iru3047 International Rectifier Corp., iru3047 Datasheet - Page 7

• Manufacturer Part Number: iru3047
• Description: Dual Synchronous Controller With Current Sharing Circuitry Controller
• Manufacturer: International Rectifier Corp.

For higher efficiency, a low ESR capacitor is recom-
mended.
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 6TPB470M 470mF, 6.3V has an ESR 40mV.
Selecting 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.
The resulting output ripple current is smaller then each
channel ripple current due to the 1808 phase shift. These
currents cancel each other. The cancellation is not the
maximum because of the different duty cycle for each
channel.
Inductor Selection
The inductor is selected based on output power, operat-
ing frequency and efficiency requirements. Low induc-
tor value causes large ripple current, resulting in the
smaller size, but poor efficiency and high output noise.
Generally, 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.
Rev. 1.0
09/09/02
For V
16TPB47M (16V, 47mF, 70mV, 1.4A)
For V
40mV, 3A).
ESR [
Where:
DV
DI
DV
O
O
O
= Output Current
=100mV and DI
= Output Voltage Ripple
IN2
I N 1
=5V, choose two 6TPB330M (6.3V, 330mF,
DV
=12V, choose two Poscap from Sanyo
DI
20mV which achieves our low ESR goal.
O
O
---(5)
O
=5A, results to ESR=20mV
www.irf.com
For the buck converter, the inductor value for desired
operating ripple current can be determined using the fol-
lowing relation:
The Coilcraft DO5022HC series provides a range of in-
ductors in different values and low profile for large cur-
rents.
Power MOSFET Selection
The selections criteria to meet power transfer require-
ments is based on maximum drain-source voltage (V
gate-source drive voltage (V
rent, On-resistance R
The MOSFET must have a maximum operating voltage
(V
The gate drive requirement is almost the same for both
MOSFETs. Caution should be taken with devices at very
low V
tary 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 total conduction loss is defined as:
DSS
V
L = (V
Where:
V
V
f
D = Duty Cycle
For Di
For Di
For L
For L
P
P
q = R
P
S
) exceeding the maximum input voltage (V
i = Inductor Ripple Current
t = Turn On Time
GS
CON(TOTAL)
IN
IN
OUT
COND
COND
= Switching Frequency
- V
= Maximum Input Voltage
to prevent undesired turn-on of the complemen-
= Output Voltage
3
4
(Upper Switch) = I
(Lower Switch) = I
DS(ON)
1
2
IN
OUT
choose DO5022P-602HC (6mH, 7.5A)
choose DO5022P-472HC (4.7mH, 8.4A)
=30% of I
=30% of I
- V
=P
= L3
OUT
CON
Temperature Dependency
(Upper Switch)q+P
)3
Dt
Di
1
2
, we get L
, we get: L
V
DS(ON)
IN
; Dt = D3
3Di3f
V
OUT
and thermal management.
2
2
LOAD
LOAD
GS
), maximum output cur-
3
S
3R
3R
=5.46mH
4
=3.47mH
1
f
DS(ON)
DS(ON)
S
CON
; D =
(Lower Switch)q
IRU3047
---(6)
3D3q
3(1 - D)3q
V
V
OUT
IN
IN
).
DSS
7
),