9

FN9282.0

March 30, 2006

extended copper plane, and buried planes for heat

spreading allows the IC to achieve its full thermal potential.

Upper MOSFET Self Turn-On Effects At Start-up

Should the driver have insufficient bias voltage applied, its

outputs are floating. If the input bus is energized at a high

dV/dt rate while the driver outputs are floating, because of

UGATE could momentarily rise up to a level greater than the

threshold voltage of the MOSFET. This could potentially turn

on the upper switch and result in damaging inrush energy.

Therefore, if such a situation (when input bus powered up

before the bias of the controller and driver is ready) could

conceivably be encountered, it is a common practice to

upper MOSFET to suppress the Miller coupling effect. The

value of the resistor depends mainly on the input voltage’s

threshold of the upper MOSFET. A higher dV/dt, a lower

FET will require a smaller resistor to diminish the effect of

the internal capacitive coupling. For most applications, a 5k

to 10k

Ω resistor is typically sufficient, not affecting normal

performance and efficiency.

The coupling effect can be roughly estimated with the

following equations, which assume a fixed linear input ramp

and neglect the clamping effect of the body diode of the

upper drive and the bootstrap capacitor. Other parasitic

components such as lead inductances and PCB

capacitances are also not taken into account. These

equations are provided for guidance purpose only.

Therefore, the actual coupling effect should be examined

using a very high impedance (10M

Ω or greater) probe to

ensure a safe design margin.

V

GS_MILLER

dV

dt

------- RC

V

–

DS

dV

dt

------- RC

⋅

iss

⋅

----------------------------------

–

⎝⎠

⎜⎟

⎜⎟

⎜⎟

⎜⎟

⎛⎞

⋅⋅

=

RR

UGPH

R

GI

+

=

C

rss

C

GD

=

C

iss

C

GD

C

GS

+

=

(EQ. 5)

FIGURE 5. GATE TO SOURCE RESISTOR TO REDUCE

UPPER MOSFET MILLER COUPLING

VIN

D

S

G

BOOT

DU

DL

PHASE

PVCC

UGATE

ISL6594D