MIC4685
3A SPAK SuperSwitcher™
Buck Regulator
General Description
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-pin SPAK package.
The thermal performance of the SPAK allows it to replace
TO-220s and TO-263s (D
2
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 a 7-pin SPAK package with a
junction temperature range of –40°C to +125°C.
Data sheets and support documentation can be found on
Micrel’s web site at www.micrel.com.
Features
•
•
•
•
•
•
•
•
•
•
•
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
•
•
•
•
•
•
•
•
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
___________________________________________________________________________________________________________
Typical Application
V
IN
8V to 30V
MIC4685_R
2
5
IN
EN
GND
BS
SW
FB
4, Tab
1
6
3
C
BS
0.33µF/50V
L1
39mH
D1
3A
40V
R1
3.01k
R2
6.49k
V
OUT
1.8V/3A
C
OUT
330µF
6.3V
C
IN
33µF
35V
1.8V Output Converter
SuperSwitcher is a trademark of Micrel, Inc
Micrel Inc. • 2180 Fortune Drive • San Jose, CA 95131 • USA • tel +1 (
408
) 944-0800 • fax + 1 (408) 474-1000 • http://www.micrel.com
January 2010
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M9999-012610
Micrel, Inc.
MIC4685
Ordering Information
Part Number
Standard
MIC4685BR
RoHS Compliant*
MIC4685WR
MIC4685WR EV
Voltage
Adj.
Adj.
Junction
Temp. Range
–40° to +125°C
Package
7-Pin SPAK
Evaluation Board
*
RoHS compliant with ‘high-melting solder’ exemption.
Pin Configuration
7
6
5
4
3
2
1
NC
SW
EN
GND
FB
IN
BS
7-Pin SPAK (R)
Pin Description
Pin Number
1
2
3
4, Tab
5
6
7
Pin Name
BS
IN
FB
GND
EN
SW
NC
Pin Function
Bootstrap Voltage Node (External Component): Connect to external boost
capacitor.
Supply (Input): Unregulated +4V to 30V supply voltage (34V transient)
Feedback (Input): Outback voltage feedback to regulator. Connect to 1.235V
tap of resistive divider.
Ground
Enable (Input): Logic high = enable; logic low = shutdown
Switch (Output): Emitter of NPN output switch. Connect to external storage
inductor and Schottky diode.
No Connect. Tie this pin-to-ground.
January 2010
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M9999-012610
Micrel, Inc.
MIC4685
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 V
IN
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 V
SAT
of 1.8V, and 75%
efficiency to a V
SAT
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. See Tables 1 and 2 for recommended
resistor values.
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 up-per 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 V
SAT
, 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
typically to a –0.5V.
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 it’s 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 (V
IN
, Pin 2)
The V
IN
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.
January 2010
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M9999-012610
Micrel, Inc.
MIC4685
Absolute Maximum Ratings
(1)
Supply Voltage (V
IN
)
(1)
.................................................+34V
Enable Voltage (V
EN
)......................................... –0.3V to V
IN
Steady-State Output Switch Voltage (V
SW
) .......... –1V to V
IN
Feedback Voltage (V
FB
) ...............................................+12V
Storage Temperature (T
s
) .........................–65°C to +150°C
EDS Rating
(3)
.................................................................. 2kV
Operating Ratings
(2)
Supply Voltage (V
IN
)
(4)
..................................... +4V to +30V
Junction Temperature (T
J
) ........................ –40°C to +125°C
Package Thermal Resistance
SPAK-7 (θ
JA
) ...................................................11.8°C/W
SPAK-7 (θ
JC
).....................................................2.2°C/W
Electrical Characteristics
V
IN
= V
EN
= 12V; V
OUT
5V; I
OUT
= 500mA; T
A
= 25°C,
bold
values indicate –40°C< T
J
< +125°C, unless noted.
Parameter
MIC4685 [Adjustable]
Feedback Voltage
(±2%)
(±3%)
8V
≤
V
IN
≤
30V, 0.1A
≤
I
LOAD
≤
1A, V
OUT
= 5V,
Note 4
Feedback Bias Current
Maximum Duty Cycle
Output Leakage Current
Quiescent Current
Bootstrap Drive Current
Bootstrap Voltage
Frequency Fold Back
Oscillator Frequency
Saturation Voltage
Short Circuit Current Limit
Shutdown Current
Enable Input Logic Level
Enable Pin Input Current
Thermal Shutdown @ T
J
Notes:
1. Exceeding the absolute maximum rating may damage the device.
2. The device is not guaranteed to function outside its operating rating.
3. Devices are ESD sensitive. Handling precautions recommended. Human body model, 1.5kΩ in series with 100pF.
4. 2.5V of headroom is required between V
IN
and V
OUT
. The headroom can be reduced by implementing a bootstrap diode as seen on the 5V to 3.3V
circuit on page 1.
Condition
Min
1.210
1.198
1.186
1.173
Typ
1.235
1.235
50
Max
1.260
1.272
1.284
1.297
Units
V
V
V
V
nA
%
V
FB
= 1.0V
V
IN
= 30V, V
EN
= 0V, V
SW
= 0V
V
IN
= 30V, V
EN
= 0V, V
SW
= 1V
V
FB
= 1.5V
V
FB
= 1.5V, V
SW
= 0V
I
BS
= 10mA, V
FB
= 1.5V, V
SW
= 0V
V
FB
= 0V
I
OUT
= 1A
V
FB
= 0V,
See Test Circuit
V
EN
= 0V
regulator on
regulator off
V
EN
= 0V (regulator off)
V
EN
= 0V (regulator on)
–1
2
3.5
250
5.5
30
180
94
5
1.4
6
380
6.2
70
200
0.59
6
150
200
0.8
16
–0.83
160
50
120
225
500
20
12
µA
mA
mA
mA
V
kHz
kHz
V
A
µA
V
V
µA
mA
°C
January 2010
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M9999-012610
Micrel, Inc.
MIC4685
Test Circuit
+12V
2
5
Device Under Test
VIN
EN
FB
GND
4, Tab 3
SW
BS
6
1
68µH
I
Current Limit Test Circuit
Shutdown Input Behavior
ON
OFF
0.8V
0V
1.25V
1.4V
2V
V
IN(max)
Enable Hysteresis
January 2010
5
M9999-012610
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