LT8708
50
Rev 0
For more information
Verify expected operation by combining Table 6(a) and
Table 6(b):
• When V
IN
< V
IN_FBIN
(12V) and V
OUT
> V
OUT_FBOUT
(12V)
B
– power is transferred from V
OUT
to V
IN
• When V
IN
< V
IN_FBIN
(12V) and V
OUT
< V
OUT_FBOUT
(12V)
B
– power is transferred from V
OUT
to V
IN
• When V
IN
> V
IN_FBIN
(12V) and V
OUT
> V
OUT_FBOUT
(12V)
C
– power is transferred from V
OUT
to V
IN
• When V
IN
> V
IN_FBIN
(12V) and VOUT < V
OUT_FBOUT
(12V)
D
– power is transferred from V
IN
to V
OUT
The results above are as expected for this design example.
R
T
Selection: Choose the R
T
resistor for the free-running
oscillator frequency using:
R
T
=
43,750
f
OSC
– 1
⎛
⎝
⎜
⎜⎜
⎞
⎠
⎟
⎟⎟
kΩ =
43,750
150
– 1
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
= 290.7kΩ
We will choose 294k for R
T
resistor.
R
SENSE
Selection: Start by calculating the maximum and
minimum duty cycle in the boost region:
DC
(MAX,M3,BOOST)
≅
1–
V
IN(MIN,BOOST)
V
OUT(MAX,BOOST)
⎛
⎝
⎜
⎜
⎜
⎞
⎠
⎟
⎟
⎟
• 100%
=
1–
8V
12V
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
•100% = 33%
DC
(ABSMIN,M3,BOOST)
≅
t
ON(M3,MIN)
• ƒ • 100%
=
200ns •150kHz • 100% = 3%
Next, from the Maximum Inductor Current Sense Voltage
vs Duty Cycle graph in the Typical Performance Charac-
RSENSE(MAX,BOOST,MAXDC)
≅
83mV
V
RSENSE(MIN,BOOST,MINDC)
≅
93mV
Next, estimate the inductor current ripples at maximum
and minimum boost duty cycles:
∆I
L(MAX,BOOST)
V
OUT(MAX,BOOST)
•I
OUT(MAX,FWD)
V
IN(MIN,BOOST)
•
100%
%Ripple
– 0.5
A
=
12V •5A
8V • 100%
40%
– 0.5
= 3.75A
∆I
L(MIN,BOOST)
I
IN(MAX,RVS)
100%
10%
– 0.5
A
=
3A
100%
10%
– 0.5
= 0.32A
≅
≅
Now calculate the maximum R
SENSE
values in the boost
region:
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