iru3138 International Rectifier Corp., iru3138 Datasheet - Page 8

• Manufacturer Part Number: iru3138
• Description: Synchronous Pwm Controller For Termination Power Supply Applications -
• Manufacturer: International Rectifier Corp.

IRU3138
For higher efficiency, a low ESR capacitor is recom-
mended. Choose three Poscap from Sanyo 6TPC150M
(6.3V, 150mF, 40mV) with a maximum allowable ripple
current of 5.7A.
Inductor Selection
The inductor is selected based on operating frequency,
transient performance and allowable output voltage ripple.
Low inductor value results to faster response to step
load (high Di/Dt) and smaller size but will cause larger
output ripple due to increase of inductor ripple current.
As a rule of thumb, select an inductor that produces a
ripple current of 10-40% of full load DC.
For the buck converter, the inductor value for desired
operating ripple current can be determined using the fol-
lowing relation:
If Di = 25%(I
The Panasonic PCCN6B series provides a range of in-
ductors in different values, low profile suitable for large
currents, 1.1mH, 16A is a good choice for this applica-
tion. This will result to a ripple approximately 22% of
output current.
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
8
I
Where:
D is the Duty Cycle, D=V
I
I
For V
V
L = (V
Where:
V
V
f
D = Duty Cycle
L = 0.91mH
RMS
RMS
OUT
S
i = Inductor Ripple Current
t = Turn On Time
IN
IN
OUT
= Switching Frequency
- V
= Maximum Input Voltage
= I
is the output current for each channel.
is the RMS value of the input capacitor current.
= Output Voltage
IN
IN
OUT
OUT
=5V, I
O
- V
), then the output inductor will be:
= L3
OUT
D3(1-D)
OUT
)3
=12A and D=0.36, the I
Dt
Di
V
IN
; Dt = D3
3Di3f
V
OUT
OUT
S
---(9)
/V
IN.
1
f
S
; D =
---(11)
RMS
V
V
OUT
=5.7A
IN
www.irf.com
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 6TPC330M, 330mF, 6.3V has an ESR 40mV. Se-
lecting three 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.
Power MOSFET Selection
The IRU3138 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:
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.
DSS
ESR [
Where:
DV
Di = Inductor Ripple Current
DV
This results to: ESR=18.9mV
P
P
q = R
) exceeding the maximum input voltage (V
COND
COND
DS(ON)
O
O
= 50mV and DI
= Output Voltage Ripple
13.3mV which achieves our low ESR goal.
(Upper Switch) = I
(Lower Switch) = I
DS(ON)
temperature dependency should be consid-
DV
DI
DS(ON)
O
O
Temperature Dependency
and thermal management.
GS
---(10)
), maximum output current, On-
22% of 12A = 2.64A
LOAD
LOAD
2
2
3R
3R
DS(ON)
DS(ON)
3D3q
3(1 - D)3q
DSS
GS
IN
), gate-
01/29/04
Rev. 1.0
to pre-
).