Datasheet

LM22677

SNVS582M –SEPTEMBER 2008–REVISED APRIL 2013

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worse than that of the internal oscillator; about +/- 25% is to be expected. Finally, an external clock can be

applied to the RT/SYNC pin to allow the regulator to synchronize to a system clock or another LM22677. The

mode is set during start-up of the regulator. When the LM22677 is enabled, or after V

is applied, a weak pull-up

is connected to the RT/SYNC pin and, after approximately 100 µs, the voltage on the pin is checked against a

threshold of about 0.8V. With the RT/SYNC pin open, the voltage floats above this threshold, and the mode is set

to run with the internal clock. With a frequency set resistor present, an internal reference holds the pin voltage at

0.8V; the resulting current sets the mode to allow the resistor to control the clock frequency. If the external circuit

forces the RT/SYNC pin to a voltage much greater or less than 0.8v, the mode is set to allow external

synchronization. The mode is latched until either the EN or the input supply is cycled.

The choice of switching frequency is governed by several considerations. As an example, lower frequencies may

be desirable to reduce switching losses or improve duty cycle limits. Higher frequencies, or a specific frequency,

may be desirable to avoid problems with EMI or reduce the physical size of external components. The flexibility

of increasing the switching frequency above 500 kHz can also be used to operate outside a critical signal

frequency band for a given application. Keep in mind that the values of inductor and output capacitor cannot be

reduced dramatically, by operating above 500 kHz. This is true because the design of the internal loop

compensation restricts the range of these components.

Frequency synchronization requires some care. First the external clock frequency must be greater than the

internal clock frequency, and less than 1 MHz. The maximum internal switching frequency is specified in the

Electrical Characteristics table. Note that the frequency adjust feature and the synchronization feature can not be

used simultaneously. The synchronizing frequency must always be greater than the internal clock frequency.

Secondly, the RT/SYNC pin must see a valid high or low voltage, during start-up, in order for the regulator to go

into the synchronizing mode (see above). Also, the amplitude of the synchronizing pulses must comport with

SYNC

levels found in the Electrical Characteristics table. The regulator will synchronize on the rising edge of the

external clock. If the external clock is lost during normal operation, the regulator will revert to the 500 kHz (typ.)

internal clock.

If the frequency synchronization feature is used, current limit foldback is not operational; see the Current Limit

section for details.

Figure 17. Switching Frequency vs RT/SYNC Resistor

Self Synchronization

It is possible to synchronize multiple LM22677 regulators together to share the same switching frequency. This

can be done by tieing the RT/SYNC pins together through a MOSFET and connecting a 1 KΩ resistor to ground

at each pin. Figure 18 shows this connection. The gate of the MOSFET should be connected to the regulator

with the highest output voltage. Also, the EN pins of both regulators should be tied to the common system

enable, in order to properly initialize both regulators. The operation is as follows: When the regulators are

enabled, the outputs are low and the MOSFET is off. The 1 kΩ resistors pull the RT/SYNC pins low, thus

enabling the synchronization mode. These resistors are small enough to pull the RT/SYNC pin low, rather than

activate the frequency adjust mode. Once the output voltage of one of the regulators is sufficient to turn on the

MOSFET, the two RT/SYNC pins are tied together and the regulators will run in synchronization mode. The two

Product Folder Links: LM22677