MICREL MIC2207

MIC2207
3mmx3mm 2MHz 3A PWM Buck
Regulator
Features
General Description
The Micrel MIC2207 is a high efficiency PWM buck
(step-down) regulators that provides up to 3A of
output current. The MIC2207 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 MIC2207 allows efficiencies over 94%,
reduces external components count and eliminates
the need for an expensive current sense resistor.
The MIC2207 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.
The MIC2207 is available in the exposed pad 3mm x
3mm MLF-12L package with a junction operating
range from –40°C to +125°C.
•
•
•
•
•
•
•
•
•
•
•
•
•
2.7 to 5.5V supply voltage
2MHz PWM mode
Output current to 3A
>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 3mm x 3mm MLF-12L package
–40°C to +125°C junction temperature range
Applications
•
•
•
•
•
5V or 3.3V Point of Load Conversion
Telecom/Networking Equipment
Set Top Boxes
Storage Equipment
Video Cards
____________________________________________________________________________________________________
Typical Application
96
3.3V
4.5V
94
EFFICIENCY (%)
MIC2207
MIC2207
Efficiency
OUT
IN
92
90
88
5V
IN
5.5V
IN
86
84
82
80
0
3A 2MHz Buck Regulator
0.5
1
1.5
2
2.5
OUTPUT CURRENT (A)
3
Micrel, Inc • 2180 Fortune Drive • San Jose, Ca 95131 • USA • tel +1 (408) 944-0800 • fax +1 (408) 474-1000 • http://www.micrel.com
April 2005
M9999-040705
www.micrel.com
Micrel
MIC2207
Ordering Information
Part Number
Output
Voltage(1)
Junction Temp. Range
Package
Lead Finish
MIC2207YML
Adj.
–40° to +125°C
3x3 MLF-12L
Pb-free
Note:
1. Other Voltage options available. Contact Micrel for details.
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 NC
3mm x 3mm MLF-12 (ML)
Pin Description
Pin Number
Pin Name
1,12
SW
Switch (Output): Internal power P-Channel MOSFET output switch
Pin Function
2,11
VIN
Supply Voltage (Input): Supply voltage for the source of the internal P-channel
MOSFET and driver.
Requires bypass capacitor to GND.
April 2005
3,10
PGND
Power Ground. Provides the ground return path for the high-side drive current.
4
SGND
Signal (Analog) Ground. Provides return path for control circuitry and internal
reference.
5
BIAS
6
FB
Feedback. Input to the error amplifier, connect to the external resistor divider
network to set the output voltage.
7
NC
No Connect. Not internally connected to die. This pin can be tied to any other pin
if desired.
8
EN
Enable (Input). Logic level low will shutdown the device, reducing the current
draw to less than 5uA.
9
PGOOD
EP
GND
Internal circuit bias supply. Must be bypassed with a 0.1uF ceramic capacitor to
SGND.
Power Good. Open drain output that is pulled to ground when the output voltage
is outside +/- 7.5% of the set regulation voltage
Connect to ground.
2
M9999-040705
www.micrel.com
Micrel
MIC2207
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
ESD Rating(3) ........................................................2kV
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
3x3 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
Quiescent Current
VFB = 0.9 * VNOM (not switching)
Shutdown Current
VEN = 0V
[Adjustable] Feedback
Voltage
± 1% ILOAD = 100mA
± 2% (over temperature) ILOAD = 100mA
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)
mV
570
900
µA
2
10
µA
0.99
0.98
1
1.01
1.02
V
3.5
5
FB pin input current
1
nA
7
%
0.5
95
200
300
Oscillator Frequency
1.8
2
2.2
Enable Threshold
0.5
0.85
1.3
Enable Hysteresis
50
Enable Input Current
0.1
Power Good Resistance
IPGOOD = 500µA
%
%
100
Power Good Range
A
mΩ
MHz
V
mV
2
µA
±7
±10
%
145
200
Ω
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. Devices are ESD sensitive. Handling precautions recommended. Human body model: 1.5kΩ in series with 100pF.
4. Specification for packaged product only.
5. Dropout voltage is defined as the input-to-output differential at which the output voltage drops 2% below its nominal value that is initially
measured at a 1V differential. For outputs below 2.7V, the dropout voltage is the input-to-output voltage differential with a minimum
input voltage of 2.7V.
April 2005
3
M9999-040705
www.micrel.com
Micrel
MIC2207
Typical Characteristics
5.5VIN
88
86
84
82
0.5
1
1.5
2
2.5
OUTPUT CURRENT (A)
1.8V
3V
91
89
87
EFFICIENCY (%)
3.6V
IN
0.5
1
1.5
2
2.5
OUTPUT CURRENT (A)
1.5V
85
83
3
MIC2207
Efficiency
OUT
5VIN 5.5V
IN
1V
85
83
81
79
77
75
73
3.3VIN
71
69
67
65
0
April 2005
MIC2207
Efficiency
3.3VIN
80
78
76
74
85
3.6VIN
0.5
1
1.5
2
2.5
OUTPUT CURRENT (A)
3.6VIN
1.8V
MIC2207
Efficiency
OUT
0.5
1
1.5
2
2.5
OUTPUT CURRENT (A)
3
86
4.5V
IN
5VIN 5.5VIN
84
5V
90
5.5V
IN
IN
0.5
1
1.5
2
2.5
OUTPUT CURRENT (A)
1.2V
MIC2207
Efficiency
OUT
3V
3.3V
IN
3.6VIN
0.5
1
1.5
2
2.5
OUTPUT CURRENT (A)
1V
MIC2207
Efficiency
5VIN
5.5VIN
0.5
1
1.5
2
2.5
OUTPUT CURRENT (A)
4
81
79
77
75
73
71
69
1.010
IN
3
1.5V
3
MIC2207
Efficiency
OUT
3VIN
3.3VIN
3.6V
IN
75
67
65
0
3
OUT
4.5V
80
85
83
IN
0.5
1
1.5
2
2.5
OUTPUT CURRENT (A)
85
70
0
3
65
60
0
88
95
4.5VIN
75
70
MIC2207
Efficiency
OUT
90
80
0
3
80
IN
2.5V
82
86
84
82
72
70
0
3
OUT
3V
80
78
76
74
90
88
75
73
71
69
0.5
1
1.5
2
2.5
OUTPUT CURRENT (A)
86
84
82
72
70
0
4.5VIN
81
79
77
67
65
0
EFFICIENCY (%)
IN
3.3VIN
77
75
0
90
88
EFFICIENCY (%)
85
83
81
79
MIC2207
Efficiency
OUT
EFFICIENCY (%)
EFFICIENCY (%)
95
93
3
EFFICIENCY (%)
80
0
82
80
0
92
3VIN
96
94
92
90
88
86
84
94
EFFICIENCY (%)
5VIN
90
MIC2207
Efficiency
OUT
EFFICIENCY (%)
92
EFFICIENCY (%)
EFFICIENCY (%)
100
98
4.5VIN
94
2.5V
EFFICIENCY (%)
96
MIC2207
Efficiency
OUT
OUTPUT VOLTAGE (V)
3.3V
0.5
1
1.5
2
2.5
OUTPUT CURRENT (A)
1.2V
3
MIC2207
Efficiency
OUT
4.5VIN
5VIN 5.5V
IN
0.5
1
1.5
2
2.5
OUTPUT CURRENT (A)
3
Load Regulation
1.005
1.000
0.995
0.990
0
VIN = 3.3V
0.5
1
1.5
2
2.5
OUTPUT CURRENT (A)
3
M9999-040705
www.micrel.com
Micrel
MIC2207
Typical Characteristics cont.
100
V
EN
120
80
100
60
1.2
0
3.3V
3.2
3.7
4.2
4.7
SUPPLY VOLTAGE (V)
IN
120
0
2.7
0.2
100
0.2
0.4
80
0.4
0.6
60
0.6
0.8
40
0.8
1.0
-20
1.0
120
100
40
40
3.2 3.7 4.2 4.7 5.2
SUPPLY VOLTAGE (V)
Enable Threshold
vs. Temperature
-40
60
0
-20
-40
120
IN
ENABLE THRESHOLD (V)
ENABLE THRESHOLD (V)
80
80
=V
1
2
3
4
5
SUPPLY VOLTAGE (V)
1.2
100
60
80
200
90
85
80
75
70
2.7
Enable Threshold
vs. Supply Voltage
120
40
100
300
0
0
140
0
60
400
vs. Temperature
20
0
500
1
2
3
4
5
SUPPLY VOLTAGE (V)
-20
20
600
120
115
110
105
100
95
20
VEN = VIN
700
DSON
vs. Supply Voltage
0
0.4
-40
1.700
1.600 V = 3.3V
IN
1.500
R
800
RDSON
P-CHANNEL RDSON (mOhm)
1.900
1.800
TEMPERATURE (°C)
P-CHANNEL RDSON (mOhm)
QUIESCENT CURRENT (µA)
FEEDBACK VOLTAGE (V)
0.6
20 3.3V
IN
0
2.200
2.100
2.000
Quiescent Current
vs. Supply Voltage
900
0.8
160
2.400
2.300
TEMPERATURE (°C)
1
0
0
-20
0.994
0.992 V = 3.3V
IN
0.990
40
0.998
0.996
Feedback Voltage
vs. Supply Voltage
0.2
FREQUENCY (MHz)
1.004
1.002
1.000
SUPPLY VOLTAGE (V)
1.2
2.500
1.008
1.006
-40
FEEDBACK VOLTAGE (V)
1.010
Frequency
vs. Temperature
20
Feedback Voltage
vs. Temperature
TEMPERATURE (°C)
TEMPERATURE (°C)
MAXIMUM OUTPUT CURRENT (A)
3.5
5VIN
3.0
2.5
Max Continuous Current
vs. Ambient Temp. 2.5V
*
3
2.5
2.0
1.5
OUT
3.5
MAX. OUTPUT CURRENT (A)
OUT
5V
IN
3.3VIN
2
Layout (1oz. Copper
0.5 and
B.O.M.
0
60 70 80 90 100 110 120
AMBIENT TEMPERATURE (°C)
April 2005
3
5V
3.3V
IN
IN
2
1.5
1
0.5
OUT
3.5
2.5
1.5
1.0 *Using Recommended
Max Continuous Current
vs. Ambient Temp. 1.8V
*
MAX. OUTPUT CURRENT (A)
Max. Continuous Output
vs. Ambient Temp. 3.3V
*
*Using recommended
layout (1oz. copper) and
B.O.M.
0
60 70 80 90 100 110 120
AMBIENT TEMPERATURE (°C)
5
1
0.5
*Using recommended
layout (1oz. copper) and
B.O.M.
0
60 70 80 90 100 110 120
AMBIENT TEMPERATURE (°C)
M9999-040705
www.micrel.com
Micrel
MIC2207
Typical Characteristics cont.
Max Continuous Current
vs. Ambient Temp. 1.0V
*
OUT
MAX. OUTPUT CURRENT (A)
3.5
3
5V
2.5
3.3V
IN
IN
2
1.5
1
0.5
*Using recommended
layout (1oz. copper) and
B.O.M.
0
60 70 80 90 100 110 120
AMBIENT TEMPERATURE (°C)
April 2005
6
M9999-040705
www.micrel.com
Micrel
MIC2207
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
EA
FB
1.0V
PGOOD
1.0V
SGND
PGND
MIC2207 Block Diagram
April 2005
7
M9999-040705
www.micrel.com
Micrel
MIC2207
INDUCTOR CURRENT
(200mA/div.)
VIN = 3.3V
VOUT = 1V
L = 1µH
COUT = 4.7µF
IOUT = 1A
TIME (200ns/div.)
TIME (200ns/div.)
Load Transient Response
Output Ripple
OUTPUT VOLTAGE
(10mV/div.)
AC COUPLED
OUTPUT CURRENT
(2A/div.)
Discontinuous Current
VIN = 3.3V
VOUT = 1V
L = 1µH
COUT = 4.7µF
IOUT = 30mA
0A
SWITCH VOLTAGE
(2V/div.)
0A
Continuious Current
SWITCH VOLTAGE
(2V/div.)
INDUCTOR CURRENT
(500mA/div.)
Functional Characteristics
VIN = 3.3V
VOUT = 1.8V
SWITCH VOLTAGE
(2V/div.)
OUTPUT VOLTAGE
(20mV/div.)
0A
IOUT = 3.0A
TIME (400µs/div.)
TIME (400ns/div.)
INPUT CURRENT
ENABLE VOLTAGE
(1A/div.)
(2V/div.)
FEEDBACK VOLTAGE INDUCTOR CURRENT
(2A/div.)
(1V/div.)
Start-Up Waveforms
April 2005
TIME (40µs/div.)
8
M9999-040705
www.micrel.com
Micrel
MIC2205
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 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 MIC2207. A 10
Ohm resistor from VIN to BIAS and a 0.1uF 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 ⎞
+ 1⎟
VOUT = VREF × ⎜
⎝ R2
⎠
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.
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.
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 fro 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
where VREF is equal to 1.0V.
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 (CFF)
can be calculated as follows:
C FF =
1
2π × R1 × 200kHz
Micrel, Inc • 2180 Fortune Drive • San Jose, Ca 95131 • USA • tel +1 (408) 944-0800 • fax +1 (408) 474-1000 • http://www.micrel.com
April 2005
M9999-040705
www.micrel.com
Micrel
MIC2207
Applications Information
The MIC2207 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 ⎥
⎢⎣
⎦
I OUT =
2MHz × 2 × L
Continuous or discontinuous operation determines
how we calculate peak inductor current.
Figure 3. On-Time
Continuous Operation
charged at the rate;
(VIN − VOUT )
Figure 2 illustrates the switch voltage and inductor
current during continuous operation.
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 =
April 2005
10
D
2MHz
M9999-040705
www.micrel.com
Micrel
MIC2207
Therefore, peak to peak ripple current is;
(V IN − VOUT ) × VOUT
VIN
Ipk −pk =
2MHz × 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 ½ the peak to peak
current.
(VIN − VOUT ) × VOUT
VIN
2 × 2MHz × 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 +
−
(VOUT
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) refer to the functional characteristics.
Discontinuous mode of operation has the advantage
over full PWM in that at light loads, the MIC2207 will
skip pulses as nessasary, reducing gate drive
losses, drastically improving light load efficiency.
+ VD )
L
The total off time can be calculated as;
TOFF =
1− D
2MHz
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 MIC2207 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.
April 2005
•
PSW = R DSON × IOUT 2 × D
11
M9999-040705
www.micrel.com
Micrel
MIC2207
Where D is the duty cycle.
Since the MIC2207 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;
Component Selection
PL = DCR × IOUT 2
Input Capacitor
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.
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 MIC2207 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 0.1uF is
recommended close to the load for high frequency
filtering. Smaller case size capacitors are
April 2005
12
M9999-040705
www.micrel.com
Micrel
MIC2207
recommended due to there lower equivalent series
ESR and ESL.
The MIC2207 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 4.7uF 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.
Inductor Selection
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, CFF can be calculated as
follows;
The MIC2207 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.
C FF =
1
2π × 200kHz × R1
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.
Loop Stability and Bode Analysis
Diode Selection
Since the MIC2207 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;
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.
⎛ R1 ⎞
+ 1⎟
VOUT = VFB ⎜
⎝ 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
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
April 2005
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
13
M9999-040705
www.micrel.com
Micrel
MIC2207
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.
+8V
MIC922BC5
Feedback
R3
1k
R4
1k
50
R1
1k
Network
Analyzer
“A” Input
Output
Network Analyzer
Source
Bode Plot
V =3.3V, V
=1.8V, I
Figure 8. Op Amp Injection
60 IN
50
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
>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, is the supply voltages need to
April 2005
OUT
GAIN (dB)
40
OUT
=3A
PHASE
210
175
140
30
105
20
70
10 L=1µH
0 COUT = 4.7µF
35
GAIN
-10 R1 = 10k
R2 = 12.4k
-20 C = 82pF
FF
-30
100
1k
10k
100k
FREQUENCY (Hz)
0
PHASE (°)
Network
Analyzer
“R” Input
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.
The following Bode analysis show the small signal
loop stability of the MIC2207. The MIC2207 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).
-35
-70
-105
1M
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.
14
M9999-040705
www.micrel.com
Micrel
MIC2207
GAIN (dB)
40
PHASE
=3A
Gain and Phase
vs. Frequency
210
175
140
30
105
20
70
10 L=1µH
0 COUT = 4.7µF
GAIN
35
0
-10 R1 = 10k
R2 = 12.4k
-20 C = 82pF
FF
-30
100
1k
10k
100k
FREQUENCY (Hz)
-70
-105
1M
5Vin, 1.8Vout at 3A load;
Phase Margin=43.1 Degrees
•
GBW= 218KHz
105
70
10 L=1µH
0 COUT = 4.7µF
0
-10 R1 = 10k
GAIN
R2 = 12.4k
-20 C = 82pF
FF
-30
100
1k
10k
100k
FREQUENCY (Hz)
35
PAHSE BOOST (°)
10
5
0
1
2
3
4
OUTPUT VOLTAGE (V)
5
Bode Plot
VIN=3.3V, V OUT=1.8V, IOUT=3A
60
50
210
PHASE
GAIN (dB)
40
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.
April 2005
VREF = 1V
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.
3.3Vin, 1.8Vout Iout=50mA;
GBW= 64.4KHz
Voltage
15
-70
•
0
1M
20
-105
1M
Phase Margin=90.5 Degrees
1k
10k
100k
FREQUENCY (Hz)
25
-35
•
5
30
140
20
10
40
35
175
30
15
45
210
PHASE
20
PHASE
50
PHASE (°)
GAIN (dB)
40
GAIN
R1 = 10k
R2 = 12.4k
CFF = 82pF
Max. Amount of Phase Boost
Obtainable using CFF vs. Output
Bode Plot
VIN=3.3V,V OUT=1.8V,IOUT=50mA
50
25
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.
Being that the MIC2207 is non-synchronous; the
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.
60
L=1µH
COUT = 4.7µF
-7
-8
-9
-10
100
-35
•
0
-1
-2
-3
-4
-5
-6
PHASE BOOST (°)
50
OUT
175
140
30
105
20
70
10 L=1µH
0 COUT = 4.7µF
0
GAIN
-10 R1 = 10k
R2 = 12.4k
-20 C = 0pF
FF
-30
100
1k
10k
100k
FREQUENCY (Hz)
35
PHASE (°)
OUT
GAIN (dB)
IN
PHASE (°)
60
Bode Plot
V =5V, V
=1.8V, I
-35
-70
-105
1M
As you 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
15
M9999-040705
www.micrel.com
Micrel
MIC2207
phase boost of 13.4 degrees (47 degrees- 33.6
Degrees = 13.4 Degrees).
dBm
Output Impedance and Transient
response
10 10 × 1mW × 50Ω × 2
∆V =
0.707
and peak to peak current;
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 =
∆VOUT
∆IOUT
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.
Output impedance for a buck regulator is the parallel
impedance of the output capacitor and the MOSFET
and inductor divided by the gain;
Output Impedance
vs. Frequency
R
+ DCR + X L
Z TOTAL = DSON
X COUT
GAIN
OUTPUT IMPEDANCE (Ohms)
1
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. Fig 9 shows a test set-up to
measure output impedance from 10Hz to 1MHz
using the MIC5190 high speed controller.
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);
VOUT=1.8V
L=1µH
COUT=4.7µF + 0.1µ
0.1
3.3VIN
0.01
0.001
10
5VIN
100 1k 10k 100k 1M
FREQUENCY (Hz)
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 =
A/sec
2π
Figure 9. Output Impedance Measurement
April 2005
16
M9999-040705
www.micrel.com
Micrel
MIC2207
Then, determine the output impedance by looking at
the output impedance vs frequency graph. Then
calculating the voltage deviation times the load step;
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.
∆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 =
April 2005
↓ R DSON + DCR + X L
↑ GAIN
X COUT
17
M9999-040705
www.micrel.com
Micrel
MIC2207
Recommended Layout\ 3A Evaluation Board
Recommended Top Layout
Recommended Bottom Layout
April 2005
18
M9999-040705
www.micrel.com
Micrel
MIC2207
MIC2207 Schematic and B.O.M for 3A Output
MIC2207 Schematic
Item
Part Number
Description
Manufacturer
Qty
C1a,C1b
C2012JB0J106K
10uF Ceramic Capacitor X5R 0805 6.3V
TDK
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
C3
C2012JB0J475K
4.7uF Ceramic Capacitor X5R 0603 6.3V
TDK
GRM188R60J475KE19
4.7uF Ceramic Capacitor X5R 0603 6.3V
Murata
06036D475MAT
4.7uF Ceramic Capacitor X5R 0603 6.3V
AVX
1
C4
VJ0402A820KXAA
82pF 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
1
744 778 9001
1uH Inductor 12mOhm 7.3mm(L)x7.3mm(W)x3.2mm(H)
Wurth Electronik
1
IHLP2525AH-01 1
1uH Inductor 17.5mΩ (L)6.47mmx(W)6.86mmx(H) 1.8mm
Vishay Dale
1
R1,R4
CRCW04021002F
10KΩ 1% 0402 resistor
Vishay Dale
1
R2
CRCW04026651F
CRCW04021242F
CRCW04022002F
CRCW04024022F
6.65kΩ 1% 0402 For 2.5VOUT
12.4kΩ 1% 0402 For 1.8 VOUT
20kΩ 1% 0402 For 1.5 VOUT
40.2kΩ 1% 0402 For 1.2 VOUT
Open
For 1.0 VOUT
Vishay Dale
Vishay Dale
Vishay Dale
Vishay Dale
Vishay Dale
1
2
1
R3
CRCW040210R0F
10Ω 1% 0402 resistor
Vishay Dale
1
U1
MIC2207BML
2MHz 3A Buck Regulator
Micrel
1
Notes:
1. Sumida Tel: 408-982-9660
2. Murata Tel: 949-916-4000
3. Vishay Tel: 402-644-4218
4. Micrel Semiconductor Tel: 408-944-0800
April 2005
19
M9999-040705
www.micrel.com
Micrel
MIC2207
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.
April 2005
20
M9999-040705
www.micrel.com
Micrel
MIC2207
Revision History
Date
April 2005
Edits by:
Revision Number
21
M9999-040705
www.micrel.com