iru3072 International Rectifier Corp., iru3072 Datasheet - Page 18

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

IRU3072
(5) Feedback resistor
LDO Regulator Component Selection
and LDO Power MOSFET Selection
The first step in selecting the power MOSFET for the
linear regulator is to select its maximum R
on the input to output dropout voltage and maximum
load current.
For V
Note that the MOSFET’s R
perature, the calculated R
R
get typical R
IRLR2703s from Internation Rectifier with D2 package,
30V, V
the requirement.
To select the heat sink for the LDO MOSFET, the first
step is to calculate the maximum power dissipation of
the device:
The junction temperature of MOSFET can be estimated
by the following formula:
18
DS(ON)
(Optional) an additional capacitor Cc2 can be
adopted, where:
The output of PWM is determined by:
Where V
R
feedback resistor.For 1.2V output, R
R
R
P
P
T
T
Where:
T
T
P
R
R
R
bient.
J
J
J
A
OUT2
T
DS(ON)MAX
DS(ON)MAX
D
D
D
uJC
uCS
uSA
= T
should be < T
DS
temperature coefficienct (about 1.5) in order to
= the estimated junction temperature.
= (V
= (3.3V-2.5V)32A = 1.4W
= the ambient temperature.
= the power disspation.
is the top feedback resistor and R
Cc2 1/(p3Rc13F
V
or
R
= the thermal resistance from junction to case.
= the thermal resistance from case to heat sink.
= the thermal resistance from heat sink to am-
logic drive and 65mV is good enough to meet
=2.5V, V
A
OUT
T
+P
IN(LDO)
= (V
DS(ON).
= V
REF
D
= (V
= (3.3V-2.5V)/2.0A = 0.4V
3(R
OUT
=0.8V
-V
REF
IN(LDO)
IN(LDO)
/V
OUT
uJC
3(R
J(MAX)
REF
+R
)3I
-V
-1)3R
=3.3V and I
T
uCS+
+R
@ 1508C
OUT2
OUT
DS(ON)
R
B
DS(ON)
)/R
S
)/I
uSA
B
)
has to be divided by the
OUT2
B
)
220pF
increases with tem-
OUT2
=2A:
T
=499V, R
B
DS(ON)
is bottom
based
B
=1K.
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The required thermal resistance of heat sink should be
In this example, the MOSFET is mounted in the copper
area more than 1 square inch. The estimated junction
temperaure is:
Where R
ambient with PCB mounted.
For IRLR2703s, R
The thermal managment can meet the requirement.
VccLDO Selection
For LDO, the LDO controller supply voltage has to sat-
isfy the following:
For this example, V
1V. Then:
LDO Feedback Resistor Selection
The output of LDO is determined by:
For 2.5V output, if R
LDO Soft-Start Capacitor
The soft-start capacitor can be estimated from equation
(1). Select start up time as 2ms:
R
T
T
T
V
Where:
V
V
old voltage
V
V
Select V
V
Where:
V
R
feedback resistor.
R
Select Rt=2.15K
C
Select C12=C
J
A
J
uSA
CC(LDO)
LDO(OUT)MAX
GS(TH)MIN
BE
CC(LDO)
OUT
REF
T
T
SS(LDO)
=T
= 35+1.5W3508C/W = 1108C < 1508C
= 358C
= (V
is the top feedback resistor and R
<(T
is the diode drop, approximately 0.6V
=0.8V
A
= V
uJA
+P
OUT
J
> 2.5V+1V+230.6V = 4.7V
= 20mA3t
> V
-T
is the thermal resistance from junction to
REF
D
CCLDO
3R
is the minimum LDO MOSFET gate thresh-
A
/V
)/P
3(R
LDO(OUT)MAX
is the maximum output voltage
REF
uJA
=12V for proper power sequence
uJA
D
SS(LDO)
-R
-1)3R
GS(TH)MIN
T
=508C/W, Assume:
B
+R
uJC
=1K then:
START
-R
B
=33nF
)/R
+V
uCS
B
= (2.5/0.8-1)31K = 2.12K
= 20mA32ms = 0.04mF
B
of MOSFET IRLR2703s, is
GS(TH)MIN
+2V
BE
B
is bottom
Rev. 1.0
3/25/04