MIC4684BM Micrel Inc, MIC4684BM Datasheet - Page 9

• Manufacturer Part Number: MIC4684BM
• Description: IC REG BUCK 2A ADJ VOLT HE 8SOIC
• Manufacturer: Micrel Inc
• Series: SuperSwitcher™r
• Type: Step-Down (Buck)r

Specifications of MIC4684BM

• Mounting Type: Surface Mount
• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: Adj to 1.235V
• Current - Output: 2A
• Frequency - Switching: 200kHz
• Voltage - Input: 4 ~ 30 V
• Operating Temperature: -40°C ~ 125°C
• Package / Case: 8-SOIC (3.9mm Width)
• No. Of Pins: 8
• Peak Reflow Compatible (260 C): No
• Current Rating: 2A
• Output Voltage Max: 1.235V
• Supply Voltage Max: 30V
• Leaded Process Compatible: No
• Voltage Regulator Type: Switching
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant, Lead free / RoHS Compliant

Block Diagrams
Functional Description
The MIC4684 is a variable duty cycle switch-mode regula-
tor with an internal power switch. Refer to the above block
diagram.
Supply Voltage
The MIC4684 operates from a +4V to +30V (34V transient)
unregulated input. Highest efficiency operation is from a supply
voltage around +12V. See the efficiency curves 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 V
Feedback
Fixed-voltage versions of the regulator have an internal resis-
tive divider from the feedback (
to the output voltage.
Adjustable versions require an external resistive voltage
divider from the output voltage to ground, center tapped to
the
tor 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
January 2010
MIC4684
fb
pin. See Table 1 and Table 2 for recommended resis-
Enable
EN
= 0V.
fb
MIC4684
) pin. Connect
Oscillator
200kHz
parator
Com-
Error
Amp
Regulator
Shutdown
Bootstrap
Thermal
Charger
Internal
Reset
fb
Adjustable Regulator
directly
1.235V
Bandgap
Reference
Current
Driver
Limit
9
waveform to produce a voltage controlled variable duty cycle
output.
A higher feedback voltage increases the error amplifier output
voltage. A higher error amplifier voltage (comparator invert-
ing 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 MIC4684 uses a voltage-mode control architecture.
Output Switching
When the internal switch is ON, an increasing current flows
from the supply V
output capacitor C
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 C
Output Capacitor
External output capacitor C
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.
IN
V
IN
SW
FB
IN,
OUT
R1 R2
V
V
OUT
REF
through external storage inductor L1, to
=
OUT
and the load. Energy is stored in the
=
=
1.235V
V
.
(
REF
V
V
R1
R2
OUT
REF
OUT
(
R2
R1
-
1
)
V
C
+
OUT
provides stabilization and
1
OUT
)
M9999-012610
Micrel, Inc.