IRU3007
P
DH
= D
MAX
3Io
2
3R
DS(MAX)
P
DH
= 0.76310
2
30.029
= 2.21W
R
DS(MAX)
= Maximum R
DS(ON)
of the MOSFET at 1258C
For diode, the maximum power dissipation happens at
minimum Vo and minimum duty cycle.
D
MIN
≈
(3.3 + 0.5) / (5.25 - 0.27 + 0.5) = 0.69
P
DD
= (1 - D
MIN
)3Io3Vf
P
DD
= (1 - 0.69)31030.5 = 1.55W
Switcher Current Limit Protection
The IRU3007 uses the MOSFET R
DS(ON)
as the sensing
resistor to sense the MOSFET current and compares to
a programmed voltage which is set externally via a re-
sistor (Rcs) placed between the drain of the MOSFET
and the “CS+” terminal of the IC as shown in the appli-
cation circuit.
For example, if the desired current limit point is set to
be 22A for the synchronous and 16A for the non syn-
chronous, and from our previous selection, the maxi-
mum MOSFET R
DS(ON)
=19mW, then the current sense
resistor Rcs is calculated as:
Vcore
Vcs = I
CL
3R
DS
= 2230.019 = 0.418V
Rcs = Vcs / I
B
= (0.418V) / (200mA) = 2.1KV
Where:
I
B
=200mA is the internal current setting of the
IRU3007
3.3V supply
Vcs = I
CL
3R
DS
= 1630.019 = 0.3V
Rcs = Vcs / I
B
= (0.3V) / (200mA) = 1.50KV
1.5V, GTL+ Supply LDO Power MOSFET Selection
The first step in selecting the power MOSFET for the
1.5V linear regulator is to select its maximum R
DS(ON)
of
the pass transistor based on the input to output Dropout
voltage and the maximum load current.
For Vo = 1.5V, V
IN
= 3.3V and I
L
= 2A:
R
DS(MAX)
= (V
IN
- Vo) / I
L
= (3.3 - 1.5) / 2 = 0.9V
Note:
Since the MOSFETs R
DS(ON)
increases with tem-
perature, this number must be divided by
≈
1.5, in order
to find the R
DS(ON)
max at room temperature. The Motorola
MTP3055VL has a maximum of 0.18V R
DS(ON)
at room
temperature, which meets our requirement.
For the 1.5V and 2A load:
P
D
= (V
IN
- Vo)3I
L
P
D
= (3.3 - 1.5)32 = 3.6W
Assuming T
J(MAX)
= 1258C:
Ts = T
J
- P
D
3(u
JC
+
ucs)
Ts = 125 - 3.63(1.8 + 0.05) = 1188C
With the maximum heat sink temperature calculated in
the previous step, the heat-sink-to-air thermal resistance
(u
SA
) is calculated as follows:
Assuming T
A
= 358C:
DT
= Ts - T
A
= 118 - 35 = 838C
Temperature Rise Above Ambient
u
SA
=
DT
/ P
D
= 83 / 3.6 = 238C/W
The same heat sink as the one selected for the switcher
MOSFETs is also suitable for the 1.5V regulator.
2.5V Clock Supply
The IRU3007 provides a complete 2.5V regulator with a
minimum of 200mA current capability. The internal regu-
lator has short circuit protection with internal thermal
shutdown.
1.5V and 2.5V Supply Resistor Divider Selection
Since the internal voltage reference for the linear regula-
tors is set at 1.26V for IRU3007, there is a need to use
external resistor dividers to step up the voltage. The re-
sistor dividers are selected using the following equations:
Vo = (1 + Rt / R
B
)3V
REF
Where:
Rt = Top resistor divider
R
B
= Bottom resistor divider
V
REF
= 1.26V typical
For 1.5V supply
Assuming R
B
= 1KV:
Rt = R
B
3[(Vo
/ V
REF
) - 1]
Rt = 13[(1.5 / 1.26) - 1] = 191V
To select the heat sink 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.
Where:
P
D
= Power Dissipation of the Linear Regulator
I
L
= Linear Regulator Load Current
Rev. 2.1
08/20/02
www.irf.com
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