FDMF6704A_09 FAIRCHILD [Fairchild Semiconductor], FDMF6704A_09 Datasheet - Page 11

• Manufacturer Part Number: FDMF6704A_09
• Description: The Xtra Small, High Performance, High Frequency DrMOS Module
• Manufacturer: FAIRCHILD [Fairchild Semiconductor]

FDMF6704A Rev. F
Power Loss and Efficiency
Measurement and Calculation
Refer to Figure 24 for power loss testing method. Power loss
calculation are as follows:
(a) P
(b) P
(c) P
(d) P
(e) P
(f) EFF
(g) EFF
PCB Layout Guideline
Figure 25 shows a proper layout example of FDMF6704A and
critical parts. All of high current flow path, such as VIN, VSWH,
V
stable current flow, heat radiation and system performance.
Following is a guideline which the PCB designer should
consider:
1. Input ceramic bypass capacitors must be close to VIN and
PGND pin of FDMF6704A to help reduce the input current ripple
component induced by switching operation.
2. The VSWH copper trace serves two purposes. In addition to
being the high frequency current path from the DrMOS package
to the output inductor, it also serves as heatsink for the lower
FET in the DrMOS package. The trace should be short and wide
enough to present a low impedance path for the high frequency,
high current flow between the DrMOS and inductor in order to
minimize losses and temperature rise. Please note that the
VSWH node is a high voltage and high frequency switching
node with high noise potential. Care should be taken to
minimize coupling to adjacent traces. Additionally, since this
copper trace also acts as heatsink for the lower FET, tradeoff
must be made to use the largest area possible to improve
DrMOS cooling while maintaining acceptable noise emission.
3. Output inductor location should be as close as possible to the
FDMF6704A for lower power loss due to copper trace. Care
should be taken so that inductor dissipation does not heat the
DrMOS.
4. The PowerTrench
very effective at minimizing ringing. In most cases, no snubber
will be required. If a snubber is used, it should be placed near
OUT
IN
SW
OUT
LOSS_MODULE
LOSS_BOARD
and GND copper, should be short and wide for better and
MODULE
BOARD
V
5V
PWM Input
= (V
= V
= V
= P
= P
= 100 x P
= 100 x P
®
SMOD#
5 MOSFETs used in the output stage are
SW
OUT
IN
IN
DISB#
IN
- P
- P
A
x I
x I
x I
SW
OUT
IN
OUT
OUT
SW
OUT
I
) + (V
5V
/P
C
/P
IN
VDRV
IN
5V
Figure 24. Power Loss Measurement Block Diagram
x I
DISB#
PWM
5V
SMOD#
VDRV
)
(W)
(W)
(W)
(W)
(W)
(%)
(%)
CGND PGND
VCIN
PHASE
VIN
VSWH
BOOT
11
the FDMF6704A. The resistor and capacitor need to be of
proper size for the power dissipation.
5. Place ceramic bypass capacitor and BOOT capacitor as
close as possible to the VCIN and BOOT pins of the
FDMF6704A to ensure clean and stable power. Routing width
and length should be considered as well.
6. Include a trace from PHASE to VSWH in order to improve
noise margin. Keep the trace as short as possible.
7. The layout should include the option to insert a small value
series boot resistor between boot cap and BOOT pin. The boot
loop size, including Rboot and Cboot, should be as small as
possible. The boot resistor is normally not required, but is
effective at improving noise operating margin in multi phase
designs that may have noise issues due to ground bounce and
high negative VSWH ringing. The VIN and PGND pins handle
large current transients with frequency components above
100 MHz. If possible, these package pins should be connected
directly to the VIN and board GND planes. The use of thermal
relief traces in series with these pins is discouraged since this
will add inductance to the power path. This added inductance in
series with the PGND pin will degrade system noise immunity
by increasing negative VSWH ringing.
8. CGND pad and PGND pins should be connected by plane
GND copper with multiple vias for stable grounding. Poor
grounding can create a noise transient offset voltage level
between CGND and PGND. This could lead to fault operation of
gate driver and MOSFET.
9. Ringing at the BOOT pin is most effectively controlled by
close placement of the boot capacitor. Do not add an additional
BOOT to PGND capacitor. This may lead to excess current flow
through the BOOT diode.
10. SMOD#, DISB# and PWM pins don’t have internal pull up or
pull down resistors. They should not be left floating. These pins
should not have any noise filter caps.
11. Use multiple vias on each copper area to interconnect top,
inner and bottom layers to help smooth current flow and heat
conduction. Vias should be relatively large and of reasonable
inductance. Critical high frequency components such as Rboot,
Cboot, the RC snubber and bypass caps should be located
close to the DrMOS module and on the same side of the PCB
as the module. If not feasible, they should be connected from
the backside via a network of low inductance vias.
R
BOOT
V V
C
BOOT
SW
C
L
VIN
OUT
I
IN
A
V
IN
C
I
OUT
OUT
A
V
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OUT