SNVS574E – JULY 2008 – REVISED MAY 2013
Boost
I CIN-RMS =
Buck-boost
' i L-PP
12
(73)
I CIN-RMS = I LED x
D MAX
1-D MAX
(74)
9. NFET
The NFET voltage rating should be at least 15% higher than the maximum NFET drain-to-source voltage (V T-
MAX ):
Buck
V T - MAX = V IN - MAX
Boost
V T - MAX = V O
Buck-boost
V T - MAX = V IN - MAX + V O
The current rating should be at least 10% higher than the maximum average NFET current (I T-MAX ):
Buck
I T-MAX = D MAX x I LED
Boost and Buck-boost
D MAX
I T-MAX = x I LED
1 - D MAX
Approximate the nominal RMS transistor current (I T-RMS ) :
Buck
I T- RMS = I LED x D
Boost and Buck-boost
(75)
(76)
(77)
(78)
(79)
(80)
I T - RMS =
I LED
D c
x D
(81)
Given an NFET with on-resistance (R DS-ON ), solve for the nominal power dissipation (P T ):
P T = I T - RMS x R DSON
2
(82)
10. DIODE
The Schottky diode voltage rating should be at least 15% higher than the maximum blocking voltage (V RD-MAX ):
Buck
V RD-MAX = V IN-MAX
(83)
36
Copyright ? 2008–2013, Texas Instruments Incorporated
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