iru3072 International Rectifier Corp., iru3072 Datasheet - Page 16

• Manufacturer Part Number: iru3072
• Description: 20-pin Synchronous Pwm Controller/ 3 Ldo Controller -
• Manufacturer: International Rectifier Corp.

IRU3072
Output capacitor selection
The voltage rating of the output capacitor is the same as
the output voltage. Typical available capacitors on the
market are electrolytic, tantalum and ceramic. If electro-
lytic or tantalum capacitors are employed, the criteria is
normally based on the value of Effective Series Resis-
tance (ESR) of total output capacitor. In most cases,
the ESR of the output capacitor is calculated based on
the following relationship:
Depending on which one is the requirement.
In this example:
The required ESR is calculated as:
Select three Sanyo POSCAP 6TPB330M with 6.3V
330mF and 40mV ESR will give about 13mV, which will
meet the specification.
Input capacitor Selection
Input capacitor is dertermined by the voltage rating and
input RMS current. For this application, the input RMS
current is given as:
The input RMS current is estimated as:
Select two Sanyo POSCAP -16TPB47M with 16V, 47mF
and 1.4A ripple current. A 1mH, 1A small input inductor
is enough for the input filer.
Power MOSFET Selection
In general, the MOSFET selection criteria depends on
the maximum drain-source voltage, RMS current and
ON resistance (R
MOSFETs, a drain-source voltage rating higher than
maximum input voltage is necessary. In the demo-board,
20V rating should be satisfied. The gate drive require-
16
ESR < DV
or
ESR < DV
ESR < 150mV/8A = 18.75mV
I
D = V
I
Where:
DV
DI
DV
droop during the transient or step load.
DI
DV
DI
IN(RMS)
IN(RMS)
PK_PK
STEPLOAD(MAX)
STEPLOAD(MAX)
RIPPLE(SPEC)
STEPLOAD(SPEC)
STEPLOAD(SPEC)
OUT
= I
= 8A3
is the current ripple.
/V
OUT
STEPLOAD(SPEC)
RIPPLE(SPEC)
IN
3
= 1.2V/12V 0.1
DS(ON)
is the maximum allowed voltage ripple.
= 8A
is the maximum step load current.
= 150mV
0.13(1-0.1)
is the maximum allowed voltage
D3(1-D)
). For both high side and low side
/DI
/DI
PK_PK
STEPLOAD(MAX)
2.4A
www.irf.com
ment for each MOSFET is almost the same. If logic-
level or 3V driver MOSFET is used, some caution should
be taken with devices at very low V
ired turn-on of the complementary MOSFET, which re-
sults a shoot-through circuit.
If output inductor current ripple is neglected, the RMS
current of high side switch is given by:
The RMS current of low side switch is given as:
For low side MOSFET, if it is driven by 5V, a logic gate
driver MOSFET is preferred. For R
it should be as small as possible in order to get highest
efficiency. A logic driver MOSFET such as IRF7460 from
International Rectifier in a SOIC 8-pin package,
R
I
Power Dissipation for MOSFETs
The power dissipation for MOSFETS typically includes
conduction loss and switching losses. For high side
switch, the conduction loss is estimated as:
The R
datasheet of IRF7460:
The switching loss is more difficult to calculate because
of the parasitic parameters. In general, the switching
loss can be estimated by the following:
tr is the rising time and tf is the falling time. From IRU3072
datasheet: tr=50ns and tf=50ns
The total disspation for the high side switch is:
For low side switch, most of the loss are conduction
loss. The low side switch power dissipation is:
DS
DS(ON)
is selected for high side and low side MOSFET.
D = V
I
I
P
R
P
P
P
P
P
P
P
P
RMS(HI)
RMS(HI)
COND(HI)
DS(ON)MAX
COND(HI)
SW
SW(HI)
SW(HI)
D(HI)
D(LO)
D(LO)
D(LO)
=10mV, 20V drain source voltage rating and 12A
DS(ON)
= 0.53V
= P
OUT
=
=
= 0.5312V38A3(50ns+50ns)3400KHz
P
P
P
= D3I
= 0.138A38A314mV
1.92W
has to consider the worst case. In the
/V
COND(LO)
SW(HI)
COND(LO)
COND(LO)
= 14mV @ Vgs = 4.5V
1-D3I
IN
D3I
= 0.1
DS
+P
OUT
3I
OUT
= (1-D)3I
= (1-0.1)38A38A314mV
COND(HI)
= 0.81W
3I
OUT
OUT
=
OUT
3(tr+tf)3F
=
3R
0.138A = 2.53A
1-0.138A = 7.6A
2W
OUT
DS(ON)MAX
3I
DS(ON)
GS
OUT
S
to prevent undes-
0.09W
3R
of the MOSFET,
DS(ON)MAX
Rev. 1.0
3/25/04