8
FN6494.0
April 25, 2008
Application Information
MOSFET and Driver Selection
The parasitic inductances of the PCB and of the power
devices’ packaging (both upper and lower MOSFETs) can
cause serious ringing, exceeding the device’s absolute
maximum ratings. The negative ringing at the edges of the
PHASE node could increase the bootstrap capacitor voltage
through the internal bootstrap diode, and in some cases, it
may overstress the upper MOSFET driver. Careful layout,
proper selection of MOSFETs and packaging, as well as the
driver can minimize such unwanted stress.
The selection of D
2
-PAK, or D-PAK packaged MOSFETs, is
a much better match (for the reasons discussed) for the
ISL6620A. Low-profile MOSFETs, such as Direct FETs and
multi-source leads devices (SO-8, LFPAK, PowerPAK), have
low parasitic lead inductances and can be driven by either
ISL6620 or ISL6620A (assuming proper layout design). The
ISL6620, missing the 3Ω integrated BOOT resistor, typically
yields slightly higher efficiency than the ISL6620A.
Layout Considerations
FA good layout helps reduce the ringing on the switching
node (PHASE) and significantly lower the stress applied to
the output drives. The following advice is meant to lead to an
optimized layout:
• Keep decoupling loops (VCC-GND and BOOT-PHASE) as
short as possible.
• Minimize trace inductance, especially on low-impedance
lines. All power traces (UGATE, PHASE, LGATE, GND,
VCC) should be short and wide, as much as possible.
• Minimize the inductance of the PHASE node. Ideally, the
source of the upper and the drain of the lower MOSFET
should be as close as thermally allowable.
• Minimize the current loop of the output and input power
trains. Short the source connection of the lower MOSFET
to ground as close to the transistor pin as feasible. Input
capacitors (especially ceramic decoupling) should be
placed as close to the drain of upper and source of lower
MOSFETs as possible.
In addition, connecting the thermal pad of the DFN package
to the power ground through a via, or placing a low noise
copper plane underneath the SOIC part is recommended for
high switching frequency, high current applications. This is to
improve heat dissipation and allow the part 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, due to self
coupling via the internal C
GD
of the MOSFET, the gate of the
upper MOSFET could momentarily rise up to a level greater
than the threshold voltage of the device, potentially turning
on the upper switch. Therefore, if such a situation could
conceivably be encountered, it is a common practice to
place a resistor (R
UGPH
) across the gate and source of the
upper MOSFET to suppress the Miller coupling effect. The
value of the resistor depends mainly on the input voltage’s
rate of rise, the C
GD
/C
GS
ratio, as well as the gate-source
threshold of the upper MOSFET. A higher dV/dt, a lower
C
DS
/C
GS
ratio, and a lower gate-source threshold upper
FET will require a smaller resistor to diminish the effect of
the internal capacitive coupling. For most applications, the
integrated 20kΩ resistor is sufficient, not affecting normal
performance and efficiency.
The coupling effect can be roughly estimated using Equation 5,
which assumes a fixed linear input ramp and neglects 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. Figure 5 provides a visual reference for this
phenomenon and its potential solution.
FIGURE 4. TYPICAL LOWER-GATE DRIVE TURN-ON PATH
LVCC
Q2
D
S
G
R
G2R
L2
R
HI2
C
DS
C
GS
C
GD
R
LO2
V
GS_MILLER
dV
dt
-------
RC
rss
1e
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
Q
UPPER
D
S
G
R
GI
R
UGPH
BOOT
DU
C
DS
C
GS
C
GD
DL
PHASE
UVCC
ISL6620, ISL6620A
C
BOOT
UGATE
ISL6620, ISL6620A
