12
FN9177.3
June 25, 2009
MOSFET Selection and Considerations
Typically, a MOSFET cannot tolerate even brief excursions
beyond their maximum drain to source voltage rating. The
MOSFETs used in the power stage of the converter should
have a maximum V
DS
rating that exceeds the sum of the
upper voltage tolerance of the input power source and the
voltage spike that occurs when the MOSFET switches off.
There are several power MOSFETs readily available that are
optimized for DC/DC converter applications. The preferred
high-side MOSFET emphasizes low switch charge so that
the device spends the least amount of time dissipating
power in the linear region. Unlike the low-side MOSFET
which has the drain-source voltage clamped by its body
diode during turn off, the high-side MOSFET turns off with
V
IN
-V
OUT
-V
L
across it. The preferred low-side MOSFET
emphasizes low r
DS(ON)
when fully saturated to minimize
conduction loss.
For the low-side MOSFET, (LS), the power loss can be
assumed to be conductive only and is written as:
For the high-side MOSFET, (HS), its conduction loss is
written as:
For the high-side MOSFET, its switching loss is written as:
Where:
-I
VALLEY
is the difference of the DC component of the
inductor current minus 1/2 of the inductor ripple current
-I
PEAK
is the sum of the DC component of the inductor
current plus 1/2 of the inductor ripple current
-t
ON
is the time required to drive the device into
saturation
-t
OFF
is the time required to drive the device into cut-off
Selecting The Bootstrap Capacitor
The selection of the bootstrap capacitor is written as:
Where:
-Q
g
is the total gate charge required to turn on the
high-side MOSFET
- V
BOOT
, is the maximum allowed voltage decay across
the boot capacitor each time the high-side MOSFET is
switched on
As an example, suppose the high-side MOSFET has a total
gate charge Q
g
, of 25nC at V
GS
= 5V, and a V
BOOT
of
200mV. The calculated bootstrap capacitance is 0.125µF; for
a comfortable margin select a capacitor that is double the
calculated capacitance, in this example 0.22µF will suffice.
Use an X7R or X5R ceramic capacitor.
Layout Considerations
As a general rule, power should be on the bottom layer of
the PCB and weak analog or logic signals are on the top
layer of the PCB. The ground-plane layer should be adjacent
to the top layer to provide shielding. The ground plane layer
should have an island located under the IC, the
compensation components, and the FSET components. The
island should be connected to the rest of the ground plane
layer at one point.
Signal Ground and Power Ground
The bottom of the ISL6269 QFN package is the signal
ground (GND) terminal for analog and logic signals of the IC.
Connect the GND pad of the ISL6269 to the island of ground
plane under the top layer using several vias, for a robust
thermal and electrical conduction path. Connect the input
capacitors, the output capacitors, and the source of the
lower MOSFETs to the power ground plane.
PGND (PIN 10)
This is the return path for the pull-down of the LG low-side
MOSFET gate driver. Ideally, PGND should be connected to
the source of the low-side MOSFET with a low-resistance,
low-inductance path.
VIN (PIN 1)
The VIN pin should be connected close to the drain of the
high-side MOSFET, using a low resistance and low
inductance path.
VCC (PIN 2)
For best performance, place the decoupling capacitor very
close to the VCC and GND pins.
PVCC (PIN 12)
For best performance, place the decoupling capacitor very
close to the PVCC and PGND pins, preferably on the same
side of the PCB as the ISL6269 IC.
(EQ. 16)
P
CON_LS
I
LOAD
2
r
DS ON_LS
1D–
(EQ. 17)
P
CON_HS
I
LOAD
2
r
DS ON_HS
D=
(EQ. 18)
P
SW_HS
V
IN
I
VALLEY
t
ON
f
SW
2
-----------------------------------------------------------------
V
IN
I
PEAK
t
OFF
f
SW
2
-------------------------------------------------------------
+=
C
BOOT
Q
g
V
BOOT
------------------------
=
(EQ. 19)
INDUCTOR
VIAS TO
GROUND
PLANE
VIN
VOUT
PHASE
NODE
GND
OUTPUT
CAPACITORS
LOW-SIDE
MOSFETS
INPUT
CAPACITORS
SCHOTTKY
DIODE
HIGH-SIDE
MOSFETS
FIGURE 8. TYPICAL POWER COMPONENT PLACEMENT
ISL6269
