ADP3208 ON Semiconductor, ADP3208 Datasheet - Page 31

• Manufacturer Part Number: ADP3208
• Description: 7-bit, Programmable, Dual-phase, Mobile, Cpu, Synchronous Buck Controller
• Manufacturer: ON Semiconductor

POWER MOSFETS
For typical 20 A per phase applications, the N-channel power
MOSFETs are selected for two high-side switches and two or
three low-side switches per phase. The main selection
parameters for the power MOSFETs are V
and R
level threshold MOSFETs must be used.
The maximum output current, I
requirement for the low-side (synchronous) MOSFETs. In the
ADP3208, currents are balanced between phases; the current in
each low-side MOSFET is the output current divided by the
total number of MOSFETs (n
dominant, the following expression shows the total power that
is dissipated in each synchronous MOSFET in terms of the
ripple current per phase (I
current (I
where:
D is the duty cycle and is approximately the output voltage
divided by the input voltage.
I
approximately
Knowing the maximum output current and the maximum
allowed power dissipation, the user can calculate the required
R
compatible MOSFETs, the junction-to-ambient (PCB) thermal
impedance is 50°C/W. In the worst case, the PCB temperature is
70°C to 80°C during heavy load operation of the notebook, and
a safe limit for P
perature. Therefore, for this example (40 A maximum), the R
per MOSFET is less than 8.5 mΩ for two pieces of low-side
MOSFETs. This R
120°C; therefore, the R
6 mΩ at room temperature, or 8.5 mΩ at high temperature.
Another important factor for the synchronous MOSFET is the
input capacitance and feedback capacitance. The ratio of the
feedback to input must be small (less than 10% is recommended)
to prevent accidentally turning on the synchronous MOSFETs
when the switch node goes high.
The high-side (main) MOSFET must be able to handle two
main power dissipation components: conduction losses and
switching losses. Switching loss is related to the time for the
main MOSFET to turn on and off and to the current and
voltage that are being switched. Basing the switching speed
I
R
DS(ON)
R
is the inductor peak-to-peak ripple current and is
=
P
DS(ON)
1 (
SF
for the MOSFET. For 8-lead SOIC or 8-lead SOIC-
−
L
=
O
D
):
×
. Because the voltage of the gate driver is 5 V, logic-
1 (
)
×
f
−
SW
V
D
SF
OUT
)
is about 0.8 W to 1.0 W at 120°C junction tem-
×
DS(SF)
⎡
⎢
⎢
⎣
⎛
⎜
⎜
⎝
n
I
is also at a junction temperature of about
DS(SF)
SF
O
⎞
⎟
⎟
⎠
R
2
) and the average total output
per MOSFET should be less than
+
SF
12
). With conduction losses being
1
O
, determines the R
×
⎛ ×
⎜
⎜
⎝
n
n
SF
I
R
GS(TH)
⎞
⎟
⎟
⎠
2
⎤
⎥
⎥
⎦
×
, Q
R
DS
G
, C
DS(ON)
(
SF
Rev. 1 | Page 31 of 38 | www.onsemi.com
ISS
)
, C
DS(SF)
RSS
(16)
,
on the rise and fall times of the gate driver impedance and
MOSFET input capacitance, the following expression provides
an approximate value for the switching loss per main MOSFET:
where:
n
R
C
The most effective way to reduce switching loss is to use lower
gate capacitance devices.
The conduction loss of the main MOSFET is given by the
following equation:
where R
Typically, a user wants the highest speed (low C
for a main MOSFET, but such a device usually has higher on
resistance. Therefore, the user must select a device that meets
the total power dissipation (about 0.8 W to 1.0 W for an 8-lead
SOIC) when combining the switching and conduction losses.
For example, an IRF7821 device can be selected as the main
MOSFET (four in total; that is, n
C
T
synchronous MOSFET (four in total; that is, n
R
power dissipation per MOSFET at I
yields 630 mW for each synchronous MOSFET and 590 mW
for each main MOSFET. A third synchronous MOSFET is an
option to further increase the conversion efficiency and reduce
thermal stress.
Finally, consider the power dissipation in the driver for each
phase. This is best described in terms of the Q
MOSFETs and is given by the following equation:
where Q
Q
The previous equation also shows the standby dissipation
(I
P
MF
G
ISS
J
DS(SF)
ISS
CC
DRV
GSF
= 120°C), and an IR7832 device can be selected as the
is the total gate resistance.
is the total number of main MOSFETs.
is the input capacitance of the main MOSFET.
= 1010 pF (maximum) and R
times the VCC) of the driver.
is the total gate charge for each synchronous MOSFET.
P
P
=
= 6.7 mΩ (maximum at T
S
C
(
(
MF
⎡
⎢
⎣
MF
DS(MF)
GMF
2
f
)
)
SW
×
= 2
is the total gate charge for each main MOSFET, and
=
n
is the on resistance of the MOSFET.
D
×
×
(
×
n
f
MF
⎡
⎢
⎢
⎣
SW
⎛
⎜
⎜
⎝
n
×
I
×
MF
O
Q
V
GMF
⎞
⎟
⎟
⎠
DC
n
2
MF
+
×
+
12
I
1
n
O
SF
J
×
MF
×
DS(MF)
= 120°C). Solving for the
×
⎛ ×
⎜
⎜
⎝
R
= 4), with approximately
n
Q
O
G
n
= 40 A and I
GSF
MF
×
I
= 18 mΩ (maximum at
n
R
)
MF
n
⎞
⎟
⎟
⎠
+
2
I
⎤
⎥
⎥
⎦
CC
×
×
G
SF
C
R
⎤
⎥
⎦
ISS
for the
ISS
= 4), with
×
DS
) device
ADP3208
R
VCC
(
MF
= 9.0 A
)
(17)
(18)
(19)