LT8301 datasheet(14/20 Pages) LINER

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LTM8067

8067fc

For more information www.linear.com/LTM8067

APPLICATIONS INFORMATION

age at the VIN pin of the LTM8067 can ring to more than

twice the nominal input voltage, possibly exceeding the

LTM8067’s rating and damaging the part. If the input

supply is poorly controlled or the user will be plugging

the LTM8067 into an energized supply, the input network

should be designed to prevent this overshoot. This can be

accomplished by installing a small resistor in series to VIN,

but the most popular method of controlling input voltage

overshoot is adding an electrolytic bulk capacitor to the

VIN net. This capacitor’s relatively high equivalent series

resistance damps the circuit and eliminates the voltage

overshoot. The extra capacitor improves low frequency

ripplefilteringandcanslightlyimprovetheefficiencyofthe

circuit, though it can be a large component in the circuit.

Thermal Considerations

The LTM8067 output current may need to be derated if it

is required to operate in a high ambient temperature. The

amount of current derating is dependent upon the input

voltage, output power and ambient temperature. The

temperature rise curves given in the Typical Performance

Characteristicssectioncanbeusedasaguide.Thesecurves

were generated by the LTM8067 mounted to a 58cm2

4-layer FR4 printed circuit board. Boards of other sizes

and layer count can exhibit different thermal behavior, so

it is incumbent upon the user to verify proper operation

over the intended system’s line, load and environmental

operating conditions.

Forincreasedaccuracyandfidelitytotheactualapplication,

many designers use FEA to predict thermal performance.

To that end, the Pin Configuration section of the data sheet

typically gives four thermal coefficients:

θJA: Thermal resistance from junction to ambient

θJCbottom: Thermal resistance from junction to the bot-

tom of the product case

θJCtop: Thermal resistance from junction to top of the

product case

θJCboard:Thermalresistancefromjunctiontotheprinted

circuit board.

While the meaning of each of these coefficients may

seem to be intuitive, JEDEC has defined each to avoid

Minimum Load

Due to the nature of the flyback regulator in general, and

the LTM8067 control scheme specifically, the LTM8067

requires a minimum load for proper operation. Other-

wise, the output may go out of regulation if the load is

too light. The most common way to address this is to

place a resistor across the output. The Minimum Load

Current vs VOUT Over Full Output Voltage Range graph

in the Typical Performance Characteristics section may

be used as a guide in selecting the resistor. Note that this

graph describes room temperature operation. If the end

applicationoperatesatacoldertemperature,theminimum

load requirement may be higher and the minimum load

condition must be characterized for the lowest operating

temperature.

If it is impractical to place a resistive load permanently

across the output, a resistor and Zener diode may be used

instead, as shown in Figure 2. While the minimum load

resistor mentioned in the prior paragraph will always draw

current while the LTM8067 output is powered, the series

resistor-Zener diode combination will only draw current if

the output is too high. When using this circuit, take care to

ensure that the characteristics of the Zener diode are ap-

propriatefortheintendedapplication’stemperaturerange.

Figure 2: Use a Resistor and Zener Diode to Meet the

Minimum Load Requirement

Hot-Plugging Safely

The small size, robustness and low impedance of ceramic

capacitors make them an attractive option for the input

bypass capacitor of the LTM8067. However, these capaci-

tors can cause problems if the LTM8067 is plugged into a

live supply (see Linear Technology Application Note 88 for

a complete discussion). The low loss ceramic capacitor

combined with stray inductance in series with the power

source forms an underdamped tank circuit, and the volt-

LTM8067

VOUTP

VOUTN

8067 F02

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