IRLR024 datasheet(13/17 Pages) IRF

13 / 17 page

IRU3007

Rev. 2.1

08/20/02

www.irf.com

DMIN

≈ (3.3 + 0.5) / (5.25 - 0.27 + 0.5) = 0.69

PDD = (1 - DMIN)

3Io3Vf

PDD = (1 - 0.69)

31030.5 = 1.55W

For Vo = 1.5V, VIN = 3.3V and IL = 2A:

RDS(MAX) = (VIN - Vo) / IL = (3.3 - 1.5) / 2 = 0.9

Vcs = ICL

3RDS = 2230.019 = 0.418V

Rcs = Vcs / IB = (0.418V) / (200

mA) = 2.1KV

PDH = DMAX

3Io23RDS(MAX)

PDH = 0.76

310230.029 = 2.21W

RDS(MAX) = Maximum RDS(ON) of the MOSFET at 125

For diode, the maximum power dissipation happens at

minimum Vo and minimum duty cycle.

Switcher Current Limit Protection

The IRU3007 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 “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 RDS(ON)=19mW, then the current sense

resistor Rcs is calculated as:

Vcore

Where:

IB=200

mA is the internal current setting of the

IRU3007

3.3V 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: Since the MOSFETs RDS(ON) increases with tem-

perature, 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

V RDS(ON) 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.

Where:

PD = Power Dissipation of the Linear Regulator

IL = Linear Regulator Load Current

For the 1.5V and 2A load:

Assuming TJ(MAX) = 125

8C:

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

8C:

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:

Where:

Rt = Top resistor divider

RB = Bottom resistor divider

VREF = 1.26V typical

For 1.5V supply

Assuming RB = 1K

Vcs = ICL

3RDS = 1630.019 = 0.3V

Rcs = Vcs / IB = (0.3V) / (200

mA) = 1.50KV

Rt = RB

[(Vo / VREF) - 1]

Rt = 1

[(1.5 / 1.26) - 1] = 191V

Vo = (1 + Rt / RB)

3VREF

DT = Ts - TA = 118 - 35 = 838C

Temperature Rise Above Ambient

uSA = DT / PD = 83 / 3.6 = 238C/W

PD = (VIN - Vo)

3IL

PD = (3.3 - 1.5)

32 = 3.6W

Ts = TJ - PD

3(uJC + ucs)

Ts = 125 - 3.6

3(1.8 + 0.05) = 1188C

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