July 2001
1
MIC4684
MIC4684
Micrel
MIC4684
2A High-Efficiency SuperSwitcherTM Buck Regulator
Final Information
General Description
The MIC4684 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 MIC4684 achieves 2A of continuous current in an 8-lead
SO (small outline) package at 60
C ambient temperature.
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, no external heat sink is required. The MIC4684,
housed in an SO-8, can replace larger TO-220 and TO-263
packages in many applications.
The MIC4684 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 protec-
tion, frequency-foldback short-circuit protection, and thermal
shutdown.
The MIC4684 is available in an 8-lead SO package with a
junction temperature range of 40
C to +125
C.
Typical Application
BS
VIN
4
1
5
2, 6, 7
3
8
SW
FB
EN
GND
MIC4684BM
C
BS
0.33
F/50V
C
IN
33
F
35V
330
F
6.3V
3A
40V
R1
3.01k
R2
3.01k
68
H
V
IN
6.5V to 25V
V
OUT
2.5V/1.5A
Adjustable Buck Converter
BS
VIN
4
1
5
2, 6, 7
3
8
SW
FB
EN
GND
1A, 20V
MIC4684BM
C
BS
0.33
F/50V
C
IN
68
F
10V
220
F
10V
2A
20V
47
H
V
IN
5V
10%
V
OUT
3.3V/1.7A
Feed forward diode
5V to 3.3V Converter
Features
SO-8 package with 2A continuous output current
Over 85% efficiency
Fixed 200kHz PWM operation
Wide 4V to 30V input voltage range
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
Simple high-efficiency step-down regulator
5V to 3.3V/1.7A converter (60
C ambient)
12V to 1.8V/2A converter (60
C ambient)
On-card switching regulator
Dual-output
5V converter
Battery charger
Micrel, Inc. 1849 Fortune Drive San Jose, CA 95131 USA tel + 1 (408) 944-0800 fax + 1 (408) 944-0970 http://www.micrel.com
0
20
40
60
80
100
0
0.5
1
1.5
2
EFFICIENCY (%)
OUTPUT CURRENT (A)
Efficiency
vs. Output Current
V
OUT
= 1.8V
V
OUT
= 2.5V
V
OUT
= 3.3V
V
IN
= 5.0V
Efficiency vs. Output Current
Ordering Information
Part Number
Voltage
Junction Temperature Range
Package
MIC4684BM
Adj
40
C to +125
C
SOP-8
MIC4684
Micrel
MIC4684
2
July 2001
Pin Description
Pin Number
Pin Name
Pin Function
1
SW
Switch (Output): Emitter of NPN output switch. Connect to external storage
inductor and Shottky diode.
2, 6, 7
GND
Ground
3
IN
Supply (Input): Unregulated +4V to 30V supply voltage (34V transient)
4
BS
Booststrap Voltage Node (External Component): Connect to external boost
capacitor.
5
FB
Feedback (Input): Outback voltage feedback to regulator. Connect to output
of supply for fixed versions. Connect to 1.23V tap of resistive divider for
adjustable versions.
8
EN
Enable (Input): Logic high = enable; logic low = shutdown
Pin Configuration
1
SW
GND
VIN
BS
8
EN
GND
GND
FB
7
6
5
2
3
4
8-Pin SOP (M)
Detailed Pin Description
Switch (SW, pin 1)
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 to a 0.5V typically.
Ground (GND, pins 2,6,7)
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 resis-
tance 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 its necessary
to place the input capacitor ground lead as close as possible
to the switching regulators ground pin.
Input Voltage (V
IN
, pin 3)
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.
Bootstrap (BS, pin 4)
The bootstrap pin in conjunction with the external bootstrap
capacitor provides a bias voltage higher than the input
voltage to the MIC4684'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 5)
The feedback pin is tied to the inverting side of a GM error
amplifier. The noninverting side is tied to a 1.235V bandgap
reference. Fixed voltage versions have an internal voltage
divider from the feedback pin. Adjustable versions require an
external resistor voltage divider from the output to ground,
with the center tied to the feedback pin.
Enable (EN, pin 8)
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 150
A. 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.
July 2001
3
MIC4684
MIC4684
Micrel
Electrical Characteristics
V
IN
= V
EN
=
12V, V
OUT
= 5V; I
OUT
= 500mA; T
A
= 25
C, unless otherwise noted. Bold values indicate 40
C
T
J
+125
C.
Parameter
Condition
Min
Typ
Max
Units
Feedback Voltage
(
2%)
1.210
1.235
1.260
V
(
3%)
1.198
1.272
V
8V
V
IN
30V, 0.1A
I
LOAD
1A, V
OUT
= 5V
1.186
1.235
1.284
V
1.173
1.297
V
Feedback Bias Current
50
nA
Maximum Duty Cycle
V
FB
= 1.0V
94
%
Output Leakage Current
V
IN
= 30V, V
EN
= 0V, V
SW
= 0V
5
500
A
V
IN
= 30V, V
EN
= 0V, V
SW
= 1V
1.4
20
mA
Quiescent Current
V
FB
= 1.5V
6
12
mA
Bootstrap Drive Current
V
FB
= 1.5V, V
SW
= 0V
250
380
mA
Bootstrap Voltage
I
BS
= 10mA, V
FB
= 1.5V, V
SW
= 0V
5.5
6.2
V
Frequency Fold Back
V
FB
= 0V
30
50
120
kHz
Oscillator Frequency
180
200
225
kHz
Saturation Voltage
I
OUT
= 1A
0.59
V
Short Circuit Current Limit
V
FB
= 0V, See
Test Circuit
2.2
A
Shutdown Current
V
EN
= 0V
150
A
Enable Input Logic Level
regulator on
2
V
regulator off
0.8
V
Enable Pin Input Current
V
EN
= 0V (regulator off)
16
50
A
V
EN
= 12V (regulator on)
1
0.83
mA
Thermal Shutdown @ T
J
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.
Note 4.
2.5V of headroom is required between V
IN
and V
OUT
. The headroom can be reduced by implementing a feed-forward diode a seen on the 5V
to 3.3V circuit on page 1.
Note 5.
Measured on 1" square of 1 oz. copper FR4 printed circuit board connected to the device ground leads.
Absolute Maximum Ratings
(Note 1)
Supply Voltage (V
IN
), Note 3 ...................................... +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
ESD Rating .............................................................. Note 3
Operating Ratings
(Note 2)
Supply Voltage (V
IN
) Note 4 ........................... +4V to +30V
Ambient Temperature (T
A
) ......................... 40
C to +85
C
Junction Temperature (T
J
) ....................... 40
C to +125
C
Package Thermal Resistance
JA
, Note 5 .......................................................... 75
C/W
JC
, Note 5 .......................................................... 25
C/W
MIC4684
Micrel
MIC4684
4
July 2001
Test Circuit
SW
68
H
I
VIN
BS
FB
EN
Device Under Test
+12V
SOP-8
5
GND
2,6,7
4
1
3
8
Current Limit Test Circuit
Shutdown Input Behavior
ON
OFF
GUARANTEED
ON
TYPICAL
ON
GUARANTEED
OFF
TYPICAL
OFF
0.8V
1.25V
0V
1.4V
V
IN(max)
2V
Enable Hysteresis
July 2001
5
MIC4684
MIC4684
Micrel
Typical Characteristics
(T
A
= 25
C unless otherwise noted)
0
10
20
30
40
50
60
70
80
90
100
0
0.5
1
1.5
2
2.5
3
EFFICIENCY (%)
OUTPUT CURRENT (A)
Efficiency vs. Output Current
with Feed Forward Diode
5V
OUT
1.8V
OUT
2.5V
OUT
3.3V
OUT
V
IN
= 12V
0
50
100
150
200
250
300
350
0
2
4
6
8 10 12 14 16 18 20
BOOTSTRAP CURRENT (mA)
INPUT VOLTAGE (V)
Bootstrap Drive Current
vs. Input Voltage
V
IN
= 12V
V
FB
= 1.5V
10.3
10.4
10.5
10.6
10.7
10.8
10.9
0
5
10 15 20 25 30 35 40
DUTY CYCLE (%)
INPUT VOLTAGE (V)
Minimum Duty Cycle
vs. Input Voltage
V
IN
= 12V
V
OUT
= 5V
V
FB
= 1.3V
1.225
1.230
1.235
1.240
1.245
1.250
1.255
0
5
10 15 20 25 30 35 40
REFERENCE VOLTAGE (V)
INPUT VOLTAGE (V)
Reference Voltage
vs. Input Voltage
V
IN
= 12V
V
OUT
= V
REF
I
OUT
= 500mA
0
1
2
3
4
5
6
7
0
5
10
15
20
25
30
BOOTSTRAP VOLTAGE (V)
INPUT VOLTAGE (V)
Bootstrap Voltage
vs. Input Voltage
V
IN
= 12V
V
FB
= 1.5V
0
20
40
60
80
100
120
140
160
180
200
0
5
10 15 20 25 30 35 40
INPUT CURRENT (
A)
INPUT VOLTAGE (V)
Shutdown Current
vs. Input Voltage
V
EN
= 0V
570
575
580
585
590
595
600
605
0
5
10 15 20 25 30 35 40
SATURATION VOLTAGE (mV)
INPUT VOLTAGE (V)
Saturation Voltage
vs. Input Voltage
I
OUT
= 1A
V
OUT
= 5V
48.5
49
49.5
50
50.5
51
51.5
0
5
10 15 20 25 30 35 40
FREQUENCY (kHz)
INPUT VOLTAGE (V)
Foldback Frequency
vs. Input Voltage
V
FB
= 0V
5.7
5.8
5.9
6
6.1
6.2
6.3
0
5
10 15 20 25 30 35 40
INPUT CURRENT (mA)
INPUT VOLTAGE (V)
Quiescent Current
vs. Input Voltage
V
EN
= 5V
50
55
60
65
70
75
80
85
90
95
100
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
EFFICIECNY (%)
OUTPUT CURRENT (A)
5V
OUT
Efficiency without Feed
Forward Diode
V
OUT
= 5V
V
IN
= 8V
V
IN
= 12V
V
IN
= 24V
50
55
60
65
70
75
80
85
90
95
100
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
EFFICIECNY (%)
OUTPUT CURRENT (A)
3.3V
OUT
Efficiency without
Feed Forward Diode
V
OUT
= 3.3V
V
IN
= 8V
V
IN
= 12V
V
IN
= 24V
50
55
60
65
70
75
80
85
90
95
100
0
0.5
1
1.5
2
EFFICIENCY (%)
OUTPUT CURRENT (A)
5V
IN
Efficiency with Feed
Forward Diode
V
OUT
= 1.8V
V
OUT
= 2.5V
V
OUT
= 3.3V
V
IN
= 5.0V
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