IRU3018
13
Rev. 1.6
07/16/02
www.irf.com
Vcs = ICL3RDS = 2230.019 = 0.418V
Rcs = Vcs / IB = (0.418V) / (200mA) = 2.1KV
RDS(MAX) = (VIN - Vo) / IL
For Vo = 1.5V, VIN = 3.3V and, IL = 2A:
RDS(MAX) = (3.3 - 1.5) / 2 = 0.9V
uSA = DT / PD = 83 / 3.6 = 238C/W
Next, a heat sink with lower uSA than the one calculated
in the previous step must be selected. One way to do
this is to 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.
Co. Part #
Thermalloy............................6078B
AAVID..................................577002
Following the same procedure for the Schottky diode
results in a heat sink with uSA=258C/W. Although it is
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 IRU3018 uses the MOSFET RDS(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 “OCSet1” terminal of the IC as shown in the
application circuit. For example, if the desired current
limit point is set to be 22A, for the synchronous and 16A
for the non-synchronous, and from our previous selec-
tion, the maximum MOSFET RDS(ON)=19mV, then the
current sense resistor Rcs is calculated as:
Where:
IB = 200mA is the internal current setting of the
IRU3018
Switcher Frequency Selection
The IRU3018 frequency is internally set at 200KHz with
no external timing resistor. However, it can be adjusted
up by using an external resistor from Rt pin to Gnd or
can be adjusted down if the resistor is connected to the
12V supply.
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 RDS(ON) of
the pass transistor based on the input to output Dropout
voltage and the maximum load current.
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 0.18V RDS(ON)
at room temperature, which meets our requirement.
To select the heat sink for the LDO MOSFET, first cal-
culate the maximum power dissipation of the device
and then follow the same procedure as for the switcher.
Where:
PD = Power Dissipation of the Linear Regulator
IL = Linear Regulator Load Current
For the 1.5V and 2A load:
Assuming TJ(MAX) = 1258C:
With the maximum heat sink temperature calculated in
the previous step, the heat-sink-to-air thermal resistance
(uSA) is calculated as follows:
Assuming TA = 35°C:
∆T = Ts - Ta = 118 - 35 = 83 °C
Temperature Rise Above Ambient
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 IRU3018 provides an internal ultra low dropout regu-
lator with a minimum of 200mA current capability that
converts 3.3V supply to a programmable regulated 2.5V
supply to power the clock chip. The internal regulator
has short circuit protection with internal thermal shut-
down.
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 IRU3018, there is a need to use
external resistor dividers to step up the voltage. The re-
sistor dividers are selected using the following equations:
Where:
Rt = Top resistor divider
RB = Bottom resistor divider
VREF = 1.26V typical
PD = (VIN - Vo)3IL
PD = (3.3 - 1.5)32 = 3.6W
Ts = TJ - PD3(uJC + ucs)
Ts = 125 - 3.63(1.8 + 0.05) = 1188C
Vo = (1 + Rt/RB)3VREF
