Linear Technology Analog Devices LT8708 Datasheet page 51

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