LM22671 datasheet(9/18 Pages) NSC

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Current Limit

When the power switch turns on, the slight capacitance load-

ing of the Schottky diode, D1, causes a leading-edge current

spike with an extended ringing period. This spike can cause

the current limit comparator to trip prematurely. A leading

edge blanking time (T

BLK) of 100 ns (typical) is used to avoid

sampling the spike.

When the switch current reaches the current limit threshold

the switch is immediately turned off. If T

ON is larger than the

minimum (100 ns typical) the switcher will hold the output

current flat at the set current limit value. But if T

ON is at or

decreases to the minimum T

ON (100 ns typical) the switching

frequency decreases to 1/5 the typical frequency. This effec-

tively causes the output current to fold back to a lower and

safe value. When the current limit condition is removed the

switching frequency is restored to nominal. This 5X frequency

fold back will result in a lower duty cycle pulse of the power

switch to minimize the overall fault condition power dissipa-

tion.

30076850

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 LM22671 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:

IN x TBLK x F < VOUT x 0.724

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 reduction of the

switching frequency will bring a design into the safe operating

area.

When synchronized to an external sync pulse, the LM22671

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 can be adjusted by se-

lecting an external soft-start capacitor. An internal 50 µA

current source charges up the external soft-start capacitor.

The generated voltage is the voltage the internal reference

limits. If no external soft-start capacitor is used, there is an

internal soft-start feature with 500 µs (typical) start-up time.

Recommended soft-start capacitor values are between 100

nF to 1 µF.

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

LM22671 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 LM22671 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 LM22671 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

LM22671 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 LM22671 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.

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