LTM4618(RevA) Ver la hoja de datos (PDF) - Linear Technology

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Fabricante

LTM4618
(Rev.:RevA)

6A DC/DC µModule Regulator with Tracking and Frequency Synchronization

Linear Technology 

LTM4618

APPLICATIONS INFORMATION

The 1.5V and 3.3V power loss curves in Figures 7 and 8

can be used in coordination with the load current derating

curves in Figures 9 to 16 for calculating an approximate

θJA thermal resistance for the LTM4618 with various heat

sinking and air flow conditions. The power loss curves

are taken at room temperature, and are increased with

multiplicative factors according to the ambient tempera-

ture. These approximate factors are: 1 for 40°C; 1.05 for

50°C; 1.1 for 60°C; 1.15 for 70°C; 1.2 for 80°C; 1.25 for

90°C; 1.3 for 100°C; 1.35 for 110°C and 1.4 for 125°C.

The derating curves are plotted with the output current

starting at 6A and the ambient temperature at 40°C. The

output voltages are 1.5V, and 3.3V. These are chosen to

include the lower and higher output voltage ranges for

correlating the thermal resistance. Thermal models are

derived from several temperature measurements in a con-

trolled temperature chamber along with thermal modeling

analysis. The junction temperatures are monitored while

ambient temperature is increased with and without air

flow. The power loss increase with ambient temperature

change is factored into the derating curves. The junctions

are maintained at 120°C maximum while lowering output

current or power with increasing ambient temperature.

The decreased output current will decrease the internal

module loss as ambient temperature is increased. The

monitored junction temperature of 120°C minus the

ambient operating temperature specifies how much mod-

ule temperature rise can be allowed. As an example, in

Figure 11 the load current is derated to ~5A at ~85°C with

no air flow or heat sink and the power loss for the 12V to

1.5V at 5A output is about 1.7W. The 1.7W loss is calcu-

lated with the ~1.4W room temperature loss from the 12V

to 1.5V power loss curve at 5A, and the 1.2 multiplying

factor at 85°C ambient. If the 85°C ambient temperature

is subtracted from the 115°C junction temperature, then

the difference of 30°C divided 1.7W equals a 17°C/W θJA

thermal resistance. Table 2 specifies a 16°C/W value which

is very close. Table 2 and Table 3 provide equivalent thermal

resistances for 1.5V and 3.3V outputs with and without air

flow and heat sinking. The derived thermal resistances in

Tables 2 and 3 for the various conditions can be multiplied

by the calculated power loss as a function of ambient

temperature to derive temperature rise above ambient,

thus maximum junction temperature. Room temperature

power loss can be derived from the efficiency curves in

the Typical Performance Characteristics section and ad-

justed with the above ambient temperature multiplicative

factors. The printed circuit board is a 1.6mm thick four

layer board with two ounce copper for the two outer lay-

ers and one ounce copper for the two inner layers. The

PCB dimensions are 95mm × 76mm. The BGA heat sink

is listed in Table 3.

Safety Considerations

The LTM4618 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 needs to be

provided to protect each unit from catastrophic failure.

2.0

5VIN

12VIN

1.5

1.0

0.5

0

0

1

2

3

4

5

6

LOAD CURRENT (A)

4618 F07

Figure 7. Power Loss at 1.5VOUT

3.0

12VIN

24VIN

2.5

2.0

1.5

1.0

0.5

0

0

1

2

3

4

5

6

LOAD CURRENT (A)

4618 F08

Figure 8. Power Loss at 3.3VOUT

4618fa

15

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