Micrel MIC2208 3mmx3mm 1mhz 3a pwm buck regulator Datasheet

MIC2208
3mmx3mm 1MHz 3A PWM Buck
Regulator
Features
General Description
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•
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The Micrel MIC2208 is a high efficiency PWM buck
(step-down) regulator that provides up to 3A of
output current. The MIC2208 operates at 1MHz and
has external compensation that allows a closed loop
bandwidth of over 100KHz.
The low on-resistance internal p-channel MOSFET
of the MIC2208 allows efficiencies over 94%,
reduces external component count and eliminates
the need for an expensive current sense resistor.
The MIC2208 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 low-dropout conditions.
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•
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The MIC2208 is available in the exposed pad 3mm x
3mm MLF-12L package with a junction operating
range from –40°C to +125°C.
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2.7 to 5.5V supply voltage
1MHz PWM mode
Output current to 3A
>90% efficiency
Adjustable output voltage option down to 1V
Ultra-fast transient response
External Compensation
Stable with a wide range of output
capacitance
Fully integrated 5A MOSFET switch
Micropower shutdown
Thermal shutdown and current limit
protection
Pb-free 3mm x 3mm MLF-10L package
–40°C to +125°C junction temperature range
Internal soft-start
Applications
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5V or 3.3V Point of Load Conversion
Telecom/Networking Equipment
Set Top Boxes
Storage Equipment
Video Cards
Typical Application
MIC2208
3A 1MHz Buck Regulator
MLF and MicroLeadFrame are trademarks of Amkor Technology
Micrel, Inc • 2180 Fortune Drive • San Jose, Ca 95131 • USA • tel +1 (408) 944-0800 • fax +1 (408) 474-1000 • http://www.micrel.com
September 2005
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Micrel, Inc.
MIC2208
Ordering Information
Part Number
Output
Voltage(1)
Junction Temp. Range
Package
Lead Finish
MIC2208YML
Adj.
–40° to +125°C
3mmx3mm MLF-10L
Pb-free
.
Pin Configuration
SW 1
12 SW
VIN 2
11 VIN
PGND 3
10 PGND
SGND 4
9 PGOOD
BIAS 5
FB 6
8 EN
EP
7 COMP
3mm x 3mm MLF-12 (ML)
Pin Description
Pin Number
Pin Name
Pin Function
1,12
SW
Switch (Output): Internal power P-Channel MOSFET output switch
2,11
VIN
Supply Voltage (Input): Supply voltage for the source of the internal P-channel
MOSFET and driver.
Requires bypass capacitor to GND.
3,10
PGND
Power Ground. Provides the ground return path for the high-side drive current.
4
SGND
Signal Ground. Provides return path for control circuitry and internal reference.
5
BIAS
Internal circuit bias supply. Must be bypassed with a 0.1uF ceramic capacitor to
SGND.
6
FB
7
COMP
8
EN
9
PGOOD
EP
GND
September 2005
Feedback. Input to the error amplifier, connect to the external resistor divider
network to set the output voltage.
Compensation. This is the internal error amplifier output. Connect external
compensation components for type II or type III compensation.
Enable (Input). Logic level low will shutdown the device, reducing the current
draw to less than 5uA.
Power Good. Open drain output that is pulled to ground when the output voltage
is within +/- 7.5% of the set regulation voltage
Connect to ground.
2
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MIC2208
Absolute Maximum Ratings(1)
Operating Ratings(2)
Supply Voltage (VIN) ...............................-0.3V to +6V
Output Switch Voltage (VSW) .....................-1V to +6V
Output Switch Current (ISW) ................................. 10A
Logic Input Voltage (VEN)......................... -0.3V to VIN
Storage Temperature (Ts)................ -60°C to +150°C
ESD Rating(3) ............................................2KV (HBM)
Supply Voltage (VIN)............................+2.7V to +5.5V
Logic Input Voltage (VEN,VLOWQ) .................. 0V to VIN
Junction Temperature (TJ) .............. –40°C to +125°C
Junction Thermal Resistance
3mmx3mm MLF-12L (θJA) ....................... 60°C/W
Electrical Characteristics (4)
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
Supply Voltage Range
Under-Voltage Lockout
Threshold
Typ
2.7
(turn-on)
2.45
UVLO Hysteresis
2.55
Max
Units
5.5
V
2.65
V
100
mV
Quiescent Current
VFB = 0.9 * VNOM (not switching)
720
950
µA
Shutdown Current
VEN = 0V
0.1
5
µA
[Adjustable] Feedback
Voltage
± 1%
± 2% (over temperature)
1
1.01
1.02
V
1
100
nA
8
10
A
0.99
0.98
FB pin input current
Current Limit in PWM Mode
VFB = 0.9 * VNOM
Output Voltage Line
Regulation
VOUT > 2.2V; VIN = VOUT+500mV to 5.5V; ILOAD= 20mA
VOUT < 2.2V; VIN = 2.7V to 5.5V; ILOAD= 20mA
0.13
Output Voltage Load
Regulation
20mA < ILOAD < 3A
0.2
PWM Switch ONResistance
ISW = 50mA VFB = 0.7VFB_NOM (High Side Switch)
95
%
1
200
300
%
mΩ
Oscillator Frequency
0.9
1
1.1
Enable Threshold
0.5
0.85
1.3
V
0.1
2
µA
Enable Input Current
Soft Start Time
MHz
450
µs
Over-Temperature
Shutdown
160
°C
over-Temperature
Hysteresis
20
°C
VOUT =10% to VOUT=90%
Power Good Range
Power Good Resistance
IPGOOD
±7
±10
%
145
200
Ω
Notes:
1. Exceeding the absolute maximum rating may damage the device.
2. The device is not guaranteed to function outside its operating rating.
3. Devices are ESD sensitive. Handling precautions recommended. Human body model: 1.5kΩ in series with 100pF.
4. Specification for packaged product only.
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MIC2208
Typical Characteristics
1.2V
OUT
85
83
4.5VIN
5V
IN
5.5VIN
73
71
69
67
65
0
0.5
1
1.5
2
2.5
OUTPUT CURRENT (A)
81
79
77
75
3
Load Regulation
1.0100
1.010
1.008
3.6V
IN
0.5
1
1.5
2
2.5
OUTPUT CURRENT (A)
Feedback Voltage
vs. Temperature
77
75
73
71
69
1.20
3
MIC2208
Efficiency
OUT
4.5VIN
5VIN
5.5V
IN
0.5
1
1.5
2
2.5
OUTPUT CURRENT (A)
3
Feedback Voltage
vs. Temperature
1.15
1.10
1.05
1.00
0.95
0.9940
0.9920 3.3V
IN
0.9900
-40
81
79
67
65
0
3
FEEDBACK VOLTAGE (V)
OUTPUT VOLTAGE (V)
3
0.5
1
1.5
2
2.5
OUTPUT CURRENT (A)
85
83
0.9980
0.9960
0.998
0.996
0.994
September 2005
OUT
1.0040
1.0020
1.0000
1.002
1.000
3.6VIN
1.2V
1.0080
1.0060
1.006
1.004
0.992 3.3V
IN
0.990
0
0.5
1
1.5
2
2.5
OUTPUT CURRENT (A)
MIC2208
Efficiency
73
71
69
67
65
0
3.3VIN
75
70
0
3
3VIN
3.3VIN
80
0.90
TEMPERATURE (°C)
4
0.85 3.3V
IN
0.80
120
77
75
MIC2208
Efficiency
0.5
1
1.5
2
2.5
OUTPUT CURRENT (A)
85
80
81
79
74
72
3VIN
100
85
83
78
76
70
0
3
5VIN
MIC2208
Efficiency
60
1.5V
82
80
90
3
OUT
40
0.5
1
1.5
2
2.5
OUTPUT CURRENT (A)
86
84
95
IN
0
IN
79
77
75
0
EFFICIENCY (%)
3.6V
4.5V
0.5
1
1.5
2
2.5
OUTPUT CURRENT (A)
20
IN
84
FEEDBACK VOLTAGE (V)
3.3V
IN
5.5VIN
5VIN
1.5V
120
83
81
IN
86
MIC2208
Efficiency
80
87
85
90
88
3V
4.5V
88
80
0
3
OUT
100
91
89
0.5
1
1.5
2
2.5
OUTPUT CURRENT (A)
1.8V
EFFICIENCY (%)
EFFICIENCY (%)
95
93
MIC2208
Efficiency
OUT
OUT
82
60
1.8V
3
IN
40
0.5
1
1.5
2
2.5
OUTPUT CURRENT (A)
3.6V
86
84
82
80
0
EFFICIENCY (%)
80
0
3.3VIN
MIC2207
Efficiency
90
-20
82
EFFICIENCY (%)
84
90
88
EFFICIENCY (%)
86
94
92
EFFICIENCY (%)
5.5VIN
88
94
2.5V
92
20
5VIN
OUT
-40
92
90
MIC2207
Efficiency
98
96
EFFICIENCY (%)
EFFICIENCY (%)
100
4.5VIN
94
2.5V
0
96
MIC2208
Efficiency
OUT
-20
3.3V
TEMPERATURE (°C)
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MIC2208
300
200
100
1
*Using recommended
layout (1oz copper)
and B.O.M.
2.5
3.5
MAX. OUTPUT CURRENT (A)
Max. Continuous Current
vs. Ambient Temp 3.3V OUT*
Max. Continuous Current
vs. Ambient Temp 2.5V OUT*
*Using
recommended
layout (1oz copper)
and B.O.M.
3
2.5
2
1.5
2
3.3VIN
1.5
1
5VIN
0
60 70 80 90 100 110 120
AMBIENT TEMPERATURE (°C)
5V
IN
1
0.5
MAX. OUTPUT CURRENT (A)
TEMPERATURE (°C)
3.5
Max. Continuous Current
vs. Ambient Temp 1.8V OUT*
3
2.5
2
3.3VIN
1.5
5V
IN
1 *Using
recommended
0.5 layout (1oz copper)
0
60 70 80 90 100 110 120
AMBIENT TEMPERATURE (°C)
3.5
0
3.2
3.7
4.2
4.7
SUPPLY VOLTAGE (V)
120
120
80
100
60
40
20
0
-20
-40
0
2.7
TEMPERATURE (°C)
3
0.2
0.2
3.3VIN
80
20
MAX. OUTPUT CURRENT (A)
0.4
0.4
40
1
0.6
0.6
60
1.2
100
80
Enable Threshold
vs. Temperature
0.8
0.8
100
3.2 3.7 4.2 4.7 5.2
SUPPLY VOLTAGE (V)
60
ENABLE THRESHOLD (V)
120
0.5
Enable Threshold
vs. Supply Voltage
1.2
140
75
70
2.7
1
2
3
4
5
SUPPLY VOLTAGE (V)
40
RDSON
vs. Temperature
160
P-CHANNEL RDSON (mOhms)
0
0
1
2
3
4
5
SUPPLY VOLTAGE (V)
ENABLE THRESHOLD (V)
0
0
0
0.2
90
85
80
20
0.4
3.5
100
95
400
0.6
-20
0.8
0
110
105
500
-40
FEEDBACK VOLTAGE (V)
600
1.0
RDSON
vs. Supply Voltage
120
115
P-CHANNEL RDSON (mOhms)
700
QUIESCENT CURRENT (µA)
1.2
Quiescent Current
vs. Supply Voltage
MAX. OUTPUT CURRENT (A)
Feedback Voltage
vs. Supply Voltage
and B.O.M.
0
60 70 80 90 100 110 120
AMBIENT TEMPERATURE (°C)
Max. Continuous Current
vs. Ambient Temp 1.0V OUT*
3
2.5
2
3.3VIN
1.5
5V
IN
1 *Using
recommended
0.5 layout (1oz copper)
September 2005
and B.O.M.
0
60 70 80 90 100 110 120
AMBIENT TEMPERATURE (°C)
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MIC2208
Functional Diagram
VIN
VIN
P-Channel
Current Limit
BIAS
HSD
SW
SW
PWM
Control
EN
Enable and
Control Logic
COMP
Bias,
UVLO,
Thermal
Shutdown
Soft
Start
EA
FB
1.0V
PGOOD
1.0V
PGND
SGND
MIC2208 Block Diagram
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MIC2208
Functional Characteristics
0A
INDUCTOR CURRENT
(200mA/div.)
0A
INDUCTOR CURRENT
(500mA/div.)
SWITCH VOLTAGE
(2V/div.)
Discontinuous Operation
SWITCH VOLTAGE
(2V/div.)
Continuous Operation
VIN = 3.3V
VOUT = 1V@1A
VIN = 3.3V
VOUT = [email protected]
TIME (200ns/div.)
TIME (200ns/div.)
INPUT CURRENT
(1A/div.)
OUTPUT VOLTAGE
ENABLE (1V/div.)
OUTPUT CURRENT
(2A/div.)
0A
VIN = 3.3V. 1V, 3A to 200mA
Enable Response
VIN = 3.3V
VOUT = 1V@3A
(5V/div.)
INDUCTOR CURRENT
(2A/div.)
OUTPUT VOLTAGE
(100mV/div.)
Transient Response
TIME (100µs/div.)
TIME (100µs/div.)
POWER GOOD
(5V/div.)
OUTPUT VOLTAGE
(1V/div.)
Power Good
TIME (100µs/div.)
September 2005
VIN = 3.3V
ENABLE
(5V/div.)
VIN = 3.3V
ENABLE
(5V/div.)
POWER GOOD
(5V/div.)
OUTPUT VOLTAGE
(1V/div.)
Power Good
TIME (40µs/div.)
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MIC2208
Pin Description
external components. Refer to the compensation
section of the datasheet for determining nessasary
component values.
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 is recommended close to VIN and
the power ground (PGND) for each pin for
bypassing. Please refer to layout recommendations.
SW
The switch (SW) pin connects directly to the inductor
and provides the switching current nessasary 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.
BIAS
The bias (BIAS) provides power to the internal
reference and control sections of the MIC2208. A 10
Ohm resistor from VIN to BIAS and a 0.1uF from
BIAS to SGND is required for clean operation.
PGOOD
Power good is an open drain pull down that
indicates when the output voltage has reached
regulation. For a power good low, 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.
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:
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 Analog ground (AGND) loop.
Refer to the layout considerations fro more details.
⎛ R1
⎞
VOUT = VREF × ⎜
+ 1⎟
R2
⎝
⎠
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.
where VREF is equal to 1.0V.
COMP
The COMP pin is the output of the internal error
amplifier. This pin is used to compensate the
MIC2208 for stability over a varying range of
September 2005
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MIC2208
Applications Information
The MIC2208 is a 3A PWM non-synchronous
buck 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 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.
During the on-time, the high side switch is turned
on, current flows from the input supply through the
inductor and to the output. The inductor current is
Figure 1.
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
⎢ VOUT - OUT ⎥
VIN ⎥⎦
⎢
IOUT = ⎣
1MHz × 2 × L
Continuous or discontinuous operation determines
how we calculate peak inductor current.
Figure 3. On-Time
Continuous Operation
charged at the rate;
Figure 2 illustrates the switch voltage and inductor
current during continuous operation.
(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;
D=
VOUT
VIN
and the On time is;
TON =
September 2005
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D
1MHz
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MIC2208
Therefore, peak to peak ripple current is;
Ipk −pk =
(VIN - VOUT ) × VOUT
VIN
1MHz × L
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 half the peak-topeak current. For those of you who have not had
enough formulas already, here it is;
(VIN - VOUT ) × VOUT
VIN
2 × 1MHz × L
Figure 4 demonstrates the off-time. During the offtime, the high-side internal P-channel MOSFET
turns off. Since the current in the inductor has to
discharge, the current flows through the freewheeling Schottky diode to the output. In this case,
the inductor discharge rate is (where VD is the diode
forward voltage);
Ipk = IOUT +
Figure 5. Discontinuous Operation
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 and all) refer to the functional
characteristics.
Discontinuous mode of operation has the advantage
over full PWM in that at light loads, the MIC2208 will
skip pulses as nessasary, reducing gate drive
losses, drastically improving light load efficiency.
+ VD )
L
The total off time can be calculated as;
−
(VOUT
TOFF =
1- D
1MHz
Efficiency Considerations
Calculating the efficiency is as simple as measuring
power out and dividing it by the power in;
P
Efficiency = OUT × 100
PIN
Where input power (PIN) is;
PIN = VIN × IIN
and output power (POUT) is calculated as;
POUT = VOUT × IOUT
The Efficiency of the MIC2208 is determined by
several factors.
Figure 4. Off-Time
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;
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.
September 2005
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MIC2208
PSW = R DSON × IOUT 2 × D
Where D is the duty cycle.
Since the MIC2208 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 (VF) and the output current. Diode
power losses can be approximated as follows:
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 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.
PD = VF × IOUT × (1 − D)
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 losses 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 × IOUT
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.
2
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 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 (Or exaggerates.)
how switching losses due to the transitions dissipate
power in the switch.
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.
Inductor Selection
The MIC2208 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
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MIC2208
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.
Gain (dB)
Dominant
Pole
Diode Selection
Since the MIC2208 is non-synchronous, a freewheeling 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;
Zero
20dB/Decade
LC
Frequency
Compensation
The MIC2208 utilizes voltage mode compensation
and has the error amplifier pin (COMP) pinned out to
allow it to be compensated using external
components. This allows the MIC2208 to be stable
with a wide range of inductor and capacitor values.
TYPE II compensation
Type II compensation can be expressed as polezero-pole. In our case, a dominant pole (R1 and C3)
followed by a zero (C3 and R4) , allowing the final
pole to be provided by the output inductor and
output capacitor (L and COUT). This mode of
compensation works well when using higher ESR
output capacitors, such as tantalum and electrolytic
dielectrics. The ESR of the capacitor, along with the
output capacitance provides a zero (COUT and ESR)
that negates one of the two poles created by the
inductor-output capacitor filter. This allows the gain
to cross the 0dB point with a -1 slope
(-20dB/decade).
⎛ 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 is available in the bill of materials on page
x. 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.
Bias filter
A small 10 Ohm resistor is recommended from the
input supply to the bias pin along with a small 0.1uF
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.
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Micrel, Inc.
MIC2208
Type II Compensation
80
288
VIN=5VIN
70
VOUT=1.0V
IOUT=3A
Gain (dB)
60
252
216
50
180
40
144
30
108
20
72
10
36
0
0
-10
-20
100
MIC2208
-36
1k
10k
100k
Frequency (KHz)
-72
1M
Bill of Materials
Item
Part Number
C1a,C1b C2012JB0J106K
Description
Manufacturer
10uF Ceramic Capacitor X5R 0805 6.3V
TDK
Qty
GRM219R60J106KE19 10uF Ceramic Capacitor X5R 0805 6.3V
Murata
08056D106MAT
10uF Ceramic Capacitor X5R 0805 6.3V
AVX
2
C2
0402ZD104MAT
0.1uF Ceramic Capacitor X5R 0402 10V
AVX
1
C3
0402ZD100MAT
100pF Ceramic Capacitor X5R 0402 10V
AVX
1
C4
TPME477M010R0030
470uF Tantalum Capacitor 10V
AVX
1
D1
SSA33L
3A Schottky 30V SMA
Vishay Semi
1
L1
RLF7030-1R0N6R4
1uH Inductor 8.8mOhm 7.1mm(L) x 6.8mm (W)x 3.2mm(H)
TDK
744 778 9001
1uH Inductor 12mOhm 7.3mm(L)x7.3mm(W)x3.2mm(H)
Wurth Electronik
IHLP2525AH-01 1
1uH Inductor 17.5mΩ (L)6.47mmx(W)6.86mmx(H) 1.8mm
Vishay Dale
1
CRCW04023012F
CRCW04022002F
30.1KΩ 1% 0402 resistor
20 kΩ 1% 0402 For 2.5VOUT
Vishay Dale
Vishay Dale
1
CRCW04023742F
37.4 kΩ 1% 0402 For 1.8 VOUT
Vishay Dale
CRCW04026042F
60.4 kΩ 1% 0402 For 1.5 VOUT
Vishay Dale
CRCW04021503F
150 kΩ 1% 0402 For 1.2 VOUT
Vishay Dale
Open
Vishay Dale
R1
R2
For 1.0 VOUT
1
R4
CRCW04024993F
499KΩ 1% 0402 resistor
Vishay Dale
1
R5
CRCW040210R0F
10Ω 1% 0402 resistor
Vishay Dale
1
R6
CRCW04021002F
MIC2208BML
10KΩ 1% 0402 resistor
Vishay Dale
Micrel
1
U1
September 2005
1MHz 3A Buck Regulator
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Micrel, Inc.
MIC2208
Type III Open Loop Gain Response
TYPE III compensation
Type III in our case, is a dominant pole (C3 and R1)
followed by a zero (C3 and R4) and an additional
zero (C5 and R4), allowing the final pole to be
provided by the output inductor and output capacitor.
This mode of compensation is required when using
low ESR output capacitors, such as ceramic
capacitors. The additional zero offsets the double
pole created by the inductor/output capacitor filter.
Gain(dB)
Dominant
Pole
LC
Frequency
Zero
Zero
20dB/Decade
Frequency (Hz)
Type III Compensation
288
80
V IN=5V IN
V OUT=1.0V
IOUT=3A
COUT=47µF
70
Gain (dB)
60
252
216
50
180
40
144
30
108
20
72
MIC2208
36
10
Gain
0
Phase
-36
-10
-20
100
1k
10k
100k
-72
1M
Frequency (KHz)
Bill of Materials
Item
C1a,C1b
Part Number
Description
Manufacturer
C2012JB0J106K
10uF Ceramic Capacitor X5R 0805 6.3V
TDK
Qty
2
GRM219R60J106KE19
10uF Ceramic Capacitor X5R 0805 6.3V
Murata
08056D106MAT
10uF Ceramic Capacitor X5R 0805 6.3V
AVX
C2
0402ZD104MAT
0.1uF Ceramic Capacitor X5R 0402 10V
AVX
1
C3
0402ZD103MAT
1nF Ceramic Capacitor X5R 0402 10V
AVX
2
C3216X5R0J476K
47uF Ceramic Capacitor X5R 1206 6.3V
C4
TDK
1
GRM32ER60J476ME20 47uF Ceramic Capacitor X5R 1206 6.3V
Murata
12106D476MAT2A
47uF Ceramic Capacitor X5R 1210 6.3V
AVX
VJ0402A330KXAA
33pF Ceramic Capacitor 0402
Vishay VT
1
D1
SSA33L
3A Schottky 30V SMA
Vishay Semi
1
L1
RLF7030-1R0N6R4
1uH Inductor 8.8mOhm 7.1mm(L) x 6.8mm (W)x 3.2mm(H)
TDK
C5
R1
744 778 9001
1uH Inductor 12mOhm 7.3mm(L)x7.3mm(W)x3.2mm(H)
Wurth Electronik
IHLP2525AH-01 1
1uH Inductor 17.5mΩ (L)6.47mmx(W)6.86mmx(H) 1.8mm
Vishay Dale
1
CRCW04024992F
49.9KΩ 1% 0402 resistor
Vishay Dale
1
September 2005
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Micrel, Inc.
MIC2208
Bill of Materials (cont.)
R2
CRCW04023322F
33.2 kΩ 1% 0402 For 2.5VOUT
Vishay Dale
CRCW04026192F
61.9 kΩ 1% 0402 For 1.8 VOUT
Vishay Dale
CRCW04021003F
100 kΩ 1% 0402 For 1.5 VOUT
Vishay Dale
CRCW04022493F
249 kΩ 1% 0402 For 1.2 VOUT
Vishay Dale
Open
Vishay Dale
For 1.0 VOUT
1
R3
CRCW04024991F
4.99KΩ 1% 0402 resistor
Vishay Dale
1
R4
CRCW04024991F
90.9KΩ 1% 0402 resistor
Vishay Dale
1
R5
CRCW040210R0F
10Ω 1% 0402 resistor
Vishay Dale
1
R6
CRCW04021002F
10KΩ 1% 0402 resistor
Vishay Dale
1
U1
MIC2208BML
1MHz 3A Buck Regulator
Micrel
1
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.
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
transformer. Figure 7 demonstrates how a
transformer is used to inject a signal into the
feedback network.
Network
Analyzer
“R” Input
+8V
MIC922BC5
Feedback
R3
1k
R4
1k
50
R1
1k
Network
Analyzer
“A” Input
Output
Network Analyzer
Source
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 50 Ohm 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 50 Ohm 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 supplies control loop. An op-amp with
Figure 7. Transformer Injection
A 50 ohm 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
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Micrel, Inc.
MIC2208
>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 opamp injection method, that the supply voltages need
to below the maximum operating voltage of the opamp. Also, the maximum output voltage for driving
50 Ohm inputs using the MIC922 is 3V. For
measuring higher output voltages, a 1MOhm 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.
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
50 Ohm 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);
Output Impedance and Transient
response
and peak to peak current;
∆V =
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 =
∆I =
× 1mW × 50Ω × 2
0.707
dBm
10 10
× 1mW × 50Ω × 2
0.707 × 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
∆IOUT
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 =
dBm
10 10
Output Impedance vs Frequency
1
R DSON + DCR + X L
X COUT
GAIN
Output Impedance (Ohms)
To measure output impedance vs. frequency, the
load current must be 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.
3.3V IN
0. 1
5V IN
0 . 01
V OUT = 1.8V
L =1µH
COUT = 4.7µF+0.1µF
0. 0 0 1
10
10 0
1k
10 k
10 0 k
1M
10 M
Frequency (Hz)
From this graph, you can see the effects of
bandwidth and output capacitance. For frequencies
<100KHz, the output impedance is dominated by the
gain and inductance. For frequencies >100KHz, the
Figure 9. Output Impedance Measurement
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MIC2208
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 nonshielded 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.
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 =
A/sec
2π
Then, determine the output impedance by looking at
the output impedance vs frequency graph. Then
calculating the voltage deviation times the load step;
∆VOUT = ∆IOUT × Z OUT
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 =
↓ R DSON + DCR + X L
↑ GAIN
X COUT
Ripple measurements
To properly measure ripple on either
input or output of a switching regulator,
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MIC2208
Package Information
12-Lead MLF™ (ML)
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.
© 2005 Micrel, Incorporated.
September 2005
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Micrel, Inc.
MIC2208
Revision History
Date
3-30-05
6-16-05
Edits by:
Martin Galinski
Martin Galinski
September 2005
Revision Number
19
M9999-092905
www.micrel.com