Linear Technology Analog Devices LT8708, Техническое описание

Страница: 53 / 64

LT8708
53
Rev 0
For more information www.analog.com
P
M2
≅
P
I2R
+ P
SWITCHING
≅
V
IN
– V
OUT
V
IN
• I
OUT(MAX,RVS)
2
• R
DS(ON)
•
ρ
τ
⎛
⎝
⎜
⎜⎜
⎞
⎠
⎟
⎟⎟
+ V
IN
• I
OUT(MAX,RVS)
• ƒ • t
RF1
( )
+ 0.5 • C
OSS(M1+M2)
• V
IN
2
• ƒ
( )
W
P
(M2,MAX)
≅
25V – 12V
25V
• 3A ( )
2
• 6.9mΩ • 1.5
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
+ 25V • 3A • 150kHz • 20ns ( )
+
(0.5 • (685P + 685P) • 25V • 25V • 150k) [ ]
= 0.13W + 0.225W + 0.064W = 0.419W
To check the power dissipation in the buck region with
V
IN
maximum and V
OUT
minimum, choose the equation
from Table 7 with positive inductor current in buck mode
which yields:
P
M1
≅
P
I2R
+ P
SWITCHING
≅
V
OUT
V
IN
• I
OUT(MAX,FWD)
⎛
⎝
⎜
⎜⎜
⎞
⎠
⎟
⎟⎟
2
• R
DS(ON)
•
ρ
τ
⎡
⎣
⎢
⎢
⎢
⎢
⎤
⎦
⎥
⎥
⎥
⎥
+ V
IN
• I
OUT(MAX,FWD)
• ƒ • t
RF1
( )
+ 0.5 • C
OSS(M1+M2)
• V
IN
2
• ƒ
(
)
W
P
(M1,MAX)
≅
12V
25V
• 5A
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
2
• 6.9mΩ • 1.5
⎡
⎣
⎢
⎢
⎢
⎤
⎦
⎥
⎥
⎥
+ 25V • 5A • 150k • 20ns ( )
+
(0.5 • (685P + 685P) • 25V • 25V • 150k) [ ]
= 0.06W + 0.38W + 0.064W = 0.504W
The maximum switching power of 0.38W can be reduced
by choosing a slower switching frequency. Since this
calculation is approximate, measure the actual rise and
fall times on the PCB to obtain a better power estimate.
Select M3 and M4: With 12V output voltage we need
MOSFETs with 20V or higher rating.
The highest dissipation of M3 and M4 occurs in the boost
region. For switch M3, the max dissipation occurs when
the I
OUT
is highest in the forward direction and V
IN
is at
the minimum 8V (see Table 7):
P
M3
≅
P
I2R
+ P
SWITCHING
≅
(V
OUT
– V
IN
) • V
OUT
V
IN
2
• I
OUT(MAX,FWD)
2
• R
DS(ON)
•
ρ
τ
⎛
⎝
⎜
⎜⎜
⎞
⎠
⎟
⎟⎟
+ V
OUT
2
• I
OUT(MAX,FWD)
• ƒ •
t
RF2
V
IN
⎛
⎝
⎜
⎜⎜
⎞
⎠
⎟
⎟⎟
+ 0.5 • C
OSS(M3+M4)
• V
OUT
2
• ƒ
( )
W
For switch M4, the max dissipation occurs when the I
IN
is
highest in the reverse direction and V
IN
is highest in the
boost region (see Table 7):
P
M4
≅
P
I2R
+ P
SWITCHING
≅
V
IN
V
OUT
• I
IN(MAX,RVS)
2
• R
DS(ON)
•
ρ
τ
⎛
⎝
⎜
⎜⎜
⎞
⎠
⎟
⎟⎟
+ V
OUT
• I
IN(MAX,RVS)
• ƒ • t
RF2
( )
+ 0.5 • C
OSS(M3+M4)
• V
OUT
2
• ƒ
( )
W
and
V
IN(MAX,BOOST)
V
OUT(MAX,BOOST)
=
1– DC
(ABSMIN,M3,BOOST)
therefore,
APPLICATIONS INFORMATION