MIC2172/3172
Micrel, Inc.
By using the MIC3172, U1 and Q1 shown in figure 5 can be
eliminated, reducing the total components count.
Synchronizing the MIC2172
Using several unsynchronized switching regulators in the
same circuit will cause beat frequencies to appear on the
inputs and outputs. These beat frequencies can be very low
making them difficult to filter.
Micrel’s MIC2172 can be synchronized to a single master
frequency avoiding the possibility of undesirable beat fre-
quencies in multiple regulator circuits. The master frequency
can be an external oscillator or a designated master MIC2172.
The master frequency should be 1.05 to 1.20 times the
slave’s 100kHz nominal frequency to guarantee synchroni-
zation.
U2
4
SYNC
U1
4
SYNC
10kΩ
MIC2172
V
SW
Slave
U3
Master
4
SYNC
MIC2172
Additional
Slaves
Slave
V
SW
5
5
MIC2172
V
SW
5
Applications Information
Using the MIC3172 Enable Control (New Designs)
For new designs requiring enable/shutdown control, connect
EN to a TTL or CMOS control signal (figure 3). The very low
driver current requirement ensures compatibility regardless
of the driver or gate used.
U1
Enable
Shutdown
4
Logic
Gate
EN
MIC3172
Figure 3. MIC3172 TTL Enable/Shutdown
Using the MIC3172 in LT1172 Applications
The MIC3172 can be used in most original LT1172 applica-
tions by adapting the MIC3172’s enable/shutdown feature to
the existing LT1172 circuit.
Unlike the LT1172 which can be shutdown by reducing the
voltage on pin 2 (V
C
) below 0.15V, the MIC3172 has a
dedicated enable/shutdown pin. To replace the LT1172 with
the MIC3172, determine if the LT1172’s shutdown feature is
used.
Circuits without Shutdown
If the shutdown feature is not being used, connect EN to V
IN
to continuously enable the MIC3172 or use an MIC2172 with
SYNC open (figure 4).
V
IN
V
IN
SYNC
MIC2172
V
IN
V
IN
EN
MIC3172
Figure 6. Master/Slave Synchronization
4
N/C
4
Figure 4. MIC2172/3172 Always Enabled
Circuits with Shutdown
If shutdown was used in the original LT1172 application,
connect EN to a logic gate that produces a TTL logic-level
output signal that matches the shutdown signal. The MIC3172
will be enabled by a logic-high input and shutdown with a
logic-low input (figure 5). The actual components performing
the functions of U1 and Q1 may vary according to the original
application.
4
Figure 6 shows a typical application where several MIC2172s
operate from the same supply voltage. U1’s oscillator fre-
quency is increased above U2’s and U3’s by connecting a
resistor from SYNC to ground. U2-SYNC and U3-SYNC are
capacitively coupled to the master’s output (V
SW
). The
slaves lock to the negative (falling edge) of U1’s output
waveform.
U1
4
SYNC
MIC2172
External
Signal
Slave
U2
4
SYNC
V
SW
5
MIC2172
V
SW
5
EN
MIC3172
COMP
Additional
Slaves
add
connection
Slave
U1
Enable
Shutdown
Figure 7. External Synchronization
Existing
Q1
VN2222
or equiv.
R1
C1
Existing
Logic
Gate
Care must be exercised to insure that the master MIC2172 is
always operating in continuous mode.
Figure 5. Adapting to the LT1172 Socket
April 2005
9
M9999-042205
MICREL [ MICREL SEMICONDUCTOR ]
