iru3046 International Rectifier Corp., iru3046 Datasheet - Page 7

• Manufacturer Part Number: iru3046
• Description: Dual Synchronous Pwm Controller With Current Sharing Circuitry And Ldo 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 6TPC150M 150mF, 6.3V has an ESR 40mV. Se-
lecting six 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. 1.9
09/27/02
For V
6TPB330M (6.3V, 330mF, 40mV, 3A)
For V
70mV, 1.4A).
ESR [
Where:
DV
DI
DV
O
O
O
= Output Current
=75mV and DI
= Output Voltage Ripple
IN2
7mV which achieves our low ESR goal.
I N 1
=12V, choose two 16TPB47M (16V, 47mF,
DV
=5V, choose two Poscap from Sanyo
DI
O
O
O
=10A, result to ESR=7.5mV
---(5)
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:
P
P
q = R
The total conduction loss is defined as:
P
CON(TOTAL)
COND
COND
DSS
V
L = (V
Where:
V
V
f
D = Duty Cycle
For Di
For Di
For L
For L
S
) exceeding the maximum input voltage (V
i = Inductor Ripple Current
t = Turn On Time
GS
IN
IN
OUT
DS(ON)
= Switching Frequency
(Upper Switch) = I
(Lower Switch) = I
- V
= Maximum Input Voltage
to prevent undesired turn-on of the complemen-
=P
= Output Voltage
1
2
1
2
IN
OUT
choose: DO5022P-222HC (2.2mH,12A)
choose: DO5022P-332HC (3.3mH,10A)
=30% of I
=30% of I
Temperature Dependency
CON
- V
= L3
(Upper Switch)q + P
OUT
)3
Dt
Di
1
2
, we get: L
, we get: L
V
DS(ON)
IN
; Dt = D3
3Di3f
LOAD
LOAD
2
2
V
OUT
and thermal management.
GS
3 R
3 R
), maximum output cur-
S
1
2
=2.18mH
=2.7mH
DS(ON)
DS(ON)
1
f
CON
S
(Lower Switch)q
; D =
IRU3046
3 D 3 q
3 (1 - D) 3 q
---(6)
V
V
OUT
IN
IN
).
DSS
7
),