iru3038 International Rectifier Corp., iru3038 Datasheet - Page 6

• Manufacturer Part Number: iru3038
• Description: Synchronous Pwm Controller For Termination Power Supply Applications
• Manufacturer: International Rectifier Corp.

IRU3038
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 150mF, 6.3V has an ESR 40mV. Se-
lecting two of these capacitors in parallel, results to an
ESR of
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:
6
ESR [
Where:
DV
DI
DV
This results to: ESR=25mV
V
L = (V
Where:
V
V
f
D = Duty Cycle
S
i = Inductor Ripple Current
t = Turn On Time
IN
IN
OUT
O
= Switching Frequency
O
O
- V
= Maximum Input Voltage
= Output Current
=100mV and DI
= Output Voltage Ripple
= Output Voltage
20mV which achieves our low ESR goal.
IN
OUT
DV
DI
- V
= L3
O
O
OUT
)3
Di
Dt
V
---(4)
IN
O
; Dt = D3
3Di3f
V
=4A
OUT
S
f
1
S
; D =
---(5)
V
V
OUT
IN
www.irf.com
If Di = 30%(I
The Coilcraft DO5022HC series provides a range of in-
ductors in different values, low profile suitable for large
currents, 3.3mH, 10A is a good choice for this applica-
tion. This will result to a ripple approximately 26.5% of
output current.
Power MOSFET Selection
The IRU3038 uses two N-Channel MOSFETs. The se-
lections criteria to meet power transfer requirements is
based on maximum drain-source voltage (V
source drive voltage (V
resistance R
The MOSFET must have a maximum operating voltage
(V
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 V
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:
P
P
q = R
The R
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.
Choose IRF7460 for both control MOSFET and synchro-
nous MOSFET. This device provides low on-resistance
in a compact SOIC 8-Pin package.
COND
COND
DSS
L = 2.6mH
) exceeding the maximum input voltage (V
DS(ON)
DS(ON)
(Upper Switch) = I
(Lower Switch) = I
Temperature Dependency
temperature dependency should be consid-
O
DS(ON)
), then the output inductor will be:
and thermal management.
GS
), maximum output current, On-
LOAD
LOAD
2
2
3 R
3 R
DS(ON)
DS(ON)
3 D 3 q
3 (1 - D) 3 q
DSS
GS
IN
), gate-
09/12/02
Rev. 2.0
to pre-
).