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LTM8055

8055fb

For more information www.linear.com/LTM8055

Switching Mode

TheMODEpinallowstheusertoselecteitherdiscontinuous

mode or forced continuous mode switching operation. In

forcedcontinuousmode,theLTM8055willnotskipcycles,

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 LTM8055 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

LTM8055 in discontinuous mode, preventing reverse cur-

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

where MODE and LL are tied together, a small capacitor

(~0.1µF) from these pins to GND may improve the light

load transient response by delaying the transition from

the discontinuous to forced continuous switching modes.

MODE may be tied to GND for the purpose of blocking

reverse current if no output current sense resistor is used.

FB Resistor Divider and Load Regulation

The LTM8055 regulates its FB pin to 1.2V, using a resistor

divider to sense the output voltage. The location at which

the output voltage is sensed affects the load regulation.

If there is a current sense resistor between VOUT and

IOUT, and the output is sensed at VOUT, the voltage at the

load will drop by the value of the current sense resistor

multiplied by the output current. If the output voltage can

be sensed at IOUT, the load regulation may be improved.

PCB Layout

Most of the headaches associated with PCB layout have

been alleviated or even eliminated by the high level of

integration of the LTM8055. The LTM8055 is neverthe-

less a switching power supply, and care must be taken to

minimize EMI and ensure proper operation. Even with the

high level of integration, you may fail to achieve specified

operation with a haphazard or poor layout. See Figure 6

for a suggested layout. Ensure that the grounding and

heat sinking are acceptable.

A few rules to keep in mind are:

1. Place the RFB and RT resistors as close as possible to

their respective pins.

APPLICATIONS INFORMATION

VIN

ZENER

DIODE

8055 F05a

LTM8055

LOAD

CURRENT

GND

VOUT

SOURCING

LOAD

VIN

RUN

8055 F05b

10µF

LTM8055

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 LTM8055 if

the Input Rises Above a Predetermined Threshold. When the

LTM8055 Turns Off, the Energy Stored in the Internal Inductor

Will Raise VIN a Small Amount Above the Threshold

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