the internal reference and
control circuitry. Tantalum and electrolytic capacitors are
not recommended for the bias, these types of capacitors
lose their ability to filter at high frequencies.
June 2008
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Micrel, Inc. MIC4723
Loop Stability and Bode Analysis
Bode analysis is an excellent way to measure small
signal stability and loop response in power supply
designs. Bode analysis monitors gain and phase of a
Network
Analyzer
“R” Input
Feedback
+8V
MIC922BC5
R1
1k
Network
Analyzer
“A” Input
Output
control loop. This is done by breaking the feedback loop
and injecting a signal into the feedback node and
comparing the injected signal to the output signal of the
control loop. This will require a network analyzer to
sweep the frequency and compare the injected signal to
the output signal. The most common method of injection
R3
1k
R4
1k
50
Network Analyzer
Source
is the use of transformer. Figure 7 demonstrates how a
transformer is used to inject a signal into the feedback
network.
Figure 8. Op Amp Injection
R1 and R2 reduce the DC voltage from the output to the
non-inverting input by half. The network analyzer is
generally a 50Ω source. R1 and R2 also divide the AC
signal sourced by the network analyzer by half. These
two signals are “summed” together at half of their
original input. The output is then gained up by 2 by R3
and R4 (the 50Ω is to balance the network analyzer’s
source impedance) and sent to the feedback signal. This
Figure 7. Transformer Injection
essentially breaks the loop and injects the AC signal on
top of the DC output voltage and sends it to the
feedback. By monitoring the feedback “R” and output
A 50Ω resistor allows impedance matching from the
network analyzer source. This method allows the DC
loop to maintain regulation and allow the network
analyzer to insert an AC signal on top of the DC voltage.
The network analyzer will then sweep the source while
monitoring A and R for an A/R measurement. While this
is the most common method for measuring the gain and
phase of a power supply, it does have significant
limitations. First, to measure low frequency gain and
phase, the transformer needs to be high in inductance.
This makes frequencies <100Hz require an extremely
large and expensive transformer. Conversely, it must be
able to inject high frequencies. Transformers with these
wide frequency ranges generally need to be custom
made and are extremely expensive (usually in the tune
of several hundred dollars!). By using an op-amp, cost
and frequency limitations used by an injection
transformer are completely eliminated. Figure 8
demonstrates using an op-amp in a summing amplifier
configuration for signal injection.
“A”, gain and phase are measured. This method has no
minimum frequency. Ensure that the bandwidth of the
op-amp being used is much greater than the expected
bandwidth of the power supplies control loop. An op-amp
with >100MHz bandwidth is more than sufficient for most
power supplies (which includes both linear and
switching) and are more common and significantly
cheaper than the injection transformers previously
mentioned. The one disadvantage to using the op-amp
injection method; is the supply voltages need to below
the maximum operating voltage of the op-amp. Also, the
maximum output voltage for driving 50Ω inputs using the
MIC922 is 3V. For measuring higher output voltages,
1MΩ input impedance is required for the A and R
channels. Remember to always measure the output
voltage with an oscilloscope to ensure the measurement
is working properly. You should see a single sweeping
sinusoidal waveform without distortion on the output. If
there is distortion of the sinusoid, reduce the amplitude
of the source signal. You could be overdriving the
feedback causing a large signal response.
June 2008
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Micrel, Inc. MIC4723
The following Bode analysis show the small signal loop
stability of the MIC4723, it utilizes type III compensation.
This is a dominant low frequency pole, followed by 2
zeros and finally the double pole of the inductor
capacitor filter, creating a final 20dB/decade roll off.
Bode analysis gives us a few important data points;
speed of response (Gain Bandwidth or GBW) and loop
stability. Loop speed or GBW determines the response
time to a load transient. Faster response times yield
smaller voltage deviations to load steps.
Instability in a control loop occurs when there is gain and
positive feedback. Phase margin is the measure of how
stable the given system is. It is measured by determining
how far the phase is from crossing zero when the gain is
equal to 1 (0dB).
V
IN
60
50
40
30
20
10
GAIN (dB)
0
-10
-20
-30
100 1k
Bode Plot
=3.3V, V
L=1µH
C
= 4.7µF
OUT
R1 = 10k
R2 = 12.4k
C
= 82pF
FF
FREQUENCY (Hz)
=1.8V, I
OUT
PHASE
GAIN
10k 100k
OUT
=3A
1M
210
175
140
105
70
35
0
-35
-70
-105
PHASE (°)
Typically for 3.3Vin and 1.8Vout at 3A;
•
Phase Margin=47 Degrees
• GBW=156KHz
Gain will also increase with input voltage. The following
graph shows the increase in GBW for an increase in
supply voltage.
Bode Plot
V
IN
60
50
40
30
20
10
L=1µH
GAIN (dB)
C
0
R1 = 10k
-10
R2 = 12.4k
-20
C
-30
100 1k
=5V, V
OUT
= 82pF
FF
=1.8V, I
OUT
PHASE
= 4.7µF
FREQUENCY (Hz)
GAIN
10k 100k
OUT
=3A
1M
210
175
140
105
70
35
0
-35
-70
-105
PHASE (°)
regulator only has the ability to source current. This
means that the regulator has to rely on the load to be
able to sink current. This causes a non-linear response
at light loads. The following plot shows the effects of the
pole created by the nonlinearity of the output drive
during light load (discontinuous) conditions.
Bode Plot
=3.3V,V
V
IN
60
50
40
30
20
10
L=1µH
GAIN (dB)
C
0
R1 = 10k
-10
R2 = 12.4k
-20
C
-30
100 1k
OUT
= 82pF
FF
=1.8V,I
OUT
PHASE
= 4.7µF
GAIN
10k 100k
FREQUENCY (Hz)
OUT
=50mA
1M
210
175
140
105
70
35
0
-35
-70
-105
PHASE (°)
3.3Vin, 1.8Vout Iout=50mA;
•
Phase Margin=90.5 Degrees
• GBW= 64.4KHz
Feed Forward Capacitor
The feedback resistors are a gain reduction block in the
overall system response of the regulator. By placing a
capacitor from the output to the feedback pin, high
frequency signal can bypass the resistor divider, causing
a gain increase up to unity gain.
Gain and Phase
vs. Frequency
0
L=1µH
-1
C
-2
R1 = 10k
-3
R2 = 12.4k
C
-4
-5
-6
GAIN (dB)
-7
-8
-9
-10
100 1k
= 4.7µF
OUT
= 82pF
FF
FREQUENCY (Hz)
GAIN
PHASE
10k 100k
1M
25
20
15
10
5
0
PHASE BOOST (°)
The graph above shows the effects on the gain and
phase of the system caused by feedback resistors and a
feedforward capacitor. The maximum amount of phase
boost achievable with a feedforward capacitor is
graphed below.
5Vin, 1.8Vout at 3A load;
•
Phase Margin=43.1 Degrees
• GBW= 218KHz
Being that the MIC4723 is non-synchronous; the
June 2008
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M9999-060308-E
Page 15
Micrel, Inc. MIC4723
Max. Amount of Phase Boost
Obtainable using C
50
45
40
35
30
25
20
15
PAHSE BOOST (°)
10
5
0
12345
Voltage
OUTPUT VOLTAGE (V)
vs. Output
FF
V
= 1V
REF
By looking at the graph, phase margin can be affected to
a greater degree with higher output voltages.
The next bode plot shows the phase margin of a 1.8V
output at 3A without a feedforward capacitor.
V
IN
60
50
40
30
20
10
GAIN (dB)
0
-10
-20
-30
100 1k
As one can see, the typical phase margin, using the
same resistor values as before without a feedforward
capacitor results in 33.6 degrees of phase margin. Our
prior measurement with a feedforward capacitor yielded
a phase margin of 47 degrees. The feedforward
Bode Plot
=3.3V, V
L=1µH
C
= 4.7µF
OUT
R1 = 10k
R2 = 12.4k
= 0pF
C
FF
FREQUENCY (Hz)
=1.8V, I
OUT
PHASE
GAIN
10k 100k
OUT
=3A
1M
210
175
140
105
70
35
0
-35
-70
-105
PHASE (°)
capacitor has given us a phase boost of 13.4 degrees
(47 degrees- 33.6 Degrees = 13.4 Degrees).
June 2008
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Micrel, Inc. MIC4723
∆=∆
Output Impedance and Transient
Response
Output impedance, simply stated, is the amount of
output voltage deviation vs. the load current deviation.
The lower the output impedance, the better.
V
∆
OUT
Z
OUT
=
∆
Output impedance for a buck regulator is the parallel
impedance of the output capacitor and the MOSFET and
inductor divided by the gain;
Z
TOTAL
=
To measure output impedance vs. frequency, the load
current must be load current must be swept across the
I
OUT
GAIN
XDCRR
++
LDSON
X
COUT
I
The following graph shows output impedance vs
frequency at 3A load current sweeping the AC current
from 10Hz to 10MHz, at 1A peak to peak amplitude.
dBm
=∆
10
Output Impedance
1
V
=1.8V
OUT
L=1µH
=4.7µF + 0.1µ
C
OUT
0.1
0.01
R707.0
×
LOAD
vs. Frequency
3.3VIN
5V
250Ω1mW10
×××
IN
frequencies measured, while the output voltage is
monitored. Figure 9 shows a test set-up to measure
output impedance from 10Hz to 1MHz using the
MIC5190 high speed controller.
OUTPUT IMPEDANCE (Ohms)
0.001
10
100
1k
10k 100k
FREQUENCY (Hz)
1M
From this graph, one can see the effects of bandwidth
and output capacitance. For frequencies <200KHz, the
output impedance is dominated by the gain and
inductance. For frequencies >200KHz, the output
impedance is dominated by the capacitance. A good
approximation for transient response can be calculated
from determining the frequency of the load step in amps
per second;
A/sec
=
Figure 9. Output Impedance Measurement
By setting up a network analyzer to sweep the feedback
current, while monitoring the output of the voltage
regulator and the voltage across the load resistance,
output impedance is easily obtainable. To keep the
current from being too high, a DC offset needs to be
applied to the network analyzer’s source signal. This can
be done with an external supply and 50Ω resistor. Make
sure that the currents are verified with an oscilloscope
first, to ensure the integrity of the signal measurement. It
is always a good idea to monitor the A and R
f
Then, determine the output impedance by looking at the
output impedance vs frequency graph. Then calculating
the voltage deviation times the load step;
The output impedance graph shows the relationship
between supply voltage and output impedance. This is
caused by the lower Rdson of the high side MOSFET
and the increase in gain with increased supply voltages.
This explains why higher supply voltages have better
transient response.
Z
TOTAL
π
2
ZIV ×
OUTOUTOUT
++↓
XDCRR
=↓
↑
GAIN
LDSON
X
COUT
measurements with a scope while you are sweeping it.
To convert the network analyzer data from dBm to
something more useful (such as peak to peak voltage
and current in our case);
dBm
V
=∆
10
707.0
250Ω1mW10
×××
and peak to peak current;
June 2008
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M9999-060308-E
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Micrel, Inc. MIC4723
Ripple measurements
To properly measure ripple on either input or output of a
switching regulator, a proper ring in tip measurement is
required. Standard oscilloscope probes come with a
grounding clip, or a long wire with an alligator clip.
Unfortunately, for high frequency measurements, this
ground clip can pick-up high frequency noise and
erroneously inject it into the measured output ripple.
The standard evaluation board accommodates a home
made version by providing probe points for both the
input and output supplies and their respective grounds.
This requires the removing of the oscilloscope probe
sheath and ground clip from a standard oscilloscope
probe and wrapping a non-shielded bus wire around the
oscilloscope probe. If there does not happen to be any
non-shielded bus wire immediately available, the leads
from axial resistors will work. By maintaining the
shortest possible ground lengths on the oscilloscope
probe, true ripple measurements can be obtained.
June 2008
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M9999-060308-E
Page 18
Micrel, Inc. MIC4723
MIC4723 Schematic and BOM for 3A Output
Item Part Number Manufacturer Description Qty
C1a,C1b
C2 0402ZD104MAT AVX 0.1µF Ceramic Capacitor X5R 0402 10V 1
C3
C2012JB0J106K TDK
GRM219R60J106KE19 Murata
08056D106MAT AVX
C2012JB0J475K TDK
GRM188R60J475KE19 Murata
10µF Ceramic Capacitor X5R 0805 6.3V 2
4.7µF Ceramic Capacitor X5R 0603 6.3V 1
06036D475MAT AVX
C4 VJ0403A820KXAA Vishay VT 82pF Ceramic Capacitor 0402 1
D1 SSA33L Vishay Semi 3A Schottky 30V SMA 1
L1
RLF7030-1R0N6R4 TDK 1µH Inductor 8.8mΩ 7.1mm(L) x 6.8mm (W)x 3.2mm(H) 1
744 778 9001 Wurth Elektronik 1µH Inductor 12mΩ 7.3mm(L)x7.3mm(W)x3.2mm(H) 1
IHLP2525AH-01 1 Visha y Dale 1µH Inductor 17.5mΩ 6.47mm(L)x6.86mm(W)x1.8mm(H) 1
R1,R4 CRCW04021002F Vishay Dale 10KΩ1% 0402 resistor 1
R2
CRCW04026651F 6.65kΩ 1% 0402 For 2.5V
CRCW04021242F 12.4kΩ 1% 0402 For 1.8 V
CRCW04022002F 20kΩ 1% 0402 For 1.5 V
CRCW04024022F
Vishay Dale
49.9kΩ 1% 0402 For 1.2 V
Open For 1.0 V
OUT
OUT
OUT
OUT
OUT
R3 CRCW040210R0F Vishay Dale 10Ω1% 0402 resistor 1
U1 MIC4723YML Micrel, Inc. 3A 2MHz Integrated Switch Buck Regulator 1
Notes:
1. TDK: www.tdk.com
2. Murata: www.murata.com
3. AVX: www.avx.com
4. Vishay: www.vishay.com
5. Wurth Elektronik: www.we-online.com
6. Micrel, Inc: www.micrel.com
1
June 2008
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M9999-060308-E
Page 19
Micrel, Inc. MIC4723
Package Information
12-Pin 3mm x 3mm MLF® (ML)
June 2008
10-Pin ePAD MSOP (MME)
19
M9999-060308-E
Page 20
Micrel, Inc. MIC4723
MICREL, INC. 2180 FORTUNE DRIVE SAN JOSE, CA 95131 USA
TEL +1 (408) 944-0800 FAX +1 (408) 474-1000 WEB http://www.micrel.com
The information furnished by Micrel in this data sheet is believed to be accurate and reliable. However, no responsibility is assumed by Micrel for its
Micrel Products are not designed or authorized for use as components in life support appliances, devices or systems where malfunction of a product
can reasonably be expected to result in personal injury. Life support devices or systems are devices or systems that (a) are intended for surgical implant
into the body or (b) support or sustain life, and whose failure to perform can be reasonably expected to result in a significant injury to the user. A
Purchaser’s use or sale of Micrel Products for use in life support appliances, devices or systems is a Purchaser’s own risk and Purchaser agrees to fully
June 2008
use. Micrel reserves the right to change circuitry and specifications at any time without notification to the customer.
indemnify Micrel for any damages resulting from such use or sale.
© 2007 Micrel, Incorporated.
20
M9999-060308-E
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