ISL6596_14 Datasheet, PDF (9/11 Pages) Intersil Corporation – Synchronous Rectified MOSFET Driver

ISL6596
Application Information
MOSFET Selection
The parasitic inductances of the PCB and of the power
devices’ packaging (both upper and lower MOSFETs) can
cause serious ringing, exceeding absolute maximum rating
of the devices. 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 can go a long
way toward minimizing such unwanted stress.
The D²-PAK, or D-PAK packaged MOSFETs, have large
parasitic lead inductances and are not recommended unless
additional circuits are implemented to prevent the BOOT and
PHASE pins from exceeding the device rating. Low-profile
MOSFETs, such as Direct FETs and multi-SOURCE leads
devices (SO-8, LFPAK, PowerPAK), have low parasitic lead
inductances and are preferred.
Layout Considerations
A good layout helps reduce the ringing on the switching
node (PHASE) and significantly lowers 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, 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 power ground plane(s) with thermal
vias. This combination of vias for vertical heat escape,
extended copper plane, and buried planes improve heat
dissipation and allow the part to achieve its full thermal
potential.
Upper MOSFET Self Turn-On Effects At Startup
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
self-coupling via the internal C_GD of the MOSFET, the
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
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, a
5kΩ to 10kΩ 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 (10MΩ or greater) probe to
ensure a safe design margin.
⎛
---------–----^V----^D-----^S-----------⎞
V _{G S _MILLER}
=
d_----V_---
dt
⋅
R
⋅
C_rs
s
⎜
⎜
⎜
1
–
e
d_----V_---
dt
⋅
R
⋅
Ci
s
⎟
s⎟
⎟
⎜
⎟
⎝
⎠
(EQ. 5)
R = R_UGPH + R_GI
C_rss = C_GD
C_iss = C_GD + C_GS
VCC
DU
DL
BOOT
C_BOOT
C_GD
VIN
D
UGATE G
R_GI
C_DS
PHASE
C_GS
S
Q_UPPER
FIGURE 5. GATE TO SOURCE RESISTOR TO REDUCE
UPPER MOSFET MILLER COUPLING
9
FN9240.1
January 22, 2010