LM22680, LM22680-Q1

SNVS595L – SEPTEMBER 2008 – REVISED NOVEMBER 2014

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Feature Description (continued)

Soft-start is reset any time the part is shut down or a thermal overload event occurs.

7.3.3 Switching Frequency Adjustment and Synchronization

The LM22680 device will operate in three different modes, depending on the condition of the RT/SYNC pin. With

the RT/SYNC pin floating, the regulator will switch at the internally set frequency of 500 kHz (typ). With a resistor

in the range of 25 k

Ω to 200 kΩ, connected from RT/SYNC to ground, the internal switching frequency can be

adjusted from 1 MHz to 200 kHz. Figure 13 shows the typical curve for switching frequency versus the external

resistance connected to the RT/SYNC pin. The accuracy of the switching frequency, in this mode, is slightly

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 LM22680. The mode is

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

threshold of about 0.8 V. 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 0.8 V; thus, 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 0.8 V, 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 ensured 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

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 13. Switching Frequency vs RT/SYNC Resistor

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