IR3621 & (PbF)
13
www.irf.com
Choose IRF7821 for control MOSFETs and IRF8113 for
synchronous MOSFETs. These devices provide low on-
resistance in a compact SOIC 8-Pin package.
The MOSFETs have the following data:
The total conduction losses for each output will be:
The switching loss is more difficult to calculate, even
though the switching transition is well understood. The
reason is the effect of the parasitic components and
switching times during the switching procedures such
as turn-on / turnoff delays and rise and fall times. The
control MOSFET contributes to the majority of the switch-
ing losses in a synchronous Buck converter. The syn-
chronous MOSFET turns on under zero voltage condi-
tions, therefore, the switching losses for synchronous
MOSFET can be neglected. With a linear approxima-
tion, the total switching loss can be expressed as:
These values are taken under a certain condition test.
For more details please refer to the IRF7821 data sheet.
By using equation (9), we can calculate the total switch-
ing losses.
Programming the Over-Current Limit
The over-current threshold can be set by connecting a
resistor (RSET) from drain of low side MOSFET to the
OCSet pin. The resistor can be calculated by using equa-
tion (3).
The RDS(on) has a positive temperature coefficient and it
should be considered for the worse case operation.
PCON(TOTAL, 2.5V) = PCON(UPPER) + PCON(LOWER)
PCON(TOTAL, 2.5V) = 1.0W
PSW(TOTAL,2.5V) = 0.18W
PSW(TOTAL,1.8V) = 0.18W
IRF7821
VDSS = 30V
RDS(on) = 9mΩ
Where:
VDS(OFF) = Drain to Source Voltage at off time
tr = Rise Time
tf = Fall Time
T = Switching Period
I
LOAD = Load Current
P
SW = ILOAD ---(9)
×
VDS(OFF)
2
tr + tf
T
×
IRF7821
tr = 2.7ns
tf = 7.3ns
RDS(on) = 6mΩ×1.5 = 9mΩ
ISET ≅ IO(LIM) = 10A×1.5 = 15A
(50% over nominal output current)
This results to:
RSET = R1=R6=6.75KΩ
V
DS
V
GS
10%
90%
t
d
(ON)
t
d
(OFF)
t
r
t
f
Figure 13 - Switching time waveforms.
From IRF7821 data sheet we obtain:
2
2
PCOND(Upper Switch) = ILOAD×RDS(on)×D×ϑ
PCOND(Lower Switch) = ILOAD×RDS(on)×(1 - D)×ϑ
ϑ = RDS(on) Temperature Dependency
The gate drive requirement is almost the same for both
MOSFETs. Logic-level transistors can be used and cau-
tion should be taken with devices at very low V
GS to pre-
vent undesired turn-on of the complementary MOSFET,
which results in a shoot-through.
The total power dissipation for MOSFETs includes con-
duction and switching losses. For the Buck converter,
the average inductor current is equal to the DC
load current. The conduction loss is defined as:
The R
DS(ON) temperature dependency should be consid-
ered for the worst case operation. This is typically given
in the MOSFET data sheet. Ensure that the conduction
losses and switching losses do not exceed the package
ratings or violate the overall thermal budget.
PCON(TOTAL, 1.8V) = PCON(UPPER) + PCON(LOWER)
PCON(TOTAL, 1.8V) = 1.0W
IRF8113
VDSS = 30V
RDS(on) = 6mΩ
This resistor must be placed close to the IC, place a
small ceramic capacitor from this pin to ground for noise
rejection purposes.
