30074150

FIGURE 2. Output Current in Foldback vs.

Nominal Duty Cycle

The percentage of output current limit fold back is affected by

duty

cycle,

inductance,

and

switching

frequency.

See Figure 2 for details.

The current limit will only protect the inductor from a runaway

condition if the LM22677 is operating in its safe operating

area. A runaway condition of the inductor is potentially catas-

trophic to the application. For every design, the safe operating

area needs to be calculated. Factors in determining the safe

operating area are the switching frequency, input voltage,

output voltage, minimum on-time and feedback voltage dur-

ing an over current condition.

As a first pass check, if the following equation holds true, a

given design is considered in a safe operating area and the

current limit will protect the circuit:

V

If the equation above does not hold true, the following sec-

ondary equation will need to hold true to be in safe operating

area:

If both equations do not hold true, a particular design will not

have an effective current limit function which might damage

the circuit during startup, over current conditions, or steady

state over current and short circuit condition. Oftentimes a

reduction of the maximum input voltage or a reduction of the

switching frequency will bring a design into the safe operating

area.

When synchronized to an external sync pulse, the LM22677

will not fold back the switching frequency in an over current

condition.

Soft-Start

The soft-start feature allows the regulator to gradually reach

the initial steady state operating point, thus reducing start-up

stresses and surges. The soft-start is fixed to 500 µs (typical)

start-up time and cannot be modified.

Switching Frequency Setting and

Synchronization

There are three different modes for the RT/SYNC pin. It can

be left floating for a 500 kHz switching frequency.

A resistor from the RT/SYNC pin to ground can be used to

adjust the switching frequency between 200 kHz and 1 MHz.

An external synchronization pulse can be applied to the RT/

SYNC pin for switching frequencies up to 1 MHz. The

LM22677 internally sets the RT/SYNC mode during start up.

Many applications are limited to lower switching frequencies

due to minimum on-time and minimum off-time requirements

as well as increased switching losses at switching frequen-

cies higher than 500 kHz. In case of synchronizing to an

external sync pulse, the clock pulse must be in a valid low or

high state when the LM22677 starts up and can be applied

after device regulation. The internal oscillator will synchronize

to the external sync pulse rising edge. If the external sync

pulse is disconnected the LM22677 switching frequency will

return to default 500 kHz (typ.).

When operating at frequencies above 500 kHz, external com-

ponents such as the inductor and the output capacitors can-

not be reduced dramatically. This is due to the fixed internal

compensation and stability requirements. Running the

LM22677 at frequencies higher than 500 kHz is intended for

applications with EMI or beat frequency concerns. The flexi-

bility of increasing the switching frequency above 500 kHz can

be used to operate outside a critical signal frequency band of

an application. This can also be used to set multiple switchers

in an application to the same frequency to reduce beat fre-

quencies and simplify filtering.

See the current limit section of this datasheet for information

about the safe operating area. When synchronizing to an ex-

ternal synchronization pulse, the LM22677 will not fold back

the switching frequency in an over current condition.

The typical curve below shows adjusted switching frequen-

cies with different frequency set resistors from the RT/SYNC

pin to ground.

30074113

FIGURE 3. Switching Frequency vs RT/SYNC Resistor

Self Synchronize

It is also possible to self-synchronize multiple LM22677 reg-

ulators to share the same switching frequency. This can be

done by attaching the RT/SYNC pins together and putting a

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