ISL6334DIRZ datasheet(23/28 Pages) INTERSIL

23 / 28 page

FN6802.2

August 31, 2010

MOSFETs

The choice of MOSFETs depends on the current each

MOSFET will be required to conduct; the switching

frequency; the capability of the MOSFETs to dissipate heat;

and the availability and nature of heat sinking and air flow.

LOWER MOSFET POWER CALCULATION

The calculation for heat dissipated in the lower MOSFET is

simple, since virtually all of the heat loss in the lower

MOSFET is due to current conducted through the channel

resistance (rDS(ON)). In Equation 21, IM is the maximum

continuous output current; IP-P is the peak-to-peak inductor

current (see Equation 1); d is the duty cycle (VOUT/VIN); and

L is the per-channel inductance.

An additional term can be added to the lower-MOSFET loss

equation to account for additional loss accrued during the

dead time when inductor current is flowing through the

lower-MOSFET body diode. This term is dependent on the

diode forward voltage at IM, VD(ON); the switching

frequency, Fsw; and the length of dead times, td1 and td2, at

the beginning and the end of the lower-MOSFET conduction

interval respectively.

Thus the total maximum power dissipated in each lower

MOSFET is approximated by the summation of PLOW,1 and

PLOW,2.

UPPER MOSFET POWER CALCULATION

In addition to rDS(ON) losses, a large portion of the upper

MOSFET losses are due to currents conducted across the

input voltage (VIN) during switching. Since a substantially

higher portion of the upper MOSFET losses are dependent on

switching frequency, the power calculation is more complex.

Upper MOSFET losses can be divided into separate

components involving the upper MOSFET switching times;

the lower MOSFET body-diode reverse-recovery charge, Qrr;

and the upper MOSFET rDS(ON) conduction loss.

When the upper MOSFET turns off, the lower MOSFET does

not conduct any portion of the inductor current until the

voltage at the phase node falls below ground. Once the

lower MOSFET begins conducting, the current in the upper

MOSFET falls to zero as the current in the lower MOSFET

ramps up to assume the full inductor current. In Equation 23,

the required time for this commutation is t1 and the

approximated associated power loss is PUP,1.

At turn-on, the upper MOSFET begins to conduct and this

transition occurs over a time t2. In Equation 24, the

approximate power loss is PUP,2.

A third component involves the lower MOSFET’s reverse

recovery charge, Qrr. Since the inductor current has fully

commutated to the upper MOSFET before the lower

MOSFET’s body diode can draw all of Qrr, it is conducted

through the upper MOSFET across VIN. The power

dissipated as a result is PUP,3 and is approximated in

Equation 25:

Finally, the resistive part of the upper MOSFET’s is given in

Equation 26 as PUP,4.

The total power dissipated by the upper MOSFET at full load

can now be approximated as the summation of the results

from Equations 23, 24, and 25. Since the power equations

depend on MOSFET parameters, choosing the correct

MOSFETs can be an iterative process involving repetitive

solutions to the loss equations for different MOSFETs and

different switching frequencies, as shown in Equation 26.

Current Sensing Resistor

The resistors connected to the ISEN+ pins determine the

gain in the channel-current balance loop and set the

overcurrent trip point. Select values for these resistors by

using Equation 27:

where RISEN is the sense resistor connected to the ISEN+

pin, N is the active channel number, RX is the resistance of

the current sense element, either the DCR of the inductor or

RSENSE depending on the sensing method, and IOCP is the

desired overcurrent trip point. Typically, IOCP can be chosen

to be 1.2x the maximum load current of the specific

application.

With integrated temperature compensation, the sensed

current signal is independent on the operational temperature

of the power stage, i.e. the temperature effect on the current

sense element RX is cancelled by the integrated

temperature compensation function. RX in Equation 27

should be the resistance of the current sense element at the

room temperature.

When the integrated temperature compensation function is

disabled by pulling the TCOMP pin to GND, the sensed

current will be dependent on the operational temperature of

PLOW 1

rDS ON

()

------

⎝⎠

⎜⎟

⎛⎞ 2

–

()

ILP-P

()

2 1d

–

()

------------------------------------

(EQ. 21)

PLOW 2

VDON

() Fsw

------

IP-P

----------

⎝⎠

⎛⎞ t

------

IP-P

----------

–

⎝⎠

⎜⎟

⎛⎞

td2

(EQ. 22)

PUP 1, VIN

------

IP-P

----------

⎝⎠

⎛⎞ t1

----

⎝⎠

⎜⎟

⎛⎞

≈

(EQ. 23)

PUP 2

VIN

------

IP-P

----------

–

⎝⎠

⎜⎟

⎛⎞ t2

----

⎝⎠

⎜⎟

⎛⎞

≈

(EQ. 24)

PUP 3,

VIN Qrr fS

(EQ. 25)

PUP 4, rDS ON

()

------

⎝⎠

⎜⎟

⎛⎞ 2

IP-P

----------d

≈

(EQ. 26)

RISEN

105 10 6

–

---------------------------

IOCP

--------------

(EQ. 27)

ISL6334D

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