MIC2172 datasheet(9/16 Pages) MICREL

9 / 16 page

1997

4-21

MIC2172/3172

Micrel

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.

Additional

Slaves

Master

10k

Ω

MIC2172

VSW

SYNC

Slave

MIC2172

VSW

SYNC

Slave

MIC2172

VSW

SYNC

Figure 6. Master/Slave Synchronization

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.

Additional

Slaves

Slave

MIC2172

VSW

SYNC

Slave

MIC2172

VSW

SYNC

External

Signal

Figure 7. External Synchronization

Care must be exercised to insure that the master MIC2172 is

always operating in continuous mode.

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.

MIC3172

Enable

Shutdown

Logic

Gate

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

to continuously enable the MIC3172 or use an MIC2172 with

SYNC open (figure 4).

MIC2172

VIN

SYNC

MIC3172

VIN

N/C

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.

MIC3172

COMP

Existing

VN2222

or equiv.

Existing

Logic

Gate

add

connection

Enable

Shutdown

Figure 5. Adapting to the LT1172 Socket

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