9

Power Dissipation

Package power dissipation is mainly a function of the

switching frequency and total gate charge of the selected

MOSFETs. Calculating the power dissipation in the driver for

a desired application is critical to ensuring safe operation.

Exceeding the maximum allowable power dissipation level

will push the IC beyond the maximum recommended

operating junction temperature of 125°C. The maximum

allowable IC power dissipation for the SO-8 package is

approximately 800mW. When designing the driver into an

application, it is recommende

d that the following calculation

be performed to ensure safe operation at the desired

frequency for the selected MOSFETs. The power dissipated

by the driver is approximated as below and plotted as in

Figure 9.

MOSFET selection and any external capacitance added to

of the driver and is typically negligible.

Layout Consideration

For heat spreading, place copper underneath the IC whether

it has an exposed pad or not. The copper area can be

extended beyond the bottom area of the IC and/or

connected to buried copper plane(s) with thermal vias. This

combination of vias for vertical heat escape, extended

copper plane, and buried planes for heat spreading allows

the IC to achieve its full thermal potential.

Place each channel power component as close to each

other as possible to reduce PCB copper losses and PCB

parasitics: shortest distance between DRAINs of upper FETs

and SOURCEs of lower FETs; shortest distance between

DRAINs of lower FETs and the power ground. Thus, smaller

amplitudes of positive and negative ringing are on the

switching edges of the PHASE node. However, some space

in between power components is required for good airflow.

The gate traces from the drivers to the FETs should be kept

short and wide to reduce the inductance of the traces and

promote clean drive signals.

+

() I

+

=

FIGURE 9. POWER DISSIPATION vs FREQUENCY

FREQUENCY (kHz)

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ISL6608