iru3072 International Rectifier Corp., iru3072 Datasheet - Page 17

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

Estimated Temperature Rise for MOSFET
The estimated junction temperature of the MOSFET is
given by:
For MOSFET IRF7460 with SOIC 8-pin package,
R
For high side MOSFET, the junction temperature is given
as:
For low side MOSFET IRF7460, the maximum junction
temperature can be calculated as:
The maximum junction temperature of both MOSFETs
is below the maximum rating of 1508C.
Controller Parameter Calculation
(1) Frequency Selection
(2) Soft-Start Capacitor
(3) Over Current Limit Setting
Rev. 1.0
3/25/04
uJA
=508C/W. Assume ambient temperature is T
T
Where:
T
T
P
R
with MOSFET on 1" square PCB board and is from
the data sheet.
T
T
From Figure 23, the frequency setting resistor can
be chosen to be Rt=47KV, which gives us approxi-
mately 400KHz frequency.
Soft-start capacitor for PWM secton is selected from
equation (1). Select start up time t
Select C11=C
The over current limit resistor can be calculated based
on Figure 17. The output current limit is set by:
The current ripple during nomral operation (400KHz)
is given by:
The over current setting I
The over current setting resistor:
J
J
A
J
J
D
uJA
= T
= T
= T
is the junction temperature.
is the ambient temperature.
is the power dissipation.
C
I
DI
DI
I
R
is the junction-to-ambient thermal resistance
O(LIM)
SET
A
A
A
SET
SS
+P
PK_PK
PK_PK
+P
+P
= 20mA3t
= I
= I
D
D
D
= 10A
3R
3R
3R
O(LIM)
= (12-1.2)31.2/(1mH3400KHz312)
SET
2.7A
SS
uJA
uJA
uJA
3R
-DI
=0.1mF
= 35+2350 = 1358C < 1508C
= 35+0.81350 = 768C < 1508C
PK_PK
DS(ON)
START
/2 = 10A-2.7A/2 8.7A
/20mA
= 20mA35ms = 0.1mF
SET
is:
START
=5ms:
A
=358C.
www.irf.com
(4) Compensation Design
http://www.irf.com/technical-info/appnotes.htm
Rc1=2p3403131.2531.2/(133123100030.8)
For low side MOSFET IRF7460, with 4.5V gate volt-
age and maximum R
Select R7=R
For electrolytic capacitor, the frequency caused by
ESR is typically at a few KHz range. A type II com-
pensator is a good option. The detailed description
is shown in application note AN-1043 from:
Select the zero crossover frequency to be 1/10 of
switching frequency that is 40KHz:
The compensation resistor can be calculated as:
Where V
and gm is the transconductance of the error ampli-
fier. From the datasheet we get V
g
tor is:
The compensator capacitor is given as:
m
=1000mmho. The calculated compensation resis-
Cc1
R
F
Rc1=
Rc1=2.98K
Select R8=Rc1=3.3K
Cc1 =
Cc1 =
Select C9=Cc1=15nF
O
SET
= 40KHz
Figure 25 - Type II compensator.
Rf1
Rf2
= 8.7A314mV/20mA = 6.09KV
OSC
V
2p3F
OUT
(ESR3V
SET
(Optional)
(L3C
is the oscillator peak to peak voltage
(1mH3450mF)/0.75/3.3K = 10nF
Rc1
Cc2
=6.8KV
O
3L3V
OUT
IN
DS(ON)
3g
)/0.75/Rc1
Comp
V
1V
IRU3072
OSC
FB
m
of 14mV, then:
3V
3V
REF
Error Amp
g
OUT
m
)
IRU3072
OSC
=1.25V and
17