參數(shù)資料
型號： IRU3072C
廠商： International Rectifier
元件分類： 基準(zhǔn)電壓源/電流源
英文描述： 20-PIN SYNCHRONOUS PWM CONTROLLER/ 3 LDO CONTROLLER
中文描述： 20引腳同步PWM控制器/ 3 LDO控制器
文件頁數(shù)： 18/24頁
文件大?。?/td> 332K
代理商： IRU3072C
18
Rev. 1.0
3/25/04
IRU3072
www.irf.com
T
J
= T
A
+P
D
×
(R
θ
JC
+R
θ
CS+
R
θ
SA
)
T
J
should be < T
J(MAX)
@ 150
8
C
Where:
T
J
= the estimated junction temperature.
T
A
= the ambient temperature.
P
D
= the power disspation.
R
θ
JC
= the thermal resistance from junction to case.
R
θ
CS
= the thermal resistance from case to heat sink.
R
θ
SA
= the thermal resistance from heat sink to am-
bient.
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
θ
JA
is the thermal resistance from junction to
ambient with PCB mounted.
For IRLR2703s, R
θ
JA
=50
8
C/W, Assume:
T
A
= 35
8
C
T
J
= 35+1.5W
×
50
8
C/W = 110
8
C < 150
8
C
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
GS(TH)MIN
of MOSFET IRLR2703s, is
1V. Then:
V
CC(LDO)
> 2.5V+1V+2
×
0.6V = 4.7V
Select V
CCLDO
=12V for proper power sequence
LDO Feedback Resistor Selection
The output of LDO is determined by:
V
OUT
= V
REF
×
(R
T
+R
B
)/R
B
For 2.5V output, if R
B
=1K then:
Select Rt=2.15K
LDO Soft-Start Capacitor
The soft-start capacitor can be estimated from equation
(1). Select start up time as 2ms:
(Optional) an additional capacitor Cc2 can be
adopted, where:
Cc2
1/(
π×
Rc1
×
F
S
)
220pF
(5) Feedback resistor
The output of PWM is determined by:
V
OUT
= V
REF
×
(R
T
+R
B
)/R
B
or
R
T
= (V
OUT
/V
REF
-1)
×
R
B
Where V
REF
=0.8V
R
T
is the top feedback resistor and R
B
is bottom
feedback resistor.For 1.2V output, R
T
=499
, R
B
=1K.
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
DS(ON)
based
on the input to output dropout voltage and maximum
load current.
For V
OUT2
=2.5V, V
IN(LDO)
=3.3V and I
OUT2
=2A:
R
DS(ON)MAX
= (V
IN(LDO)
-V
OUT2
)/I
OUT2
R
DS(ON)MAX
= (3.3V-2.5V)/2.0A = 0.4
Note that the MOSFET’s R
DS(ON)
increases with tem-
perature, the calculated R
DS(ON)
has to be divided by the
R
DS(ON)
temperature coefficienct (about 1.5) in order to
get typical R
DS(ON).
IRLR2703s from Internation Rectifier with D2 package,
30V, V
DS
logic drive and 65m
is good enough to meet
the requirement.
To select the heat sink for the LDO MOSFET, the first
step is to calculate the maximum power dissipation of
the device:
P
D
= (V
IN(LDO)
-V
OUT
)
×
I
OUT
P
D
= (3.3V-2.5V)
×
2A = 1.4W
The junction temperature of MOSFET can be estimated
by the following formula:
R
θ
SA
<(T
J
-T
A
)/P
D
-R
θ
JC
-R
θ
CS
T
J
=T
A
+P
D
×
R
θ
JA
V
CC(LDO)
> V
LDO(OUT)MAX
+V
GS(TH)MIN
+2V
BE
Where:
V
LDO(OUT)MAX
is the maximum output voltage
V
GS(TH)MIN
is the minimum LDO MOSFET gate thresh-
old voltage
V
BE
is the diode drop, approximately 0.6V
Where:
V
REF
=0.8V
R
T
is the top feedback resistor and R
B
is bottom
feedback resistor.
R
T
= (V
OUT
/V
REF
-1)
×
R
B
= (2.5/0.8-1)
×
1K = 2.12K
C
SS(LDO)
= 20
μ
A
×
t
START
= 20
μ
A
×
2ms = 0.04
μ
F
Select C12=C
SS(LDO)
=33nF
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