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IRU3037/IRU3037A & (PbF)
www.irf.com
Assuming the following:
By using equation (3), CIN = 193.3µF
For higher efficiency, low ESR capacitor is recommended.
Choose two 100µF capacitors.
The Sanyo TPB series PosCap capacitor 100µF, 10V
with 55mΩ ESR is a good choice.
Output Capacitor Selection
The criteria to select the output capacitor is normally
based on the value of the Effective Series Resistance
(ESR). In general, the output capacitor must have low
enough ESR to meet output ripple and load transient
requirements, yet have high enough ESR to satisfy sta-
bility requirements. The ESR of the output capacitor is
calculated by the following relationship:
The Sanyo TPC series, PosCap capacitor is a good
choice. The 6TPC150M 150µF, 6.3V has an ESR 40mΩ.
Selecting two of these capacitors in parallel, results to
an ESR of ≅ 20mΩ which achieves our low ESR goal.
The capacitor value must be high enough to absorb the
inductor's ripple current. The larger the value of capaci-
tor, the lower will be the output ripple voltage.
Inductor Selection
The inductor is selected based on output power, operat-
ing frequency and efficiency requirements. Low inductor
value causes large ripple current, resulting in the smaller
size, but poor efficiency and high output noise. Gener-
ally, the selection of inductor value can be reduced to
desired maximum ripple current in the inductor (∆i). The
optimum point is usually found between 20% and 50%
ripple of the output current.
For the buck converter, the inductor value for desired
operating ripple current can be determined using the fol-
lowing relation:
If ∆i = 20%(IO), then the output inductor will be:
The Toko D124C series provides a range of inductors in
different values, low profile suitable for large currents,
10µH, 4.2A is a good choice for this application. This
will result to a ripple approximately 14% of output cur-
rent.
Power MOSFET Selection
The IRU3037 uses two N-Channel MOSFETs. The se-
lections criteria to meet power transfer requirements is
based on maximum drain-source voltage (V
DSS), gate-
source drive voltage (VGS), maximum output current, On-
resistance RDS(ON) and thermal management.
The MOSFET must have a maximum operating voltage
(VDSS) exceeding the maximum input voltage (VIN).
The gate drive requirement is almost the same for both
MOSFETs. Logic-level transistor can be used and cau-
tion should be taken with devices at very low VGS to pre-
vent undesired turn-on of the complementary MOSFET,
which results a shoot-through current.
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 cur-
rent. The conduction loss is defined as:
The RDS(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.
2
2
PCOND (Upper Switch) = ILOAD × RDS(ON) × D × ϑ
PCOND (Lower Switch) = ILOAD × RDS(ON) × (1 - D) × ϑ
ϑ = RDS(ON) Temperature Dependency
Where:
∆VO = Output Voltage Ripple
∆IO = Output Current
∆VO=100mV and ∆IO=4A
Results to ESR=25mΩ
ESR ≤ ---(4)
∆VO
∆IO
Where:
V
IN = Maximum Input Voltage
VOUT = Output Voltage
∆i = Inductor Ripple Current
fS = Switching Frequency
∆t = Turn On Time
D = Duty Cycle
V
IN - VOUT = L× ; ∆t = D× ; D =
∆i
∆t
1
fS
VOUT
VIN
L = (VIN - VOUT)× ---(5)
VOUT
VIN×∆i×fS
L = 7µH
∆t = D ×
∆t = 3.3µs
1
fS
IIN =
IIN = 2.93A
∆V = 1%(VIN), Efficiency(η) = 90%
V
O × IO
η × VIN
