LTM4600EVPBF datasheet(14/24 Pages) LINER

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LTM4600

4600fa

APPLICATIO S I FOR ATIO

Thermal Considerations and Output Current Derating

The power loss curves in Figures 8 and 13 can be used

in coordination with the load current derating curves in

Figures 9 to 12, and Figures 14 to 15 for calculating an

approximate θJA for the module with various heatsink-

ing methods. Thermal models are derived from several

temperature measurements at the bench, and thermal

modeling analysis. Application Note 103 provides a detailed

explanation of the analysis for the thermal models, and the

derating curves. Tables 3 and 4 provide a summary of the

equivalent θJA for the noted conditions. These equivalent

θJA parameters are correlated to the measure values, and

improved with air-ﬂow. The case temperature is maintained

at 100°C or below for the derating curves. This allows for

4W maximum power dissipation in the total module with

top and bottom heatsinking, and 2W power dissipation

through the top of the module with an approximate θJC

between 6°C/W to 9°C/W. This equates to a total of 124°C

at the junction of the device.

Safety Considerations

The LTM4600 modules do not provide isolation from VIN to

VOUT. There is no internal fuse. If required, a slow blow fuse

with a rating twice the maximum input current should be

provided to protect each unit from catastrophic failure.

In the application where the light load efﬁciency is im-

portant, tying the FCB pin above 0.6V threshold enables

discontinuous operation where the bottom MOSFET turns

off when inductor current reverses. Therefore, the conduc-

tion loss is minimized and light load efﬁcient is improved.

The penalty is that the controller may skip cycle and the

output voltage ripple increases at light load.

Paralleling Operation with Load Sharing

Two or more LTM4600 modules can be paralleled to provide

higher than 10A output current. Figure 7 shows the neces-

sary interconnection between two paralleled modules. The

OPTI-LOOP™ current mode control ensures good current

sharing among modules to balance the thermal stress.

The new feedback equation for two or more LTM4600s

in parallel is:

OUT

SET

100

.•

where N is the number of LTM4600s in parallel.

Figure 7. Parallel Two µModules with Load Sharing

VIN

VOUT

VIN

VOUT

(20AMAX)

4600 F07

LTM4600

PGND

SGND

COMP VOSET

RSET

VIN

VOUT

LTM4600

PGND

SGND

COMP VOSET

OPTI-LOOP is a trademark of Linear Technology Corporation.

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