Datasheet

LT8705
26
8705fb
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applicaTions inForMaTion
are greatest or in the boost region when V
IN
is smallest,
V
OUT
is highest and M1 is always on. Switch M1 power
consumption can be approximated as:
P
M1
=P
I
2
R
+P
SWITCHING
≅
V
OUT
V
IN
•I
OUT


 

2
•R
DS(ON)
•ρ
τ

 

+ V
IN
•I
OUT
• f• t
RF1
(
)
W
where:
the P
SWITCHING
term is 0 in the boost region
t
RF1
is the average of the SW1 pin rise and fall times.
Typical values are 20ns to 40ns depending on the
MOSFET capacitance and V
IN
voltage.
ρ
τ
is a normalization factor (unity at 25°C) accounting
for the significant variation in MOSFET on-resistance
with temperature, typically about 0.4%/°C, as shown
in Figure 9. For a maximum junction temperature of
125°C, using a value ρ
τ
= 1.5 is reasonable.
Since the switching power (P
SWITCHING
) often dominates,
look for MOSFETs with lower C
RSS
or consider operating
at a lower frequency to minimize power loss and increase
efficiency.
JUNCTION TEMPERATURE (°C)
–50
ρ
τ
NORMALIZED ON-RESISTANCE (Ω)
1.0
1.5
150
8705 F09
0.5
0
0
50
100
2.0
Figure 9. Normalized MOSFET R
DS(ON)
vs Temperature
Switch M2: In most cases the switching power dissipa-
tion in the M2 switch is quite small and I
2
R power losses
dominate. I
2
R power is greatest in the buck region where
the switch operates as the synchronous rectifier. Higher
V
IN
and lower V
OUT
causes the M2 switch to be “on” for
the most amount of time, leading to the highest power
consumption. The M2 switch power consumption in the
buck region can be approximated as:
P
(M2,BUCK)
≅
V
IN
– V
OUT
V
IN
•I
OUT(MAX)
2
•R
DS(ON)
•ρ
τ




W
Switch M3: Switch M3 operates in the boost and buck-
boost regions as a control switch. Similar to the M1
switch, the power dissipation comes from I
2
R power and
switching power. The maximum power dissipation is when
V
IN
is the lowest and V
OUT
is the highest. The following
expression approximates the power dissipation in the M3
switch under those conditions:
P
M3
=P
I
2
R
+P
SWITCHING
≅
V
OUT
– V
IN
(
)
• V
OUT
V
IN
2
•I
O
UT
2
•R
DS(ON)
•ρ
τ






+ V
OUT
2
•I
OUT
• f•
t
RF2
V
IN





W

where the total power is 0 in the buck region.