MICREL MIC4744YML

MIC4744
4 MHz Dual 2A Integrated Switch
Buck Regulator
General Description
Features
The Micrel MIC4744 is a high efficiency dual PWM buck
(step-down) regulator that provides dual 2A current output.
The MIC4744 operates at 4MHz. A proprietary internal
compensation technique allows a closed loop bandwidth of
over 200kHz.
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The low on-resistance internal P-Channel MOSFET of the
MIC4744 allows efficiencies over 90%, reduces external
components count and eliminates the need for an
expensive current sense resistor.
The MIC4744 operates from 2.9V to 5.5V input and the
output can be adjusted down to 0.6V. The device can
operate with a maximum duty cycle of 100% for use in lowdropout conditions.
The MIC4744 is available in the exposed pad 16-pin
3mm x 3mm MLF® and ETSSOP-16 packages with
junction operating temperature ranging from –40°C to
+125°C.
All support documentation can be found on Micrel’s web
site at: www.micrel.com.
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2.9 to 5.5V supply voltage
4MHz PWM mode
2A Dual output
Greater than 90% efficiency
100% maximum duty cycle
Output voltage adjustable down to 0.6V
Ultra-fast transient response
Ultra-small external components
Stable with a 0.47µH inductor and a 10µF output
capacitor
Fully integrated 2A MOSFET switches
Micro-power shutdown
Thermal shutdown and current limit protection
Available in 3mm x 3mm 16-pin MLF® and 16-pin
TSSOP packages
–40°C to +125°C junction temperature range
Applications
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Broadband: xDSL modems
Automotive satellite radios
HD STB, DVD/TV recorder
Computer peripherals: printers and graphic cards
FPGA/ASIC
General point of load
Typical Application
5.0Vin 3.3Vout Efficiency
100
98
EFFICIENCY (%)
96
94
92
90
88
86
84
82
80
0.0
0.4
0.8
1.2
1.6
2.0
OUTPUT CURRENT (A)
2A 4MHz Dual Buck Regulator
MLF and MicroLeadFrame are registered trademarks of Amkor Technology, Inc.
Micrel Inc. • 2180 Fortune Drive • San Jose, CA 95131 • USA • tel +1 (408) 944-0800 • fax + 1 (408) 474-1000 • http://www.micrel.com
March 2009
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M9999-030209-C
Micrel, Inc.
MIC4744
Ordering Information
Output Voltage
Junction Temp. Range
MIC4744YML
Part Number
Adj.
–40° to +125°C
16-Pin 3mm x 3mm MLF®
Package
Lead Finish
Pb-Free
MIC4744YTSE
Adj.
–40° to +125°C
16-Pin EPAD TSSOP
Pb-Free
Note:
®
MLF is a GREEN RoHS-compliant package. Lead finish is NiPdAu. Mold compound is Halogen Free.
Pin Configuration
16-Pin 3mm x 3mm MLF® (YML)
March 2009
16-Pin EPAD-TSSOP (YTSE)
2
M9999-030209-C
Micrel, Inc.
MIC4744
Pin Description
Pin Number
®
(MLF )
Pin Number
(EPAD-TSSOP)
Pin Name
1
7
FB2
Feedback for output 2 (Input). Input to the error amplifier, connect to the
external resistor divider network to set the output voltage.
2(1)
8(1)
EN2
Enable for output 2 (Input). Logic level low, will shutdown the device, reducing
the current draw to less than 5µA (both EN1 and EN2 are low).
3,4
9,10
SW2
Switch for output 2 (Output): Internal power P-Channel MOSFET output switch
5
11
PGND2
Pin Function
Power Ground for output 2. Provides the ground return path for the high-side
drive current.
PGND1 pin and PGND2 pin are internally connected by anti-parallel diodes.
6,15
5,12
VIN2
Supply Voltage for output 2 (Input): Supply voltage for the source of the
internal P-Channel MOSFET and driver. Requires bypass capacitor to GND.
VIN1 pins and VIN2 pins are internally connected by anti-parallel diodes.
7,14
4,13
VIN1
Supply Voltage for output 1 (Input): Supply voltage for the source of the
internal P-Channel MOSFET and driver. Requires bypass capacitor to GND.
VIN1 pins and VIN2 pins are internally connected by anti-parallel diodes.
8
14
PGND1
Power Ground for output 1. Provides the ground return path for the
high-side drive current.
PGND1 pin and PGND2 pin are internally connected by anti-parallel diodes.
9,10
15,16
SW1
Switch for output 1 (Output): Internal power P-Channel MOSFET output switch
11(1)
1(1)
EN1
Enable for output 1 (Input). Logic level low, will shutdown the device, reducing
the current draw to less than 5µA (both EN1 and EN2 are low).
12
2
FB1
Feedback for output 1 (Input). Input to the error amplifier, connect to the
external resistor divider network to set the output voltage.
13
3
SGND
Signal (Analog) Ground. Provides return path for control circuitry and internal
reference.
16
6
BIAS
Internal circuit bias supply. Must be bypassed with a 0.1µF ceramic capacitor
to SGND. Biased through a 10Ω resistor to VIN.
EPAD
EPAD
GND
Connect to ground.
Note:
1. Do not float Enable Input.
March 2009
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Micrel, Inc.
MIC4744
Absolute Maximum Ratings(1)
Operating Ratings(2)
Supply Voltage (VIN) ......................................... -0.3V to +6V
Output Switch Voltage (VSW1, VSW2).................. -0.3V to +6V
Output Switch Current (ISW1, ISW2)………...Internally Limited
Input Voltage (VEN), (VFB), (VBIAS) ...................... -0.3V to VIN
Storage Temperature (Ts) .........................–60°C to +150°C
Junction Temperature ................................................ 150°C
Lead Temperature (soldering, 10sec.)....................... 260°C
ESD Rating(3) ................................................................. 2KV
Supply Voltage (VIN)..................................... +2.9V to +5.5V
Logic Input Voltage (VEN) ....................................... 0V to VIN
Junction Temperature (TJ) ........................ –40°C to +125°C
Junction Thermal Resistance
MLF® (θJA)..........................................................60°C/W
EPAD TSSOP (θJA)............................................35°C/W
Electrical Characteristics(4)
VIN = VEN = 3.6V; L = 1.0µH; COUT = 4.7µF; TA = 25°C, unless noted. Bold values indicate –40°C< TJ < +125°C.
Parameter
Condition
Min
2.9
Supply Voltage Range
Under-Voltage Lockout Threshold
2.45
Turn-on
UVLO Hysteresis
Quiescent Current
Typ
2.6
Max
Units
5.5
V
2.7
100
VFB = 0.9 * VNOM (not switching); VIN = 5.5V
1.4
V
mV
3.0
mA
VFB = 0.9 * VNOM (not switching); VIN = 3.6V
1.0
Shutdown Current
VEN = 0V
1.6
10
µA
[Adjustable] Feedback Voltage
± 2% (over temperature) ILOAD = 100µA
0.6
0.612
V
0.588
FB pin input current
mA
1
2.5
nA
Current Limit
VFB = 0.9 * VNOM
4.3
A
Output Voltage Line Regulation
VIN = 2.9V to 5.5V; ILOAD= 100µA
0.07
%
Output Voltage Load Regulation
20mA < ILOAD < 2A
0.2
%
Maximum Duty Cycle
VFB ≤ 0.9 * VNOM
Switch ON-Resistance
ISW = 50mA VFB =GND
100
%
155
3.0
Oscillator Frequency
Switching Phase
3.8
mΩ
4.4
180
0.5
Enable Threshold
Enable Hysteresis
0.9
MHz
°C
1.3
55
V
mV
2
Enable Input Current
0.1
Over-Temperature Shutdown
153
µA
°C
Over-Temperature Hysteresis
18
°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, 1.5k in series with 100pF.
4. Specification for packaged product only.
March 2009
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Micrel, Inc.
MIC4744
Typical Characteristics
5.0Vin 1.0Vout Efficiency
74
74
78
72
72
76
70
68
66
64
62
3.0Vin
3.3Vin
58
EFFICIENCY (%)
80
60
70
68
66
64
62
60
4.5Vin
58
5.0Vin
3.6Vin
0.4
0.8
1.2
1.6
0.4
OUTPUT CURRENT (A)
68
66
0.8
1.2
1.6
3.3Vin 1.5Vout Efficiency
80
72
70
68
66
4.5Vin
64
78
76
74
72
70
76
74
72
2
0
0.4
0.8
1.2
1.6
2
0
86
92
84
84
90
82
82
88
76
74
72
70
EFFICIENCY (%)
86
EFFICIENCY (%)
94
78
80
78
76
74
3.0Vin
72
3.3Vin
70
4.5Vin
5.0Vin
3.6Vin
0.4
94
90
92
88
90
86
84
82
80
0.8
1.2
1.6
0
0.4
80
78
4.5Vin
1.6
2
1.005
1.000
0.995
Vin=3.3V
5.5Vin
76
2.0
1.2
5.0Vin
5.5Vin
74
0.8
Load Regulation
82
5.0Vin
March 2009
3.3Vin
1.010
84
76
OUTPUT CURRENT (A)
3.0Vin
76
OUTPUT CURRENT (A)
86
4.5Vin
1.6
78
2
88
78
1.2
80
OUTPUT VOLTAGE (V)
92
0.8
82
5.0Vin 3.3Vout Efficiency
96
EFFICIENCY (%)
EFFICIENCY (%)
5.0Vin 2.5Vout Efficiency
94
0.4
84
OUTPUT CURRENT (A)
OUTPUT CURRENT (A)
0.0
86
74
0
2
2
3.6Vin
68
1.6
1.6
5.5Vin
68
1.2
1.2
3.3Vin 2.5Vout Efficiency
88
80
0.8
OUTPUT CURRENT (A)
5.0Vin 1.8Vout Efficiency
3.3Vin 1.8Vout Efficiency
0.8
0.4
OUTPUT CURRENT (A)
88
0.4
5.0Vin
64
OUTPUT CURRENT (A)
0
4.5Vin
5.5Vin
64
1.6
2
78
66
3.6Vin
60
1.6
68
3.3Vin
66
5.5Vin
1.2
70
3.0Vin
68
5.0Vin
62
EFFICIENCY (%)
82
80
EFFICIENCY (%)
82
76
74
0.8
5.0Vin 1.5Vout Efficiency
78
1.2
0.4
OUTPUT CURRENT (A)
84
0.8
3.6Vin
0
84
0.4
3.3Vin
2
80
0
3.0Vin
OUTPUT CURRENT (A)
5.0Vin 1.2Vout Efficiency
EFFICIENCY (%)
70
60
0
2
72
62
56
0
74
64
5.5Vin
56
EFFICIENCY (%)
3.3Vin 1.2Vout Efficiency
76
EFFICIENCY (%)
EFFICIENCY (%)
3.3Vin 1.0Vout Efficiency
76
0.990
0.0
0.4
0.8
1.2
1.6
OUTPUT CURRENT (A)
5
2.0
0
0.4
0.8
1.2
1.6
2
OUTPUT CURRENT (A)
M9999-030209-C
Micrel, Inc.
MIC4744
Typical Characteristics (continue)
Feedback Voltage
vs. Temperature
Frequency
vs. Temperature
1.0030
0.605
1.0028
0.604
4.2
1.0026
0.603
4.1
1.0024
1.0022
1.0020
1.0018
1.0016
1.0014
4.3
FREQUENCY (MHz)
FEEDBACK VOLTAGE (V)
OUTPUT VOLTAGE (V)
Line Regulation
0.602
0.601
0.600
0.599
0.598
4.0
3.9
3.8
3.7
3.6
0.597
3.5
1.0012
0.596
3.4
1.0010
0.595
2.9
3.3
3.7
4.1
4.5
4.9
5.3
3.3
-40
-20
0
SUPPLY VOLTAGE (V)
20
40
60
80
100
120
-40
-20
TEMPERATURE (°C)
Feedback Voltage
vs. Supply Voltage
20
40
60
80
100
120
TEMPERATURE (°C)
Quiescent Current
vs. Supply Voltage
0.8
0
Rdson
vs. Supply Voltage
1600
180
0.4
0.2
1200
160
Rdson (mΩ)
QUIESCENT CURRENT (μA)
FEEDBACK VOLTAGE (V)
170
0.6
800
2
3
4
5
100
0
1
SUPPLY VOLTAGE (V)
2
3
4
2.9
5
1.2
1.2
1.0
1.0
140
130
120
110
ENABLE THRESHOLD (V)
ENABLE THRESHOLD (V)
Rdson (mΩ)
150
0.8
0.6
0.4
0.2
0.0
100
0
20
40
60
TEMPERATURE (°C)
March 2009
80
100
120
4.1
4.5
4.9
5.3
Enable Threshold
vs. Temperature
170
160
3.7
SUPPLY VOLTAGE (V)
Enable Threshold
vs. Supply Voltage
180
-20
3.3
SUPPLY VOLTAGE (V)
Rdson
vs. Temperature
-40
130
110
0
1
140
120
400
0.0
0
150
0.8
0.6
0.4
0.2
0.0
2.9
3.3
3.7
4.1
4.5
4.9
SUPPLY VOLTAGE (V)
6
5.3
-40
-20
0
20
40
60
80
100
120
TEMPERATURE (°C)
M9999-030209-C
Micrel, Inc.
MIC4744
Functional Characteristics
March 2009
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Micrel, Inc.
MIC4744
Functional Diagram
MIC4744 Block Diagram
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Micrel, Inc.
MIC4744
Pin Description
SW1/SW2
The switch pins (SW1 and SW2) connect directly to the
inductor and provide the switching current necessary to
operate in PWM mode. Due to the high speed switching
on these pins, the switch nodes should be routed away
from sensitive nodes. These pins also connect to the
cathodes of the free-wheeling diodes.
VIN1/VIN2
VIN pins (two pins for VIN1 and two pins for VIN2)
provide power to the source of the internal P-Channel
MOSFET along with the current limiting sensing. VIN1
pins and VIN2 pins are internally connected by antiparallel diodes. The VIN operating voltage range is from
2.9V 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 for more details.
PGND1/PGND2
Power ground pins (PGND1 and PGND2) are the ground
paths for the MOSFET drive current. PGND1 pin and
PGND2 pin are internally connected by anti-parallel
diodes. 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
recommendation for more details.
BIAS
The bias (BIAS) provides power to the internal reference
and control sections of the MIC4744. A 10Ω resistor
from VIN to BIAS and a 0.1µF from BIAS to SGND are
required for clean operation.
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 recommendation for
more details.
EN1/EN2
The enable pins (EN1 and EN2) provides a logic level
control of the outputs 1 and 2. In the off state, supply
current of the device is greatly reduced (typically <2µA).
Do not drive the enable pin above the supply voltage.
FB1/FB2
The feedback pins (FB1 and FB2) provides the control
path to control the outputs 1 and 2. A resistor divider
connecting the feedback to the output is used to adjust
the desired output voltage. The output voltage is
calculated as follows:
EPAD
The exposed pad on the bottom of the part must be
connected to ground.
⎛ R1 ⎞
VOUT = VREF × ⎜
+ 1⎟
⎝ R2
⎠
where VREF is equal to 0.6V.
A feed-forward capacitor is recommended for most
designs. To reduce current draw, 10K feedback resistors
are recommended from the outputs to the FB pins (R1 in
the equation). Also, feed-forward capacitors should be
connected between the outputs and feedback pins
(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 feed-forward capacitor, these effects can be
significantly reduced. Feed-forward capacitance (CFF)
can be calculated as follows:
C FF =
March 2009
1
2π × R1 × 200kHz
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M9999-030209-C
Micrel, Inc.
MIC4744
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, Figure 3 illustrates the high side
switch is turned on, current flows from the input supply
through the inductor and to the output. The inductor
current is charged at the rate;
Application Information
The MIC4744 is a dual 2A 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.
(VIN − VOUT )
L
Figure 1. Example of Non-synchronous Buck Converter
For a non-synchronous buck converter, there are two
modes of operation; continuous and discontinuous.
Continuous or discontinuous refer to the inductor
current. If current is continuously flowing through the
inductor throughout the switching cycle, it is in
continuous operation. If the inductor current drops to
zero during the off time, it is in discontinuous operation.
Critically continuous is the point where any decrease in
output current will cause it to enter discontinuous
operation. The critically continuous load current can be
calculated as follows;
Figure 3. On-Time
2⎤
⎡
V
⎢VOUT − OUT ⎥
VIN ⎥⎦
⎢
I OUT = ⎣
fsw × 2 × 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=
Continuous or discontinuous operation determines how
we calculate peak inductor current.
VOUT
VIN
and the On time is;
Continuous Operation
Figure 2 illustrates the switch voltage and inductor
current during continuous operation.
TON =
D
fsw
Therefore, peak to peak ripple current is;
I pk − pk =
fsw × L
VIN
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 1/2 the peak-to-peak current.
Figure 2. Continuous Operation
March 2009
(V IN −VOUT )× VOUT
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M9999-030209-C
Micrel, Inc.
MIC4744
I pk = IOUT +
(VIN − VOUT )× VOUT
2 × fsw × L
VIN
Figure 4 demonstrates the off-time. During the off-time,
the high-side internal P-Channel MOSFET turns off.
Since the current in the inductor has to discharge, the
current flows through the free-wheeling Schottky diode
to the output. In this case, the inductor discharge rate is
(where VD is the diode forward voltage);
−
(VOUT
+ VD )
L
The total off time can be calculated as;
TOFF =
1− D
fsw
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 MIC4744 will skip
pulses as necessary, reducing gate drive losses,
drastically improving light load efficiency.
Efficiency Considerations
Calculating the efficiency is as simple as measuring
power out and dividing it by the power in;
P
Efficiency = OUT × 100
PIN
Where input power (PIN) is;
Figure 4. Off-Time
PIN = VIN × IIN
and output power (POUT) is calculated as;
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.
POUT = VOUT × IOUT
The Efficiency of the MIC4744 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:
PSW = R DSON × IOUT 2 × D
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MIC4744
Switching losses occur twice each cycle, when the
switch turns on and when the switch turns off. This is
caused by a non-ideal world where switching transitions
are not instantaneous, and neither are currents. Figure 6
demonstrates how switching losses due to the
transitions dissipate power in the switch.
Where D is the duty cycle.
Since the MIC4744 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;
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;
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.
PL = DCR × IOUT 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.
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MIC4744
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 to 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 recommendation for proper
layout of the input capacitor.
Diode Selection
Since the MIC4744 is non-synchronous, a free-wheeling
diode is required for proper operation. A Schottky diode is
recommended due to the low forward voltage drop and
their fast reverse recovery time. The diode should be rated
to be able to handle the average output current. Also, the
reverse voltage rating of the diode should exceed the
maximum input voltage. The lower the forward voltage drop
of the diode the better the efficiency. Please refer to the
layout recommendation 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 0.6V. Calculating the set output voltage
is as follows:
Output Capacitor
The MIC4744 is designed for a 4.7µF output capacitor and
a 1µH inductor or a 10µF output capacitor and a 0.47µH
inductor. 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 or a 10µF, a small 0.1µF is
recommended close to the load for high frequency filtering.
Smaller case size capacitors are recommended due to
there lower equivalent series ESR and ESL.
The MIC4744 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 or a 10µF output capacitor
is not sufficient, other values of capacitance can be used
but the original LC filter pole frequency determined by COUT
= 10µF and L = 0.47µH (which is approximately 73.4KHz)
must remain fixed. Increasing COUT forces L to decrease
and vice versa.
⎛ 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 on page 17. Although the range of
resistance for the FB resistors is very wide, R1 is
recommended to be 10K. This minimizes the effect the
parasitic capacitance of the FB node.
Feedforward Capacitor (CFF)
A capacitor across the resistor from the output to the
feedback pin (R1) is recommended for most designs. This
capacitor can give a boost to phase margin and increase
the bandwidth for transient response. Also, large values of
feedforward capacitance can slow down the turn-on
characteristics, reducing inrush current. For maximum
phase boost, CFF can be calculated as follows:
C FF =
Inductor Selection
The MIC4744 is designed for use with a 0.47µH inductor
and a 10µF output capacitor or a 1µH inductor and a 4.7µF
output capacitor. 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.
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1
2π × 200kHz × R1
Large values of feedforward capacitance may introduce
negative FB pin voltage during load shorting, which will
cause latch-off. In that case, a Schottky diode from FB pin
to the ground is recommended.
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
for the bias, these types of capacitors lose their ability to
filter at high frequencies.
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MIC4744
Loop Stability and Bode Analysis
GAIN (dB)
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.
60
210
50
175
40
140
30
105
20
70
10
35
0
0
-10
-35
-20
-70
-30
100
1000
10000
100000
PHASE (°)
Bode Plot
Vin=3.6V Vout=1.8V Iout=2A
-105
1000000
FREQUENCY (Hz)
Typically for 3.6Vin and 1.8Vout at 2A:
•
•
Phase Margin=80.7 Degrees
GBW=210KHz
Being that the MIC4744 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.
Figure 7. Transformer Injection
March 2009
GAIN (dB)
A 50Ω resistor allows impedance matching from the
network analyzer source. This method allows the DC
loop to maintain regulation and allow the network
analyzer to insert an AC signal on top of the DC voltage.
The network analyzer will then sweep the source while
monitoring A and R for an A/R measurement.
The following Bode analysis show the small signal loop
stability of the MIC4744, it utilizes type III compensation.
This is a dominant low frequency pole, followed by 2
zeros and finally the double pole of the inductor
capacitor filter, creating a final 20dB/decade roll off.
Bode analysis gives us a few important data points;
speed of response (Gain Bandwidth or GBW) and loop
stability. Loop speed or GBW determines the response
time to a load transient. Faster response times yield
smaller voltage deviations to load steps.
Instability in a control loop occurs when there is gain and
positive feedback. Phase margin is the measure of how
stable the given system is. It is measured by determining
how far the phase is from crossing zero when the gain is
equal to 1 (0dB).
60
210
50
175
40
140
30
105
20
70
10
35
0
0
-10
-35
-20
-70
-30
100
1000
10000
100000
PHASE (°)
Bode Plot
Vin=3.6V Vout=1.8V Iout=0.1A
-105
1000000
FREQUENCY (Hz)
3.6Vin, 1.8Vout Iout=0.1A:
•
•
Phase Margin=68.3 Degrees
GBW= 24.0kHz
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.
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M9999-030209-C
Micrel, Inc.
MIC4744
Gain and Phase
vs. Frequency
35
0
60
Max. Amount of Phase Boost
Obtainable Using Cff vs. Output
Voltage
30
20
-10
15
10
-15
PHASE BOOST (°)
GAIN (dB)
25
PHASE BOOST (°)
50
-5
1000
10000
100000
0
1000000
20
0
0
FREQUENCY (Hz)
1
2
3
4
5
OUTPUT VOLTAGE (V)
By looking at the graph, phase margin can be affected to
a greater degree with higher output voltages.
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.
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10
5
-20
100
40
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M9999-030209-C
Micrel, Inc.
MIC4744
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.
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MIC4744
Bill of Materials
MIC4744YML Schematic for 2A Output
Item
Part Number
C11,C12
GRM188R60J106M
C21,C22
C1608X5R0J106M
C15,C25
C13*,C23*
C14, C24
C16,C26,C7
C18,C28
D1,D2
L1,L2
06036D106KMAT2A
VJ0603A681KXXCW
Manufacturer
Murata
TDK(2)
AVX
Vishay(4)
VJ0603A82KXXCW
Vishay
82pF Ceramic Capacitor NPO 0603 10V
2
VJ0603Y104KXXAT
Vishay(4)
0.1µF Ceramic Capacitor X7R 0603 25V
5
SS2P3L
Vishay(4)
SSA23L
Vishay(4)
2A Schottky 30V
2
0.47µH Inductor 20mΩ 4.45mm(L) x 4.06mm(W) x 1.2mm(H)
2
10kΩ, 1%, Size, 0603 Resistor
2
(5)
B230A
Diodes
IHLP1616AB-01 R47
Vishay(4)
CRCW06033K16FKXX
(4)
Vishay
3.16kΩ, 1%, Size 0603, For 2.5VOUT
CRCW06034K99FKXX
CRCW06036K65FKXX
4.99kΩ, 1%, Size 0603, For 1.8 VOUT
Vishay(4)
6.65kΩ, 1%, Size 0603, For 1.5 VOUT
CRCW060310K0FKXX
15kΩ, 1%, Size 0603, For 1.0 VOUT
CRCW06032R70FKXX
Vishay(4)
CRCW060349K9FKXX
(4)
CRCW060310R0FKXX
MIC4744YML
2
10kΩ, 1%, Size 0603, For 1.2 VOUT
CRCW060315K0FKXX
U1
6
2
R12,R22
R5
10µF Ceramic Capacitor X5R 0603 6.3V
(3)
680pF Ceramic Capacitor NPO 0603 6.3V
CRCW060310K0FKXX
R14, R24
Qty
(4)
R11,R12
R13*, R23*
Description
(1)
Vishay
(4)
Vishay
Micrel, Inc.
(6)
2.7Ω, 1%, Size 0603, Resistor
2
49.9kΩ, 1%, Size 0603, Resistor
2
10Ω, 1%, Size 0603, Resistor
1
Dual 2A 4MHz Integrated Switch Buck Regulator
1
* Only need for ultra-low noise applications.
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M9999-030209-C
Micrel, Inc.
MIC4744
Notes:
1. Murata: www.murata.com.
2. TDK: www.tdk.com.
3. AVX: www.avx.com.
4. Vishay: www.vishay.com.
5. Diode: www.diodes.com.
6. Micrel, Inc: www.micrel.com.
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Micrel, Inc.
MIC4744
MIC4744YML Layout Recommendation: 2A Evaluation Board
Recommended TOP Layout
Recommended Bottom Layout
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Micrel, Inc.
MIC4744
Recommended Mid-Layer 1 Layout
Recommended Mid-Layer 2 Layout
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Micrel, Inc.
MIC4744
Package Information
16-Pin 3mm x 3mm MLF® (ML)
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Micrel, Inc.
MIC4744
16-Pin ETSSOP
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MIC4744
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.
© 2008 Micrel, Incorporated.
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M9999-030209-C