MIC28304
70V 3A Power Module
Hyper Speed Control™ Family
Hyper Speed Control and Any Capacitor are trademarks of Micr e l , In c.
HyperLight Load is a registered tradem ark 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
March 25, 2014
Revision 1.1
General Description
Micrel’s MIC28304 is synchronous step-down regulator
module, featuring a unique adaptive ON-time control
architecture. The m odule inc orporates a DC /DC co ntroller,
power MOSFETs, bootstrap diode, bootstrap capacitor
and an inductor in a single package. The MIC28304
operates over an inp ut supply range fr om 4.5V to 70V and
can be used to supply up to 3A of output current. The
output voltage is adjustable down to 0.8V with a
guaranteed accuracy of ±1%. The device operates with
programmable switching frequency from 200kHz to
600kHz.
Micrel’s HyperLight Load® architecture provides the same
high-efficiency and ultra-fast transient response as the
Hyper Speed Control™ architecture under the medium to
heavy loads, but also maintains high efficiency under light
load conditions by transitioning to variable frequency,
discontinuous-mode operation.
The MIC28304 offers a full suite of protection features.
These include undervoltage lockout, internal soft-start,
foldback current limit, “hiccup” mode short-circuit
protection, and thermal shutdown.
Datasheets and support documentation are available on
Micrel’s web site at: www.micrel.com.
Hyper Speed Control™
Features
• Easy to use
− Stable with low-ESR ceramic output capacitor
− No compensation and no inductor to choose
• 4.5V to 70V input voltage
• Single-suppl y operat ion
• Power Good (PG) output
• Low radiated emission (EMI) per EN55022, Class B
• Adjustable current limit
• Adjustable output voltage from 0.9V to 24V
(also limited by duty cycle)
• 200kHz to 600kHz, programmable switching frequency
• Supports safe start-up into a pre-bias ed out put
• –40°C to +125°C junction temperature range
• Available in 64-p in, 12m m × 12mm × 3mm QFN
package
Applications
• Distributed power systems
• Industrial, medical, telecom, and automotive
Typical Application
Micrel, Inc.
MIC28304
March 25, 2014
2 Revision 1.1
Ordering Information
Part Number Switching
Frequency Features Package Junction
Temperature
Range
Lead
Finish
MIC28304-1YMP 200kHz to 600kHz HyperLight Load 64-pin 12mm × 12mm Q FN –40°C to +125°C Pb-Free
MIC28304-2YMP 200kHz to 600kHz Hyper Speed Control 64-pin 12mm × 12mm QFN –40°C to +125°C Pb-Free
Pin Configuration
64-Pin 12mm × 12mm QFN (MP)
(Top View)
Pin Description
Pin Number Pin Name Pin Function
1, 2, 3, 54, 64 GND Analog Ground. Ground for internal controller and feedback resistor network. The analog ground
return path should be separate from the power ground (PGND) return path.
4 ILIM Current Limit Setting. Connect a resistor from SW (pin #4) to ILIM to set the over-current threshold
for the converter.
5, 60 VIN Supply Voltage for Controller. The VIN operating voltage range is from 4.5V to 70V. A 0.47μF
ceramic capacitor from VIN (pin # 60) to AGND is required for decoupling. The pin # 5 should be
externally connected to either PVIN or pin # 60 on PCB.
6, 40 to 48, 51 SW
Switch Node and Current-Sense Input. High current output driver return. The SW pin connects
directly to the switch node. Due to the high-speed switching on this pin, the SW pin should be rout ed
away from sensitive nodes. The SW pin also senses the current by monitoring the voltage across
the low-side MOSFET during OFF time.
Micrel, Inc.
MIC28304
March 25, 2014
3 Revision 1.1
Pin Description (Continued)
Pin Number Pin Name Pin Function
7, 8 FREQ Switching Frequency Adjust Input. Leaving this pin open will set the switching frequency to 600kHz.
Alternatively a resistor from this pin to ground can be used to lower the switching frequency.
9 to 13 PGND
Power Ground. PGND is the return path for the buck converter power stage. The PGND pin
connects to the sources of low-side N -Channel external MOSFET, the negative terminals of input
capacitors, and the negative terminals of output capacitors. The return path for the power ground
sh ould be as small as possibl e and separate fr om the analog ground (GN D) return path .
14 to 22 PVIN Power Input Voltage. Connection to the drain of the internal high-side power MOSFET.
23 to 38 VOUT Output Voltage. Connection with the internal inductor, the output capacitor should be connected
from this pin to PGND as close to the module as possible.
39 NC No Connection. Leave it floating.
49, 50 ANODE Anode Bootstrap Diode Input. Anode connection of internal bootstrap diode, this pin should be
connected to the PVDD pin.
52, 53 BSTC Bootstrap Capacitor. Connection to the internal bootstrap capacitor. Leave floating, no connect.
55, 56 BSTR Bootstrap Resistor. Connection to the internal bootstrap resistor and high-side power MOSFET
drive circuitry. Leave floating, no connect.
57 FB Feedback Input. Input to the transconductance amplifier of the control loop. The FB pin is regulated
to 0.8V. A resistor divider connecting the feedback to the output is used to set the desired output
voltage.
58 PGOOD Power Good Output. Open drain output, an external pull-up resistor to external power rails is
required.
59 EN Enable Input. A logic signal to enable or disable the buck converter operation. The EN pin is CMOS
compatible. Logic high enables the device, logic low shutdowns the regulator. In the disable mode,
the input supply current for the device is minimized to 4µA typically. Do not pull EN to PVDD.
61, 62 PVDD Internal +5V Linear Regulator Output. PVDD is the internal supply bus for the device. In the
applications with VIN < +5.5V, PVDD should be tied to VIN to by-pass the linear regulator.
63 NC No Connection. Leave it floating.
Micrel, Inc.
MIC28304
March 25, 2014
4 Revision 1.1
Absolute Maximum Ratings(1)
PVIN, VIN to PGND ...................................... –0.3V to +76V
PVDD, VANODE to PGND .................................. –0.3V to +6V
VSW, VFREQ, VILIM, VEN ........................ −0.3V to (PVIN +0.3V)
VBSTC/BSTR to VSW ................................................ −0.3V to 6V
VBSTC/BSTR to PGN D .......................................... −0.3V to 82V
VFB, VPG to PGND ......................... −0.3V to (PVDD + 0.3V)
PGND to AGND............................................ −0.3V to +0.3V
Junction Temperature .............................................. +150°C
Storage Temperature (TS) ......................... −65°C to +150°C
Lead Temperature (soldering, 10s) ............................ 260°C
ESD Rating(3) ................................................. ESD Sensitive
Operating Ratings(2)
Supply Voltage (PVIN, VIN) .............................. 4.5V to 70V
Enable Input (VEN) ................................................. 0V to VIN
VSW, VFEQ, VILIM, VEN .............................................. 0V to VIN
Power Good (VPGOOD)..………………..……… ... 0V to PVDD
Junction Temperature (TJ) ........................ −40°C to +125°C
Junction Thermal Resistance
12mm × 12mm QFN-64 (θJA) ............................ 20°C/W
12mm × 12mm QFN-64 (θJC)............................... 5°C/W
Electrical Characteristics(4)
PVIN = VIN = 12V, VOUT = 5V, VBST – VSW = 5V; TA = 25°C, unless noted. Bold values indicate −40°C ≤ TJ ≤ +125°C.
Parameter Condition Min. Typ. Max. Units
Power Supply Input
Input Voltage Range (PVIN, VIN) 4.5 70 V
Controller Supply Current(5)
Current into Pin 60; VFB = 1.5V (MIC28304-1) 0.4 0.75 mA
Current into Pin 60;VFB = 1.5V (MIC28304-2) 2.1 3
Current into Pin 60;VEN = 0V 0.1 10 µA
Operating Current IOUT = 0A (MIC28304-1) 0.7 mA
IOUT = 0A (MIC28304-2) 27
Shutdown Supply Current PVIN = VIN = 12V, VEN = 0V 4 µA
PVDD Supply(5)
PVDD Output Voltage VIN = 7V to 70V, IPVDD = 10mA 4.8 5.2 5.4 V
PVDD UVLO Threshold PVDD rising 3.8 4.2 4.7 V
PVDD UVLO Hysteresis 400 mV
Load Regulation IPVDD = 0 to 40mA 0.6 2 3.6 %
Reference
(5)
Feedback Reference Voltage TJ = 25°C (±1.0%) 0.792 0.8 0.808 V
−40°C ≤ TJ ≤ 125°C (±2%) 0.784 0.8 0.816
FB Bias Current VFB = 0.8V 5 500 nA
Notes:
1. Exceeding the absolute maximum ratings may damage the device.
2. The device is not guarant eed to function outside its operat i ng ratings.
3. Devices are ESD sensitive. Handling precautions are recommended. Human body model, 1.5kΩ in series with 100pF.
4. Specific at i on for pack aged product only.
5. IC tested prior to assembly.
Micrel, Inc.
MIC28304
March 25, 2014
5 Revision 1.1
Electrical Characteristics(4) (Continued)
PVIN = VIN = 12V, VOUT = 5V, VBST – VSW = 5V; TA = 25°C, unless noted. Bold values indicate −40°C ≤ TJ ≤ +125°C.
Parameter Condition Min. Typ. Max. Units
Enable Control
EN Logic Level High 1.8 V
EN Logic Level Low 0.6 V
EN Hysteresis 200 mV
EN Bias Current VEN = 12V 5 20 µA
Oscillator
Switching Frequency FREQ pin = open 400 600 750 kHz
RFREQ = 100kΩ (FREQ pin-to-GND) 300
Maximum Duty Cycle 85 %
Minimum Duty Cycle VFB > 0.8V 0 %
Minimum Off-Time 140 200 260 ns
Soft-Start(5)
Soft-Start Time 5 ms
Short-Circuit Protection (5)
Current-Limit Thre shold (VCL) VFB = 0.79V −30 −14 0 mV
Short-Circuit Threshold VFB = 0V −23 −7 9 mV
Current-Limit Source Current VFB = 0.79V 60 80 100 µA
Short-Circuit Source Current VFB = 0V 27 36 47 µA
Leakage
SW, BSTR Leakage Current 50 µA
Power Good
(5)
Power Good Threshold Voltage Sweep VFB from low-to-high 85 90 95 %VOUT
Power Good Hysteresis Sweep VFB from high-to-low 6 %VOUT
Power Good Delay Time Sweep VFB from low-to-high 100 µs
Power Good Low Voltage VFB < 90% x VNOM, IPG = 1mA 70 200 mV
Thermal Protection
Overtemperature Shutdown TJ Rising 160 °C
Overtemperature Shutdown Hysteresis 4 °C
Micrel, Inc.
MIC28304
March 25, 2014
6 Revision 1.1
Electrical Characteristics(4) (Continued)
PVIN = VIN = 12V, VOUT = 5V, VBST – VSW = 5V; TA = 25°C, unless noted. Bold values indicate −40°C ≤ TJ ≤ +125°C.
Parameter Condition Min. Typ. Max. Units
Output Characteristic
Output Voltage Ripple IOUT = 3A 16 mV
Line Regulation PVIN = VIN = 7V to 70V, IOUT = 3A 0.36 %
Load Regulation IOUT = 0A to 3A PVIN= VIN =12V (MIC28304-1) 0.75 %
IOUT = 0A to 3A PVIN= VIN =12V (MIC28304-2) 0.05
Output Voltage Deviation from Load Step
IOUT from 0A to 3A at 5A/µs (MIC28304-1) 400
mV
IOUT from 3A to 0A at 5A/µs (MIC28304-1) 500
IOUT from 0A to 3A at 5A/µs (MIC28304-2) 400
IOUT from 3A to 0A at 5A/µs (MIC28304-2) 500
Micrel, Inc.
MIC28304
March 25, 2014
7 Revision 1.1
Typical Characteris tics
−
275kHz Switching Frequency
Table 1. Recommended Component Values for 275kHz Switching Frequency
VOUT VIN R3
(Rinj) R19 R15 R1
(Top Feedback
Resistor)
R11
(Bottom Feedback
Resistor)
C10
(Cinj) C12
(Cff) COUT
5V 7V to 18V 16.5kΩ
75kΩ
3.57k 10kΩ 1.9kΩ 0.1µF 2.2nF 2x47µF/6.3V
5V 18V to 70V 39.2kΩ
75kΩ
3.57k 10kΩ 1.9kΩ 0.1µF 2.2nF 2x 47µF/6.3V
3.3V 5V to 18V 16.5kΩ
75kΩ
3.57k 10kΩ 3.24kΩ 0.1µF 2.2nF 2x 47µF/6.3V
3.3V 18V to 70V 39.2kΩ 75kΩ 3.57k 10kΩ 3.24kΩ 0.1µF 2.2nF 2x 47µF/6.3V
50
55
60
65
70
75
80
85
90
95
100
00.5 11.5 22.5 3
EFFICIENCY (%)
OUTPUT CURRENT (A)
Efficie nc y vs. Output Cur rent
(MIC28304-1)
V
OUT
= 5V
F
SW
= 275kHz
48VIN
24VIN
12VIN
36VIN
30
40
50
60
70
80
90
100
00.5 11.5 22.5 3
EFFICIENCY (%)
OUTPUT CURRENT (A)
Efficie nc y vs. Output Cur rent
(MIC28304-2)
V
OUT
= 5V
F
SW
= 275kHz
48VIN
24VIN
12VIN
36VIN
0
1
2
3
25 40 55 70 85 100 115
LOAD CURRENT (A)
MAXIMUM AMBIENT TEMPERATURE
(°C)
Thermal Derating
V
OUT
= 5V
F
SW
= 275kHz
T
j_MAX
=125°C
MIC28304-2
VIN = 12V
VIN = 24V
VIN = 48V
Micrel, Inc.
MIC28304
March 25, 2014
8 Revision 1.1
Typical Characteris tics
0.00
0.40
0.80
1.20
1.60
2.00
510 15 20 25 30 35 40 45 50 55 60 65 70
SUPPLY CURRENT (mA)
INPUT VOLTAGE (V)
VIN Operating Supply Current
vs. Input Voltage (MIC28304-1)
V
OUT
= 5V
I
OUT
= 0A
F
SW
= 600kHz
-1.0%
0.0%
1.0%
2.0%
3.0%
4.0%
5.0%
712 17 22 27 32 37 42 47 52 57 62 67
TOTAL REGULATION (%)
INPUT VOLTAGE (V)
Output Regulation
vs. Input Voltage (MIC28304-1)
V
OUT
= 5.0V
I
OUT
= 0A to 3A
F
SW
= 600kHz
0.792
0.797
0.802
0.807
0.812
0.817
510 15 20 25 30 35 40 45 50 55 60 65 70
FEEDBACK VOLTAGE (V)
INPUT VOLTAGE (V)
Feedback Voltage
vs. Input Voltage (MIC28304-1)
V
OUT
= 5.0V
I
OUT
= 0A
F
SW
= 600kHz
4.90
4.92
4.94
4.96
4.98
5.00
5.02
5.04
5.06
5.08
510 15 20 25 30 35 40 45 50 55 60 65 70
OUTPUT VOLTAGE (V)
INPUT VOLTAGE (V)
Output Volt age
vs. Input Voltage (MIC28304-1)
V
OUT
= 5V
IOUT = 0A
FSW = 600kHz
0.00
0.40
0.80
1.20
1.60
2.00
-50 -25 025 50 75 100 125
SUPPLY CURRENT (mA)
TEMPERATURE (°C)
VIN Operating Supply Current
vs. Temperature (MIC28304-1)
VIN = 12V
V
OUT
= 5.0V
I
OUT
= 0A
F
SW
= 600kHz
0.792
0.796
0.800
0.804
0.808
-50 -25 025 50 75 100 125
FEEBACK VOLTAGE (V)
TEMPERATURE (°C)
Feedback Voltage
vs. Temperature (MIC28304-1)
VIN= 12V
V
OUT
= 5.0V
I
OUT
= 0A
F
SW
= 600kHz
0.0%
0.2%
0.4%
0.6%
0.8%
1.0%
1.2%
-50 -25 025 50 75 100 125
LOAD REGULATI ON (%)
TEMPERATURE (°C)
Load Regulation
vs. Temperature (MIC28304-1)
VIN = 12V
V
OUT
= 5.0V
I
OUT
= 0A to 3A
F
SW
= 600kHz
-0.6%
-0.5%
-0.4%
-0.3%
-0.2%
-0.1%
0.0%
0.1%
0.2%
0.3%
0.4%
0.5%
0.6%
0.7%
0.8%
-50 -25 025 50 75 100 125
LINE REGULATION (%)
TEMPERATURE (°C)
Line Regulation
vs. Temperature (MIC28304-1)
VIN = 7V to 70V
V
OUT
= 5.0V
I
OUT
= 0A
F
SW
= 600kHz
-0.6%
-0.5%
-0.4%
-0.3%
-0.2%
-0.1%
0.0%
0.1%
0.2%
0.3%
0.4%
0.5%
0.6%
0.7%
0.8%
-50 -25 025 50 75 100 125
LINE REGULATION (%)
TEMPERATURE (°C)
Line Regulation
vs. Temperature (MIC28304-1)
VIN = 7V to 70V
V
OUT
= 5.0V
I
OUT
= 3A
F
SW
= 600kHz
Micrel, Inc.
MIC28304
March 25, 2014
9 Revision 1.1
Typical Characteristics (Continued)
0.792
0.796
0.800
0.804
0.808
0.0 0.5 1.0 1.5 2.0 2.5 3.0
FEEDBACK VOLTAGE (V)
OUTPUT CURRENT (A)
Feedback Voltage
vs. Output Current (MIC28304-1)
VIN = 12V
V
OUT
= 5.0V
F
SW
= 600kHz
-3.0%
-2.5%
-2.0%
-1.5%
-1.0%
-0.5%
0.0%
0.5%
1.0%
0.0 0.5 1.0 1.5 2.0 2.5 3.0
LINE REGULATION (%)
OUTPUT CURRENT (A)
Line Regulation
vs. Output Current (MIC28304-1)
VIN = 12V to 75V
V
OUT
= 5.0V
F
SW
= 600kHz
10
20
30
40
50
60
70
80
90
100
0.01 0.1 110
EFFICIENCY (%)
OUTPUT CURRENT (A)
Efficiency (VIN = 12V)
vs. Output Current (MIC28304-1)
5.0V
3.3V
2.5V
1.8V
1.2V
0.8V
F
SW
= 600kHz
CCM
10
20
30
40
50
60
70
80
90
100
0.01 0.1 110
EFFICIENCY (%)
OUTPUT CURRENT (A)
Efficiency (VIN = 18V)
vs. Output Current (MIC28304-1)
5.0V
3.3V
2.5V
1.8V
1.2V
0.8V
F
SW
= 600kHz
CCM
10
20
30
40
50
60
70
80
90
100
0.01 0.1 110
EFFICIENCY (%)
OUTPUT CURRENT (A)
Efficiency (VIN = 24V)
vs. Output Current (MIC28304-1)
5.0V
3.3V
2.5V
1.8V
1.2V
0.8V
F
SW
= 600kHz
CCM
10
20
30
40
50
60
70
80
90
100
0.01 0.1 110
EFFICIENCY (%)
OUTPUT CURRENT (A)
Efficiency (VIN = 38V)
vs. Output Current (MIC28304-1)
5.0V
3.3V
2.5V
1.8V
1.2V
0.8V
F
SW
= 600kHz
CCM
10
20
30
40
50
60
70
80
90
100
0.01 0.1 110
EFFICIENCY (%)
OUTPUT CURRENT (A)
Efficiency (VIN = 48V)
vs. Output Current (MIC28304-1)
5.0V
3.3V
2.5V
1.8V
1.2V
0.8V
F
SW
= 600kHz
CCM
10
20
30
40
50
60
70
80
90
100
0.01 0.1 110
EFFICIENCY (%)
OUTPUT CURRENT (A)
Efficiency (VIN = 70V)
vs. Output Current (MIC28304-1)
5.0V
3.3V
2.5V
1.8V
1.2V
0.8V
F
SW
= 600kHz
CCM
50
55
60
65
70
75
80
85
90
95
100
0.01 0.1 110
EFFICIENCY (%)
OUTPUT CURRENT (A)
Efficiency
vs. Output Current (MIC28304-1)
V
OUT
= 12V
F
SW
= 600kHz
CCM
R3 = 23.2kΩ
18VIN
24VIN
36VIN
48VIN
70VIN
Micrel, Inc.
MIC28304
March 25, 2014
10 Revision 1.1
Typical Characteristics (Continued)
0
10
20
30
40
50
510 15 20 25 30 35 40 45 50 55 60 65 70
SUPPLY CURRENT (mA)
INPUT VOLTAGE (V)
VIN Operating Supply Current
vs. Input Voltage (MIC28304-2)
V
OUT
= 5V
I
OUT
= 0A
F
SW
= 600kHz
0.792
0.796
0.800
0.804
0.808
0.812
712 17 22 27 32 37 42 47 52 57 62 67
FEEDBACK VOLTAGE (V)
INPUT VOLTAGE (V)
Feedback Voltage
vs. Input Voltage (MIC28304-2)
V
OUT
= 5.0V
I
OUT
= 0A
F
SW
= 600kHz
-1.0%
-0.8%
-0.6%
-0.4%
-0.2%
0.0%
0.2%
0.4%
0.6%
0.8%
1.0%
712 17 22 27 32 37 42 47 52 57 62 67
OUTPUT REGULATION (%)
INPUT VOLTAGE (V)
Output Regulation
vs. Input Voltage (MIC28304-2)
V
OUT
= 5.0V
I
OUT
= 0A TO 3A
F
SW
= 600kHz
0
5
10
15
20
25
30
35
40
45
50
510 15 20 25 30 35 40 45 50 55 60 65 70
SHUTDOWN CURRENT (µA)
INPUT VOLTAGE (V)
VIN Shutdown Current
vs. Input Voltage
V
EN
= 0V
R16 = OPEN
F
SW
= 600kHz
0
2
4
6
8
10
712 17 22 27 32 37 42 47 52 57 62 67
VDD VOLTAGE (V)
INPUT VOLTAGE (V)
PVDD Volta ge
vs. Input Voltage
V
OUT
= 5.0V
F
SW
= 600kHz
I
PVDD
= 10mA
I
PVDD
= 40mA
0
2
4
6
8
10
712 17 22 27 32 37 42 47 52 57 62 67
CURRENT LIMIT (A)
INPUT VOLTAGE (V)
Output Peak Current Limit
vs. Input Voltage
V
OUT
= 5.0V
F
SW
= 600kHz
400
450
500
550
600
650
700
750
800
712 17 22 27 32 37 42 47 52 57 62 67
SWITCHING FREQUENCY (kHz)
INPUT VOLTAGE (V)
Switching Frequency
vs. Input Voltage
V
OUT
= 5.0V
I
OUT
= 2A
0.00
0.30
0.60
0.90
1.20
1.50
510 15 20 25 30 35 40 45 50 55 60 65 70
ENABLE THRESHOLD (V)
INPUT VOLTAGE (V)
Enable Threshold
vs. Input Voltage
FALLING
RISING
F
SW
= 600kHz
0
1
2
3
4
5
6
7
8
9
10
-50 -25 025 50 75 100 125
SHUTDOWN CURRENT (µA)
TEMPERATURE (°C)
VIN Shutdown Current
vs. Temperature
VIN = 12V
V
EN
= 0V
I
OUT
= 0A
F
SW
= 600kHz
Micrel, Inc.
MIC28304
March 25, 2014
11 Revision 1.1
Typical Characteristics (Continued)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
-50 -25 025 50 75 100 125
PVDD VOLTAGE (V)
TEMPERATURE (°C)
PVDD Volta ge
vs. Temperature
VIN = 12V
I
OUT
= 0A
F
SW
= 600kHz
I
PVDD
= 40mA
I
PVDD
= 10mA
3.3
3.4
3.5
3.6
3.7
3.8
3.9
4.0
4.1
4.2
4.3
4.4
-50 -25 025 50 75 100 125
VDD THRESHOLD (V)
TEMPERATURE (°C)
PVDD UVLO Threshold
vs. Temperature
RISING
FALLING
V
IN
= 12V
I
OUT
= 0A
F
SW
= 600kHz
0
2
4
6
8
10
-50 -25 025 50 75 100 125
CURRENT LIMIT (A)
TEMPERATURE (°C)
Output Peak Current Limit
vs. Temperature
VIN = 12V
V
OUT
= 5.0V
F
SW
= 600kHz
0
20
40
60
80
100
-50 -25 025 50 75 100 125
EN BIAS CURRENT (µA)
TEMPERATURE (°C)
EN Bias Current
vs. Temperature
VIN = 12V
V
EN
= 0V
F
SW
= 600kHz
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
-50 -25 025 50 75 100 125
ENABLE THRESHOLD (V)
TEMPERATURE (°C)
Enable Threshold
vs. Temperature
FALLING
RISING
VIN = 12V
V
OUT
= 5V
F
SW
= 600kHz
0
4
8
12
16
20
24
28
32
36
40
-50 -25 025 50 75 100 125
SUPPLY CURRENT (mA)
TEMPERATURE (°C)
VIN Operating Supply Current
vs. Temperature (MIC28304-2)
VIN = 12V
V
OUT
= 5.0V
I
OUT
= 0A
F
SW
= 600kHz
0.792
0.796
0.800
0.804
0.808
0.812
-50 -25 025 50 75 100 125
FEEBACK VOLTAGE (V)
TEMPERATURE (°C)
Feedback Voltage
vs. Temperature (MIC28304-2)
V
IN
= 12V
V
OUT
= 5.0V
I
OUT
= 0A
F
SW
= 600kHz
-0.3%
-0.2%
-0.1%
0.0%
0.1%
0.2%
0.3%
0.4%
-50 -25 025 50 75 100 125
LOAD REGULATI ON (%)
TEMPERATURE (°C)
Load Regulation
vs. Temperature (MIC28304-2)
VIN = 12V
V
OUT
= 5.0V
I
OUT
= 0A TO 3A
F
SW
= 600kHz
-1.00%
-0.50%
0.00%
0.50%
1.00%
-50 -25 025 50 75 100 125
LINE REGULATION (%)
TEMPERATURE (°C)
Line Regulation
vs. Temperature (MIC28304-2)
VIN = 7V TO 70V
VOUT = 5.0V
IOUT = 0A
FSW = 600kHz
Micrel, Inc.
MIC28304
March 25, 2014
12 Revision 1.1
Typical Characteristics (Continued)
-1.0%
-0.5%
0.0%
0.5%
1.0%
-50 -25 025 50 75 100 125
LINE REGULATION (%)
TEMPERATURE (°C)
Line Regulation
vs. Temperature (MIC28304-2)
VIN = 7V TO 70V
VOUT = 5.0V
IOUT = 3A
FSW = 600kHz
100
150
200
250
300
350
400
450
500
550
600
650
700
-50 -25 025 50 75 100 125
SWITCHING FREQUENCY (kHz)
TEMPERATURE (°C)
Switching Frequency
vs. Temperature (MIC28304-2)
VIN = 12V
V
OUT
= 5V
I
OUT
= 0A
0.792
0.796
0.800
0.804
0.808
0.0 0.5 1.0 1.5 2.0 2.5 3.0
FEEDBACK VOLTAGE (V)
OUTPUT CURRENT (A)
Feedback Voltage
vs. Output Current (MIC28304-2)
VIN = 12V
V
OUT
= 5.0V
F
SW
= 600kHz
0.0%
0.1%
0.1%
0.2%
0.2%
0.3%
0.3%
0.4%
0.4%
0.0 0.5 1.0 1.5 2.0 2.5 3.0
LINE REGULATION (%)
OUTPUT CURRENT (A)
Line Regulation
vs. Output Current (MIC28304-2)
VIN = 12V to 70V
V
OUT
= 5.0V
F
SW
= 600kHz
30
40
50
60
70
80
90
100
00.5 11.5 22.5 33.5 4
EFFICIENCY (%)
OUTPUT CURRENT (A)
Efficiency (VIN =12V)
vs. Output Current (MIC28304-2)
5.0V
3.3V
2.5V
1.8V
1.2V
0.8V
F
SW
= 600kHz
30
40
50
60
70
80
90
100
00.5 11.5 22.5 33.5 4
EFFICIENCY (%)
OUTPUT CURRENT (A)
Efficiency (VIN = 18V)
vs. Output Current (MIC28304-2)
5.0V
3.3V
2.5V
1.8V
1.2V
0.8V
F
SW
= 600kHz
30
40
50
60
70
80
90
100
00.5 11.5 22.5 33.5 4
EFFICIENCY (%)
OUTPUT CURRENT (A)
Efficiency (VIN = 24V)
vs. Output Current (MIC28304-2)
5.0V
3.3V
2.5V
1.8V
1.2V
0.8V
F
SW
= 600kHz
30
40
50
60
70
80
90
100
00.5 11.5 22.5 33.5 4
EFFICIENCY (%)
OUTPUT CURRENT (A)
Efficiency (VIN = 38V)
vs. Output Current (MIC28304-2)
5.0V
3.3V
2.5V
1.8V
1.2V
0.8V
F
SW
= 600kHz
30
40
50
60
70
80
90
100
00.5 11.5 22.5 33.5 4
EFFICIENCY (%)
OUTPUT CURRENT (A)
Efficiency (VIN = 48V)
vs. Output Current (MIC28304-2)
5.0V
3.3V
2.5V
1.8V
1.2V
0.8V
F
SW
= 600kHz
Micrel, Inc.
MIC28304
March 25, 2014
13 Revision 1.1
Typical Charact eri s tics (Continued)
* Case Temperature: The temperature measurem ent was taken at the hottest point on the MIC28304 case mounted on a 5 square inch PCB (see
Thermal Measurement s ection). Actual results will depend upon the size of the PCB, ambient temperature and proximity to ot her heat-emitting
components.
30
40
50
60
70
80
90
100
00.5 11.5 22.5 33.5 4
EFFICIENCY (%)
OUTPUT CURRENT (A)
Efficiency (VIN = 70V)
vs. Output Current (MIC28304-2)
5.0V
3.3V
2.5V
1.8V
1.2V
0.8V
F
SW
= 600kHz
0
20
40
60
80
100
120
140
0.0 0.5 1.0 1.5 2.0 2.5 3.0
DIE TEMPERATURE (°C)
OUTPUT CURRENT (A)
Die Temperature* (VIN = 12V)
vs. Output Curr e nt (M IC28304-2)
VIN = 12V
V
OUT
= 5.0V
F
SW
= 600kHz
0
20
40
60
80
100
120
140
0.0 0.5 1.0 1.5 2.0 2.5 3.0
DIE TEMPERATURE (°C)
OUTPUT CURRENT (A)
Die Temperature* (VIN = 48V)
vs. Output Curr e nt (M IC28304-2)
VIN = 48V
V
OUT
= 5.0V
F
SW
= 600kHz
`
0
20
40
60
80
100
120
140
0.0 0.5 1.0 1.5 2.0 2.5 3.0
DIE TEMPERATURE (°C)
OUTPUT CURRENT (A)
Die Temperature* (VIN = 70V)
vs. Output Curr e nt (M IC28304-2)
VIN = 70V
V
OUT
= 5.0V
F
SW
= 600kHz
0
100
200
300
400
500
600
700
800
10.00 100.00 1000.00 10000.00
SW FREQ (kHz)
R19 (k Ohm)
Switching Frequency
V
OUT
= 5V
I
OUT
= 2A
VIN =48V
VIN = 12V
0
0.5
1
1.5
2
2.5
0123
IC POWER DISSIP ATION (W)
OUTPUT CURRENT (A)
IC Power Dissipation
vs. Output Current (MIC28304-2)
VIN = 12V
F
SW
= 600kHz
V
OUT
= 5V
V
OUT
= 0.8V
V
OUT
= 5V, 3.3V, 2.5V, 1.8V, 1.2V, 0.8V
0
0.5
1
1.5
2
2.5
3
3.5
0 1 2 3
IC POWER DISSIP ATION (W)
OUTPUT CURRENT (A)
IC Power Dissipation
vs. Output Current (MIC28304-2)
VIN = 24V
F
SW
= 600kHz
V
OUT
= 5V
V
OUT
= 0.8V
V
OUT
= 5V, 3.3V, 2.5V, 1.8V, 1.2V, 0.8V
0
1
2
3
4
5
6
0 1 2 3
IC POWER DISSIP ATION (W)
OUTPUT CURRENT (A)
IC Power Dissipation
vs. Output Current (MIC28304-2)
VIN = 48V
F
SW
= 600kHz
V
OUT
= 5V
V
OUT
= 0.8V
V
OUT
= 5V, 3.3V, 2.5V, 1.8V, 1.2V, 0.8V
0
1
2
3
4
5
6
7
8
9
0123
IC POWER DISSIP ATION (W)
OUTPUT CURRENT (A)
IC Power Dissipation
vs. Output Current (MIC28304-2)
VIN = 70V
F
SW
= 600kHz
V
OUT
= 5V
V
OUT
= 0.8V
V
OUT
= 5V, 3.3V, 2.5V, 1.8V, 1.2V, 0.8V
Micrel, Inc.
MIC28304
March 25, 2014
14 Revision 1.1
Typical Characteristics (Continued)
0
1
2
3
25 40 55 70 85 100
LOAD CURRENT (A)
MAXIMUM AMBIENT TEMPERATURE
(°C)
Thermal Derating
V
OUT
= 5V
F
SW
= 600kHz
MIC28304-2
T
j_MAX
= 125°C
VIN = 18V
VIN = 12V
VIN = 24V
VIN = 48V
0
1
2
3
25 40 55 70 85 100
LOAD CURRENT (A)
MAXIMUM AMBIENT TEMPERATURE
(°C)
Thermal Derating
V
OUT
= 3.3V
F
SW
= 600kHz
MIC28304-2
T
j_MAX
= 125°C
VIN = 18V
VIN = 12V
VIN = 24V
VIN = 48V
0
1
2
3
25 40 55 70 85 100
LOAD CURRENT (A)
MAXIMUM AMBIENT TEMPERATURE
(°C)
Thermal Derating
V
OUT
= 2.5V
F
SW
= 600kHz
MIC28304-2
T
j_MAX
= 125°C
VIN =18V
VIN = 12V
VIN = 24V
VIN = 48V
0
1
2
3
25 40 55 70 85 100
LOAD CURRENT (A)
MAXIMUM AMBIENT TEMPERATURE
(°C)
Thermal Derating
V
OUT
= 1.8V
F
SW
= 600kHz
MIC28304-2
T
j_MAX
=125°C
VIN = 12V
VIN = 24V
VIN = 48V
VIN =18V
0
1
2
3
25 40 55 70 85 100
LOAD CURRENT (A)
MAXIMUM AMBIENT TEMPERATURE
(°C)
Thermal Derating
V
OUT
= 1.2V
F
SW
= 600kHz
MIC28304-2
T
j_MAX
=125°C
VIN = 12V
VIN = 24V
VIN = 48V
VIN =18V
0
1
2
3
25 40 55 70 85 100
LOAD CURRENT (A)
MAXIMUM AMBIENT TEMPERATURE
(°C)
Thermal Derating
V
OUT
= 0.8V
F
SW
= 600kHz
MIC28304-2
T
j_MAX
=125°C
VIN =18V
VIN = 12V
VIN = 48V
VIN = 24V
0
1
2
3
25 40 55 70 85 100
LOAD CURRENT (A)
MAXIMUM AMBIENT TEMPERATURE
(°C)
Thermal Derating
V
OUT
= 12V
F
SW
= 600kHz
MIC28304-2
R3 = 23.2kΩ
T
j_MAX
=125°C
48VIN
18VIN
VIN = 48V
VIN = 24V
0
1
2
3
4
5
6
7
8
9
0123 4
IC POWER DISSIP ATION (W)
OUTPUT CURRENT (A)
IC Power Dissipation
vs. Output Current (MIC28304-2)
V
OUT
= 12V
R3 = 23.2kΩ
F
SW
= 600kHz 70VIN
48VIN
36VIN
24VIN
18VIN
50
55
60
65
70
75
80
85
90
95
100
00.6 1.2 1.8 2.4 33.6
EFFICIENCY (%)
OUTPUT CURRENT (A)
Efficiency
vs. Output Current (MIC28304-2)
V
OUT
= 12V
F
SW
= 600kHz
R3 = 23.2kΩ
18VIN
24VIN
36VIN
48VIN
70VIN
Micrel, Inc.
MIC28304
March 25, 2014
15 Revision 1.1
Functional Characteristics
−
600kHz Switching Frequency
Micrel, Inc.
MIC28304
March 25, 2014
16 Revision 1.1
Functional Characteristics
−
600kHz Switching Frequency (Continued)
Micrel, Inc.
MIC28304
March 25, 2014
17 Revision 1.1
Functional Characteristics
−
600kHz Switching Frequency (Continued)
Micrel, Inc.
MIC28304
March 25, 2014
18 Revision 1.1
Functional Characteristics
−
600kHz Switching Frequency (Continued)
Micrel, Inc.
MIC28304
March 25, 2014
19 Revision 1.1
Functional Characteristics
−
600kHz Switching Frequency (Continued)
Micrel, Inc.
MIC28304
March 25, 2014
20 Revision 1.1
Functional Characteristics
Micrel, Inc.
MIC28304
March 25, 2014
21 Revision 1.1
Functional Diagram
Micrel, Inc.
MIC28304
March 25, 2014
22 Revision 1.1
Functional Description
The MIC28304 is an adaptive on-time synchronous buck
regulator m odule built for h igh-input voltage to low-output
voltage conversion applications. The MIC28304 is
designed to operate over a wide input voltage range,
from 4.5V to 70V, and the output is adjustable with an
external resistor divider. An adaptive on-time control
scheme is employed to obtain a constant switching
frequency and to simplify the control compensation.
Hiccup mode over-current protection is implemented by
sensing low-side MOSFET ’s RDS(ON). The device features
internal soft-start, enable, UVLO, and thermal shutdown.
The module has integrated switching FETs, inductor,
bootstrap diode, resistor and capacitor.
Theory of Operation
Per the Functional Diagram of the MIC 283 04 m odule, t he
output voltage is sensed by the MIC28304 feedback pin
FB via the voltage divider R1 and R11, and compared to
a 0.8V reference voltage VREF at the error comparator
through a low-gain transconductance (gm) amplifier. If
the feedback voltage decreases and the amplifier output
is below 0.8V, then the error comparator will trigger the
control logic and generate an ON-time period. The ON-
time period length is predetermined by the “Fixed tON
Estimator” circuitry:
SWIN
OUT
)ESTIMATED(ON fVV
t×
=
Eq. 1
where VOUT is the output voltage, VIN is the power stage
input voltag e, and fSW is the switching frequency.
At the end of the ON-time period, the internal high-side
driver turns off the high-side MOSFET and the low-side
driver turns on the low-side MOSFET. The OFF-time
period length dep ends upo n the f eedback voltage in m ost
cases. When the feedback voltage decreases and the
output of the gm amplifier is below 0.8V, the ON-time
period is triggered and the OFF-time period ends. If the
OFF-time period determined by the feedback voltage is
less than the minimum OFF-time tOFF(MIN), which is about
200ns, the MIC28304 control logic will apply the tOFF(MIN)
instead. tOFF(MIN) is required to maintain enough energy in
the boost capacitor (CBST) to drive the high-side
MOSFET.
The maximum duty cycle is obtained from the 200ns
tOFF(MIN):
SS
)MIN(OFFS
MAX
tns200
1
t
tt
D−=
−
=
Eq . 2
Where:
tS = 1/fSW. It is not recommended to use MIC28304 with
an OFF-time close to tOFF(MIN) during steady-state
operation.
The adaptive ON-time control scheme results in a
constant switching frequency in the MIC28304. The
actual ON-time and resulting switching frequency will
vary with the different rising and falling times of the
external MOSFETs. Also, the minimum tON results in a
lower switching frequency in high VIN to VOUT applications.
During load transients, the switching frequency is
changed due to the varying OFF-time.
To illustrate the control loop operation, we will analyze
both the steady-state and load transient scenarios. For
easy analysis, the gain of the gm amplifier is assumed to
be 1. W ith this ass um ption, the i nvertin g input of the er ror
comparator is the same as the feedback voltage.
Figure 1 shows the MIC28304 control loop timing during
steady-state operation. During steady-state, the gm
amplifier senses the feedback voltage ripple, which is
proportional to the output voltage ripple plus injected
voltage ri ppl e, t o tr i gger the ON-t im e per iod . T he O N -time
is predetermined by the tON estimator. The termination of
the OFF-time is controlled b y the feed bac k voltage. At the
valley of the feedback voltage ripple, which occurs when
VFB falls below VREF, the OFF period ends and the next
ON-time period is triggered through the control logic
circuitry.
Micrel, Inc.
MIC28304
March 25, 2014
23 Revision 1.1
Figure 1. MIC283 04 Control Loop Timing
Figure 2 shows the operation of the MIC28304 during a
load transient. The output voltage drops due to the
sudden load increase, which causes the VFB to be less
than VREF. This will cause the error comparator to trigger
an ON-time period. At the end of the ON-time period, a
minimum OFF-time tOFF(MIN) is generated to charge the
bootstrap capacitor (CBST) since the feedback voltage is
still below VREF. Then, the next ON-time period is
triggered due to the low feedback voltage. Therefore, the
switching frequency changes during the load transient,
but returns to the nominal fixed frequency once the
output has stabilized at the new load current level. With
the varying duty cycle and switching frequency, the
output recovery time is fast and the output voltage
deviation is small.
Figure 2. MIC28304 Load Transient Response
Unlike true current-mode control, the MIC28304 uses the
output voltage ripple to trigger an ON-time period. The
output voltage ripple is proportional to the inductor
current ripple if the ESR of the output capacitor is large
enough.
In order to m eet the st ability requ irement s, the MIC28304
feedback voltage ripple should be in phase with the
inductor c ur rent r i pple and ar e larg e e nou gh t o be s e n s ed
by the gm amplifier and the error comparator. The
recommended feedback voltage ripple is 20mV~100mV
over full input voltage range. If a low ESR output
capacitor is selected, then the feedback voltage ripple
may be too small to be sensed by the gm amplifier and
the error comparator. Also, the output voltage ripple and
the feedback voltage ripple are not necessarily in phase
with the inductor current ripple if the ESR of the output
capacitor is very low. In these cases, ripple injection is
required to ensure proper operation. Please refer to
“Ripple Injec t ion” subs ec t io n in Applicatio n Informat ion for
more details about the ripple injection technique.
Discontinuous Mode (MIC28304-1 only)
In continuous mode, the inductor current is always
greater t han zero; ho wever, at light loads, the MIC28304-
1 is able to force the inductor current to operate in
discontinuous mode. Discontinuous mode is where the
inductor current falls to zero, as indicated by trace (IL)
shown in Figure 3. During this period, the efficiency is
optimized by shutting down all the non-essential circuits
and minimizing the supply current. The MIC28304-1
wakes up and turns on the high-side MOSFET when the
feedback voltage VFB drops below 0.8V.
The MIC28304-1 has a zero crossing comparator (ZC)
that monitors the inductor current by sensing the voltage
drop across the low-side MOSFET during its ON-time. If
the VFB > 0.8V and the inductor current goes slightly
negative, then the MIC28304-1 automatically powers
down most of the IC circuitry and goes into a low-power
mode.
Once the MIC28304-1 goes into discontinuous mode,
both DL and DH are low, which turns off the high-side
and low-side MOSFETs. The load current is supplied by
the output capacitors and VOUT drops. If the drop of VOUT
causes VFB to go below VREF, then all the circuits will
wake up into normal continuous mode. First, the bias
currents of most circuits reduced during the
discontinuous m ode are restored, and then a tON pulse is
triggered before the drivers are turned on to avoid any
possible glitches. Finally, the high-side driver is turned
on. Figure 3 shows the control loop timing in
discontinuous mode.
Micrel, Inc.
MIC28304
March 25, 2014
24 Revision 1.1
Figure 3. MIC283 02-1 Control Loop Timing
(Discontinu ous Mode )
During discontinuous mode, the bias current of most
circuits is substantially reduced. As a result, the total
power supply current during discontinuous mode is only
about 400μA, allowing the MIC28304-1 to achieve high
efficiency in light load applications.
Soft-Start
Soft-s tart r educ es the inp ut po wer s u pp l y sur ge cur r en t at
startup by controlling the output voltage rise time. The
input surg e appe ars while the o utput c apacit or is c harged
up. A slo wer o utp ut ris e t im e wi ll dra w a l o wer i np ut s u rge
current.
The MIC28304 im plements an inter nal digita l soft-s tart by
making the 0.8V reference voltage VREF ramp from 0 to
100% in about 5ms with 9.7mV steps. Therefore, the
output voltage is controlled to increase slowly by a stair-
case VFB ramp. Onc e the sof t-s tar t c ycle ends, the rel ated
circuitry is disabled to reduce current consumption.
PVDD must be powered up at the same time or after VIN
to make the soft-start function correctly.
Curren t L imit
The MIC28304 uses the RDS(ON) of the low side
MOSEFET and external resistor connected from ILIM pin
to SW node to decide the current limit.
Figure 4. MIC28304 Current-Limiting Circuit
In each switching cycle of the MIC28304, the inductor
current is sensed by monitoring the low-side MOSFET in
the OFF peri od. The sens ed volta ge V(ILIM) is com pared
with the power ground (PGND) after a blanking time of
150ns. In this way the drop voltage over the resistor R15
(VCL) is compared with the drop over the bottom FET
generating the short current limit. The small capacitor
(C6) connected from ILIM pin to PGND filters the
switching node ringing during the off-time allowing a
better short limit measurement. The time constant
created by R15 and C6 should be much less than the
minimum off time.
The VCL drop allows programming of short limit through
the value of the r esis t or (R15) , If the abs o lut e v alu e of the
voltage drop on the bottom FET is greater than VCL. In
that case the V(ILIM) is lower than PGND and a short
circuit e vent is trigger ed. A hiccup c ycle to tr eat the short
event is generated. The hiccup sequence including the
soft start reduces the stress on the switching FETs and
protects the load and supply for severe short conditions.
Micrel, Inc.
MIC28304
March 25, 2014
25 Revision 1.1
The short-circuit current limit can be programmed by
using Equation 3.
( )
CL
CL)ON(DS
PP
L
CLIM IVR)5
.0
I
I(
R15 +××
D
−
=
Eq. 3
Where:
ICLIM = Desired cur re nt limit
RDS(ON) = On-resistance of low-side power MOSFET,
57mΩ typically
VCL = Current-limit threshold (typical absolute value is
14mV per the Electrical Characteristics(4))
ICL = Current-limit source current (typical value is 80µA,
per the Electrical Characteristics table).
ΔIL(PP) = Inductor cur rent peak -to-peak, since the induc tor
is integrated use Equation 4 to calculate the inductor
ripple current.
The peak-to-peak inductor current ripple is:
L f V )V(VV
I
swIN(max)
OUTIN(max)OUT
L(PP)
××
−×
=D
Eq. 4
The MIC28304 has 4.7µH inductor integrated into the
module. T he t ypical value of RWINDING(DCR) of this particular
inductor is in the range of 45mΩ.
In case of hard short, the short limit is folded down to
allow an indefinite hard short on the output without any
destructive effect. It is mandatory to make sure that the
inductor current used to charge the output capacitance
during soft start is under the folded short limit; otherwise
the supply will go in hiccup mode and may not be
finishing the soft start successfully.
The MOSFET RDS(ON) varies 30% to 40% with
temperature; therefore, it is recommended to add a 50%
mar gin to ICLIM in Equation 3 to a void fals e current lim iting
due to increased MOSFET junction temperature rise.
Table 2 shows typical output current limit value for a
given R15 with C6 = 10pF.
Table 2. Typical Output Current-Limit Value
R15 Typical Output Current Limit
1.81kΩ 3A
2.7kΩ 6.3A
Micrel, Inc.
MIC28304
March 25, 2014
26 Revision 1.1
Application Information
Simpli fied Input Transient Circuitry
The 76V absolute maximum rating of MIC28304 allows
simplifying the transient voltage suppressor on the input
supply side which is very common in industrial
applications. The input supply voltage VIN Figure 5 may
be operating at 12V input rail most of the time, but can
encounter noise spike of 60V for a short duration. By
using MIC28304, which has 76V absolute maximum
voltage rating, the input transient suppressor is not
needed. Which saves on component count, form factor,
and ultimately the system becomes less expensive.
Figure 5. Simplified Input Transient Circuitry
Setting the Switching Frequen cy
The MIC28304 switching frequency can be adjusted by
changing the value of resistor R19. The top resistor of
100kΩ is internal to module and is connected between
VIN and FREQ pin, so the value of R19 sets the
switching frequency. The switching frequency also
depends upon VIN, VOUT and load conditions.
Figure 6. Switching Frequency Adjustment
Equation 5 gives the estimated switching frequency:
Ω
+
×= k10019R 19R
ff OADJ_SW
Eq. 5
Where:
fO = Switching freque ncy when R19 is open
For more precise setting, it is recommended to use
Figure 7:
Figure 7. Switching Frequency vs. R19
Output Capacitor Selection
The type of the outp ut capacitor is usually determ ined by
the application and its equivalent series resistance
(ESR). Voltage and RMS current capability are two other
important factors for selecting the output capacitor.
Recomm ended c apaci tor t ypes are MLC C, tant alum , low-
ESR aluminum electrolytic, OS-CON and POSCAP. The
output capacitor’s ESR is usually the main cause of the
output ripple. The MIC28304 requires ripple injection and
the output capacitor ESR effects the control loop from a
stability point of view.
0
100
200
300
400
500
600
700
800
10.00 100.00 1000.00 10000.00
SW FREQ (kHz)
R19 (k Ohm)
Switching Frequency
V
OUT
= 5V
I
OUT
= 2A
VIN =48V
VIN = 12V
Micrel, Inc.
MIC28304
March 25, 2014
27 Revision 1.1
The maximum value of ESR is calculated as in Eq uat i on
6:
L(PP)
OUT(pp)
CΔI
ΔV
ESR OUT ≤
Eq. 6
Where:
ΔVOUT(pp) = Peak-to-peak output voltage ripple
ΔIL(PP) = Peak-to-peak inductor current ripple
The total output ripple is a combination of the ESR and
output capacitance. The total ripple is calculated in
Equation 7:
( )
2
CL(PP)
2
SWOUT
L(PP)
OUT(pp) OUT
ESRΔI
8fC
ΔI
ΔV ×+
××
=
Eq. 7
Where:
D = Duty cycle
COUT = Output capacitance value
fsw = Switching frequency
As described in the “Theory of Operation” subsection in
Functional Description, the MIC28304 requires at least
20mV peak-to-peak ripple at the FB pin to make the gm
amplif ier and the error c omparator be have properl y. Also,
the output voltage ripple should be in phase with the
inductor current. Therefore, the output voltage ripple
caused by the output capacitors value should be much
smaller than the ripple caused by the output capacitor
ESR. If low-ESR capacitors, such as ceramic capacitors,
are selected as the output capacitors, a ripple injection
method should be applied to provide enough feedback
voltage ripple. Please refer to the “Ripple Injection”
subsection for more details.
The voltage rating of the capacitor should be twice the
output voltage for a tantalum and 20% greater for
aluminum electrolytic or OS-CON.
The output capacitor RMS current is calculated in
Equation 8:
12
ΔI
IL(PP)
(RMS)COUT =
Eq. 8
The power dissipated in the output capacitor is:
OUTOUTOUT
C
2
(RMS)C)DISS(C
ESRIP ×=
Eq. 9
Input Capacitor Selection
The input capacitor for the power stage input PVIN
should be selected for ripple current rating and voltage
rating. T anta lum input c ap a citor s may fail when s ubjected
to high inrush currents, caused by turning the input
supply on. A tantalum input capacitor’s voltage rating
should be at least two times the maximum input voltage
to maximize reliability. Aluminum electrolytic, OS-CON,
and multilayer polymer film capacitors can handle the
higher inrush currents without voltage de-rating. The
input voltage ripple will primarily depend on the input
capacitor’s ESR. The peak input current is equal to the
peak inductor current, so:
ΔVIN = IL(pk) × ESRCIN Eq. 10
The input capacitor must be rated for the input current
ripple. The RMS value of input capacitor current is
determ ined at t he max im um output curr ent. Assuming the
peak-to-peak inductor current ripple is low:
D)(1DII OUT(max)CIN(RMS) −××≈
Eq.11
The power dissipated in the input capacitor is:
PDISS(CIN) = ICIN(RMS)2 × ESRCIN Eq. 12
The general rule is to pick the capacitor with a ripple
current rating equal to or greater than the calculated
worst (VIN_MAX) case RMS capacitor current. Its voltage
rating should be 20% to 50% higher than the maximum
input voltage. Typically the input ripple (dV) needs to be
kept down to less than ±10% of input voltage. The ESR
also increases the input ripple.
Micrel, Inc.
MIC28304
March 25, 2014
28 Revision 1.1
Equation 13 should be used to calculate the input
capacitor. Also it is recommended to keep some margin
on the calculated value:
dVF )D1(I
C
SW
OUT(max)
IN
×
−×
≈
Eq. 13
Where:
dV = The input ripple and FSW is the switching frequency
Output Voltage Setting Components
The MIC28304 requires two resistors to set the output
voltage as shown in Figure 8:
Figure 8. Voltage-Divider Configuration
The output voltage is determined by Equation 14:
+×= 11
1
1VV FB
OUT R
R
Eq. 14
Where:
VFB = 0.8V
A typical value of R1 used on the standard evaluation
board is 10kΩ. If R1 is too large, it may allow noise to be
introduced into the voltage feedback loop. If R1 is too
small in value, it will decrease the efficiency of the power
supply, especially at light loads. Once R1 is selected,
R11 can be calculated using Equation 15:
FBOUT
FB VV R1V
R11 −
×
=
Eq. 15
Ripple Injection
The VFB ripple required for proper operation of the
MIC28304 gM amplifier and error comparator is 20mV to
100mV. However, the output voltage ripple is generally
designed as 1% to 2% of the output voltage. For a low
output voltage, such as a 1V, the output voltage ripple is
only 10mV to 20mV, and the feedback voltage ripple is
less tha n 20m V. If the feed back volta ge ripple is so s mall
that the gM amplifier and error comparator cannot sense
it, then the MIC28304 will lose control and the output
voltage is no t regu lated. I n or der to hav e som e am ount of
VFB ripple, a ripple injection method is applied for low
output voltage ripple applications. The table 2
summarizes the ripple injection component values for
ceramic output capacitor.
The applications are divided into three situations
according to the amount of the feedback voltage ripple:
1. Enough ripple at the feedback voltage due to the
large ESR of the output capacitors (Figure 9):
Figure 9. Enough Ripple at FB
As sho wn in Fi gure 1 0, the conver t er is s tabl e witho ut
any ripple injection.
Figure 10. Inadequate Ripple at FB
Micrel, Inc.
MIC28304
March 25, 2014
29 Revision 1.1
The feedback voltage ripple is:
L(PP)CFB(PP)
ΔIESR
R11R1R11
ΔV
OUT
××
+
=
Eq. 16
Where:
ΔIL(PP) = The peak-to-peak value of the inductor
current ripple
2. Inadequate ripple at the feedback voltage due to the
small ESR of the output capacitors, such is the case
with ceramic output capacitor.
The output voltage ripple is fed into the FB pin
through a feed-forward capacitor Cff in this situation,
as shown in Figure 11. The typical Cff value is
between 1nF and 100 nF .
Figure 11. Invisible Ripple at FB
W ith the feed-f or war d c a pa citor , th e f eed bac k voltage
ripple is very close to the output voltage ripple:
L(PP)FB(PP) ΔIESRΔV ×≈
Eq. 17
3. Virtually no ripple at the FB pin voltage due to the
very-low ESR of the output capacitors.
In this situation, the output voltage ripple is less than
20mV. T herefore, additiona l ripple is injected into the
FB pin from the switching node SW via a resistor Rinj
and a capacitor Cinj, as shown in Figure 11. The
injected ripple is:
τ
×
×××
×=
SW
div
INFB(pp)
f1
D)-(1D
KVΔV
Eq. 18
R1//R11R R1//R11
K
inj
div
+
=
Eq. 19
Where:
VIN = Power sta ge inp ut vol tage
D = Duty cycle
fSW = Switching frequency
τ = (R1//R11//Rinj) × Cff
In Equations 18 and 19, it is assumed that the time
constant associated with Cff must be much greater than
the switching period:
1
T
f1
SW <<=
×
ττ
Eq. 20
If the voltage divider resistors R1 and R11 are in the kΩ
range, then a Cff of 1nF to 100nF can easily satisfy the
large tim e constant require ments. Also, a 100nF inject ion
capacitor Cinj is used in order to be considered as short
for a wide range of the frequencies.
The process of sizing the ripple injection resistor and
capacitors is:
Step 1. Select Cff to feed all output ripples into the
feedback pin and make sure the large time constant
assumption is satisfied. Typical choice of Cff is 1nF to
100nF if R1 and R11 are in kΩ range.
Step 2. Select Rinj according to the expected feedback
voltage ripple using Equation 22:
D)
(1Df
V
ΔV
KSW
IN
FB(pp)
div −×
×
×=
τ
Eq. 21
Then the value of Rinj is obtained as:
1)
K1
((R1//R11)R div
inj −×=
Eq. 22
Step 3. Select Cinj as 100nF, which could be considered
as short for a wide range of the frequencies.
Table 3 summarizes the typical value of components for
particular input and output voltage, and 600kH z
switching frequency design, for details refer to the Bill of
Materials section.
Micrel, Inc.
MIC28304
March 25, 2014
30 Revision 1.1
Table 3. Recommended Component Values for 600kHz Switching Frequency
VOUT VIN R3
(Rinj)
R1
(Top Feedback
Resistor)
R11
(Bottom Feedback
Resistor)
R19 C10
(Cinj) C12
(Cff) COUT
0.9V 5V to 70V 16.5kΩ 10kΩ 80.6kΩ
DNP 0.1µF 2.2nF 47µF/6.3V
or 2 x 22µF
1.2V 5V to 70V 16.5kΩ 10kΩ 20kΩ
DNP 0.1µF 2.2nF 47µF/6.3V
or 2 x 22µF
1.8V 5V to 70V 16.5kΩ 10kΩ 8.06kΩ
DNP 0.1µF 2.2nF 47µF/6.3V
or 2 x 22µF
2.5V 5V to 70V 16.5kΩ 10kΩ 4.75kΩ
DNP 0.1µF 2.2nF 47µF/6.3V
or 2 x 22µF
3.3V 5V to 70V 16.5kΩ 10kΩ 3.24kΩ
DNP 0.1µF 2.2nF 47µF/6.3V
or 2 x 22µF
5V 7V to 70V 16.5kΩ 10kΩ 1.9kΩ
DNP 0.1µF 2.2nF 47µF/6.3V
or 2 x 22µF
12V 18V to 70V 23.2kΩ 10kΩ 715Ω
DNP 0.1µF 2.2nF 47µF/16V
or 2 x 22µF
Thermal Measurements and Safe Operating Area
Measuring th e IC’s case te mperature is recommended to
ensure it is within its operating limits. Although this might
seem like a very elementary task, it is easy to get
erroneous results. The most common mistake is to use
the standard thermal couple that comes with a thermal
meter. This thermal couple wire gauge is large, typically
22 gauge, and behaves like a heatsink, resulting in a
lower case measurement.
Two methods of temperature measurement are using a
smaller th er mal coupl e wire or an inf r ared ther mom eter. If
a thermal couple wire is used, it must be constructed of
36-gauge wire or higher (smaller wire size) to minimize
the wire heat-sinking effect. In addition, the thermal
couple tip must be covered in either thermal grease or
thermal glue to make sure that the thermal couple
junction is making good contact with the case of the IC.
Omega brand thermal couple (5SC-TT-K-36-36) is
adequate for most applications.
Wherever possible, an infrared thermometer is
recommended. The measurement spot size of most
infrared thermometers is too large for an accurate
reading on a small form factor ICs.
However, an IR thermom eter fr om O ptris has a 1m m spot
size, which makes it a good choice for measuring the
hottest point on the case. An optional stand makes it
easy to hold the beam on the IC for long periods of time.
The saf e operatin g ar ea (S O A) of the MIC 2830 4 is s ho wn
in the Typical Characteristics
−
275kHz Switching
Frequency section. These thermal measurements were
taken on MIC28304 evaluation board. Since the
MIC28304 is an entire system comprised of switching
regulator controller, MOSFETs and inductor, the part
needs to be considered as a system. The SOA curves
will give guidance to reasonable use of the MIC28304.
Emission Characteristics of MIC28304
The MIC28304 integrates switching components in a
single package, so the MIC28304 has reduced emission
compared to standard buck regulator with external
MOSFETS and inductors. The radiated EMI scans for
MIC28304 are shown in the Typical Characteristics
section. The limit on the graph is per EN55022 Class B
standard.
Micrel, Inc.
MIC28304
March 25, 2014
31 Revision 1.1
PCB Layout Guidelines
Warning: To minimize EMI and output noise, follow
these layou t recommendations .
PCB layout is critical to achieve reliable, stable and
efficient performance. A ground plane is required to
control EMI and m inimize the inductanc e in power, signa l
and return paths.
The following figures optimized from small form factor
point of view shows top and bottom layer of a four layer
PCB. It is recommended to use mid layer 1 as a
continuous ground plane.
Figure 12. Top And Bottom Layer of a Four-Layer Board
The following guidelines should be followed to insure
proper operation of the MIC28304 converter:
IC
• The analog ground pin (GND) must be connected
directl y to the gr o und p la ne s. D o not route the GND p i n
to the PGND pin on the top layer.
• Place the IC close to the point of load (POL).
• Use fat traces to route the input and output power
lines.
• Analog and power grounds should be kept separate
and connected at only one location.
Input Capacitor
• Place the input capacitors on the same side of the
board and as close to the IC as possible.
• Place several vias to the ground plane close to the
input capacitor ground terminal.
• Use either X7R or X5R dielectric input capacitors. Do
not use Y5V or Z5U type capacitors.
• Do not replace the ceramic input capacitor with any
other type of capacitor. Any type of capacitor can be
placed in parallel with the input capacitor.
• If a Tantalum input capacitor is placed in parallel with
the input capacitor, it must be recommended for
switching regulator applications and the operating
voltage must be derated by 50%.
• In “Hot-Plug” applications, a Tantalum or Electrolytic
bypass c apac itor mus t be u s ed to limit the o ver -voltage
spike seen on the input supply with power is suddenly
applied.
RC Snubber
• Place the RC snubber on the same side of the board
and as close to the SW pin as possible.
SW Node
• Do not route any digital lines underneath or close to
the SW node.
• Keep the switch node (SW) away from the feedback
(FB) pin.
Output Capacitor
• Use a wide trace to connect the output capacitor
ground terminal to the input capacitor ground terminal.
• Phase margin will change as the output capacitor value
and ESR changes. Contact the factory if the output
capacitor is dif ferent from what is shown in the BOM.
• The f eedback trace s hould be s eparate f rom the po wer
trace an d connecte d as c lose as p ossible t o the output
capacitor. Sensing a long high-current load trace can
degrade the DC load regulation.
Micrel, Inc.
MIC28304
March 25, 2014
32 Revision 1.1
Evaluation Board Schematics
Figure 13. Schematic of MIC28304 Evaluation Board
(J1, J8, J10, J11, J12, J13, R14, R20, and R21 are for Testing Purposes)
Micrel, Inc.
MIC28304
March 25, 2014
33 Revision 1.1
Evaluation Board Schematics (Continued)
Figure 14. Schematic of MIC28304 Evaluation Board
(Optimized for Smallest Footprint)
Micrel, Inc.
MIC28304
March 25, 2014
34 Revision 1.1
Bill of Materials
Item Part Number Manufacturer
Description Qty.
C1 EEU-FC2A101 Panasonic(6) 100µF Aluminum Capacitor, 100V 1
C2, C3
GRM32ER72A225K Murata(7)
2.2µF/100V Ceramic Capacitor, X7R, Size 1210 2 C3225X7R2A225K TDK(8)
12101C225KAT2A AVX(9)
C6 GCM1885C2A100JA16D Murata 10pF, 100V, 0603, NPO 1
06031A100JAT2A AVX
C8
GRM188R70J105KA01D Murata
1µF/6.3V Ceramic Capacitor, X7R, Size 0603 1 06036C105KAT2A AVX
C1608X5R0J105K TDK
C9 GRM21BR72A474KA73 Murata 0.47µF/100V Ceramic Capacitor, X7R, Size 0805 1
08051C474KAT2A AVX
C10, C17 GRM188R72A104KA35D Murata 0.1µF/100V Ceramic Capacitor, X7R, Size 0603 2
C1608X7S2A104K TDK 0.1µF/100V, X7S, 0603
C11
GRM188R72A102KA01D Murata
1nF/100V Ceramic Capacitor, X7R, Size 0603 1 06031C102KAT2A AVX
C1608X7R2A102K TDK
C12
GRM188R72A222KA01D Murata
2.2nF/100V Ceramic Capacitor, X7R, Size 0603 1 06031C222KAT2A AVX
C1608X7R2A222K TDK
C14 GRM31CR60J476ME19K Murata 47µF/6.3V Ceramic Capacitor, X5R, Size 1210 1
12106D476MAT2A AVX
C16
GRM188R71H104KA93D Murata
0.1µF/6.3V Ceramic Capacitor, X7R, Size 0603 1 06035C104KAT2A AVX
C1608X7R1H104K TDK
C4, C5, C7, C13, C15 DNP
Notes:
6. Panasonic: www.panasonic.com.
7. Murata: www.murata.com.
8. TDK: www.tdk.com.
9. AVX: www.avx.com.
Micrel, Inc.
MIC28304
March 25, 2014
35 Revision 1.1
Bill of Materials (Continued)
Item Part Number Manufacturer Description Qty.
R1 CRCW060310K0FKEA Vishay Dale(10) 10kΩ Resistor, Size 0603, 1% 1
R2 CRCW08051R21FKEA Vishay Dale 1.21Ω Resistor, Size 0805, 5% 1
R3 CRCW06031652F Vishay Dale 16.5kΩ Resistor, Size 0603, 1% 1
R10 CRCW06033K24FKEA Vishay Dale 3.24kΩ Resistor, Size 0603, 1% 1
R11 CRCW06031K91FKEA Vishay Dale 1.91kΩ Resistor, Size 0603, 1% 1
R12 CRCW0603715R0FKEA Vishay Dale 715Ω Resistor, Size 0603, 1% DNP
R14, R20 CRCW06030000FKEA Vishay Dale 0Ω Resistor, Size 0603, 5% 2
R15 CRCW04022K70JNED Vishay Dale 2.7kΩ Resistor, Size 0603, 1% 1
R16 CRCW0603100KFKEAHP Vishay Dale 100kΩ Resistor, Size 0603, 1% 1
R18 CRCW060349K9FKEA Vishay Dale 49.9kΩ Resistor, Size 0603, 1% 1
R21 CRCW060349R9FKEA Vishay Dale 49.9Ω Resistor, Size 0603, 1% 1
R23 CRCW06031R21FKEA Vishay Dale 1.21Ω Resistor, Size 0603, 1% 1
R4, R19 DNP
All reference
designators ending
with “A” Open
U1 MIC28304-1YMP Micrel, Inc.(11) 70V, 3A Power Module
−
Hyper Speed Control
Family 1
MIC28304-2YMP
Notes:
10. Vishay: www.vishay.com.
11. Micrel, Inc.: www.micrel.com.
Micrel, Inc.
MIC28304
March 25, 2014
36 Revision 1.1
PCB Layout Recommendations
Evaluation Board Top Layer
Evaluation Board Mid-Layer 1 (Ground Plane)
Micrel, Inc.
MIC28304
March 25, 2014
37 Revision 1.1
PCB Layout Recommendations (Continued)
Evaluation Board Mid-Layer 2
Evaluation Board Bottom Layer
Micrel, Inc.
MIC28304
March 25, 2014
39 Revision 1.1
Recommended Land Pattern
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