LT8685SRVPBF datasheet(20/24 Pages) AD

20 / 24 page

LT8685S

Rev. 0

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APPLICATIONS INFORMATION

the via is a much better conductor than the solder which

may or may not fill the via volume. The planes will dis-

tribute heat over a large area.

Power dissipated within the LT8685S will result in a

junction temperature rise beyond the ambient tempera-

ture in proportion to the package thermal resistance, θJA

(°C/W). The power dissipation within the LT8685S can

be estimated from an efficiency measurement by calcu-

lating the total power loss, then subtracting power loss

in components external to the LT8685S, such as inductor

DCR loss. The maximum operating junction temperature

is then estimated by multiplying the estimated LT8685S

power loss by the package θJA and summing the result

with the maximum application ambient temperature.

A good board design can achieve a θJA of 17 °C/W. If the

calculation of maximum junction temperature indicates

the LT8685S will operate near or above the allowed junc-

tion temperature, more precise thermal modelling may be

required or design changes must be made to reduce the

die junction temperature. Design changes may include

reducing VVINx, reducing fSW, or reducing the operating

load current. Load current reduction may be achieved with

either direct load current reduction or reducing the duty

cycle or duration over which the maximum load current

occurs. Die junction temperature can also be reduced by

lowering the ambient temperature or the addition of air

flow. Figure 5 shows the approximate allowable on-chip

power dissipation for a given ambient temperature using

a θJA =17 °C/W.

Figure 5. Maximum Loss vs Ambient Temperature

θJA = 17°C/W

AMBIENT TEMPERATURE TA (°C)

100

125

150

1.0

2.0

3.0

4.0

5.0

6.0

8685S F05

The LT8685S contains an internal thermal shutdown fea-

ture that will stop switching if the die temperature rises

to approximately 177°C. Switching will resume when the

temperature falls approximately 5°C. This feature is not

production tested and is intended as a failsafe only.

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