LT8708
34
Rev 0
For more information
The maximum reverse inductor current in the boost region
for applications in which V
OUT(MAX)
≥ 2•V
IN(MAX)
is:
I
L(MAX,BOOST,RVS)
≅
I
IN(MAX,RVS)
+
V
IN(MAX,BOOST)
4
•L • ƒ
⎛
⎝
⎜
⎜⎜
⎞
⎠
⎟
⎟⎟
A
For applications in which V
OUT(MAX)
< 2•V
IN(MAX)
, the
maximum reverse inductor current is smaller than the
value given by the above equation. The following equation
can be used to calculate the reverse inductor current for
given combinations of V
IN
and V
OUT
.
I
L(MAX,BOOST,RVS)
≅
I
IN(MAX,RVS)
+
V
IN
•(V
OUT
– V
IN
)
2
•L • ƒ • V
OUT
⎛
⎝
⎜
⎜⎜
⎞
⎠
⎟
⎟⎟
A
where:
V
OUT
> V
IN
The maximum positive inductor current in the buck region
for applications in which V
IN(MAX)
≥ 2•V
OUT(MAX)
is:
I
L(MAX,BUCK,FWD)
≅
I
OUT(MAX,FWD)
+
V
OUT(MAX,BUCK)
4
•L • ƒ
⎛
⎝
⎜
⎜⎜
⎞
⎠
⎟
⎟⎟
A
For applications with V
IN(MAX)
< 2•V
OUT(MAX)
, the maxi-
mum forward inductor current is smaller than the value
given by the above equation. The following equation can
be used to calculate the forward inductor current for given
combinations of V
IN
and V
OUT
.
I
L(BUCK,FWD)
≅
I
OUT(MAX,FWD)
+
V
OUT
•(V
IN
– V
OUT
)
2
•L • ƒ • V
IN
⎛
⎝
⎜
⎜⎜
⎞
⎠
⎟
⎟⎟
A
where:
V
IN
> V
OUT
The maximum reverse inductor current when operating
in the buck region is:
I
L(MAX,BUCK,RVS)
≅
I
IN(MAX,RVS)
•
V
IN(MAX,BUCK)
V
OUT(MIN,BUCK)
+
V
OUT(MIN,BUCK)
•
DC
(MAX,M2,BUCK)
100%
2 •L • ƒ
⎛
⎝
⎜
⎜
⎜
⎜
⎜
⎜⎜
⎞
⎠
⎟
⎟
⎟
⎟
⎟
⎟⎟
A
where:
DC
(MAX,M2,BUCK)
is the maximum duty cycle percent-
age of the M2 switch in the buck region (see R
in the Buck Region section).
Note that the inductor current can be higher when there are
load transients or the load current exceeds the expected
maximum amount. It can also be higher during start-up
if inadequate soft-start capacitance is used, or during
output shorts. Consider using the I
IN
and/or I
OUT
current
limiting to help prevent the inductor current from becom-
ing excessive. I
IN
and I
OUT
current limiting are discussed
later in the I
Current Monitoring and Limiting
section. Careful board evaluation of the maximum inductor
current is recommended.
POWER MOSFET SELECTION
The LT8708 requires four external N-channel power
MOSFETs, two for the top switches (switches M1 and
M4, shown in Figure 3) and two for the bottom switches
(switches M2 and M3, shown in Figure 3). Important
parameters for the power MOSFETs are the breakdown
voltage V
BR,DSS
, threshold voltage V
GS,TH
, on-resistance
R
DS(ON)
, output capacitance C
OSS
, and maximum current
I
DS(MAX)
. The gate drive voltage is set by the 6.3V GATEV
CC
supply. Consequently, logic-level threshold MOSFETs must
be used in LT8708 applications.
APPLICATIONS INFORMATION
