Datasheet

LM5642, LM5642X
www.ti.com SNVS219K –JUNE 2003–REVISED APRIL 2013
MOSFET Selection
BOTTOM FET SELECTION
During normal operation, the bottom FET is switching on and off at almost zero voltage. Therefore, only
conduction losses are present in the bottom FET. The most important parameter when selecting the bottom FET
is the on-resistance (R
DS-ON
). The lower the on-resistance, the lower the power loss. The bottom FET power loss
peaks at maximum input voltage and load current. The equation for the maximum allowed on-resistance at room
temperature for a given FET package, is:
where
• Tj_max is the maximum allowed junction temperature in the FET
• Ta_max is the maximum ambient temperature
• R
θja
is the junction-to-ambient thermal resistance of the FET
• TC is the temperature coefficient of the on-resistance which is typically in the range of 4000ppm/°C (25)
If the calculated R
DS-ON (MAX)
is smaller than the lowest value available, multiple FETs can be used in parallel.
This effectively reduces the I
max
term in the above equation, thus reducing R
DS-ON
. When using two FETs in
parallel, multiply the calculated R
DS-ON (MAX)
by 4 to obtain the R
DS-ON (MAX)
for each FET. In the case of three
FETs, multiply by 9.
(26)
If the selected FET has an Rds value higher than 35.3Ω, then two FETs with an R
DS-ON
less than 141 mΩ (4 x
35.3 mΩ) can be used in parallel. In this case, the temperature rise on each FET will not go to Tj_max because
each FET is now dissipating only half of the total power.
TOP FET SELECTION
The top FET has two types of losses: switching loss and conduction loss. The switching losses mainly consist of
crossover loss and losses related to the low-side FET body diode reverse recovery. Since it is rather difficult to
estimate the switching loss, a general starting point is to allot 60% of the top FET thermal capacity to switching
losses. The best way to precisely determine switching losses is through bench testing. The equation for
calculating the on resistance of the top FET is thus:
(27)
Example: Tj_max = 100°C, Ta_max = 60°C, Rqja = 60°C/W, Vin_min = 5.5V, Vnom = 5V, and Iload_max = 3.6A.
Copyright © 2003–2013, Texas Instruments Incorporated Submit Documentation Feedback 25
Product Folder Links: LM5642 LM5642X