MIC4690 Evaluation Board

MIC4690 Evaluation Board
SuperSwitcher™ SOIC-8 Buck Switching
Regulator (500kHz 4V to 30V/1A)
General Description
The MIC4690 SuperSwitcher™ is an easy-to-use fixed or
adjustable output voltage step-down (buck) switch-mode
voltage regulator. The 500kHz MIC4690 achieves up to 1.3A
of continuous output current over a wide input range in an 8-pin
SOIC.
The MIC4690 has an input voltage range of 4V to 30V, with
excellent line, load, and transient response. The regulator
performs cycle-by-cycle current limiting and thermal shutdown
for protection under fault conditions. In shutdown mode, the
regulator draws less than 1.5µA of standby current.
The MIC4690 SuperSwitcher™ regulator requires a minimum
number of external components and can operate using a
standard series of inductors and capacitors. Frequency
compensation is provided internally for fast transient response
and ease of use.
The MIC4690 is available in the 8-pin SOIC with a –40°C to +
125°C junction temperature range.
Describe significant voltage/current/other versions or
capabilities. Do not include applications information or theory of
operation in this general description.
(+), the SW pin voltage drops until the freewheeling diode is
forward biased. During this portion of the cycle, current flows
through the diode, inductor and load. Figure 2 shows the 5V
output efficiency versus input voltage and output current.
Precautions
MIC4690 has no protection from reversed polarity being
applied to its input. Any momentary reversal of the dc power
supply connections can cause permanent damage to the
circuit. Use extreme care with these connections. The safest
way to power up the MIC4690 evaluation board is to set the
power supply to zero volts, and then gradually increase the
supply voltage. Monitor the input supply current while
increasing the input voltage. If the circuit draws excessive
current with no load applied (greater than 100mA) then there is
probably a problem with the set-up. Immediately shut off the
main power supply and check for proper power supply
connections. This simple procedure can avoid most
catastrophic failures.
Warning: Tantalum capacitors may explode if improperly
connected. Always wear safety glasses when operating the
evaluation board
Requirements
The MIC4690 evaluation board requires a power supply
capable of at least 1.7A at up to 30V. The load should be
capable delivering 1.3A under normal operation or 3A incurrent
limit.
Operation
Figure 1 shows the schematic of the evaluation board circuit.
When the internal high-side switch turns on, one side of the
inductor is fed from the input voltage, charging the inductor (+)
and (–). During this period, current flows from the input,
through the internal switch, output inductor and load. When the
output switch turns off, the inductor polarity switches to (–) and
J1
VIN
4V to +30V
(34V transients)
C1
22µF
35V
J3
GND
Figure 2.
U1 MIC4690BM/YM
2
C2
0.1µF
50V
C2
0.1µF
50V
OFF
ON
1
IN
SHDN
JP1
SW
3
FB
4
GND
SOIC-8
5—8
* C3 can be used to provide additional stability and
improved transient response.
J2
VOUT
1A
L1
18µH
D1
2A
40V
C3* 1800pF / 50V
optional
R1
3.01k
R6
optional
R2
6.49k
1
2
JP2a
1.8V
R3
2.94k
3
4
JP2b
2.5V
R4
1.78k
5
6
JP2c
3.3V
R5
976%
7
8
JP2d
5.0V
C4
220µF
10V
C5
0.1µF
50V
J4
GND
Figure 1. Evaluation Board Schematic
SuperSwitcher is a trademark of Micrel, 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
July 2006
M9999-072006
Micrel, Inc.
MIC4690 Evaluation Board
A higher feedback voltage increases the error amplifier
output voltage. A higher error amplifier voltage
(comparator inverting input) causes the comparator to
detect only the peaks of the sawtooth, reducing the duty
cycle of the comparator output. A lower feedback voltage
increases the duty cycle. The MIC4690 uses voltage mode
control architecture.
Functional Characteristics
The MIC4690 is a variable duty cycle switch-mode
regulator with an internal power switch.
Supply Voltage
The MIC4690 operates from a +4V to +30V unregulated
input. Highest efficiency operation is from a supply voltage
around +12V. See Figure 2.
Output Switching
When the internal switch is on, an increasing current flows
from the supply VIN, through external storage inductor L1,
to output capacitor COUT and the load. Energy is stored in
the inductor as the current increases with time.
When the internal switch is turned off, the collapse of the
magnetic field in L1 forces current to flow through fast
recovery diode D1, charging COUT.
Enable/Shutdown
The shutdown (SHDN) input is TTL compatible. Ground
the input if unused. A logic-low enables the regulator. A
logic-high shuts down the internal regulator which reduces
the current to typically 1.5µA when VSHDN = VIN = 12V and
30µA when VSHDN = 5V.
Feedback
Fixed-voltage versions of the regulator have an internal
resistive divider from the feedback (FB) pin. Connect FB
directly to the output voltage.
Adjustable versions require an external resistive voltage
divider from the output voltage to ground, center tapped to
the FB pin. See Table 1 for recommended resistor values.
Output Capacitor
External output capacitor COUT provides stabilization and
reduces ripple.
Return Paths
During the on-portion of the cycle, the output capacitor and
load currents return to the supply ground. During the off
portion of the cycle, current is being supplied to the output
capacitor and load by storage inductor L1, which means
that D1 is part of the high-current return path. See Figure
3.
Duty Cycle Control
A fixed-gain error amplifier compares the feedback signal
with a 1.23V band gap voltage reference. The resulting
error amplifier output voltage is compared to a 500kHz
sawtooth waveform to produce a voltage controlled
variable duty cycle output.
CIN
Power
SOIC-8
2
MIC4690BM/YM
VIN
SW
L1
3
VOUT
COUT
1
SHDN
FB
R1
4
D1
GND
R2
Load
V IN
+4V to +30V
(34V transients)
5 6 7 8
GND
Figure 3. Critical Traces for Layout
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MIC4690 Evaluation Board
Functional Characteristics
Load Transient
1.5A
IOUT
(1A/div)
Normal
Operation
500kHz
0A
100mV
VOUT
(100mV/div)
VSW (SHORTED)
12V IN, 0V OUT
VSW (NORMAL)
12V IN, 5V/1A OUT
Switching Frequency Foldback
Short
Circuit
Operation
VIN = 12V
VOUT = 5V
190kHz
TIME
TIME (100µs/div)
CONTINUOUS OUTPUT CURRENT (A)
Frequency Foldback
The MIC4690 folds the switching frequency back during a
hard short-circuit condition to reduce the energy per cycle
and protect the device.
*
SOA for MIC4690*
1.4
1.2
TA = 25°C
1.0
0.8
0.6
0.4
VOUT = 5V
TA = 50°C
TJ = 125°C
0.2
0
0
5
10 15 20 25 30
INPUT VOLTAGE (V)
35
SOA measured on the MIC4690 evaluation board
For higher currents (> 1A) at input voltages above 15V, use the snubber circuit shown below. For higher currents with out the snubber
circuit, refer to the MIC4684.
MIC4690BM/YM
IN
SW
SHDN
FB
3
1N4148
82%
D1
2.2nF
GND
5 6 7 8
Snubber Circuit
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Micrel, Inc.
MIC4690 Evaluation Board
Application Information
V IN
⎛ R1 ⎞
VOUT = VREF ⎜
+ 1⎟
⎝ R2
⎠
(2)
⎛V
⎞
R1 = R2⎜⎜ OUT − 1⎟⎟
V
⎝ REF
⎠
VIN
SW
3
SHUTDOWN
ENABLE
FB
4
VOUT
L1
R1
CIN
Adjustable Regulators
Adjustable regulators require a 1.23V feedback signal.
Recommended voltage-divider resistor values for
common output voltages are included in Table 1.
For other voltages, the resistor values can be
determined using the following formulas:
(1)
MIC4690BM/YM
2
1
SHDN
GND
D1
R2
COUT
5—8
Figure 4. Adjustable Regulator Circuit
VREF = 1.23V
Bill of Materials Matrix
VOUT
R1(1)
R2(1)
VIN
CIN
D1
L1
COUT
IOUT
5.0V
3.01k
976Ω
6.8V–30V
22µF, 35V
Vishay-Dale
595D226X0035D2T
Micro Commercial
2A, 40V
Schotty
SS24
18µH
Sumida
CDRH6D38-180ML
220µF, 10V
Vishay-Dale
594D227X0010D2T
see
SOA
5.0V
3.01k
976Ω
6.8V–14V
47µF, 20V
Vishay-Dale
595D476X0020C2T
Micro Commercial
2A, 20V
Schotty
SS22
18µH
Sumida
CDRH6D38-180ML
100µF, 6.3V
Vishay-Dale
595D107X06R3C2T
1.0A
3.3V
3.01k
1.78k
4.9V–14V
47µF, 20V
Vishay-Dale
595D476X0020C2T
Micro Commercial
2A, 20V
Schotty
SS22
15µH
Sumida
CDRH6D38-150ML
120µF, 4.0V
Vishay-Dale
595D127X0004C2T
1.0A
2.5V
3.01k
2.94k
4.25V–14V
47µF, 20V
Vishay-Dale
595D476X0020C2T
Micro Commercial
2A, 20V
Schotty
SS22
10µH
Sumida
CDRH6D38-100ML
120µF, 4.0V
Vishay-Dale
595D127X0004C2T
1.0A
1.8V
3.01k
6.49k
4.0V–14V
47µF, 20V
Vishay-Dale
595D476X0020C2T
Micro Commercial
2A, 20V
Schotty
SS22
10µH
Sumida
CDRH6D38-100ML
120µF, 4.0V
Vishay-Dale
595D127X0004C2T
1.0A
Note 1.
All resistors 1%
Table 1. Recommended Components for Common Output Voltages
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Micrel, Inc.
MIC4690 Evaluation Board
Thermal Considerations
The MIC4690 SuperSwitcher features the power-SOIC8. This package has a standard 8-pin small-outline
package profile but with much higher power dissipation
than a standard SOIC-8. The MIC4690 SuperSwitcher is
the first dc-to-dc converter to take full advantage of this
package.
Determining Ground-Plane Heat-Sink Area
There are two methods of determining the minimum
ground plane area required by the MIC4690.
Quick Method
Make sure that MIC4690 pins 5 though 8 are connected
to a ground plane with a minimum area of 6cm2. This
ground plane should be as close to the MIC4690 as
possible. The area maybe distributed in any shape
around the package or on any pcb layer as long as there
is good thermal contact to pins 5 though 8. This ground
plane area is more than sufficient for most designs.
When designing with the MIC4690, it is a good practice
to connect pins 5 through 8 to the largest ground plane
that is practical for the specific design.
SOIC-8
Checking the Maximum Junction Temperature
For this example, with an output power (POUT) of 5W, (5V
output at 1A maximum with VIN = 12V) and 50°C
maximum ambient temperature, what is the maximum
junction temperature?
Referring to the “Figure 2, 5V Output Efficiency” graph,
read the efficiency (η) for 1A output current at VIN = 12V
or perform you own measurement.
η = 75%
The efficiency is used to determine how much of the
output power (POUT) is dissipated in the regulator circuit
(PD).
JA
JC
CA
AM
BIE
NT
ground plane
heat sink area
printed circuit board
Figure 5. Power SOIC-8 Cross Section
The power SOIC-8 has higher power dissipation (lower
thermal resistance) because pins 5 though 8 and the
die-attach paddle are a single piece of metal. The die is
attached to the paddle with thermally conductive
adhesive. This provides a low thermal resistance path
from the junction of the die to the ground pins. This
design significantly improves package power dissipation
by allowing excellent heat transfer through the ground
leads to the printed circuit board.
One of the limitation to the maximum output current on
any MIC4690 design is the junction-to-ambient thermal
resistance (θJA) of the design (package and ground
plane).
Examining θJA in more detail:
θJA = (θJC + θCA)
where:
θJC = junction-to-case thermal resistance
θCA = case-to-ambient thermal resistance
θJC is a relatively constant 20°C/W for a power SOIC-8.
θCA is dependent on layout and is primarily governed by
the connection of pins 5 though 8 to the ground plane.
The purpose of the ground plane is to function as a heat
sink.
θJA is ideally 63°C/W but will vary depending on the size
of the ground plane to which the power SOIC-8 is
attached.
July 2006
PD =
POUT
− POUT
η
PD =
5W
− 5W
0.75
PD = 1.67W
A worst-case rule of thumb is to assume that 80% of the
total output power dissipation is in the MIC4690 (PD(IC))
and 20% is in the diode-inductor-capacitor circuit.
PD(IC) = 0.8 PD
PD(IC) = 0.8 × 1.67W
PD(IC) = 1.336W
Calculate the worst-case junction temperature:
TJ = PD(IC) θJC + (TC – TA) + TA(max)
where:
TJ = MIC4690 junction temperature
PD(IC) = MIC4690 power dissipation
θJC = junction-to-case thermal resistance.
The θJC for the MIC4690’s power-SOIC-8 is
approximately 20°C/W. (Also see Figure 5.)
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Micrel, Inc.
MIC4690 Evaluation Board
TC = “pin” temperature measurement taken at
the entry point of pins 6 or 7 into the plastic
package at the ambient temperature (TA) at
which TC is measured.
TA = ambient temperature at which TC is
measured.
=
maximum
ambient
operating
TA(max)
temperature for the specific design.
Calculating the maximum junction temperature given a
maximum ambient temperature of 65°C:
TJ = 1.336 × 20°C/W + (63°C – 25°C) + 50°C
TJ = 114.72°C
This value is within the allowable maximum operating
junction temperature of 125°C as listed in “Operating
Ratings.” Typical thermal shutdown is 160°C and is
listed in “Electrical Characteristics.”
July 2006
Layout Considerations
Layout is very important when designing any switching
regulator. Rapidly changing switching currents through
the printed circuit board traces and stray inductance can
generate voltage transients which can cause problems.
To minimize stray inductance and ground loops, keep
trace lengths, indicated by the heavy lines in Figure 6, as
short as possible. For example, keep D1 close to pin 3
and pins 5 through 8, keep L1 away from sensitive node
FB, and keep CIN close to pin 2 and pins 5 though 8.
The feedback pin trace from the output back to the IC
should be kept as far way from the switching elements
(usually L1and D1) as possible.
Circuits with sample layouts are provided. See Figure 6a
though 6c.
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MIC4690 Evaluation Board
Printed Circuit Board Layouts
Figure 6a. Top-Side Layer
Figure 6b. Bottom-Side Layer
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MIC4690 Evaluation Board
J1
VIN
4V to +30V
(34V transients)
C1
22µF
35V
J3
GND
U1 MIC4690BM/YM
2
C2
0.1µF
50V
C2
0.1µF
50V
OFF
ON
1
IN
SW
SHDN
JP1
FB
GND
SOIC-8
5—8
J2
VOUT
1A
L1
3
18µH
4
D1
2A
40V
R6
optional
R2
6.49k
1
JP2a
1.8V
2
* C3 can be used to provide additional stability and
improved transient response.
C3* 1800pF / 50V
optional
R1
3.01k
R3
2.94k
3
JP2b
2.5V
4
R4
1.78k
5
JP2c
3.3V
6
C4
220µF
10V
R5
976%
7
8
C5
0.1µF
50V
JP2d
5.0V
J4
GND
Figure 6c. Evaluation Board Schematic
Bill of Material
Reference
C1
C2, C6
C3
C4
C5
D1
J1–J4
JP1
JP2
JP3(Note 1)
R1
R2
R3
R4
Part Number
595D226X0035D2T
ECE-A1HFS470
TPSD226M035R0300
VJ0805Y104KXAMB
Manufacturer
Vishay Sprague1
Panasonic
AVX2
Vishay Vitramon1
594D227X0010D2T
Vishay Sprague1
SS24
B240A
2551-2-00-01-00-00-07-0
S1012-02-ND
S2012-04-ND
SNT-100-BL-G
Micro Commercial Corp.3
Diodes Inc.4
MillMax
Sullins
Sullins
Samtec
Description
22µF, 35V
47µF 50V, 8mm x 11.5mm
22µF, 35V
0.1µF 50V
option
220µF 10V
option
2A/40V Schottky
R5
R6
1
1
1
turret pins
straight single-row male header
straight dual-row male header
jumper header female
3.01k 1/10W 1%, size 0805
6.49k 1/10W 1%, size 0805
2.94k 1/10W 1%, size 0805
1.78k 1/10W 1%, size 0805
4
1
1
976Ω 1/10W 1%, size 0805
1
1
1
1
1
optional, size 0805
L1
CDRH6D38-180MC
U1
Note 1.
Qty
1
MIC4690BM/YM
Sumida5
Micrel, Inc.
6
18µH, 1.5A ISAT
1
1A 200kHz power-SOIC-8 buck regulator
1
Voltage selector.
1. Vishay Inc.: www.vishay.com
2. AVX: www.avxcorp.com
3. Micro Commercial Corp.: www.mccsemi.com
4. Diodes Inc.: www.diodes.com
5. Sumida: www.sumida.com
6. Micrel, Inc.: www.micrel.com
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MIC4690 Evaluation Board
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.
© 2001 Micrel, Incorporated.
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