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LTM8056

8056fa

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the LTM8056 delivers. In cases where the LTM8056 is

the largest or only power converter, this may not be true

and some means may need to be devised to prevent the

LTM8056’s input from rising too high. Figure 5a shows a

passive crowbar circuit that will dissipate energy during

momentaryinputovervoltageconditions.Thebreak-down

voltage of the Zener diode is chosen in conjunction with

the resistor R to set the circuit’s trip point. The trip point

is typically set well above the maximum VIN voltage under

normal operating conditions. This circuit does not have

a precision threshold, and is subject to both part-to-part

and temperature variations, so it is most suitable for ap-

plications where the maximum input voltage is much less

than the 60VIN absolute maximum. As stated earlier, this

type of circuit is best suited for momentary overvoltages.

Figure 5a is a crowbar circuit, which attempts to prevent

the input voltage from rising above some level by dumping

energy to GND through a power device. In some cases,

it is possible to simply turn off the LTM8056 when the

input voltage exceeds some threshold. An example of this

circuit is shown in Figure 5b. When the power source on

the output drives VIN above a predetermined threshold,

the comparator pulls down on the RUN pin and stops

switching in the LTM8056. When this happens, the input

capacitance needs to absorb the energy stored within the

LTM8056’s internal inductor, resulting in an additional

voltage rise. This voltage rise depends upon the input

capacitor size and how much current is flowing from the

LTM8056 output to input.

Switching Mode

TheMODEpinallowstheusertoselecteitherdiscontinuous

mode or forced continuous mode switching operation. In

forcedcontinuousmode,theLTM8056willnotskipcycles,

evenwhentheinternalinductorcurrentfallstozerooreven

reverses direction. This has the advantage of operating at

the same fixed frequency for all load conditions, which can

be useful when designing to EMI or output noise speci-

fications. Forced continuous mode, however, uses more

current at light loads, and allows current to flow from the

load back into the input if the output is raised above the

regulation point. This reverse current can raise the input

voltage and be hazardous if the input is allowed to rise

uncontrollably. Please refer to Input Precautions in this

section for a discussion of this behavior.

Forced continuous operation may provide improved

output regulation when the LTM8056 transitions from

buck, buck-boost or boost operating modes, especially at

lighter loads. In such a case, it can be desirable to oper-

ate in forced continuous mode except when the internal

inductor current is about to reverse. If so, apply a current

sense resistor between VOUT and IOUT and tie the LL and

MODE pins together. The LL pin is low when the current

through the output sense resistor is about one-tenth the

full-scale maximum. When the output current falls to this

level, the LL pin will pull the MODE pin down, putting the

LTM8056 in discontinuous mode, preventing reverse cur-

rent from flowing from the output to the input. In the case

APPLICATIONS INFORMATION

VIN

ZENER

DIODE

8056 F05a

LTM8056

LOAD

CURRENT

GND

VOUT

SOURCING

LOAD

VIN

RUN

8056 F05b

10µF

LTM8056

LOAD

CURRENT

GND

VOUT

SOURCING

LOAD

EXTERNAL

REFERENCE

VOLTAGE

–

Figure 5a. The MOSFET Q Dissipates Momentary Energy to

GND. The Zener Diode and Resistor Are Chosen to Ensure That

the MOSFET Turns On Above the Maximum VIN Voltage Under

Normal Operation

Figure 5b. This Comparator Circuit Turns Off the LTM8056 if

the Input Rises Above a Predetermined Threshold. When the

LTM8056 Turns Off, the Energy Stored in the Internal Inductor

Will Raise VIN a Small Amount Above the Threshold

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