LT8708
36
Rev 0
For more information
Figure 11. Normalized MOSFET RDS(ON) vs Temperature
Switching Component Equations for M1 and M2:
P
SW[M1,BUCK]
or P
SW[M2,BUCK]
≅
P
SWA
+P
SWB
≅
(V
IN
•|I
OUT
| •ƒ • t
RF1
)
+(0.5 • C
OSS M1
+
M2
(
)
• V
IN
2
• ƒ) W
Switching Component Equations for M3 and M4:
P
SW[M3,BOOST]
or P
SW[M4,BOOST
]
≅
P
SWA
+P
SWB
≅
V
OUT
2
•|I
OUT
| •ƒ •
t
RF2
V
IN
⎛
⎝
⎜
⎜⎜
⎞
⎠
⎟
⎟⎟
+(0.5 • C
OSS M3
+
M4
(
)
• V
OUT
2
• ƒ) W
where:
t
RF1
is the average of the SW1 pin rise and fall times.
Typical values are 20 – 40ns depending on the MOSFET
capacitance and V
IN
voltage.
t
RF2
is the average of the SW2 pin rise and fall times
and, similar to t
RF1
, is typically 20ns – 40ns depending
on the MOSFET capacitance and V
OUT
voltage.
R
DS(ON)
is the “on” resistance of the MOSFET at 25°C
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 11. For a maximum junction temperature of
125°C, using a value = 1.5 is reasonable.
Switch M1: For positive conduction, the maximum power
dissipation in M1 occurs either in the buck region when
V
IN
is highest, V
OUT
is highest, and switching power losses
are greatest, or in the boost region when V
IN
is smallest,
V
OUT
is highest and M1 is always on.
In most cases of negative conduction, the M1 switching
power dissipation is quite small and I
2
R power losses
dominate. In negative conduction, M1 I
2
R power is great-
est in the boost region due to the lower V
IN
and higher
V
OUT
that cause the M1 switch to be “on” for the most
amount of time.
Switch M2: In most cases of positive conduction, the M2
switching power dissipation is quite small and I
2
R power
losses dominate. In positive conduction, M2 I
2
R power is
greatest in the buck region due to the higher V
IN
and lower
V
OUT
that cause M2 to be “on” for the most amount of time.
For negative conduction, the maximum power dissipation
in M2 occurs in the buck region when V
IN
is highest and
V
OUT
is lowest.
Switch M3: If the inductor current is positive, the maxi-
mum power dissipation in M3 occurs when V
IN
is lowest
and V
OUT
is highest.
In most cases of negative conduction, the M3 switching
power dissipation is quite small and I
2
R power losses
dominate. In negative conduction, M3 I
2
R power is great-
est in the boost region due to the lower V
IN
and higher
V
OUT
that cause the M3 switch to be “on” for the most
amount of time.
Switch M4: If the inductor current is positive, in most
cases the switching power dissipation in the M4 switch
is quite small and I
2
R power losses dominate. I
2
R power
is greatest in the boost region due to the lower V
IN
and
higher V
OUT
that cause M4 switch to be “on” for the most
amount of time.
If the inductor current is negative, the maximum power
dissipation in the M4 switch occurs either in the boost
region when V
IN
is highest, V
OUT
is highest, and switching
power losses are greatest, or in the buck region when V
IN
is highest, V
OUT
is lowest and M4 is always on.
APPLICATIONS INFORMATION
JUNCTION TEMPERATURE (°C)
–50
ρ
T
NORMALIZED ON-RESISTANCE (Ω)
1.0
1.5
150
8708 F11
0.5
0
0
50
100
2.0
