MICREL MIC4721

MIC4721
1.5A 2MHz Integrated Switch
Buck Regulator
General Description
Features
The Micrel MIC4721 is a high efficiency PWM buck (stepdown) regulators that provides up to 1.5A of output
current. The MIC4721 operates at 2MHz and has
proprietary internal compensation that allows a closed loop
bandwidth of over 200KHz.
The low on-resistance internal p-channel MOSFET of the
MIC4721 allows efficiencies up to 94%, reduces external
components count and eliminates the need for an
expensive current sense resistor.
The MIC4721 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 MIC4721 is available in the 10-pin MSOP package
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.
•
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•
•
•
•
•
•
•
•
•
•
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2.7 to 5.5V supply voltage
2MHz PWM mode
Output current to 1.5A
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 1.5A MOSFET switch
Micropower shutdown
Power Good pin
Thermal shutdown and current limit protection
Pb-free 10-pin 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
5V IN to 3.3VO Efficiency
94
92
90
88
86
84
82
80
78
1.5A 2MHz Buck Regulator
76
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
OUTPUT CURRENT (A)
Micrel Inc. • 2180 Fortune Drive • San Jose, CA 95131 • USA • tel +1 (408) 944-0800 • fax + 1 (408) 474-1000 • http://www.micrel.com
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Micrel, Inc.
MIC4721
Ordering Information
Part Number
Voltage
Temperature Range
Package
Lead Finish
MIC4721YMM
Adj.
–40° to +125°C
10-Pin MSOP
Pb-Free
Pin Configuration
SW
1
10 SW
VIN
2
9
VIN
SGND
3
8
PGND
BIAS
4
7
PGOOD
FB
5
6
EN
10-Pin MSOP (MM)
Pin Description
Pin Number
Pin Name
Pin Function
1, 10
SW
Switch (Output): Internal power P-Channel MOSFET output switch.
2, 9
VIN
Supply Voltage (Input): Supply voltage for the source of the internal P-channel
MOSFET and driver.
3
SGND
4
BIAS
5
FB
Feedback. Input to the error amplifier, connect to the external resistor divider
network to set the output voltage.
6
EN
Enable (Input). Logic level low will shutdown the device, reducing the current
draw to less than 5µA.
7
PGOOD
8
PGND
Requires bypass capacitor to GND.
May 2007
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.
Power Good. Open drain output that is pulled to ground when the output voltage
is within ±7.5% of the set regulation voltage.
Power Ground. Provides the ground return path for the high-side drive current.
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MIC4721
Absolute Maximum Ratings(1)
Operating Ratings(2)
Supply Voltage (VIN) .......................................................+6V
Output Switch Voltage (VSW) ..........................................+6V
Output Switch Current (ISW)............................................11A
Logic Input Voltage (VEN) .................................. –0.3V to VIN
Storage Temperature (Ts) .........................–60°C to +150°C
Supply Voltage (VIN)..................................... +2.7V to +5.5V
Logic Input Voltage (VEN) ....................................... 0V to VIN
Junction Temperature (TJ) ........................ –40°C to +125°C
Junction Thermal Resistance
MSOP-10L (θJA)...............................................130°C/W
MSOP-10L (θJC) ................................................43°C/W
Electrical Characteristics(3)
VIN = VEN = 3.6V; L = 1µH; COUT = 4.7µF; TA = 25°C, unless noted. Bold values indicate –40°C< TJ < +125°C.
Parameter
Condition
Min
(turn-on)
2.45
Max
Units
5.5
V
2.55
2.65
V
2.7
Supply Voltage Range
Under-Voltage Lockout
Threshold
Typ
UVLO Hysteresis
100
Quiescent Current
VFB = 0.9 * VNOM (not switching)
Shutdown Current
VEN = 0V
[Adjustable] Feedback
Voltage
± 2% (over temperature) ILOAD = 100mA
900
µA
2
10
µA
1.02
V
0.98
FB pin input current
3.5
mV
570
1
nA
5
A
Current Limit in PWM Mode
VFB = 0.9 * VNOM
Output Voltage Line
Regulation
VOUT > 2V; VIN = VOUT+500mV to 5.5V; ILOAD= 100mA
VOUT < 2V; VIN = 2.7V to 5.5V; ILOAD= 100mA
0.07
%
Output Voltage Load
Regulation
20mA < ILOAD < 3A
0.2
%
Maximum Duty Cycle
VFB ≤ 0.4V
PWM Switch ONResistance
ISW = 50mA; VFB = 0.7VFB_NOM (High Side Switch)
100
%
95
200
300
mΩ
mΩ
Oscillator Frequency
1.8
2
2.2
MHz
Enable Threshold
0.5
0.85
1.3
V
Enable Hysteresis
50
Enable Input Current
0.1
2.3
µA
Power Good Range
±7
±10
%
145
250
Ω
Power Good Resistance
IPGOOD = 500µA
mV
Over-Temperature
Shutdown
160
°C
Over-Temperature
Hysteresis
20
°C
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.
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MIC4721
Typical Characteristics
3.3V IN to 2.5VO Efficiency
94
93
92
91
90
3.3V IN to 1.8VO Efficiency
3.3VIN to 1.5VO Efficiency
90
86
88
84
86
82
84
80
89
88
82
78
87
80
76
86
85
78
74
84
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
OUTPUT CURRENT (A)
76
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
OUTPUT CURRENT (A)
72
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
OUTPUT CURRENT (A)
3.3V IN to 1.2VO Efficiency
3.3V IN to 1VO Efficiency
5V IN to 3.3VO Efficiency
82
90
94
80
80
92
78
70
90
60
88
50
86
40
84
30
82
70
20
80
68
10
78
76
74
72
66
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
OUTPUT CURRENT (A)
0
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
OUTPUT CURRENT (A)
76
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
OUTPUT CURRENT (A)
5V IN to 1.8VO Efficiency
5VIN to 1.5VO Efficiency
5V IN to 2.5VO Efficiency
100
90
90
90
80
80
80
70
70
70
60
60
60
50
40
50
50
40
40
30
30
30
20
10
20
20
10
10
0
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
OUTPUT CURRENT (A)
0
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
OUTPUT CURRENT (A)
5V IN to 1.2VO Efficiency
5V IN to 1VO Efficiency
80
80
70
70
60
60
50
50
40
40
30
30
20
20
10
10
0
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
OUTPUT CURRENT (A)
May 2007
0
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
OUTPUT CURRENT (A)
0
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
OUTPUT CURRENT (A)
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MIC4721
Typical Characteristics
Line Regulation
Load Regulation
0.02
0.000
-0.005
-0.010
0.00
1.005
1.000
-0.015
-0.020
-0.025
-0.030
-0.02
-0.04
-0.06
-0.08
VO = 1.8V
VIN = 3.3V
-0.10
0
0.5
1.0
1.5
OUTPUT CURRENT (A)
2.0
Reference Voltage
vs. Input Voltage
1.002
1.001
2.02
1.000
2.00
0.999
1.98
0.998
1.96
0.997
1.94
0.996
1.92
0.995
2.5
1.90
0.995
IOUT = 100mA
0.990
0.985
IOUT = 1.5A
-0.035
-0.040
-0.045
VO = 1.8V
-0.050
2
3
4
5
INPUT VOLTAGE (V)
2.04
Reference Voltage
vs. Temperature
0.980
0.975
6
Switching Frequency
vs. Temperature
0.970
VIN = 3.3V
20 40 60 80
TEMPERATURE (°C)
Switching Frequency
vs. Input Voltage
2.20
2.15
2.10
2.05
2.00
3
3.5 4 4.5 5 5.5
INPUT VOLTAGE (V)
6
Quiescent Current
vs. Temperature
3.5
1.4
3.0
1.2
2.5
1.0
2.0
1.95
1.90
1.85
1.80
2.5
20 40 60 80
TEMPERATURE (°C)
Quiescent Current
vs. Input Voltage
200
120
100
0.8
0.6
1.0
0.4
80
60
40
VIN = 3.3V
0.5
0.2
0
180
0
0
20 40 60 80
TEMPERATURE (°C)
Switch On-Resistance
vs. Input Voltage
1.10
1.05
170
160
150
2
3
4
5
INPUT VOLTAGE (V)
6
1.00
RON @ IOUT = 0.1A
100
2.5
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3.5 4 4.5 5 5.5
INPUT VOLTAGE (V)
6
0.60
2.5
Switch On-Resistance
vs. Temperature
RON @ 2.7VIN
RON @ 5VIN
RON @ 3.3VIN
20 40 60 80
TEMPERATURE (°C)
Enable Threshold
vs. Temperature
0.90
Turn-on
0.85
0.80
Turn-off
Turn-off
0.75
0.70
0.70
0.65
110
6
0.95
Turn-on
0.80
0.75
130
20
0
Enable Threshold
vs. Input Voltage
0.90
0.85
140
120
1
1.00
0.95
RON @ IOUT = 1.5A
3.5 4 4.5 5 5.5
INPUT VOLTAGE (V)
180
160
140
VIN = 2.5V
1.5
3
0.65
3
3.5 4 4.5 5 5.5
INPUT VOLTAGE (V)
5
6
0.60
VIN = 3.3V
20 40 60 80
TEMPERATURE (°C)
M9999-052907-A
Micrel, Inc.
MIC4721
Functional Characteristics
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MIC4721
Functional Diagram
VIN
VIN
P-Channel
Current Limit
BIAS
HSD
SW
SW
PWM
Control
EN
Enable and
Control Logic
Bias,
UVLO,
Thermal
Shutdown
Soft
Start
FB
EA
1.0V
PGOOD
1.0V
PGND
SGND
MIC4721 Block Diagram
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MIC4721
Pin Descriptions
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. Due to the high switching speeds, a 10µF
capacitor should be placed close to VIN and power
ground (PGND) at each pin for bypassing. Please refer
to the section on layout recommendations.
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
The power good pin pulls low to indicate the output
voltage is within its regulation range. 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.
BIAS
The bias (BIAS) provides power to the internal reference
and control sections of the MIC4721. A 10Ω resistor
from VIN to BIAS and a 0.1µF from BIAS to SGND is
required.
EN
The enable pin provides a logic level control of the
output. At a low level, the output is shut off and supply
current at the VIN pin is greatly reduced (typically <2µA).
A high level turns the output on. The enable pin must not
be driven above the supply voltage.
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
section on layout recommendations for more details.
FB
The feedback pin (FB) connects to the negative input of
the internal error amplifier. The pin is connected to an
external resistor divider, which sets the output voltage.
A feedforward capacitor across the upper resistor in the
divider is recommended for most designs. To reduce
current draw, a 10K feedback resistor is recommended
from the output to the FB pin (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.
May 2007
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 section on layout
recommendations for more details.
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MIC4721
Application Information
The MIC4721 is a 1.5A PWM non-synchronous buck
regulator. A regulated DC output voltage is obtained by
switching an input voltage supply, and filtering the
switched voltage through an inductor and capacitor.
Figure 1 shows a simplified example of a non-synchronous buck converter and its input/output voltage.
Figure 2. Continuous Operation
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.
As seen in Figure 3, the high side switch is turned on
(on-time) and current flows from the input supply through
the inductor and to the output.
Figure 1. Simplified Buck Converter
For a non-synchronous buck converter, there are two
modes of operation; continuous and discontinuous. The
mode refers to the state of current in the inductor. If
current continuously flows 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.
VOUT
VIN
2 × L × fS
2
VOUT −
I OUT _ CRITICAL =
Where: fS is the switching frequency (2MHz for the
MIC4721).
L is the output inductance (Henry).
When IOUT is less than IOUT_CRITICAL, the buck converter
operates in discontinuous mode and the inductor current
goes to zero before the end of each switching cycle.
When IOUT is greater than IOUT_CRITICAL, the converter
operates in continuous mode and current always flows in
the inductor. Continuous or discontinuous operation
determines how peak inductor current is calculated.
Figure 3. On-Time
The inductor current is charged at the rate:
(VIN
− VOUT )
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:
Continuous Operation
Figure 2 illustrates the switch voltage and inductor
current during continuous operation.
D=
VOUT
VIN
and the On time is:
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Ton =
MIC4721
Discontinuous Operation
Discontinuous operation is when the inductor current
discharges to zero at sometime during the off cycle.
Figure 5 demonstrates the switch voltage and inductor
currents during discontinuous operation.
D
fS
therefore, peak to peak ripple current is:
(VIN
I PK −PK = I OUT +
− VOUT ) ×
VOUT
VIN
fS × L
Peak-to-peak ripple current is used in calculating output
voltage ripple.
The peak current (or maximum inductor current) is equal
to the output current plus ½ the peak-to-peak current.
(VIN
I PK = I OUT +
VOUT
VIN
2 × fS × L
− VOUT ) ×
The peak inductor current is used when selecting a
suitable output inductor.
Figure 4 demonstrates circuit activity during the off-time.
When the high-side internal P-channel MOSFET turns
off, current must flow through the free-wheeling diode,
since inductor current must remain continuous. In this
case, the inductor discharge rate is:
+ VD )
L
The total off time can be calculated as:
−
(VOUT
Toff =
Figure 5. Discontinuous Operation
1− D
fS
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 MIC4721 will skip
pulses as necessary, reducing gate drive losses,
drastically improving light load efficiency.
Duty Cycle Considerations
The P-Channel MOSFET inside the MIC4721 allows the
FET to remain in the on-state indefinitely (100% duty
cycle). This feature is useful for maintaining output
voltage regulation in battery powered and other
applications where the input voltage drops close to the
output voltage.
When the MOSFET is operating at the 2MHz switching
frequency, the maximum operating duty cycle is typically
82%. In situations where a duty cycle greater than the
maximum operating duty cycle is required, the MIC4721
will keep the FET turned on for additional cycles to
maintain output voltage regulation. This effectively
decreases the switching frequency, allowing a larger
duty cycle. The output voltage ripple increases slightly
when the MIC4721 is in pulse skipping mode.
Figure 4. Off-Time
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MIC4721
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 current
transitions. Figure 6 demonstrates (or exaggerates…)
how switching losses due to the transitions dissipate
power in the switch.
Efficiency Considerations
Calculating the efficiency is as simple as measuring
power out and dividing it by the power in.
Efficiency =
POUT
× 100
PIN
Where input power (PIN) is:
PIN = VIN × IIN
and output power (POUT) is calculated as:
POUT = VOUT × IOUT
The Efficiency of the MIC2207 is determined by several
factors.
•
RDSON (Internal P-channel Resistance)
•
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:
Figure 6. Switching Transition Losses
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 (VS-D) and the current through the switch (IS-D)
are at midpoint of their excursions and cause 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.
PSW = RDSON × IOUT2 × D
Where D is the duty cycle.
Since the MIC4721 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, thus
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 (VF) and the output current. Diode power
losses can be approximated as follows:
PD = VF × IOUT × (1 – D)
Component Selection
For this reason, the low forward voltage drop Schottky
diode is the rectifier of choice. The low 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 majority of power loss on the
Schottky diode.
Inductor conduction losses (PL) can be calculated by
multiplying the DC resistance (DCR) times the square of
the output current:
PL = DCR × IOUT2
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 voltage and are
therefore not recommended. Also, tantalum and
electrolytic capacitors alone are not recommended
because of their reduced RMS current handling,
reliability, and higher ESR. Smaller case size capacitors
are recommended due to their lower ESL (equivalent
series
inductance).
Please
refer
to
layout
recommendations for proper layout of the input
capacitors.
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
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MIC4721
Output Capacitor
The MIC4721 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 voltage and are therefore, not
recommended. Smaller case size capacitors are
recommended due to their lower ESL. The MIC4721
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, the MIC2208
offers the ability to externally control the compensation,
allowing for a wide range of output capacitor types and
values.
calculated by:
Inductor Selection
The MIC4721 is designed for use with a 1µH inductor.
Proper selection should ensure the inductor can handle
the maximum RMS 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 since at higher
temperatures, the inductor will saturate at a lower
current.
Bias filter
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 since these types of capacitors aren’t
as effective at filtering high frequencies.
R2 =
R1 × VFB
VOUT − VFB
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, CFF can be calculated as follows:
C FF =
1
2π × 200KHz × R1
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
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
is the use of a transformer. Figure 7 demonstrates how a
transformer is used to inject a signal into the feedback
network.
Diode Selection
Since the MIC4721 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 fast reverse recovery time. The diode should be
rated to handle the average output current. Also, the
reverse voltage rating of the diode should exceed the
maximum input voltage. 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
⎞
VOUT = VFB ⎜
+ 1⎟
⎝ R2
⎠
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 at the
end of this specification. Although the resistance range
of the FB resistors is very wide, R1 is recommended to
be 10K. This minimizes the effect the parasitic
capacitance of the FB node. Resistor R2 can be
May 2007
Figure 7. Transformer Injection
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.
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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 to the tune
of several hundred dollars!). By using an op-amp, cost
and frequency limitations caused by an injection
transformer are completely eliminated. Figure 8
demonstrates using an op-amp in a summing amplifier
configuration for signal injection.
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.
The following Bode analysis show the small signal loop
stability of the MIC4721. The MIC4721 utilizes a type III
compensation. This is a dominant low frequency pole,
followed by 2 zero’s 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).
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 amplified by 2 by R3
and R4 (the 50Ω is to balance the network analyzer’s
source impedance) and sent to the feedback signal. This
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”, 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 supply’s 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 be
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, a 1M input impedance is required for the A and
May 2007
Typically for 3.3VIN and 1.8VOUT at 1.5A;
•
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.
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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 and 1.8VOUT at 1.5A load;
•
Phase Margin = 43.1 Degrees
• GBW = 218KHz
The non-synchronous MIC4721 regulator only has the
ability to source current. This means that the regulator
must rely on the load to sink current. This causes a nonlinear 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.
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
1.5A without a feedforward capacitor.
3.3VIN and 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.
May 2007
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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
capacitor has given us a phase boost of 13.4 degrees
(47 degrees – 33.6 Degrees = 13.4 Degrees).
10
∆V =
and peak to peak current:
10
∆V =
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.
Z OUT =
dBm
× 1mW × 50Ω × 2
10
.707
dBm
× 1mW × 50Ω × 2
10
.707 × R LOAD
The following graph shows output impedance vs
frequency at 2A load current sweeping the AC current
from 10Hz to 10MHz, at 1A peak-to-peak amplitude.
∆VOUT
∆I OUT
Output impedance for a buck regulator is the parallel
impedance of the output capacitor and the MOSFET and
inductor divided by the gain:
ZTOTAL =
R DSON + DCR + X L
|| X COUT
GAIN
To measure output impedance vs. frequency, the load
current must be swept across the 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.
From this graph, you 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:
f =
Then, determine the output impedance by looking at the
output impedance vs frequency graph. Next, calculate
the voltage deviation times the load step;
∆VOUT = ∆IOUT ×ZOUT
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.
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
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):
May 2007
A / sec
2π
↓ ZTOTAL =
↓ R DSON + DCR + X L
↑ GAIN
|| X COUT
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.
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MIC4721
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.
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Recommended Layout MIC4721 1.5A Evaluation Board
Top
Bottom
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MIC4721 Schematic and B.O.M. for 1.5A Output
Bill of Materials
Item
C1, C2
Part Number
Manufacturer
C2012JB0J106K
TDK(1)
GRM219R60J106KE19
Murata(2)
C9
AVX
0402ZD104MAT
AVX
C2012JB0J475K
TDK(1)
GRM188R60J475KE19
Murata(2)
C8
(3)
06036D475MAT
AVX
VJ0402A820KXAA
Vishay VT
(4)
1
82pF Ceramic Capacitor 0402
1
1
1
R1, R4
CRCW04021002F
Vishay Dale
(4)
10KΩ1% 0402 resistor
2
6.65 kΩ 1% 0402 For 2.5VOUT
12.4 kΩ 1% 0402 For 1.8 VOUT
20 kΩ 1% 0402 For 1.5 VOUT
40.2 kΩ 1% 0402 For 1.2 VOUT
Open
For 1.0 VOUT
1
10Ω 1% 0402 resistor
1.5 2MHz Integrated Switch Buck Regulator
1
1
CRCW04026651F
Vishay Dale(4)
CRCW04024022F
U1
4.7µF Ceramic Capacitor X5R 0603 6.3V
3A Schottky 30V SMA
Vishay Dale
CRCW040210R0F
MIC4721YMM
1
1µH Inductor 17.5mΩ(L)6.47mm x (W)6.86mm x (H)1.8mm
Vishay Semi
R3
0.1µF Ceramic Capacitor X5R 0402 10V
(4)
SSA33L
IHLP2525AH-01 1
CRCW04022002F
2
(4)
L1
CRCW04021242F
10µF Ceramic Capacitor X5R 0805 6.3V
(3)
D1
R2
Qty.
(3)
08056D106MAT
C7
Description
(4)
Vishay Dale
Micrel, Inc.(5)
Notes:
1. TDK: www.tdk.com
2. Murata: www.murata.com
3. AVX: www.avx.com
4. Vishay: www.vishay.com
5. Micrel, Inc.: www.micrel.com
May 2007
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MIC4721
Package Information
10-Pin MSOP (MM)
MICREL, INC. 2180 FORTUNE DRIVE SAN JOSE, CA 95131 USA
TEL +1 (408) 944-0800 FAX +1 (408) 474-1000 WEB http://www.micrel.com
The information furnished by Micrel in this data sheet is believed to be accurate and reliable. However, no responsibility is assumed by Micrel for its
use. Micrel reserves the right to change circuitry and specifications at any time without notification to the customer.
Micrel Products are not designed or authorized for use as components in life support appliances, devices or systems where malfunction of a product
can reasonably be expected to result in personal injury. Life support devices or systems are devices or systems that (a) are intended for surgical implant
into the body or (b) support or sustain life, and whose failure to perform can be reasonably expected to result in a significant injury to the user. A
Purchaser’s use or sale of Micrel Products for use in life support appliances, devices or systems is a Purchaser’s own risk and Purchaser agrees to fully
indemnify Micrel for any damages resulting from such use or sale.
© 2007 Micrel, Incorporated.
May 2007
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