ISL6615A_14 Datasheet, PDF (8/12 Pages) Intersil Corporation – High-Frequency 6A Sink Synchronous MOSFET Drivers with Protection Features

ISL6615A
where the gate charge (Q_G1 and Q_G2) is defined at a particular
gate to source voltage (V_GS1and V_GS2) in the corresponding
MOSFET datasheet; I_Q is the driver’s total quiescent current with
no load at both drive outputs; N_Q1 and N_Q2 are the number of
upper and lower MOSFETs, respectively; PVCC is the drive voltage
for both upper and lower FETs. The I_Q*VCC product is the
quiescent power of the driver without capacitive load and is
typically 200mW at 300kHz and VCC = PVCC = 12V.
The total gate drive power losses are dissipated among the
resistive components along the transition path. 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 transition path. The power dissipation on the driver
can be roughly estimated, as shown in Equation 4.
P_DR = P_{DR_UP} + P_{DR_LOW} + I_Q • VCC
P _{D R _UP}
=
⎛
⎜
⎝
------------^R----^H----^I--¹-------------
R_HI1 + R_EXT1
+
R^-----_L^---_O^---₁^{-R----+L---O--}R^--1--_E^---_X^----_T^--₁^--_{⎠⎟⎞}
• -^P---^Q----^g---^_---^Q---¹--
2
(EQ. 4)
P _{D R _LOW}
=
⎛
⎜
⎝
------------^R----^H----^I--²-------------
R_HI2 + R_EXT2
+
-_R----_L---_O---₂-^R----+^L---^O--_R--²--_E---_X----_T---₂-_{⎠⎟⎞}
• ^P----^Q----^g---^_---^Q---²--
2
R_EXT1
=
R_G1
+
R_{-----G---I--1--}
N_Q1
R_EXT2
=
R_G2
+
R_{-----G---I--2--}
N_Q2
PVCC
BOOT
D
R_HI1
R_LO1
C_GD
G
R_G1
R_GI1
C_GS
S
C_DS
Q1
PHASE
FIGURE 3. TYPICAL UPPER-GATE DRIVE TURN-ON PATH
PVCC
R_HI2
R_LO2
C_GD
G
R_G2
R_GI2
C_GS
S
D
C_DS
Q2
FIGURE 4. TYPICAL LOWER-GATE DRIVE TURN-ON PATH
Application Information
Layout Considerations
The parasitic inductances of the PCB and of the power devices’
packaging (both upper and lower MOSFETs) can cause serious
ringing, exceeding the absolute maximum ratings of the devices.
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 and performance:
• Keep decoupling loops (VCC-GND, PVCC-GND and BOOT-PHASE)
short and wide (at least 25 mils). Avoid using vias on decoupling
components other than their ground terminals, which should be
on a copper plane with at least two vias.
• Minimize trace inductance, especially on low-impedance lines.
All power traces (UGATE, PHASE, LGATE, GND, PVCC, VCC,
GND) should be short and wide (at least 25 mils). Try to place
power traces on a single layer, otherwise, two vias on
interconnection are preferred where possible. For no
connection (NC) pins on the QFN part, connecting them to the
adjacent net (LGATE2/PHASE2) can reduce trace inductance.
• Shorten all gate drive loops (UGATE-PHASE and LGATE-GND)
and route them closely spaced.
• 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.
• Avoid routing relatively high impedance nodes (such as PWM
and ENABLE lines) close to high dV/dt UGATE and PHASE
nodes.
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 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 due to the 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
8
FN6608.2
April 13, 2012