MIC28500 4A Evaluation Board
75V/4A Hyper Speed Control™
Synchronous DC/DC Buck Regulator
General Description
The MIC28500 DC/DC regulator operates over an input
supply range of 30V to 75V and provides a regulated
output at up to 4A of output current. The output voltage is
adjustable down to 0.8V with a typical accuracy of ±1% .
The switching frequency is adjustable and once adjusted
the switching frequency remains fairly constant with
changes in input voltage and output load.
The basic parameters of the evaluation board are:
1. Input
−
30V to 75V
2. Output
−
0.8V to 5V at 4A
(1)
, 12V at 2A
(2)
3. 250kHz Switching Frequency (Adjustable 100kHz
to 500kHz)
Notes:
1,2.
Refer to thermal de-rating curves shown in Typical
Characteristics section.
Datasheets and support documentation can be found
on Micrel’s web site at:
www.micrel.com
.
Requirements
The MIC28500 evaluation board requires only a single
power supply with at least 10A current capability. A
linear regulator, which includes a Zener and an NPN
transistor, has been installed on the board to provide
housekeeping (V
DD
) for the MIC28500. The output
load can either be a passive or an active load.
Precautions
The MIC28500 evaluation board does not have
reverse polarity protection. Applying a negative
voltage to the PVIN and GND terminals may damage
the device. The maximum PVIN of the board is rated
at 75V. Exceeding 75V on the PVIN could damage the
device
Getting Started
1.
V
IN
Supply
Connect a supply to the Vin and GND terminals,
paying careful attention to the polarity and the supply
range (30V < V
IN
< 75V). Monitor I
IN
with a current
Hyper Speed Control is a trademark of Micrel, Inc
meter and V
IN
at Vin and GND terminals with
voltmeter. Do not apply power until Step 5.
2.
Connect Load and Monitor Output
Connect a load to the Vout and GND terminals. The
load can be either a passive (resistive) or an active
(as in an electronic load) type. A current meter may be
placed between the Vout terminal and load to monitor
the output current. Ensure the output voltage is
monitored at the Vout terminal.
3.
V
IN
30V Limiter Circuit
To prevent overstressing of the internal top FET of the
MIC28500, the minimum input voltage at full load
should be limited to 30V. This is implemented by
using external VIN limiter circuit installed on either EN
pin (dash-line block#1) or VDD Pin (dash-line block#2)
as shown in the Evaluation board schematic. On the
MIC28500 evaluation board the limiter circuit on EN
Pin is installed, which shuts off the MIC28500 until the
input supply voltage reaches 30V.
4.
Enable Input
The EN pin has an internal 100k pull-up resistor to
VDD, which allows the output to be turned on when
VDD exceeds its UVLO threshold. An EN connector is
provided on the evaluation board for users to easily
access the enable feature. Applying an external logic
signal on the EN pin to pull it low or using a jumper to
short the EN pin to GND will shut off the output of the
MIC28500 evaluation board. If in certain application
the EN needs to be controlled externally the V
IN
limiter
circuit should be implemented on VDD pin.
5.
Turn on the Power
Turn on the VIN supply and verify that the output
voltage is regulated to 3.3V.
Ordering Information
Part Number
MIC28500 4A EV
MIC28500 2A EV
Description
Evaluation Board up to 5V Output
Evaluation Board 12V Output
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 2011
M9999-071911
hbwhelp@micrel.com
or (408) 955-1690
Micrel, Inc.
MIC28500 4A Evaluation Board
Features
Feedback Resistors
The output voltage on the MIC28500 evaluation
board, which is preset to 3.3V, is determined by the
feedback divider:
⎛
R4
V
OUT
=
V
REF
× ⎜
1
+
⎜
R
BOTTOM
⎝
⎞
⎟
⎟
⎠
where V
REF
= 0.8V, and R
BOTTOM
is one of R5, R6, R7,
R8, R9, R10, R11, R12, which corresponds to 0.9V,
1.0V, 1.2V, 1.5V, 1.8V, 2.5V, 3.3V, or 5V. Leaving the
R
BOTTOM
open gives a 0.8V output voltage. All other
voltages not listed above can be set by modifying
R
BOTTOM
value according to:
R
BOTTOM
=
R4
×
V
REF
V
OUT
−
V
REF
Note that the output voltage should not be set to
exceed 5V due to the 6.3V voltage rating on the
output capacitors.
If higher than 5V output is desired, it is recommended
to use the designs shown in Figure 2, where the
output capacitors, L1, R3 and R17 are optimized for
12V/2A output.
SW Node
Test point J11 (VSW) is placed for monitoring the
switching waveform, one of the most critical
waveforms for the converter.
Current Limit
The MIC28500 has internal FETs, and the current limit
is implemented by sensing the R
DS-ON
of bottom FET.
The MIC28500 has a fixed current limit of 7A (TYP).
Loop Gain Measurement
The resistor, R13, is placed in series with the regulator
feedback path. The control loop gain can be
measured by connecting an impedance analyzer
across the resistor and selecting the resistor value in
between 20
Ω
to 50
Ω
.
July 2011
2
M9999-071911
hbwhelp@micrel.com
or (408) 955-1690
Micrel, Inc.
MIC28500 4A Evaluation Board
MIC28500 0.8V to 5V/4A Evaluation Board Typical Characteristics
Efficiency (V
IN
= 48V)
vs. Output Current (D02)
100
90
80
5.0V
3.3V
2.5V
1.8V
1.5V
1.2V
1.0V
0.9V
0.8V
Thermal Derating
4
V
IN
= 30V
3
V
OUT
= 5V
f
SW
=250kHz
L=10uH
V
OUT
= 3.3V
Thermal Derating
4
V
IN
= 48V
Load Current (A)
EFFICIENCY (%)
70
60
50
40
30
20
10
0
1
2
3
4
V
IN
= 48V
f
SW
= 250kHz
Load Current (A)
3
V
OUT
= 5V
f
SW
=250kHz
L=10uH
2
2
V
OUT
= 3.3V
1
1
0
5
6
25
40
55
70
85
100
0
25
40
55
70
85
100
OUTPUT CURRENT (A)
Maximum Ambient Temperature (°C)
Maximum Ambient Temperature (°C)
Thermal Derating
4
V
IN
= 75V
3
f
SW
=250kHz
L=10uH
Load Current (A)
2
V
OUT
= 3.3V
1
V
OUT
= 5V
0
25
40
55
70
85
100
Maximum Ambient Temperature (°C)
Die Temperature* :
The temperature measurement was taken at the hottest point on the MIC28500 case mounted on a 5 square inch 4 layer, 0.62”,
FR-4 PCB with 2oz. finish copper weight per layer, see Thermal Measurement section. Actual results will depend upon the size of the PCB, ambient
temperature and proximity to other heat emitting components.
July 2011
3
M9999-071911
hbwhelp@micrel.com
or (408) 955-1690
Micrel, Inc.
MIC28500 4A Evaluation Board
MIC28500 0.8V to 5V/4A Output Evaluation Board Schematic
Figure 1. MIC28500 Evaluation board for 0.8V to 5V/4A Output
July 2011
4
M9999-071911
hbwhelp@micrel.com
or (408) 955-1690
Micrel, Inc.
MIC28500 4A Evaluation Board
Bill of Materials 0.8V to 5V/4A Output
Item
C1
C2, C3
C13
Part Number
EEU-FC2A101B
GRM32ER72A225KA35L
C3225X7R2A225KT5
GRM32ER60J107ME20L
12106D107MAT2A
06035C104KAT2A
C6
GRM188R71H104KA93D
C1608X7R1H104K
C10
GRM188R72A104KA35D
C1608X7S2A104K
0805ZC225MAT2A
C8, C9
GRM21BR71A225KA01L
C2012X7R1A225K
GRM188R72A102KA01D
C11
C1608X7R2A102K
06031C102KAT2A
GRM188R71H472KA01D
C12
C1608X7R2A472K
06035C472KAT2A
C16
C4, C5, C7
C14, C15
D1
D2
D5
D6
D4
L1
Q1
R1
R2, R16
R3
R4
R5
GRM21BR71A105KA01L
C2012X7R1A105K
Open
Open
BAT46W-TP
BAT46W-7-F
MMXZ5232B-TP
CMDZ5L6
CMDZ24L-MIC
CMDZ3L6-MIC
Open
DR125-100-R
FCX493
CRCW06034R75FKEA
CRCW08051R21FKEA
CRCW060319K6FKEA
CRCW060310K0FKEA
CRCW060380K6FKEA
Cooper Bussmann
Vishay Dale
(9)
Vishay Dale
(9)
Vishay Dale
(9)
Vishay Dale
(9)
(8)
Manufacturer
Panasonic
(1)
Murata
(2)
TDK
(3)
Description
100µF Aluminum Capacitor, SMD, 100V
2.2µF Ceramic Capacitor, X7R, Size 1210, 100V
100µF Ceramic Capacitor, X5R, Size 1210, 6.3V
Qty.
1
2
1
Murata
AVX
(4)
AVX
(4)
Murata
(2)
TDK
TDK
(3)
0.1µF Ceramic Capacitor, X7R, Size 0603, 50V
1
Murata
(3)
0.1µF Ceramic Capacitor, X7R, Size 0603, 100V
1
AVX
(4)
Murata
(2)
TDK
(3)
2.2µF Ceramic Capacitor, X7R, Size 0805, 10V
2
Murata
(2)
TDK
(3)
AVX
(4)
Murata
TDK
(2)
(3)
1nF Ceramic Capacitor, X7R, Size 0603, 100V
1
4.7nF Ceramic Capacitor, X7R, Size 0603, 50V
1
AVX
(4)
Murata
(2)
TDK
(3)
1µF Ceramic Capacitor, X7R, Size 0805, 10V
1
MCC
(5)
Diodes Inc.
(6)
MCC
(5)
Central Semi
(7)
Central Semi
(7)
Central Semi
(7)
Small Signal Schottky Diode
5.6V Zener Diode
24V Zener Diode
3.6V Zener Diode
10µH Inductor, 5.35A RMS, 7A Saturation Current
4.75Ω Resistor, Size 0603, 1%
1.21Ω Resistor, Size 0805, 1%
19.6kΩ Resistor, Size 0603, 1%
10kΩ Resistor, Size 0603, 1%
80.6kΩ Resistor, Size 0603, 1%
1
1
1
1
1
1
1
2
1
1
1
Diodes Inc/ZETEX
(6)
100V NPN Transistor
Vishay Dale
(9)
July 2011
5
M9999-071911
hbwhelp@micrel.com
or (408) 955-1690
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