MIC4680 Evaluation Board

MIC4680 Evaluation Board
Micrel
MIC4680 Evaluation Board
SuperSwitcher™ SOP-8 Buck Switching Regulator
200kHz 4V to 34V/1A
shows the 5V output efficiency versus input voltage and
output current.
Precautions
MIC4680 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 MIC4680 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.
General Description
The MIC4680 SuperSwitcher is a series of easy-to-use fixed
and adjustable BiCMOS step-down (buck) switching regulators. The 200kHz MIC4680 achieves 1A of continuous output
current over the entire input voltage and temperature range,
(–40°C to +125°C) and up to 60°C ambient respectively, in a
tiny SOP-8 package. It has a logic compatible enable that
provides 5µA of quiescent current (typical) in shutdown
mode. Efficiencies up to 81% peak are also possible. The
MIC4680 will also achieve up to 1.3A of continuous output
current over a VIN range of 10V to 20V and a 5V output. The
MIC4680 features a 200kHz switching frequency that reduces the inductor of the popular 52kHz, LM257x by a factor
of 2, freeing up precious board space. The MIC4680 is a thirdgeneration simple step-down switching regulator, with its
lineage traced back to the popular LM257x, but has been
enhanced with 200kHz operation, tighter current limit and
thermal shutdown. Other improvements includes 1.8V of
head room (VSAT) and 10µA instead of 200µA (typical)
quiescent current in shutdown mode.
Requirements
The MIC4680 evaluation board requires a power supply
capable of at least 1.7A at up to 34V. The load should be
capable delivering 1.3A under normal operation or 3A in
current limit.
5V Output
Efficiency
90
EFFICIENCY (%)
80
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 (+), 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
J1
VIN
4V to +34V
C1
15µF
35V
J3
GND
2
OFF
ON
C2
0.1µF
50V
1
U1 MIC4680BM
IN
SW
SHDN
S1
NKK G12AP
FB
GND
SOP-8
5–8
3
70
7V
60
12V
24V
50
40
30
20
10
0
0
0.2 0.4 0.6 0.8 1.0 1.2 1.4
OUTPUT CURRENT (A)
Figure 2.
J2
VOUT
1A
L1
68µH
R1
3.01k
4
D1
R6
B260A optional
or
1
SS26
2
* C3 can be used to provide additional stability
and improved transient response.
R2
6.49k
3
JP1a
1.8V
C3*
optional
R3
2.94k
5
JP1b
2.5V
4
6
R4
1.78k
7
JP1c
3.3V
8
C4
220µF
10V
R5
976Ω
JP1d
5.0V
C5
0.1µF
50V
J4
GND
Figure 1. Evaluation Board Schematic
SuperSwitcher is a trademark of Micrel, Inc.
Micrel, Inc. • 1849 Fortune Drive • San Jose, CA 95131 • USA • tel + 1 (408) 944-0800 • fax + 1 (408) 944-0970 • http://www.micrel.com
August 2000
1
MIC4680 Evaluation Board
MIC4680 Evaluation Board
Micrel
waveform to produce a voltage controlled variable duty cycle
output.
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 MIC4680 uses a voltage-mode control architecture.
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.
Output Capacitor
External output capacitor COUT provides stabilization and
reduces ripple. See “Bode Plots” for additional information.
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 4.
Functional Description
The MIC4680 is a variable duty cycle switch-mode regulator
with an internal power switch.
Supply Voltage
The MIC4680 operates from a +4V to +34V unregulated
input. Highest efficiency operation is from a supply voltage
around +15V. See Figure 2.
Enable/Shutdown
The shutdown (SHDN) input is TTL compatible. Ground the
input if unused. A logic-low enables the regulator. A logichigh 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.
Duty Cycle Control
A fixed-gain error amplifier compares the feedback signal
with a 1.23V bandgap voltage reference. The resulting error
amplifier output voltage is compared to a 200kHz sawtooth
MIC4680BM
IN
SW
3
1N4148 82Ω
SHDN
D1
FB
2.2nF
GND
5 6 7 8
Figure 3. Increasing Maximum Output Current at High Input Voltages
2
CIN
Power
SOP-8
MIC4680BM
IN
SW
L1
3
VOUT
68µH
COUT
1
SHDN
FB
R1
4
D1
GND
R2
Load
VIN
+4V to +34V
5 6 7 8
GND
Figure 4. Critical Traces for Layout
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MIC4680 Evaluation Board
Micrel
Bode Plots
The following bode plots show that the MIC4680 is stable over all conditions using a 68µF inductor (L) and a 220µF output
capacitor (COUT). To assure stability, it is a good practice to maintain a phase margin of greater than 35°.
No-Load Stability
Phase Margin = 106°
Full-Load Stability
Phase Margin = 113°
L = 68µF
COUT = 220µF
L = 68µF
COUT = 220µF
VIN = 7V
VOUT = 5.0V
IOUT = 0.0A
VIN = 7V
VOUT = 5.0V
IOUT = 1.0A
TIME (100ms/div.)
TIME (100ms/div.)
No-Load Stability
Phase Margin = 117°
Full-Load Stability
Phase Margin = 69°
L = 68µF
COUT = 220µF
L = 68µF
COUT = 220µF
VIN = 12V
VOUT = 5.0V
IOUT = 0.0A
VIN = 12V
VOUT = 5.0V
IOUT = 1.0A
TIME (100ms/div.)
TIME (100ms/div.)
Functional Characteristics
Safe
Operating Area
1.5
Minimum
Current Limit
1.4
1.3
1.2
Normal
Operation
Note
1.1
OUTPUT CURRENT (A)
VSW (SHORTED)
12V IN, 0V OUT
VSW (NORMAL)
12V IN, 5V/1A OUT
Switching Frequency Foldback
200kHz
Short
Circuit
Operation
60kHz
1.0
0.9
0.8
0.7
0.6
0.5
0.4
VOUT = 5V
TA = 60°C
Demonstration
board layout
0.3
TIME
0.2
Frequency Foldback
0.1
The MIC4680 folds the switching frequency back during a hard shortcircuit condition to reduce the energy per cycle and protect the device.
August 2000
0
3
0
5
10
15
20
25
INPUT VOLTAGE (V)
30
35
MIC4680 Evaluation Board
MIC4680 Evaluation Board
Micrel
Applications Information
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)
 R1 
VOUT = VREF 
+ 1
 R2 
(2)
V

R1 = R2  OUT − 1
 VREF

VIN
MIC4680BM
2
IN
SW
3
FB
4
R1
CIN
SHUTDOWN
ENABLE
VOUT
L1
1
SHDN
GND
COUT
D1
R2
5–8
Figure 5. Adjustable Regulator Circuit
VREF = 1.23V
VOUT
R1*
R2*
CIN
D1
L1
COUT
1.8V 3.01k 6.49k
68µH 1.5A
Coiltronics UP2-680
15µF 35V
2A 60V Schottky
or
220µF 10V
3.3V 3.01k 1.78k
AVX TPSE156035R0200 General Semiconductor SS26 Sumida CDRH125-680MC** AVX TPSE227010R0060
5.0V 3.01k 976Ω
or
Sumida CDRH124-680MC**
6.0V 3.01k 787Ω
2.5V 3.01k 2.94k
* All resistors 1%
** shielded magnetics for low RFI applications
Table 1. Recommended Components for Common Ouput Voltages
Thermal Considerations
The MIC4680 SuperSwitcher features the power-SOP-8.
This package has a standard 8-lead small-outline package
profile but with much higher power dissipation than a standard SOP-8. The MIC4680 SuperSwitcher is the first dc-to-dc
converter to take full advantage of this package.
The power SOP-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
MIC4680 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 SOP-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 SOP-8 is attached.
SOP-8
θJA
θJC
θCA
ground plane
heat sink area
AM
BIE
NT
printed circuit board
Figure 6. Power SOP-8 Cross Section
MIC4680 Evaluation Board
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MIC4680 Evaluation Board
Micrel
Determining Ground-Plane Heat-Sink Area
There are two methods of determining the minimum ground
plane area required by the MIC4680.
Quick Method
Make sure that MIC4680 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 MIC4680 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.
Minimum Copper/Maximum Current Method
Using Figure 7, for a given input voltage range, determine the
minimum ground-plane heat-sink area required for the
application’s maximum output current. Figure 5 assumes a
constant die temperature of 75°C above ambient.
1.5
OUTPUT CURRENT (I)
8V
1.0
PD(IC) = 0.8 PD
PD(IC) = 0.8 × 1.05W
PD(IC) = 0.84W
Calculate the worst-case junction temperature:
TJ = PD(IC) θJC + (TC – TA) + TA(max)
where:
TJ = MIC4680 junction temperature
PD(IC) = MIC4680 power dissipation
θJC = junction-to-case thermal resistance.
The θJC for the MIC4680’s power-SOP-8 is approximately
20°C/W. (Also see Figure 1.)
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.
TA(max) = maximum ambient operating temperature
for the specific design.
Calculating the maximum junction temperature given a
maximum ambient temperature of 65°C:
TJ = 0.84 × 20°C/W + (45°C – 25°C) + 65°C
TJ = 101.8°C
This value is less than the allowable maximum operating
junction temperature of 125°C as listed in “Operating Ratings.” Typical thermal shutdown is 160°C.
Increasing the Maximum Output Current
The maximum output current at high input voltages can be
increased for a given board layout. The additional three
components shown in Figure 3 will reduce the overall loss in
the MIC4680 by about 20% at high VIN and high IOUT.
Even higher output current can be achieved by using the
MIC4680 to switch an external FET. See data sheet.
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 7, 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 L1
and D1) as possible.
A circuit with sample layouts are provided. See Figure 8b
though 8e.
12V
24V
34V
TA = 50°C
0.5
Minimum Current Limit = 1.3A
0
0
5
10
15
20
25
AREA (cm2)
Figure 7. Output Current vs. Ground Plane Area
When designing with the MIC4680, it is a good practice to
connect pins 5 through 8 to the largest ground plane that is
practical for the specific design.
Checking the Maximum Junction Temperature:
For this example, with an output power (POUT) of 5W, (5V
output at 1A maximum with VIN = 12V) and 65°C maximum
ambient temperature, determine the maximum junction temperature.
Referring to the Figure 2, read the efficiency (η) for 1A output
current at VIN = 12V or perform you own measurement.
η = 79%
The efficiency is used to determine how much of the output
power (POUT) is dissipated in the regulator circuit (PD).
PD = POUT (1 – η)
PD = 5W (1 – 0.79)
PD = 1.05W
A worst-case rule of thumb is to assume that 80% of the total
output power dissipation is in the MIC4680 (PD(IC)) and 20%
is in the diode-inductor-capacitor circuit.
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MIC4680 Evaluation Board
MIC4680 Evaluation Board
Micrel
Printed Circuit Board Layouts
Figure 8a. Top-Side Silk Screen
Figure 8c. Bottom-Side Silkscreen
Figure 8b. Top-Side Copper
Figure 8d. Bottom-Side Copper
MIC4680 Evaluation Board
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Bill of Material
Reference
Part Number
Manufacturer
Description
C1
TPSD156M035R0300
ECE-A1HFS470
AVX
Panasonic
15µF 35V
47µF 50V, 8mm × 11.5mm
1
C2, C5
08055C104KAT1A
AVX
0.1µF 50V
2
AVX or other
optional 3300pF
C3
Qty
(1)
C4
TPSD327M010R0150
AVX
220µF 10V
1
D1
SS26
General Semiconductor
Schottky
1
J1–J4
2551-2-00-01-00-00-07-0 MillMax
turret pins
4
JP1
929836-09036-ND
straight dual-row male header
1
JP2(Note 1)
929950-00-ND
female jumper header
1
R1
3.01k 1/10W 1%, size 0805
1
R2
6.49k 1/10W 1%, size 0805
1
R3
2.94k 1/10W 1%, size 0805
1
R4
1.78k 1/10W 1%, size 0805
1
R5
976Ω 1/10W 1%, size 0805
1
R6
optional, size 0805
S1
G12AP
NKK Switches
SPDT
1
L1
UP2-680
Coiltronics
68µH, 1.5A
1
U1
MIC4680BM
Micrel Semiconductor
1A 200kHz power-SO-8 buck regulator
1
Note 1.
Voltage selector.
J1
VIN
4V to +34V
C1
15µF
35V
J3
GND
2
OFF
ON
C2
0.1µF
50V
1
U1 MIC4680BM
IN
SW
SHDN
S1
NKK G12AP
FB
GND
SOP-8
5–8
J2
VOUT
1A
L1
3
68µH
R1
3.01k
4
D1
SS26
R6
optional
1
2
* C3 can be used to provide additional stability
and improved transient response.
R2
6.49k
3
JP1a
1.8V
C3*
optional
R3
2.94k
5
JP1b
2.5V
4
6
R4
1.78k
7
JP1c
3.3V
8
C4
220µF
10V
R5
976Ω
JP1d
5.0V
C5
0.1µF
50V
J4
GND
Figure 8e. Evalution Board Schematic
August 2000
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MIC4680 Evaluation Board
MIC4680 Evaluation Board
Micrel
MICREL INC. 1849 FORTUNE DRIVE SAN JOSE, CA 95131 USA
TEL
+ 1 (408) 944-0800
FAX
+ 1 (408) 944-0970
WEB
http://www.micrel.com
This information is believed to be accurate and reliable, however no responsibility is assumed by Micrel for its use nor for any infringement of patents or
other rights of third parties resulting from its use. No license is granted by implication or otherwise under any patent or patent right of Micrel Inc.
© 2000 Micrel Incorporated
MIC4680 Evaluation Board
8
August 2000