LTM4657 datasheet(21/32 Pages) AD

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LTM4703

Rev. 0

For more information www.analog.com

Use Equation 16 for solving for temperature.

T(KELVIN) =

∆VD

K'D

(

°CELSIUS)= T(KELVIN) –273.15 (16)

Where,

300°K = 27°C

means that you take the difference in voltage across the

diode measured at two currents with a ratio of 10. The

resulting voltage is 198μV per Kelvin of the junction with

a zero intercept at 0 Kelvin.

The diode-connected NPN transistor across the TSENSE+

and the TSENSE− pins can be used to monitor the internal

temperature of the LTM4703.

Hot-Plugging Safely

The small size, robustness, and low impedance of ceramic

capacitors make them an attractive option for the input

bypass capacitor of LTM4703. However, these capaci-

tors can cause problems if the LTM4703 is plugged into

a live supply (Refer to the Analog Devices 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 voltage at the VIN pins of the LTM4703 can ring

to more than twice the nominal input voltage, possibly

exceeding the LTM4703’s rating and damaging the part.

If the input supply is poorly controlled or the LTM4703 is

hot-plugged 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 cap 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

ripple filtering and can slightly improve the efficiency of

the circuit, though it is likely to be the largest compo-

nent in the circuit.

Figure 7 shows a temperature plot of the LTM4703 with

12V input, 1V output at 12A without a heat sink and no

airflow condition.

Figure 7. Thermal Image at 12VIN, 1V, 12A Output, No Airflow

Thermal Considerations

The LTM4703 output current may need to be derated if

it is required to operate at a high ambient temperature.

The amount of current derating is dependent upon the

input voltage, output power, and ambient temperature.

The derating curves shown in the Typical Performance

Characteristics section can be used as a guide. These

curves were generated by the LTM4703 mounted to a

75cm2 4-layer FR4 printed circuit board. Boards of other

sizes and layer-count can exhibit different thermal behav-

iors, it is your responsibility to verify proper operation

over the intended system’s line, load, and environmental

operating conditions.

For increased accuracy and fidelity to the actual applica-

tion, many designers use FEA (finite element analysis) to

predict thermal performance. Therefore, below are the

thermal coefficients.

θJA – Thermal resistance from junction to ambient.

θJCbot – Thermal resistance from the junction to the

bottom of the product case.

θJCtop – Thermal resistance from junction to top of the

product case.

While the meaning of each of these coefficients may

seem to be intuitive, JEDEC has defined each to avoid

APPLICATIONS INFORMATION

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