MIC2172/3172
Micrel, Inc.
MIC2172/3172
100kHz 1.25A Switching Regulators
slave’s. The master MIC2172’s oscillator frequency is in-
creased up to 135kHz by connecting a resistor from SYNC to
ground (see applications information).
General Description
The MIC2172 and MIC3172 are complete 100kHz SMPS
current-mode controllers with internal 65V 1.25A power
switches. The MIC2172 features external frequency syn-
chronization or frequency adjustment, while the MIC3172
features an enable/shutdown control input.
The MIC2172/3172 is available in an 8-pin plastic DIP or
SOIC for –40°C to +85°C operation.
Features
Although primarily intended for voltage step-up applications,
the floating switch architecture of the MIC2172/3172 makes
it practical for step-down, inverting, and Cuk configurations
as well as isolated topologies.
Operating from 3V to 40V, the MIC2172/3172 draws only
7mA of quiescent current making it attractive for battery
operated supplies.
• 1.25A, 65V internal switch rating
• 3V to 40V input voltage range
• Current-mode operation
• Internal cycle-by-cycle current limit
• Thermal shutdown
• Low external parts count
• Operates in most switching topologies
• 7mA quiescent current (operating)
• <1µA quiescent current, shutdown mode (MIC3172)
• TTL shutdown compatibility (MIC3172)
• External frequency synchronization (MIC2172)
• External frequency trim (MIC2172)
• Fits most LT1172 sockets (see applications info)
The MIC3172 is for applications that require on/off control of
the regulator. The MIC3172 is externally shutdown by
applyingaTTLlowsignaltoEN(enable). Whendisabled, the
MIC3172 draws only leakage current (typically less than
1µA). ENmustbehighfornormaloperation. Forapplications
not requiring control, EN must be tied to V or TTL high.
IN
Applications
The MIC2172 is for applications requiring two or more SMPS
regulators that operate from the same input supply. The
MIC2172 features a SYNC input which allows locking of its
internal oscillator to an external reference. This makes it
possibletoavoidtheaudiblebeatfrequenciesthatresultfrom
the unequal oscillator frequencies of independent SMPS
regulators.
A reference signal can be supplied by one MIC2172 desig-
natedasamaster. Toinsurelockingoftheslave’soscillators,
the reference oscillator frequency must be higher than the
• Laptop/palmtop computers
• Toys
• Hand-held instruments
• Off-line converter up to 50W
(requires external power switch)
• Predriver for higher power capability
• Master/slave configurations (MIC2172)
Typical Applications
VOUT
VIN
4V to 6V
+5V
5V, 0.25A
T1
(4.75V min.)
D2
C1
R4*
C3*
D1*
C1*
L1
1N5818
22µF
22µF
R1
27µH
C4
3.74k
1%
VOUT
470µF
VIN
D1
VIN
+12V, 0.14A
Enable
VSW
N/C
SYNC
MIC2172
VSW
EN
Shutdown
R1
1:1.25
PRI = 100µH
1N5822
10.7k
1%
L
MIC3172
COMP
FB
COMP
FB
GND
R2
GND
R3
1k
R2
C2
P1 P2
S
R3
1k
1.24k
P1 P2
S
1.24k
1%
470µF
1%
C3
C2
1µF
1µF
* Locate near MIC2172 when supply leads > 2"
* Optional voltage clipper (may be req’d if T1 leakage inductance too high)
Figure 1.
Figure 2.
MIC2172 5V to 12V Boost Converter
MIC3172 5V Flyback Converter
Micrel, Inc. • 2180 Fortune Drive • San Jose, CA 95131 • USA • tel + 1 (408) 944-0800 • fax + 1 (408) 474-1000 • http://www.micrel.com
M9999-042205
April 2005
1