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LM5050-1
,
LM5050-1-Q1
www.ti.com
SNVS629F –MAY 2011–REVISED DECEMBER 2019
Product Folder Links: LM5050-1 LM5050-1-Q1
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Application Information (continued)
8.1.1 MOSFET Selection
The important MOSFET electrical parameters are the maximum continuous Drain current ID, the maximum
Source current (that is, body diode) IS, the maximum drain-to-source voltage VDS(MAX), the gate-to-source
threshold voltage VGS(TH), the drain-to-source reverse breakdown voltage V(BR)DSS, and the drain-to-source On
resistance RDS(ON).
The maximum continuous drain current, ID, rating must exceed the maximum continuous load current. The rating
for the maximum current through the body diode, IS, is typically rated the same as, or slightly higher than the
drain current, but body diode current only flows while the MOSFET gate is being charged to VGS(TH).
Gate Charge Time = Qg/ IGATE(ON)
1. The maximum drain-to-source voltage, VDS(MAX), must be high enough to withstand the highest differential
voltage seen in the application. This would include any anticipated fault conditions.
2. The drain-to-source reverse breakdown voltage, V(BR)DSS, may provide some transient protection to the OUT
pin in low voltage applications by allowing conduction back to the IN pin during positive transients at the OUT
pin.
3. The gate-to-source threshold voltage, VGS(TH), should be compatible with the LM5050-1 gate drive
capabilities. Logic level MOSFETs, with RDS(ON) rated at VGS(TH) at 5 V, are recommended, but sub-Logic
level MOSFETs having RDS(ON) rated at VGS(TH) at 2.5 V, can also be used.
4. The dominate MOSFET loss for the LM5050-1 active OR-ing controller is conduction loss due to source-to-
drain current to the output load, and the RDS(ON) of the MOSFET. This conduction loss could be reduced by
using a MOSFET with the lowest possible RDS(ON). However, contrary to popular belief, arbitrarily selecting a
MOSFET based solely on having low RDS(ON) may not always give desirable results for several reasons:
1. Reverse transition detection. Higher RDS(ON) will provide increased voltage information to the LM5050-1
Reverse Comparator at a lower reverse current level. This will give an earlier MOSFET turnoff condition
should the input voltage become shorted to ground. This will minimize any disturbance of the redundant
bus.
2. Reverse current leakage. In cases where multiple input supplies are closely matched it may be possible
for some small current to flow continuously through the MOSFET drain to source (that is, reverse)
without activating the LM5050-1 Reverse Comparator. Higher RDS(ON) will reduce this reverse current
level.
3. Cost. Generally, as the RDS(ON) rating goes lower, the cost of the MOSFET goes higher.
5. The dominate MOSFET loss for the LM5050-1 active OR-ing controller is conduction loss due to source-to-
drain current to the output load, and the RDS(ON) of the MOSFET. This conduction loss could be reduced by
using a MOSFET with the lowest possible RDS(ON). However, contrary to popular belief, arbitrarily selecting a
MOSFET based solely on having low RDS(ON) may not always give desirable results for several reasons:
a. Selecting a MOSFET with an RDS(ON) that is too large will result in excessive power dissipation.
Additionally, the MOSFET gate will be charged to the full value that the LM5050-1 can provide as it
attempts to drive the Drain to Source voltage down to the VSD(REG) of 22 mV typical. This increased Gate
charge will require some finite amount of additional discharge time when the MOSFET needs to be
turned off.
b. As a guideline, it is suggest that RDS(ON) be selected to provide at least 22 mV, and no more than 100
mV, at the nominal load current.
c. (22 mV / ID)≤RDS(ON) ≤(100 mV / ID)
d. The thermal resistance of the MOSFET package should also be considered against the anticipated
dissipation in the MOSFET to ensure that the junction temperature (TJ) is reasonably well controlled,
because the RDS(ON) of the MOSFET increases as the junction temperature increases.
6. PDISS = ID2× (RDS(ON))
7. Operating with a maximum ambient temperature (TA(MAX)) of 35°C, a load current of 10 A, and an RDS(ON) of
10 mΩ, and desiring to keep the junction temperature under 100°C, the maximum junction-to-ambient
thermal resistance rating (θJA) must be:
a. RθJA ≤(TJ(MAX) - TA(MAX))/(ID2× RDS(ON))
b. RθJA ≤(100°C - 35°C)/(10 A × 10 A × 0.01 Ω)