15 A Dual Low-Side RF MOSFET Driver

IXRFD615X2

8

Applications Information

Introduction

Circuits capable of very high switching speeds and

high frequency operation require close attention to sev-

eral important issues. Key elements include circuit loop

inductance, Vcc bypassing, and grounding.

Circuit Loop Inductance

The power supply to ground current path defines the

loop that generates the inductive term. This loop must

be kept as short as possible. The output lead must be

no further than 0.375 inches (9.5 mm) from the gate of

the MOSFET. Furthermore, the output ground leads

must provide a balanced symmetric coplanar ground

return for optimum operation.

Vcc Bypassing

In order to turn a MOSFET on properly, the

IXRFD615X2 must be able to draw up to 15 A of cur-

rent per driver from the Vcc power supply in 4 to 8 ns

(depending upon the input capacitance of the MOSFET

being driven). Good performance requires very low im-

pedance between the driver and the power supply. The

most common method of achieving this low impedance

is to bypass the power supply at the driver with a ca-

pacitance value much larger than the load capacitance.

Usually, this is achieved by placing two or three differ-

ent types of bypassing capacitors, with complementary

impedance curves, very close to the driver itself.

(These capacitors should be carefully selected for low

inductance, low resistance, and high pulse current ser-

vice.) Care should be taken to keep the lengths of the

leads between these bypass capacitors and the

IXRFD630 to an absolute minimum.

The bypassing should be comprised of several values

of MLC (Multi-Layer Ceramic) capacitors symmetrically

placed on either side of the IC. Recommended values

are 0.01uF and 0.47uF for bypass and at least two

4.7uF tantalums for bulk storage.

Grounding

In order for the design to turn the load off properly, the

IXRFD615X2 must be able to drain 15 A of current per

driver into an adequate grounding system. There are

two paths for returning current that need to be consid-

ered: Path one is between the IXRFD615X2 and its

load, and path two is between the IXRFD615X2 and its

power supply. Both of these paths should be as low in

resistance and inductance as possible, and thus as

short as practical.

Output Lead Inductance

Of equal importance to supply bypassing and

grounding are issues related to the output lead

inductance. Every effort should be made to keep

the leads between the driver and its load as short

and wide as possible, and treated as coplanar

transmission lines. In configurations where the

optimum configuration of circuit layout and by-

passing cannot be used, a series resistance of a

few ohms in the gate lead may be necessary to

dampen ringing.

Heat Sinking

For high power operation, the bottom side metal-

lized substrate should be placed in compression

against an appropriate heat sink. The substrate

is metalized for improved heat dissipation, and is

not electrically connected to the device or to

ground. See the technical note “DE-Series

MOSFET and IC Mounting Instructions” on the

IXYS Colorado website at www.ixyscolorado.com

for detailed mounting instructions.

An

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Fort Collins, CO USA 80525

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Email: sales@ixyscolorado.com

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