iru3072 International Rectifier Corp., iru3072 Datasheet - Page 15

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

Switching frequency
The switching frequency of IRU3072 can be selected by
the following figure.
Rev. 1.0
3/25/04
Figure 23 - Soft-start with output is short to ground.
Figure 24 - Switching frequency versus resistor Rt.
500
450
400
350
300
250
200
150
100
Figure 22 - Operation of PWM output and LDO
50
0
0
when LDO V
50
Rt (K 8 8 )
OUT2
100
is short.
150
www.irf.com
200
Design Example
Input voltage for buck converter: V
Output voltage for buck converter: V
Nominal output current from switching regulator: I
Output current limit is 10A.
Switching frequency: F
The maximum dynamic output voltage droop at 8A step
load is 150mV.
LDO specification
Output inductor selection
The inductor is selected based on the inductor current
ripple, operation frequency and efficiency consideration.
In general, a large inductor results in a small output ripple
and higher efficiency but large size. A small value induc-
tor causes large current ripple and poor efficiency but
small size. Generally, the inductor is selected based on
the output current ripple. The optimum point is usually
found between 20% and 50% ripple of output inductor
current.
Suppose the ripple is selected as 40% of the total out-
put current.
The current ripple is calculated as:
Combining of above two equations, the inductance can
be selected by:
In this example,
Select inductor from Panasonic so that L=1mH. The ripple
current is calculated as:
DI
DI
L > V
L > 1.2V3(12 - 1.2)/(400KHz312V30.438A)
L > 0.8mH
DI
DI
LDO input voltage: V
LDO output1: V
LDO output2: V
LDO output3: V
PK_PK
PK_PK
PK_PK
PK_PK
OUT
/I
= (V
= (12 - 1.2)31.2/(1mH3400KHz312)
OUT
3(V
2.7A
IN
= 40%
-V
IN
-V
OUT
OUT2
OUT3
OUT4
OUT
)3V
S
)/(F
=2.5V @ 2A
=1.8V @ 2A
=1.5V @ 2A
=400KHz
IN(LDO)
OUT
S
3V
/(L3F
=3.3V
IN
340%3I
IN
=12V
S
OUT
3V
IRU3072
=1.2V
IN
)
OUT
)
OUT
15
=8A