ADP3110 Analog Devices, ADP3110 Datasheet - Page 9

• Manufacturer Part Number: ADP3110
• Description: 12V MOSFET Driver
• Manufacturer: Analog Devices

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The MOSFET vendor should provide a maximum voltage slew
rate at drain current rating such that this can be designed
around. The next step is to determine the expected maximum
current in the MOSFET. This can be done by
D
driver. Note this current gets divided roughly equally between
MOSFETs if more than one is used (assume a worst-case
mismatch of 30% for design margin). L
inductor value.
When producing the design, there is no exact method for
calculating the dV/dt due to the parasitic effects in the external
MOSFETs as well as the PCB. However, it can be measured to
determine if it is safe. If it appears the dV/dt is too fast, an
optional gate resistor can be added between DRVH and the
high-side MOSFET. This resistor slows down the dV/dt, but it
also increases the switching losses in the high-side MOSFET.
The ADP3110 is optimally designed with an internal drive
impedance that works with most MOSFETs to switch them
efficiently yet minimize dV/dt. However, some high speed
MOSFETs may require this external gate resistor, depending on
the currents being switched in the MOSFET.
Low-Side (Synchronous) MOSFETs
The low-side MOSFETs are usually selected to have a low on
resistance to minimize conduction losses. This usually implies a
large input gate capacitance and gate charge. The first concern is
to make sure the power delivery from the ADP3110’s DRVL
does not exceed the thermal rating of the driver.
The next concern for the low-side MOSFETs is to prevent them
from inadvertently being switched on when the high-side
MOSFET turns on. This occurs due to the drain-gate (Miller,
also specified as C
drain of the low-side MOSFET is switched to VCC by the high-
side turning on (at a rate dV/dt), the internal gate of the low-
side MOSFET is pulled up by an amount roughly equal to
VCC × (C
the MOSFET into conduction.
Another consideration is the nonoverlap circuitry of the
ADP3110, which attempts to minimize the nonoverlap period.
During the state of the high-side turning off to low-side turning
on, the SW pin and the conditions of SW prior to switching are
monitored to adequately prevent overlap.
However, during the low-side turn off to high-side turn on, the
SW pin does not contain information for determining the
proper switching time, so the state of the DRVL pin is monitored
to go below one sixth of V
the Miller capacitance and internal delays of the low-side
MOSFET gate, one must ensure the Miller-to-input capacitance
I
MAX
MAX
is determined for the VR controller being used with the
=
I
DC
rss
/C
(
per
iss
). It is important to make sure this does not put
phase
rss
) capacitance of the MOSFET. When the
)
+
CC
(
VCC
and then a delay is added. Due to
−
V
OUT
)
OUT
×
f
is the output
MAX
D
MAX
×
L
OUT
(7)
Rev. 0 | Page 9 of 12
ratio is low enough and the low-side MOSFET internal delays
are not large enough to allow accidental turn on of the low-side
MOSFET when the high-side MOSFET turns on.
Contact Sales for an updated list of recommended low-side
MOSFETs.
PC BOARD LAYOUT CONSIDERATIONS
Use the following general guidelines when designing printed
circuit boards.
1.
2.
3.
4.
5.
The circuit in Figure 6 shows how four drivers can be combined
with the ADP3181 to form a total power conversion solution for
generating V
compliant.
Figure 5 shows an example of the typical land patterns based on
the guidelines given previously. For more detailed layout
guidelines for a complete CPU voltage regulator subsystem,
refer to the Layout and Component Placement section in the
ADP3181 data sheet.
Trace out the high current paths and use short, wide
(>20 mil) traces to make these connections.
Minimize trace inductance between the DRVH and DRVL
outputs and the MOSFET gates.
Connect the PGND pin of the ADP3110 as closely as
possible to the source of the lower MOSFET.
The V
possible to the VCC and PGND pins.
Use vias to other layers when possible to maximize thermal
conduction away from the IC.
D1
CC
Figure 5. External Component Placement Example
CC(CORE)
bypass capacitor should be located as closely as
C
VCC
C
C
BST1
BST2
for an Intel CPU that is VRD 10.x
R
BST
ADP3110