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參數資料
| 型號： | IRU3005CW |
| 文件頁數： | 12/14頁 |
| 文件大小： | 83K |
| 代理商： | IRU3005CW |
4-14
Rev. 1.2
12/8/00
IRU3004, IRU3005
Following the same procedure for the Schottky diode
results in a heatsink with
θ
sa = 25
°
C/W. Although it is
possible to select a slightly smaller heatsink, for sim-
plicity the same heatsink as the one for the high side
MOSFET is also selected for the synchronous MOSFET.
Switcher Current Limit Protection
The PWM controller uses the MOSFET RDS(on) as the
sensing resistor to sense the MOSFET current and com-
pares to a programmed voltage which is set externally
via a resistor (Rcs) placed between the drain of the
MOSFET and the “CS+” terminal of the IC as shown in
the application circuit. For example, if the desired cur-
rent limit point is set to be 22A and from our previous
selection, the maximum MOSFET RDS(on)=19m
, then
the current sense resistor, Rcs is calculated as:
Vcs=IcL*Rds=22*0.019=0.418V
Rcs=Vcs/Ib=(0.418V)/(200uA)=2.1k
Where: Ib=200
μ
A is the internal current setting of the
device
Switcher Timing Capacitor Selection
The switching frequency can be programmed using an
external timing capacitor. The value of Ct can be ap-
proximated using the equation below:
LDO Power MOSFET Selection
The first step in selecting the power MOSFET for the
linear regulators is to select its maximum RDS(on) based
on the input to output Dropout voltage and the maximum
load current.
RDS(max) = (Vin - Vo) / IL
For Vo = 1.5V, and Vin = 3.3V , IL=2A
RDS(max) = (3.3 - 1.5)/2= 0.9
Note that since the MOSFETs RDS(on) increases with
temperature, this number must be divided by
≈
1.5, in
order to find the RDS(on) max at room temperature. The
Motorola MTP3055VL has a maximum of 0.18
RDS(on)
at room temperature, which meets our requirement.
To select the heatsink for the LDO MOSFET the first
step is to calculate the maximum power dissipation of
the device and then follow the same procedure as for the
switcher.
Pd = ( Vin - Vo ) * IL
Where :
Pd = Power Dissipation of the Linear Regulator
IL = Linear Regulator Load Current
For the 1.5V and 2A load:
Pd = (3.3 - 1.5)*2=3.6 W
Assuming Tj-max=125
°
C
Ts = Tj - Pd * (
θ
jc +
θ
cs)
Ts = 125 - 3.6 * (1.8 + 0.05) = 118
°
C
With the maximum heat sink temperature calculated in
the previous step, the heat-sink-to-air thermal resistance
(
θ
sa) is calculated as follows:
Assuming Ta=35
°
C
T = Ts - Ta = 118 - 35 = 83
°
C Temperature Rise
Above Ambient
θ
sa =
T/Pd
θ
sa = 83 / 3.6 = 23
°
C/W
The same heat sink as the one selected for the switcher
MOSFETs is also suitable for the 1.5V regulator. It is
also possible to use TO-263 package or even the
MTD3055VL in D-Pak if the load current is less than
1.5A. For the 2.5V regulator since the dropout voltage is
only 0.8V and the load current is less than 0.5A, for
most applications the same MOSFET without heat sink
or for low cost applications, one can use PN2222A in
TO-92 or SOT-23 package.
LDO Regulator Component Selection
Since the internal voltage reference for the linear regula-
tors is set at 1.5V for all devices, there is no need to
divide the output voltage for the 1.5V, GTL+ regulator.
F
C
Where
Where:
Cr = Timing Capacitor
F
SW
= Switching Frequency
SW
T
≈
35 10
.
5
C
pF
T
35 10
200 10
175
5
3
≈
=
.
If, F
SW
= 200kHz:
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