Linear Technology Analog Devices LT8708, Datasheet

Page: 54 / 64

LT8708
54
Rev 0
For more information www.analog.com
P
M4
≅
P
I2R
+ P
SWITCHING
=
(1– DC
(ABSMIN,M3,BOOST)
) • 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
The Fairchild FDMS7672 can also be used for M3 and M4.
Assuming 20ns rise and fall times, the calculated power
loss is then 0.48W for M3 and 0.21W for M4.
Select R
SENSE2
, R
IMON_OP
and R
IMON_ON
: The I
OUT(MAX,FWD)
= 5A and I
IN(MAX,RVS)
= 3A, with a 20%margin, the I
OUT
current limit is set to 6A in the forward and the I
IN
current
limit is set to 3.6A in the reverse directions, respectively.
Choose R
IMON_OP
to be 17.4k, so that the V
CSPOUT-CSNOUT
limit becomes 50mV, and the R
SENSE2
is calculated to be:
R
SENSE2
=
50mV
6A
≅
8mΩ
Using the equation given in the I
IN
and I
OUT
Current Moni-
toring and Limiting section, R
IMON_ON
is calculated to be:
R
IMON _ ON
=
1.21
I
(OUT,RVS,LIMIT)
• 1m
A
V
•R
SENSE2
+ 20µA
Ω
=
1.21
3.6A •1m
A
V
• 8mΩ + 20µA
=
24.9kΩ
V
OUT
Voltage: V
OUT
voltage is 12V. Select R
FBOUT2
as
20k. R
FBOUT1
is:
R
FBOUT1
=
V
OUT
1.207V
– 1
⎛
⎝
⎜⎜
⎞
⎠
⎟⎟
•R
FBOUT2
Select R
FBOUT1
as 178k. Both R
FBOUT1
and R
FBOUT2
should
have a tolerance of no more than 1%.
V
IN
Voltage: Input voltage is 12V. Select R
FBIN2
as 20k.
R
FBIN1
is:
R
FBIN1
=
V
IN
1.207V
– 1
⎛
⎝
⎜⎜
⎞
⎠
⎟⎟
•R
FBIN2
Select R
FBIN1
as 178k. Both R
FBIN1
and R
FBIN2
should have
a tolerance of no more than 1%.
Capacitors: A low ESR (5mΩ) capacitor network with
30μF ceramic capacitors for C
IN
is selected. In this mode,
the maximum ripple is:
∆V
(BUCK,CAP)
≅
I
OUT(MAX,FWD)
•
V
OUT
V
IN
•ESR
CERAM
•
1– exp
– V
OUT
V
IN
• ƒ •ESR
CERAM
•C
IN–CERAM
⎛
⎝
⎜
⎜⎜
⎞
⎠
⎟
⎟⎟
⎛
⎝
⎜
⎜⎜
⎞
⎠
⎟
⎟⎟
≅
5A •
12V
24V
• 5mΩ
• 1– exp
– 12V
24V • 150kHz • 5mΩ • 30µF
⎛
⎝
⎜
⎜⎜
⎞
⎠
⎟
⎟⎟
⎛
⎝
⎜
⎜⎜
⎞
⎠
⎟
⎟⎟
=
12.5mV
Having 5mΩ of ESR with 66μF ceramic capacitor for the
C
OUT
network sets the maximum output voltage ripple at:
∆V
(Boost,CAP)
≅
I
OUT(MAX,FWD)
•ESR
CERAM
•
1– exp
V
IN
– V
OUT
V
OUT
• ƒ •ESR
CERAM
•C
OUT–CERAM
⎛
⎝
⎜
⎜⎜
⎞
⎠
⎟
⎟⎟
⎛
⎝
⎜
⎜⎜
⎞
⎠
⎟
⎟⎟
≅
5A • 5mΩ
• 1– exp
8V – 12V
12V •150kHz • 5mΩ • 66µF
⎛
⎝
⎜
⎜⎜
⎞
⎠
⎟
⎟⎟
⎛
⎝
⎜
⎜⎜
⎞
⎠
⎟
⎟⎟
=
25mV
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