IRU3007
PDH = DMAX×Io2×RDS(MAX)
PDH = 0.76×102×0.029 = 2.21W
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.
RDS(MAX) = Maximum RDS(ON) of the MOSFET at 1258C
For diode, the maximum power dissipation happens at
minimum Vo and minimum duty cycle.
Where:
PD = Power Dissipation of the Linear Regulator
IL = Linear Regulator Load Current
DMIN » (3.3 + 0.5) / (5.25 - 0.27 + 0.5) = 0.69
PDD = (1 - DMIN)×Io×Vf
For the 1.5V and 2A load:
PD = (VIN - Vo)×IL
PDD = (1 - 0.69)×10×0.5 = 1.55W
Switcher Current Limit Protection
The IRU3007 uses the MOSFET RDS(ON) as the sensing
PD = (3.3 - 1.5)×2 = 3.6W
resistor to sense the MOSFET current and compares to Assuming TJ(MAX) = 1258C:
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.
Ts = TJ - PD×(θJC + θcs)
Ts = 125 - 3.6×(1.8 + 0.05) = 1188C
With the maximum heat sink temperature calculated in
the previous step, the heat-sink-to-air thermal resistance
For example, if the desired current limit point is set to (θSA) is calculated as follows:
be 22A for the synchronous and 16A for the non syn-
chronous, and from our previous selection, the maxi- Assuming TA = 358C:
mum MOSFET RDS(ON)=19mW, then the current sense
resistor Rcs is calculated as:
∆T = Ts - TA = 118 - 35 = 838C
Temperature Rise Above Ambient
θSA = ∆T / PD = 83 / 3.6 = 238C/W
Vcore
The same heat sink as the one selected for the switcher
MOSFETs is also suitable for the 1.5V regulator.
Vcs = ICL×RDS = 22×0.019 = 0.418V
Rcs = Vcs / IB = (0.418V) / (200µA) = 2.1KΩ
Where:
2.5V Clock Supply
IB=200µA is the internal current setting of the The IRU3007 provides a complete 2.5V regulator with a
IRU3007
minimum of 200mA current capability. The internal regu-
lator has short circuit protection with internal thermal
shutdown.
3.3V supply
Vcs = ICL×RDS = 16×0.019 = 0.3V
Rcs = Vcs / IB = (0.3V) / (200µA) = 1.50KΩ
1.5V and 2.5V Supply Resistor Divider Selection
Since the internal voltage reference for the linear regula-
1.5V, GTL+ Supply LDO Power MOSFET Selection tors is set at 1.26V for IRU3007, there is a need to use
The first step in selecting the power MOSFET for the external resistor dividers to step up the voltage. The re-
1.5V linear regulator is to select its maximum RDS(ON) of sistor dividers are selected using the following equations:
the pass transistor based on the input to output Dropout
Vo = (1 + Rt / RB)×VREF
voltage and the maximum load current.
Where:
Rt = Top resistor divider
RB = Bottom resistor divider
For Vo = 1.5V, VIN = 3.3V and IL = 2A:
RDS(MAX) = (VIN - Vo) / IL = (3.3 - 1.5) / 2 = 0.9Ω
Note: Since the MOSFETs RDS(ON) increases with tem-
perature, this number must be divided by » 1.5, in order
VREF = 1.26V typical
to find the RDS(ON) max at room temperature. The Motorola For 1.5V supply
MTP3055VL has a maximum of 0.18Ω RDS(ON) at room Assuming RB = 1KΩ:
temperature, which meets our requirement.
Rt = RB×[(Vo / VREF) - 1]
Rt = 1×[(1.5 / 1.26) - 1] = 191Ω
Rev. 2.1
08/20/02
www.irf.com
13
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