8
FN6601.2
March 19, 2009
The total gate drive power losses are dissipated among the
resistive components along the transition path, as outlined in
Equation 4. The drive resistance dissipates a portion of the
total gate drive power losses, the rest will be dissipated by the
external gate resistors (R
G1
and R
G2
) and the internal gate
resistors (R
GI1
and R
GI2
) of MOSFETs. Figures 3 and 4 show
the typical upper and lower gate drives turn-on current path.
Application Information
Layout Considerations
During switching of the devices, the parasitic inductances of
the PCB and the power devices’ packaging (both upper and
lower MOSFETs) leads to ringing, possibly in excess of the
absolute maximum rating of the devices. Careful layout can
help minimize such unwanted stress. The following advice is
meant to lead to an optimized layout:
• Keep decoupling loops (LVCC-GND and BOOT-PHASE)
as short as possible.
• Minimize trace inductance, especially low-impedance
lines: all power traces (UGATE, PHASE, LGATE, GND,
LVCC) 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 input current loop: connect the source 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 improved heat dissipation, 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 surface copper islands, and buried planes
combine to allow the IC and the power switches to achieve
their 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, 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 with
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 3. TYPICAL UPPER-GATE DRIVE TURN-ON PATH
FIGURE 4. TYPICAL LOWER-GATE DRIVE TURN-ON PATH
P
DR
P
DR_UP
P
DR_LOW
I
Q
VCC•++=
(EQ. 4)
P
DR_UP
R
HI1
R
HI1
R
EXT1
+
--------------------------------------
R
LO1
R
LO1
R
EXT1
+
----------------------------------------
+
⎝⎠
⎜⎟
⎛⎞
P
Qg_Q1
2
---------------------
•=
P
DR_LOW
R
HI2
R
HI2
R
EXT2
+
--------------------------------------
R
LO2
R
LO2
R
EXT2
+
----------------------------------------
+
⎝⎠
⎜⎟
⎛⎞
P
Qg_Q2
2
---------------------
•=
R
EXT1
R
G1
R
GI1
N
Q1
-------------
+=
R
EXT2
R
G2
R
GI2
N
Q2
-------------
+=
Q1
D
S
G
R
GI1
R
G1
BOOT
R
HI1
C
DS
C
GS
C
GD
R
LO1
PHASE
UVCC
LVCC
Q2
D
S
G
R
GI2
R
G2
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)
ISL6622A
