FAN5231 Datasheet, PDF (14/17 Pages) Fairchild Semiconductor – Precision Dual PWM Controller And Linear Regulator for Notebook CPUs

FAN5231
MOSFET Selection and Considerations
Requirements for upper and lower MOSFETs are different
in mobile applications. The reason for this is the 10:1 differ-
ence in conduction time of the lower and the upper MOS-
FETs driven by a difference between the input voltage which
is nominally in the range from 8V to 20V and output voltage
which is about 1.5V.
Requirements for the lower MOSFET are simpler than those
for the upper one. The lower the Rdson of this device the
lower the conduction losses, and the higher converter’s
efficiency. Switching losses and gate drive losses are not
significant because of zero-voltage switching conditions
inherent for this device in the buck converter. Important is
low reverse recovery charge of the body diode which causes
shoot-trough current spikes when the upper MOSFET turns
on. Also, important is to verify that the lower MOSFET gate
voltage does not reach threshold when high dV/dt transition
occurs on the phase node. Specially for that reason,
FAN5231 is equipped with a low, 1.0Ω typical, pull-down
resistance of low side driver.
Requirements for the upper MOSFET Rdson are less
stringent than for the lower MOSFET because its conduction
time is significantly shorter while switching losses can
dominate especially at higher input voltages. It is recom-
mended to have equal conduction and switching losses in the
upper MOSFET at the nominal input voltage and load
current. In this case maximum converter efficiency is tuned
to the operation point that it is most desired.
Precise calculation of power dissipation in the MOSFETs is
very complex because many parameters affecting turn-on
and turn-off times such as gate reverse transfer charge, gate
internal resistance, body diode reverse recovery charge,
package and layout impedances and their variation with the
operation conditions are not available to a designer. Follow-
ing equations are provided only for crude estimation of the
power losses and should be accompanied by a detail bread-
board evaluation. Attention should be paid to the input
voltage extremes where power dissipation in the MOSFETs
is usually higher.
Pupper
=
-^I--^o---²----^×-----^R----^d--_V--^s--_io--_n--ⁿ----^×-----^V----^o---^u----^t
+
-^I--^o----^×-----^V----ⁱ--ⁿ-----^×-----^F----^s---^×-----⁽---^t--^o---ⁿ-----+----^t--^o----^f--^f--⁾
2
Plower = Io² × Rdson × _1 – ^V--_V---^o-_i--^un---^t_
14
REV. 1.1.1 8/15/01