LT8607 datasheet(19/24 Pages) AD

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LT8609S

Rev. D

For more information www.analog.com

Thermal Considerations and Peak Current Output

For higher ambient temperatures, care should be taken in

the layout of the PCB to ensure good heat sinking of the

LT8609S. The exposed pad on the bottom of the package

must be soldered to a ground plane. This ground should be

tied to large copper layers below with thermal vias; these

layers will spread heat dissipated by the LT8609S. Placing

additional vias can reduce thermal resistance further. The

maximum load current should be derated as the ambient

temperature approaches the maximum junction rating.

Power dissipation within the LT8609S can be estimated

by calculating the total power loss from an efficiency mea-

surement and subtracting the inductor loss. The die tem-

perature is calculated by multiplying the LT8609S power

dissipation by the thermal resistance from junction to

ambient. The LT8609S will stop switching and indicate

a fault condition if safe junction temperature is exceeded.

Temperature rise of the LT8609S is worst when operating

at high load, high VIN, and high switching frequency. If

the case temperature is too high for a given application,

then either VIN, switching frequency or load current can

be decreased to reduce the temperature to an acceptable

level. Figure 5 shows how case temperature rise can be

managed by reducing VIN.

The LT8609S’s internal power switches are capable of

safely delivering up to 3A of peak output current. However,

due to thermal limits, the package can only handle 3A

loads for short periods of time. This time is determined by

how quickly the case temperature approaches the maxi-

mum junction rating. Figure 6 shows an example of how

case temperature rise changes with the duty cycle of a

10Hz pulsed 3A load. Junction temperature will be higher

than case temperature.

APPLICATIONS INFORMATION

Figure 5. Case Temperature Rise vs Load Current

Figure 6. Case Temperature Rise vs 3A Pulsed Load

LOAD CURRENT (A)

0 0.25 0.50 0.75 1 1.25 1.50 1.75 2

8609S F05

VIN = 12V

VIN = 24V

VOUT = 5V

fSW = 2MHz

DUTY CYCLE (%)

0 10 20 30 40 50 60 70 80 90 100

8609S F06

VIN = 12V

VIN = 24V

STANDBY LOAD = 50mA

PULSED LOAD = 3A

VOUT = 5V

fSW = 2MHz

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