MICREL MIC4685

MIC4685
Micrel
MIC4685
3A SPAK SuperSwitcher™ Buck Regulator
Final
General Description
Features
The MIC4685 is a high-efficiency 200kHz stepdown (buck)
switching regulator. Power conversion efficiency of above
85% is easily obtainable for a wide variety of applications.
The MIC4685 achieves 3A of continuous current in the 7-lead
SPAK package.
The thermal performance of the SPAK allows it to replace
TO-220s and TO-263s (D2 PAKs) in many applications. The
SPAK saves board space with a 36% smaller footprint than
TO-263.
High efficiency is maintained over a wide output current range
by utilizing a boost capacitor to increase the voltage available
to saturate the internal power switch. As a result of this high
efficiency, only the ground plane of the PCB is needed for a
heat sink.
The MIC4685 allows for a high degree of safety. It has a wide
input voltage range of 4V to 30V (34V transient), allowing it to
be used in applications where input voltage transients may be
present. Built-in safety features include over-current protection, frequency-foldback short-circuit protection, and thermal
shutdown.
The MIC4685 is available in an 7-lead SPAK package with a
junction temperature range of –40°C to +125°C.
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•
•
Low 2mm profile SPAK package
3A continuous output current
Wide 4V to 30V input voltage range (34V transient)
Fixed 200kHz PWM operation
Over 85% efficiency
Output voltage adjustable to 1.235V
All surface mount solution
Internally compensated with fast transient response
Over-current protection
Frequency foldback short-circuit protection
Thermal shutdown
Applications
•
•
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Point of load power supplies
Simple high-efficiency step-down regulators
5V to 3.3V/2A conversion
12V to 5V/3.3V/2.5V/1.8V 3A conversion
Dual-output ±5V conversion
Base stations
LCD power supplies
Battery chargers
Ordering Information
Part Number
Voltage
Junction Temperature Range
Package
MIC4685BR
Adj
–40°C to +125°C
SPAK-07L
Typical Applications
VIN
8V to 30V
2
5
MIC4685BR
IN
BS
EN
CIN
33µF
35V
1
SW
6
FB
3
GND
4, Tab
DBS
3A, 20V
VIN
5V
±10%
CBS
0.33µF/50V
VOUT
1.8V/3A
L1
39µH
D1
3A
40V
R1
3.01k
R2
6.49k
2
5
CIN
68µF
10V
COUT
330µF
6.3V
MIC4685BR
IN
BS
EN
1
SW
6
FB
3
GND
4, Tab
1.8V Output Converter
CBS
0.33µF/50V
VOUT
3.3V/2A
L1
39µH
D1
3A
20V
R1
3.01k
R2
1.78k
330µF
6.3V
5V to 3.3V Converter
Micrel, Inc. • 1849 Fortune Drive • San Jose, CA 95131 • USA • tel + 1 (408) 944-0800 • fax + 1 (408) 944-0970 • http://www.micrel.com
September 2002
1
MIC4685
MIC4685
Micrel
Pin Configuration
TAB, GND
7
6
5
4
3
2
1
NC
SW
EN
GND
FB
IN
BS
SPAK-07L (R)
Pin Description
Pin Number
Pin Name
Pin Function
1
BS
Bootstrap Voltage Node (External Component): Connect to external boost
capacitor.
2
IN
Supply (Input): Unregulated +4V to 30V supply voltage (34V transient)
3
FB
Feedback (Input): Outback voltage feedback to regulator. Connect to 1.235V
tap of resistive divider.
4, Tab
GND
5
EN
Enable (Input): Logic high = enable; logic low = shutdown
6
SW
Switch (Output): Emitter of NPN output switch. Connect to external storage
inductor and Schottky diode.
7
NC
No Connect. Tie this pin to ground.
Ground
Bootstrap (BS, Pin 1)
The bootstrap pin in conjunction with the external bootstrap
capacitor provides a bias voltage higher than the input
voltage to the MIC4685’s main NPN pass element. The
bootstrap capacitor sees the dv/dt of the switching action at
the SW pin as an AC voltage. The bootstrap capacitor then
couples the AC voltage back to the BS pin plus the dc offset
of VIN where it is rectified and used to provide additional drive
to the main switch, in this case a NPN transistor.
This additional drive reduces the NPN’s saturation voltage
and increases efficiency, from a VSAT of 1.8V, and 75%
efficiency to a VSAT of 0.5V and 88% efficiency respectively.
Feedback (FB, Pin 3)
The feedback pin is tied to the inverting side of an error
amplifier. The noninverting side is tied to a 1.235V bandgap
reference. An external resistor voltage divider is required
from the output to ground, with the center tied to the feedback
pin.
Enable (EN, Pin 5)
The enable (EN) input is used to turn on the regulator and is
TTL compatible. Note: connect the enable pin to the input if
unused. A logic-high enables the regulator. A logic-low shuts
down the regulator and reduces the stand-by quiescent input
current to typically 150µA. The enable pin has an upper
threshold of 2.0V minimum and lower threshold of 0.8V
maximum. The hysterisis provided by the upper and lower
thresholds acts as an UVLO and prevents unwanted turn on
of the regulator due to noise.
Detailed Pin Description
Switch (SW, Pin 6)
The switch pin is tied to the emitter of the main internal NPN
transistor. This pin is biased up to the input voltage minus the
VSAT of the main NPN pass element. The emitter is also
driven negative when the output inductor’s magnetic field
collapses at turn-off. During the OFF time the SW pin is
clamped by the output Schottky diode to a –0.5V typically.
Ground (GND, Pin 4, Tab)
There are two main areas of concern when it comes to the
ground pin, EMI and ground current. In a buck regulator or
any other non-isolated switching regulator the output
capacitor(s) and diode(s) ground is referenced back to the
switching regulator’s or controller’s ground pin. Any resistance between these reference points causes an offset
voltage/IR drop proportional to load current and poor load
regulation. This is why its important to keep the output
grounds placed as close as possible to the switching regulator’s
ground pin. To keep radiated EMI to a minimum it is necessary to place the input capacitor ground lead as close as
possible to the switching regulator’s ground pin.
Input Voltage (VIN, Pin 2)
The VIN pin is the collector of the main NPN pass element.
This pin is also connected to the internal regulator. The output
diode or clamping diode should have its cathode as close as
possible to this point to avoid voltage spikes adding to the
voltage across the collector.
MIC4685
2
September 2002
MIC4685
Micrel
Absolute Maximum Ratings (Note 1)
Operating Ratings (Note 2)
Supply Voltage (VIN), Note 1 ...................................... +34V
Enable Voltage (VEN) .................................... –0.3V to +VIN
Steady-State Output Switch Voltage (VSW) ....... –1V to VIN
Feedback Voltage (VFB) .............................................. +12V
Storage Temperature (TS) ....................... –65°C to +150°C
ESD Rating Note 3 ....................................................... 2kV
Supply Voltage (VIN) Note 4 ........................... +4V to +30V
Junction Temperature (TJ) ....................... –40°C to +125°C
Package Thermal Resistance
θJA, SPAK-7 Lead ............................................ 11.8°C/W
θJC, SPAK-7 Lead .............................................. 2.2°C/W
Electrical Characteristics
VIN = VEN = 12V, VOUT = 5V; IOUT = 500mA; TA = 25°C, unless otherwise noted. Bold values indicate –40°C ≤ TJ ≤ +125°C.
Parameter
Condition
Min
Typ
Max
Units
Feedback Voltage
(±2%)
(±3%)
1.210
1.198
1.235
1.260
1.272
V
V
8V ≤ VIN ≤ 30V, 0.1A ≤ ILOAD ≤ 1A, VOUT = 5V, Note 4
1.186
1.173
1.235
1.284
1.297
V
V
Feedback Bias Current
50
nA
%
Maximum Duty Cycle
VFB = 1.0V
94
Output Leakage Current
VIN = 30V, VEN = 0V, VSW = 0V
5
500
µA
VIN = 30V, VEN = 0V, VSW = –1V
1.4
20
mA
6
12
mA
Quiescent Current
VFB = 1.5V
Bootstrap Drive Current
VFB = 1.5V, VSW = 0V
250
380
mA
Bootstrap Voltage
IBS = 10mA, VFB = 1.5V, VSW = 0V
5.5
6.2
V
Frequency Fold Back
VFB = 0V
30
70
120
kHz
180
200
225
kHz
Oscillator Frequency
Saturation Voltage
IOUT = 1A
Short Circuit Current Limit
VFB = 0V, See Test Circuit
Shutdown Current
VEN = 0V
Enable Input Logic Level
regulator on
0.59
3.5
150
6
A
200
µA
2
V
regulator off
Enable Pin Input Current
V
VEN = 0V (regulator off)
16
VEN = 12V (regulator on)
–1
Thermal Shutdown @ TJ
0.8
V
50
µA
–0.83
mA
160
°C
Note 1.
Exceeding the absolute maximum rating may damage the device.
Note 2.
The device is not guaranteed to function outside its operating rating.
Note 3.
Devices are ESD sensitive. Handling precautions recommended. Human body model, 1.5kΩ in series with 100pF.
Note 4.
2.5V of headroom is required between VIN and VOUT. The headroom can be reduced by implementing a bootstrap diode as seen on the 5V to
3.3V circuit on page 1.
September 2002
3
MIC4685
MIC4685
Micrel
Test Circuit
+12V
2
5
Device Under Test
6
VIN
SW
EN
BS
GND
68µH
1
I
FB
4, Tab
3
Current Limit Test Circuit
Shutdown Input Behavior
ON
OFF
GUARANTEED
OFF
0V
TYPICAL
OFF
0.8V
2V
1.25V
1.4V
GUARANTEED
ON
TYPICAL
ON
VIN(max)
Enable Hysteresis
MIC4685
4
September 2002
MIC4685
Micrel
Typical Characteristics
(TA = 25°C unless otherwise noted)
90
80
VIN = 30V
50
40
Standard
Configuration
VOUT = 5.0V
30
20
VIN = 24V
50
VIN = 30V
VIN = 12V
40
30
Standard
Configuration
VOUT = 1.8V
20
10
0
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
OUTPUT CURRENT (A)
70
60
50
40
30
20
Bootstrap
Configuration
VOUT = 5.0V
90
80
VIN = 12V
EFFICIENCY (%)
EFFICIENCY (%)
70
60
VIN = 5V
VIN = 16V
50
40
30
20
Bootstrap
Configuration
VOUT = 2.5V
70
60
50
BOOTSTRAP CURRENT (mA)
BOOTSTRAP VOLTAGE (V)
5
4
3
2
VIN = 12V
VFB = 1.5V
September 2002
VIN = 12V
70
60
50
40
VIN = 5V
VIN = 16V
30
20
Bootstrap
Configuration
10
VOUT = 3.3V
0
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
OUTPUT CURRENT (A)
Quiescent Current
vs. Input Voltage
30
20
Bootstrap
Configuration
VOUT = 1.8V
6.2
6.1
6
5.9
5.8
VEN= 5V
5.7
0
30
350
12
300
10
250
200
150
100
VIN = 12V
VFB = 1.5V
50
0
0 2 4 6 8 10 12 14 16 18 20
INPUT VOLTAGE (V)
5
5
10 15 20 25 30 35 40
INPUT VOLTAGE (V)
Minimum Duty Cycle
vs. Input Voltage
Bootstrap Drive Current
vs. Input Voltage
6
10 15 20 25
INPUT VOLTAGE (V)
VIN = 16V
40
Bootstrap Voltage
vs. Input Voltage
5
VIN = 4.5V
6.3
VIN = 12V
0
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
OUTPUT CURRENT (A)
7
0
0
VIN = 5V
10
10
0
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
OUTPUT CURRENT (A)
1
Efficiency
vs. Output Current
Efficiency
vs. Output Current
100
VIN = 12V
Standard
Configuration
10
VOUT = 2.5V
0
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
OUTPUT CURRENT (A)
10
0
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
OUTPUT CURRENT (A)
Efficiency
vs. Output Current
90
80
VIN = 30V
30
20
90
80
VIN = 12V
VIN = 16V
VIN = 24V
50
40
100
VIN = 7.5V
90
80
EFFICIENCY (%)
EFFICIENCY (%)
60
Standard
Configuration
VOUT = 3.3V
70
60
Efficiency
vs. Output Current
100
VIN = 8V
70
VIN = 12V
10
0
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
OUTPUT CURRENT (A)
Efficiency
vs. Output Current
80
VIN = 30V
50
40
10
0
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
OUTPUT CURRENT (A)
90
90 VIN = 8V
80
EFFICIENCY (%)
30
20
70
60
100
VIN = 8V
INPUT CURRENT (mA)
EFFICIENCY (%)
70
60
100
VIN = 12V
Efficiency
vs. Output Current
DUTY CYCLE (%)
VIN = 8V
90
80
EFFICIENCY (%)
100
Efficiency
vs. Output Current
EFFICIENCY (%)
Efficiency
vs. Output Current
8
6
4
2
0
0
VOUT = 1.8V
5
10 15 20 25
INPUT VOLTAGE (V)
30
MIC4685
MIC4685
Micrel
Shutdown Current
vs. Input Voltage
200
1.245
180
160
1.240
1.230
100
80
1.225
1.220
IOUT = 10mA
VOUT = 1.8V
30
1.248
5
1.238
1.228
IOUT = 10mA
VIN = 12V
VOUT = 1.8V
1.198
-40 -20 0 20 40 60 80 100120140
TEMPERATURE (°C)
3
2
1
OFF
0
-1
-50
IOUT = 1A
VOUT = 5V
575
0
5
10 15 20 25 30 35 40
INPUT VOLTAGE (V)
0
50
100 150
TEMPERATURE (°C)
200
1.808
1.807
1.806
1.805
1.804
1.803
1.802
0
VOUT = 13V
0.5 1 1.5 2 2.5 3
OUTPUT CURRENT (A)
3.5
1.80
1.79
1.78
IOUT = 0.100A
35
1.14
1.12
Lower Threshold
1.10
1.08
1.06
1.04
1.02
1.00
VIN = 12V
VOUT = 5V
IOUT = 100mA
-60
1.81
Upper Threshold
140
1.18
1.16
THRESHOLD TRIP POINTS
OUTPUT VOLTAGE (V)
1.20
MIC4685
580
Enable Threshold
vs. Temperature
1.82
5 10 15 20 25 30
OUTPUT CURRENT (A)
585
Load Regulation
ON
4
1.83
1.76
0
590
1.809
Line Regulation
1.77
595
570
10 15 20 25 30 35 40
INPUT VOLTAGE (V)
OUTPUT VOLTAGE (V)
6
1.208
600
Shutdown Hysteresis vs.
Temperature
1.258
OUTPUT VOLTAGE (V)
FEEDBACK VOLTAGE (V)
Feedback Voltage
vs. Temperature
5
100
120
10 15 20 25
INPUT VOLTAGE (V)
VEN = 0V
40
60
80
5
20
0
0
0
20
1.210
60
40
-40
-20
1.215
1.218
605
140
120
1.235
1.205
0
Saturation Voltage
vs. Input Voltage
SATURATION VOLTAGE (mV)
1.250
INPUT CURRENT (µA)
FEEDBACK VOLTAGE (V)
Feedback Voltage
vs. Input Voltage
TEMPERATURE (°C)
6
September 2002
MIC4685
Micrel
Typical Safe Operating Area (SOA)
(SOA measured on the MIC4685 Evaluation Board*)
5V Output SOA
Standard Configuration
3.3V Output SOA
Standard Configuration
TA = 60°C
TJ = 125°C
D = Max
1.5
1.0
0.5
0.0
0
5
10 15 20 25 30
INPUT VOLTAGE (V)
5.0
4.5 TA = 25°C
4.0 TJ = 125°C
3.5 D = Max
4.5 TA = 25°C
4.0 TJ = 125°C
3.5 D = Max
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0
35
T = 25°C
4.5 A
T = 125°C
4.0 J
D = Max
3.5
3.0
2.5
2.0
TA = 60°C
TJ = 125°C
D = Max
5
10 15 20 25 30
INPUT VOLTAGE (V)
TA = 60°C
TJ = 125°C
D = Max
3
5
TA = 60°C
TJ = 125°C
D = Max
0
7 9 11 13 15 17
INPUT VOLTAGE (V)
5
10 15 20 25 30
INPUT VOLTAGE (V)
35
3.3V Output SOA
Bootstrap Configuration
2.5
2.0
TA = 60°C
TJ = 125°C
D = Max
1.5
1.0
0.5
8
10 12 14 16
INPUT VOLTAGE (V)
18
TA = 25°C
TJ = 125°C
3.5 D = Max
3.0
4.0
2.5
2.0
TA = 60°C
TJ = 125°C
D = Max
1.5
1.0
0.5
0.0
3
5
7 9 11 13 15 17
INPUT VOLTAGE (V)
1.8V Output SOA
Bootstrap Configuration
5.0
T = 25°C
4.5 A
T = 125°C
4.0 J
D = Max
3.5
3.0
OUTPUT CURRENT (A)
OUTPUT CURRENT (A)
T = 25°C
4.5 A
T = 125°C
4.0 J
D = Max
3.5
3.0
0.5
0.0
1.5
1.0
4.5
TA = 25°C
TJ = 125°C
3.5 D = Max
3.0
0.0
6
35
5.0
1.5
1.0
35
4.0
2.5V Output SOA
Bootstrap Configuration
2.5
2.0
10 15 20 25 30
INPUT VOLTAGE (V)
2.5
2.0
0.5
0.0
4.5
OUTPUT CURRENT (A)
OUTPUT CURRENT (A)
5.0
0.5
0.0
0
5
3.0
5.0V Output SOA
Bootstrap Configuration
1.8V Output SOA
Standard Configuration
1.5
1.0
TA = 60°C
TJ = 125°C
D = Max
OUTPUT CURRENT (A)
2.5
2.0
5.0
OUTPUT CURRENT (A)
T = 25°C
4.5 A
TJ = 125°C
4.0
D = Max
3.5
3.0
OUTPUT CURRENT (A)
OUTPUT CURRENT (A)
5.0
2.5V Output SOA
Standard Configuration
2.5
2.0
TA = 60°C
TJ = 125°C
D = Max
1.5
1.0
0.5
0.0
3
5
7 9 11 13 15 17
INPUT VOLTAGE (V)
* IOUT < 3A, D1: Diode Inc. B340 (3A/40V)
IOUT > 3A, D1: SBM1040 (10A/40V)
September 2002
7
MIC4685
MIC4685
Micrel
Functional Characteristics
Load Transient
VOUT
(100mV/div.)
Normal
Operation
200kHz
VIN = 12V
VOUT = 5V
IOUT = 1.0A to 0.1A
Short
Circuit
Operation
Typical
5.1V
5V
1A
IOUT
(500mA/div.)
VSW (SHORTED)
12V IN, 0V OUT
VSW (NORMAL)
12V IN, 5V/1A OUT
Switching Frequency Foldback
0A
70kHz
TIME (25µs/div.)
TIME
Frequency Foldback
The MIC4685 folds the switching frequency back during a
hard short circuit condition to reduce the energy per cycle and
protect the device.
MIC4685
8
September 2002
MIC4685
Micrel
Block Diagrams
VIN
IN
Bootstrap
Charger
Enable
Internal
Regulator
200kHz
Oscillator
Thermal
Shutdown
 R1 
VOUT = VREF 
+ 1
 R2 
V

R1 = R2  OUT − 1
 VREF

Current
Limit
VREF = 1.235V
Comparator
VOUT
SW
Driver
COUT
Reset
R1
FB
Error
Amp
1.235V
Bandgap
Reference
R2
MIC4685
FIgure 1. Adjustable Regulator
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 MIC4685 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.
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.
Functional Description
The MIC4685 is a variable duty cycle switch-mode regulator
with an internal power switch. Refer to the above block
diagram.
Supply Voltage
The MIC4685 operates from a +4V to +30V (34V transient)
unregulated input. Highest efficiency operation is from a
supply voltage around +12V. See the efficiency curves in the
Typical Characteristics section on page 5.
Enable/Shutdown
The enable (EN) input is TTL compatible. Tie the input high
if unused. A logic-high enables the regulator. A logic-low
shuts down the internal regulator which reduces the current
to typically 150µA when VEN = 0V.
Feedback
An external resistive voltage divider is required from the
output voltage to ground, center tapped to the FB pin. See
Table 1 and Table 2 for recommended resistor values.
Duty Cycle Control
A fixed-gain error amplifier compares the feedback signal
with a 1.235V bandgap voltage reference. The resulting error
amplifier output voltage is compared to a 200kHz sawtooth
waveform to produce a voltage controlled variable duty cycle
output.
September 2002
9
MIC4685
MIC4685
Micrel
The efficiency is used to determine how much of the output
power (POUT) is dissipated in the regulator circuit (PD).
Applications Information
Adjustable Regulators
Adjustable regulators require a 1.235V 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:
PD =
η
− POUT
7.5W
− 7.5W
0.84
PD = 1.43W
A worst-case rule of thumb is to assume that 80% of the total
output power dissipation is in the MIC4685 (PD(IC)) and 20%
is in the diode-inductor-capacitor circuit.
PD(IC) = 0.8 PD
PD(IC) = 0.8 × 1.43W
PD(IC) = 1.14W
Calculate the worst-case junction temperature:
TJ = PD(IC) θJC + (TC – TA) + TA(max)
where:
TJ = MIC4685 junction temperature
PD(IC) = MIC4685 power dissipation
θJC = junction-to-case thermal resistance.
PD =
 R1 
VOUT = VREF 
+ 1
 R2 
V

R1 = R2  OUT − 1
 VREF

VREF = 1.235V
Thermal Considerations
The MIC4685 is capable of high current due to the thermally
optimized SPAK package.
One limitation of the maximum output current on any MIC4685
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 2.2°C/W for a 7-lead SPAK.
θCA is dependent on layout and is primarily governed by the
connection of pins 4, and Tab to the ground plane. The
purpose of the ground plane is to function as a heat sink.
Checking the Maximum Junction Temperature:
For this example, with an output power (POUT) of 7.5W, (5V
output at 1.5A with VIN = 12V) and 60°C maximum ambient
temperature, what is the junction temperature?
Referring to the “Typical Characteristics: 5V Output Efficiency” graph, read the efficiency (η) for 1.5A output current
at VIN = 12V or perform you own measurement.
η = 84%
MIC4685
POUT
The θJC for the MIC4685’s 7-lead SPAK is approximately
2.2°C/W.
TC = “pin” temperature measurement taken at the
Tab.
TA = ambient temperature
TA(max) = maximum ambient operating temperature
for the specific design.
Calculating the maximum junction temperature given a
maximum ambient temperature of 60°C:
TJ = 1.14 × 2.2°C + (46°C – 25°C) + 60°C
TJ = 83.5°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. Also see Typical Safe Operating Area (SOA)
graphs on page 7.
10
September 2002
MIC4685
Micrel
Layout Considerations
Layout is very important when designing any switching regulator. Rapidly changing 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 as short as possible. For example, keep D1 close to
pin 6 and pin 4, and Tab, keep L1 away from sensitive node
FB, and keep CIN close to pin 2 and pin 4, and Tab. See
Applications Information: Thermal Considerations for ground
plane layout.
The feedback pin 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 7.
Gerber files are available upon request.
2
5
CIN
7-lead
SPAK
MIC4685BR
IN
BS
EN
1
SW
6
FB
3
VOUT
L1
39µH
GND
COUT
D1
4, Tab
R1
R2
Load
VIN
+4V to +30V
(34V transient)
Bootstrap Diode
The bootstrap diode provides an external bias source directly
to the main pass element, this reduces VSAT thus allowing the
MIC4685 to be used in very low head-room applications i.e.
5VIN to 3.3VOUT with high efficiencies. Bootstrap diode not for
use if VIN exceeds 16V, VIN. See Figure 3.
GND
Figure 2. Critical Traces for Layout
September 2002
11
MIC4685
MIC4685
Micrel
Recommended Components for a Given Output Voltage (Bootstrap Configuration)
*
VOUT IOUT*
5.0V 2.1A
R1
R2
VIN
C1
D1
3.01k
976Ω
3.3V
2.2A
3.01k
2.5V
2.0A
1.8V
2.0A
D2
L1
C4
7.5V–16V
47µF, 20V
Vishay-Dale
595D476X0020D2T
3A, 30V 1A, 20V
39µH
330µF, 6.3V
Schottky Schottky Sumida
Vishay-Dale
B330A
MBRX120 CDRH127R-390MC 594D337X06R3D2T
1.78k
6.0V–16V
47µF, 20V
Vishay-Dale
595D476X0020D2T
3A, 30V 1A, 20V
39µH
330µF, 6.3V
Schottky Schottky Sumida
Vishay-Dale
B330A
MBRX120 CDRH127R-390MC 594D337X06R3D2T
3.01k
2.94k
5.0V–16V
47µF, 20V
Vishay-Dale
595D476X0020D2T
3A, 30V 1A, 20V
39µH
330µF, 6.3V
Schottky Schottky Sumida
Vishay-Dale
B330A
MBRX120 CDRH127R-390MC 594D337X06R3D2T
3.01k
6.49k
5.0V–16V
47µF, 20V
Vishay-Dale
595D476X0020D2T
3A, 30V 1A, 20V
39µH
330µF, 6.3V
Schottky Schottky Sumida
Vishay-Dale
B330A
MBRX120 CDRH127R-390MC 594D337X06R3D2T
Maximum output current at minimum input voltage. See SOA curves for maximum output current vs. input voltage.
Table 1. Recommended Components for Common Ouput Voltages
D2
MBRX120
1A/20V
JP3
J1
VIN
C1
47µF
20V
J3
GND
2
ON
C2
0.1µF
50V
5
U1 MIC4685BR
IN
SW
EN
L1
39µH
BS
1
FB
3
OFF
GND
4, Tab
J2
VOUT
6
C3
0.33µF
50V
D1
B330A
or
SS33
R1
R2
C4*
optional
C5
330µF
6.3V
C7
0.1µF
50V
J4
GND
* C4 can be used to provide additional stability
and improved transient response.
Note: optimized for 5VOUT
Figure 3. Schematic Diagram
MIC4685
12
September 2002
MIC4685
Micrel
Recommended Components for a Given Output Voltage (Standard Configuration)
*
VOUT
IOUT*
R1
R2
VIN
C1
D1
L1
C5
5.0V
2.0A
3.01k
976Ω
8V–30V
33µF, 35V
Vishay-Dale
595D336X0035R2T
3A, 40V
Schottky
B340A
39µH
Sumida
CDRH127-390MC
330µF, 6.3V
Vishay-Dale
594D337X06R3D2T
3.3V
2.4A
3.01k
1.78k
8V–26V
33µF, 35V
Vishay-Dale
595D336X0035R2T
3A, 40V
Schottky
B340A
39µH
Sumida
CDRH127-390MC
330µF, 6.3V
Vishay-Dale
594D337X06R3D2T
2.5V
2.35A
3.01k
2.94k
7V–23V
33µF, 35V
Vishay-Dale
595D336X0035R2T
3A, 40V
Schottky
B340A
39µH
Sumida
CDRH127-390MC
330µF, 6.3V
Vishay-Dale
594D337X06R3D2T
1.8V
2.0A
3.01k
6.49k
6V–16V
47µF, 25V
Vishay-Dale
595D476X0025D2T
3A, 40V
Schottky
B340A
39µH
Sumida
CDRH127-390MC
330µF, 6.3V
Vishay-Dale
594D337X06R3D2T
Maximum output current at minimum input voltage. See SOA curves for maximum output current vs. input voltage.
Table 2. Recommended Components for Common Ouput Voltages
JP3
J1
VIN
(34V transient)
C1
33µF
35V
J3
GND
2
ON
C2
0.1µF
50V
5
D2***
B340
U1 MIC4685BR
IN
SW
EN
6
BS
1
FB
3
OFF
GND
J2
VOUT
2A
L1
39µH
C3
0.33µF
50V
R1
3.01k
D1
B340A
4, Tab
R2
6.49k
1
2
3
JP1a
1.8V
4
C4*
optional
R3
2.94k
5
JP1b
2.5V
R4
1.78k
7
JP1c
3.3V
6
8
R5
976Ω
JP1d
5.0V
C5
330µF
6.3V
C6**
C7
0.1µF
50V
J4
GND
*
C4 can be used to provide additional stability
and improved transient response.
Note: optimized for 5VOUT
** C6 Optional
*** D2 is not used for standard configuration and JP3 is open.
Figure 4. Evaluation Board Schematic Diagram
September 2002
13
MIC4685
MIC4685
Micrel
Printed Circuit Board
Figure 5a. Top Silk Screen
Figure 5b. Bottom Silk Screen
Figure 5c. Top Side Copper
Figure 5d. Bottom Side Copper
Abbreviated Bill of Material (Critical Components)
Reference
Manufacturer
C1
594D336X0035R2T
Vishay
C2, C7
VJ0805Y104KXAAB
C3
GRM426X7R334K50
C4*
Optional
C5
1
Part Number
594D337X06R3D2T
Description
Sprague1
Qty
33µF 35V
1
Vitramon
0.1µF 50V
2
Murata
0.33µF, 50V ceramic capacitor
Vishay
Sprague1
Inc2
1800pF, 50V ceramic
(1)
330µF, 6.3V, tantalum
1
Schottky 3A 40V
1
D1
B340A
Diode
D2
B340A
MBRX120
Diode Inc2
Micro Commercial Component5
Schottky 3A 40V
Schottky 1A 22V
1
L1
CDRH127-390MC
Sumida3
39µH
1
U1
MIC4685BR
Micrel Semiconductor4
3A 200kHz SPAK buck regulator
1
Vishay Sprague, Inc., tel: 207-490-7256, http://www.vishay.com
2
Diodes Inc, tel: 805-446-4800, http://www.diodes.com
3
Sumida, tel: 510-668-0660, http://www.sumida.com
4
Micrel, tel: 408-944-0800, httzp://www.micrel.com
5
Micro Commercial Component, tel: 818-701-4933, http://www.mccsemi.com
MIC4685
14
September 2002
MIC4685
Micrel
Package Information
DIMENSIONS:
INCH (MM)
0.375 (9.52)
0.365 (9.27)
0.360 (9.14)
0.350 (8.89)
0.050 (1.27)
0.030 (0.76)
0.080 (2.03)
0.070 (1.78)
0.256 BSC
(6.50 BSC)
0.010 BSC
(0.25 BSC)
0.045 (1.14)
0.035 (0.89)
0.316 BSC
(8.03 BSC)
0.420 (10.67)
0.410 (10.41)
0.320 (8.13)
0.310 (7.87)
0.050 BSC
(1.27 BSC)
0.031 (0.79)
0.025 (0.63)
0.005 (0.13)
0.001 (0.03)
0.031 (0.89)
0.041 (1.14)
0.010 BSC
(0.25 BSC)
6¡
0¡
SCALE 20:1
SPAK-07L (R)
MICREL INC.
TEL
1849 FORTUNE DRIVE SAN JOSE, CA 95131
+ 1 (408) 944-0800
FAX
+ 1 (408) 944-0970
WEB
USA
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.
© 2002 Micrel Incorporated
September 2002
15
MIC4685