Datasheet

LT3791
21
3791fa
Power MOSFET Selections and Efficiency
Considerations
The LT3791 requires four external N-channel power MOS-
FETs, two for the top switches (switch M1 and M4, shown in
Figure 1) and two for the bottom switches (switch M2 and
M3 shown in Figure 1). Important parameters for the power
MOSFETs are the breakdown voltage, V
BR(DSS)
, threshold
voltage, V
GS(TH)
, on-resistance, R
DS(ON)
, reverse transfer
capacitance, C
RSS
, and maximum current, I
DS(MAX)
.
The drive voltage is set by the 5V INTV
CC
supply. Con-
sequently, logic-level threshold MOSFETs must be used
in LT3791 applications. If the input voltage is expected
to drop below the 5V, then sub-logic threshold MOSFETs
should be considered.
In order to select the power MOSFETs, the power dis-
sipated by the device must be known. For switch M1, the
maximum power dissipation happens in boost operation,
when it remains on all the time. Its maximum power dis-
sipation at maximum output current is given by:
P
M1(BOOST)
=
I
LED
• V
OUT
V
IN






2
• ρ
T
•R
DS(ON)
where ρ
T
is a normalization factor (unity at 25°C)
accounting for the significant variation in on-resistance
with temperature, typically 0.4%/°C as shown in Figure
10. For a maximum junction temperature of 125°C, using
a value of ρ
T
= 1.5 is reasonable.
Switch M2 operates in buck operation as the synchronous
rectifier. Its power dissipation at maximum output current
is given by:
P
M2(BUCK)
=
V
IN
– V
OUT
V
IN
•I
LED
2
• ρ
T
•R
DS(ON)
Switch M3 operates in boost operation as the control
switch. Its power dissipation at maximum current is
given by:
P
M3(BOOST)
=
V
OUT
– V
IN
( )
• V
OUT
V
IN
2
•I
LED
2
• ρ
T
•R
DS(ON)
+ k • V
OUT
3
•
I
LED
V
IN
• C
ROSS
• f
where C
RSS
is usually specified by the MOSFET manufac-
turers. The constant k, which accounts for the loss caused
by reverse-recovery current, is inversely proportional to
the gate drive current and has an empirical value of 1.7.
For switch M4, the maximum power dissipation happens
in boost operation, when its duty cycle is higher than
50%. Its maximum power dissipation at maximum output
current is given by:
P
M4(BOOST)
=
V
IN
V
OUT
•
I
LED
• V
OUT
V
IN






2
• ρ
T
•R
DS(ON)
For the same output voltage and current, switch M1 has
the highest power dissipation and switch M2 has the low-
est power dissipation unless a short occurs at the output.
applicaTions inForMaTion