LT8708
31
Rev 0
Next, the inductor ripple current in the buck region must
be determined. If the main inductor L is not known, the
maximum ripple current ∆I
L(MAX,BUCK)
can be estimated
by choosing ∆I
L(MAX,BUCK)
to be 30% to 50% of the maxi-
mum peak inductor current in the buck region as follows:
∆I
L(MAX,BUCK)
V
IN(MAX,BUCK)
•I
IN(MAX,RVS)
V
OUT(MIN,BUCK)
•
100%
%Ripple
– 0.5
A
≅
where:
I
IN(MAX,RVS)
is the maximum V
IN
load current in the
reverse direction required in the buck region.
%Ripple is 30% to 50%
Otherwise, if the inductance is already known then
∆I
L(MAX,BUCK)
can be more accurately calculated as follows:
∆I
L(MAX,BUCK)
≅
DC
(MAX,M2,BUCK)
100%
⎛
⎝
⎜
⎜⎜
⎞
⎠
⎟
⎟⎟
• V
OUT(MIN,BUCK)
ƒ •L
A
where:
DC
(MAX,M2,BUCK)
is the maximum duty cycle percent-
age in the buck region as calculated previously
ƒ is the switching frequency
L is the inductance of the main inductor
After the maximum ripple current is known, the maximum
allowed R
SENSE
in the buck region while in reverse conduc-
tion can be calculated as follows:
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
APPLICATIONS INFORMATION
where:
V
RSENSE(MIN,BUCK,MAXDC)
is the minimum inductor
current sense voltage at the maximum duty cycle.
This value is determined in a similar manner to
V
RSENSE(MAX,BOOST,MAXDC)
discussed previously in the
(Forward Conduction) section.
R
SENSE
Selection: Final R
SENSE
Value
The final R
SENSE
value should be lower than all four
maximum R
SENSE
values, R
SENSE(MAX,BOOST,FWD)
,
R
SENSE(MAX,BOOST,RVS)
, R
SENSE(MAX,BUCK,FWD)
and
R
SENSE(MAX,BUCK,RVS)
. A margin of 20% to 30% is rec-
ommended.
Figure 8 shows approximately how the maximum posi-
tive I
OUT
and inductor currents would vary with V
IN
/V
OUT
while all other operating parameters remain constant
(frequency = 120kHz, inductance = 10µH, R
SENSE
= 1mΩ).
This graph is normalized and accounts for changes in
maximum current due to the slope compensation ramps
and the effects of changing ripple current. The curve is
theoretical but can be used as a guide to predict relative
changes in maximum currents over a range of V
IN
/V
OUT
voltages. Similarly, when in reverse conduction, Figure 9
shows approximately how the maximum negative I
IN
and
inductor currents would vary with V
IN
/V
OUT
.
Figure 8. Currents vs V
IN
/V
OUT
Ratio in Forward Conduction
V
IN
/V
OUT
(V/V)
0.1
1
10
0
0.20
0.40
0.60
0.80
1.00
NORMALIZED CURRENT
8708 F08
MAXIMUM
OUTPUT
INDUCTOR
CURRENT
MAXIMUM
CURRENT
