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LTM8055

8055fb

For more information www.linear.com/LTM8055

still operate properly. Some examples of this are provided

in the Typical Applications section.

Soft-Start

Soft-startreducestheinputpowersources’surgecurrents

bygraduallyincreasingthecontroller’scurrent.Asindicated

in the Block Diagram, the LTM8055 has an internal soft-

start RC network. Depending upon the load and operating

conditions, the internal network may be sufficient for the

application. To increase the soft-start time, simply add a

capacitor from SS to GND.

Output Current Limit (IOUT)

The LTM8055 features an accurate average output current

limit set by an external sense resistor placed between VOUT

and IOUT as shown in Figure 3. VOUT and IOUT internally

connect to a differential amplifier that limits the current

when the voltage VOUT-IOUT reaches 58mV. The current

limit is:

IOUT(LIM) =

58mV

RSENSE

where RSENSE is the value of the sense resistor in ohms.

Most applications should use an output sense resistor as

shown in Figure 3, if practical. The internal buck-boost

power stage is current limited, but is nonetheless capable

of delivering large amounts of current in an overload

condition, especially when the output voltage is much

lower than the input and the power stage is operating as

a buck converter.

tying the LL and MODE pins together can improve perfor-

mance—see Switching Mode in this section.

In high step-down voltage regulator applications, the

internal current limit can be quite high to allow proper

operation. This can potentially damage the LTM8055

in overload or short-circuit conditions. Apply an output

current sense resistor to set an appropriate current limit

to protect the LTM8055 against these fault conditions.

Output Current Limit Control (CTL)

Use the CTL input to reduce the output current limit from

thevaluesetbytheexternalsenseresistorappliedbetween

VOUT and IOUT. The typical control range is between 0V

and 1.2V. The CTL pin does not directly affect the input

current limit. If this function is not used, leave CTL open.

Drive CTL to less than about 50mV to stop switching. The

CTL pin has an internal pull-up resistor to 2V.

Input Current Limit (IIN)

Some applications require that LTM8055 draw no more

than some predetermined current from the power source.

Current limited power sources and power sharing are two

examples.TheLTM8055featuresanaccurateinputcurrent

limit set by an external sense resistor placed between IIN

and VINasshowninFigure4.VINandIINinternallyconnect

to a differential amplifier that limits the current when the

voltage IIN-VIN reaches 50mV. The current limit is:

IIN(LIM) =

50mV

RSENSE

where RSENSE is the value of the sense resistor in ohms.

If input current limiting is not required, simply tie IINtoVIN.

APPLICATIONS INFORMATION

IOUT

LTM8055

VOUT

RSENSE

8055 F03

LOAD

Figure 3. Set The LTM8055 Output Current Limit with an External

Sense Resistor

Figure 4. Set the LTM8055 Input Current Limit with an

External Sense Resistor

IIN

LTM8055

RSENSE

VIN

POWER

SOURCE

8055 F04

When the voltage across the output sense resistor falls

to about 1/10th of full scale, the LL pin pulls low. If there

is no output sense resistor, and IOUT is tied to VOUT, LL

will be active low. Applying an output sense resistor and

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