Linear Technology Analog Devices LT8708 Datasheet page 36

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