iru3004 International Rectifier Corp., iru3004 Datasheet - Page 12

• Manufacturer Part Number: iru3004
• Description: 5-bit Programmable Synchronous Buck Controller Ic With Dual Ldo Controller
• Manufacturer: International Rectifier Corp.

IRU3004
With the maximum heat sink temperature calculated in
the previous step, the heat-sink-to-air thermal resistance
(u
Assuming T
Next, a heat sink with lower u
in the previous step must be selected. One way is to
simply look at the graphs of the “Heat Sink Temp Rise
Above the Ambient” vs. the “Power Dissipation” given in
the heat sink manufacturers’ catalog and select a heat
sink that results in lower temperature rise than the one
calculated in previous step. The following heat sinks from
AAVID and Thermalloy meet this criteria.
Company
Thermalloy............................6078B
AAVID..................................577002
Following the same procedure for the Schottky diode
results in a heat sink with u
possible to select a slightly smaller heat sink, for sim-
plicity, the same heat sink 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 R
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 R
the current sense resistor, Rcs is calculated as:
12
SA
) is calculated as follows:
DT = Ts - T
Temperature Rise Above Ambient
u
Where:
I
vice
Vcs = I
Rcs =
B
SA
= 200mA is the internal current setting of the de-
=
DT
P
Vcs
D
CL
A
I
B
=
= 358C:
3 R
3.82
A
=
83
= 118 - 35 = 838C
DS
0.418V
200mA
= 228C/W
= 22 3 0.019 = 0.418V
= 2.1KV
SA
SA
Part #
=258C/W. Although it is
than the one calculated
DS(ON)
=19mV, then
DS(ON)
as the
www.irf.com
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:
If Fsw = 200KHz:
The first step in selecting the power MOSFET for the
linear regulators is to select its maximum R
on the input to output Dropout voltage and the maximum
load current.
For Vo = 1.5V, V
Note that since the MOSFETs R
temperature, this number must be divided by
order to find the R
Motorola MTP3055VL has a maximum of 0.18V R
at room temperature, which meets our requirement.
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.
For the 1.5V and 2A load:
Assuming T
LDO Power MOSFET Selection
Fsw y y
Where:
Ct = Timing Capacitor
Fsw = Switching Frequency
Ct y y
R
P
Where:
P
I
P
Ts = T
Ts = 125 - 3.6 3 (1.8 + 0.05) = 118 C
L
DS(max)
D
D
D
= Linear Regulator Load Current
= (3.3 - 1.5) 3 2 = 3.6W
= (V
= Power Dissipation of the Linear Regulator
3.5 3 10
200 3 10
J
=
3.5 3 10
IN
- P
J(max)
- Vo) 3 I
(V
D
Ct
IN
3 (u
= 1258C then:
IN
I
- Vo)
L
DS(on)
= 3.3V and I
-5
3
-5
JC
= 175pF
L
+ u
max at room temperature. The
=
(3.3 - 1.5)
CS
)
2
L
= 2A:
DS(ON)
= 0.9V
increases with
DS(ON)
07/16/02
Rev. 1.7
1.5, in
based
DS(ON)