MIC4607 Datasheet, PDF (28/42 Pages) Microchip Technology – 85V, Three-Phase MOSFET Driver with Adaptive Dead-Time, Anti-Shoot-Through and Overcurrent Protection

MIC4607
two functions: it provides decoupling for the high-side
circuitry and also provides current to the high-side cir-
cuit while the high-side external MOSFET is on.
Ceramic capacitors are recommended because of their
low impedance and small size. Z5U type ceramic
capacitor dielectrics are not recommended because of
the large change in capacitance over temperature and
voltage. A minimum value of 0.1 μF is required for C_B
(xHB to xHS capacitors) and 1 μF for the V_DD capacitor,
regardless of the MOSFETs being driven. Larger MOS-
FETs may require larger capacitance values for proper
operation. The voltage rating of the capacitors depends
on the supply voltage, ambient temperature and the
voltage derating used for reliability. 25V rated X5R or
X7R ceramic capacitors are recommended for most
applications. The minimum capacitance value should
be increased if low voltage capacitors are used
because even good quality dielectric capacitors, such
as X5R, will lose 40% to 70% of their capacitance value
at the rated voltage.
Placement of the decoupling capacitors is critical. The
bypass capacitor for V_DD should be placed as close as
possible between the V_DD and V_SS pins. The bypass
capacitor (C_B) for the xHB supply pin must be located
as close as possible between the xHB and xHS pins.
The etch connections must be short, wide, and direct.
The use of a ground plane to minimize connection
impedance is recommended. Refer to the “Grounding,
Component Placement and Circuit Layout” sub-section
for more information.
The voltage on the bootstrap capacitor drops each time
it delivers charge to turn on the MOSFET. The voltage
drop depends on the gate charge required by the MOS-
FET. Most MOSFET specifications specify gate charge
versus V_GS voltage. Based on this information and a
recommended ΔV_HB of less than 0.1V, the minimum
value of bootstrap capacitance is calculated as:
EQUATION 6-13:
C
B

Q_{-----G----A---T---E-}
V _HB
Where:
Q_GATE = Total gate charge at V_HB.
ΔV_HB = Voltage drop at the HB pin.
If the high-side MOSFET is not switched but held in an
on state, the voltage in the bootstrap capacitor will drop
due to leakage current that flows from the HB pin to
ground. This current is specified in the Electrical Char-
acteristics table. In this case, the value of C_B is calcu-
lated as:
EQUATION 6-14:
C
B

-^I--^H----^B---^S----^-----^t--^O----^N--
V _HB
DS20005610A-page 28
Where:
I_HBS = Maximum xHB pin leakage current.
t_ON = maximum high-side FET on-time.
The larger value of C_B from Equation 6-13 or
Equation 6-14 should be used.
6.10 Grounding, Component
Placement and Circuit Layout
Nanosecond switching speeds and ampere peak cur-
rents in and around the MIC4607 driver require proper
placement and trace routing of all components.
Improper placement may cause degraded noise immu-
nity, false switching, excessive ringing, or circuit
latch-up.
Figure 6-9 shows the critical current paths of the high-
and low-side driver when their outputs go high and turn
on the external MOSFETs. It also helps demonstrate
the need for a low impedance ground plane. Charge
needed to turn-on the MOSFET gates comes from the
decoupling capacitors C_VDD and C_B. Current in the
low-side gate driver flows from C_VDD through the inter-
nal driver, into the MOSFET gate, and out the source.
The return connection back to the decoupling capacitor
is made through the ground plane. Any inductance or
resistance in the ground return path causes a voltage
spike or ringing to appear on the source of the MOS-
FET. This voltage works against the gate drive voltage
and can either slow down or turn off the MOSFET
during the period when it should be turned on.
Current in the high-side driver is sourced from capaci-
tor C_B and flows into the xHB pin and out the xHO pin,
into the gate of the high side MOSFET. The return path
for the current is from the source of the MOSFET and
back to capacitor C_B. The high-side circuit return path
usually does not have a low-impedance ground plane
so the etch connections in this critical path should be
short and wide to minimize parasitic inductance. As
with the low-side circuit, impedance between the MOS-
FET source and the decoupling capacitor causes neg-
ative voltage feedback that fights the turn-on of the
MOSFET.
It is important to note that capacitor C_B must be placed
close to the xHB and xHS pins. This capacitor not only
provides all the energy for turn-on but it must also keep
xHB pin noise and ripple low for proper operation of the
high-side drive circuitry.
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