Linear Technology Analog Devices LT8708, Datasheet, Page: 51 / 64

LT8708
51
Rev 0
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R
SENSE(MAX,BOOST,FWD)
=
2 • V
RSENSE(MAX,BOOST,MAXDC)
• V
IN(MIN,BOOST)
2 •I
OUT(MAX,FWD)
• V
OUT(MAX,BOOST)
( )
+ ∆I
L(MAX,BOOST)
• V
IN(MIN,BOOST)
( )
Ω
=
2 • 83mV • 8V
2 • 5A • 12V ( ) + 3.75A • 8V ( )
= 8.85mΩ
R
SENSE(MAX,BOOST,RVS)
=
2•| V
RSENSE(MIN,BOOST,MINDC)
|
2•|I
IN(MAX,RVS)
|
( )
– ∆I
L(MIN,BOOST)
Ω
=
2 • 93mV
2 • 3A ( ) – 0.32A
= 32.7mΩ
Next, calculate the maximum and minimum duty cycle in
the buck region:
DC
(ABSMIN,M2,BUCK)
≅
t
ON(M2,MIN)
• ƒ •100%
=
200ns •150kHz •100% = 3%
DC
(MAX,M2,BUCK)
≅
1–
V
OUT(MIN,BUCK)
V
IN(MAX,BUCK)
⎛
⎝
⎜
⎜
⎜
⎞
⎠
⎟
⎟
⎟
•100%
=
1–
12V
25V
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
•100% = 52%
Next, from the Maximum Inductor Current Sense Voltage
vs Duty Cycle graph in the Typical Performance Charac-
teristics section:
V
RSENSE(MAX,BUCK,MINDC)
≅
82mV
V
RSENSE(MIN,BUCK,MAXDC)
≅
65mV
Next, estimate the inductor current ripples at maximum
and minimum buck duty cycles:
∆I
L(MIN,BUCK)
I
OUT(MAX,FWD)
100%
10%
– 0.5
A =
5A
100%
10%
– 0.5
=
0.526A
∆I
L(MAX,BUCK)
V
IN(MAX,BUCK)
•I
IN(MAX,RVS)
V
OUT(MIN,BUCK)
•
100%
%Ripple
– 0.5
A
=
25V •3A
12V •
100%
40%
– 0.5
=
3.125A
≅
≅
























Now calculate the maximum R
SENSE
values in the boost
region:
R
SENSE(MAX,BUCK,FWD)
=
2 • V
RSENSE(MAX,BUCK,MINDC)
2 •I
OUT(MAX,FWD)
( )
– ∆I
L(MIN,BUCK)
Ω
=
2 • 82mV
2 • 5A ( ) – 0.53A
= 17.3mΩ
R
SENSE(MAX,BUCK,RVS)
=
2• | V
RSENSE(MIN,BUCK,MAXDC)
| •V
OUT(MIN,BUCK)
2• |I
IN(MAX,RVS)
| •V
IN(MAX,BUCK)
( )
+ ∆I
L(MAX,BUCK)
• V
OUT(MIN,BUCK)
( )
Ω
=
2 • 65mV •12V
2 • 3A • 25V
( )
+ 3.125A •12V
( )
= 8.3mΩ
Choose the smallest calculated R
SENSE
and add an ad-
ditional 30% margin, choose R
SENSE
to be 8.3mΩ/1.3 =
6.3mΩ
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