Datasheet

LT3791
16
3791fa
applicaTions inForMaTion
For high efficiency, choose an inductor with low core
loss. Also, the inductor should have low DC resistance to
reduce the I
2
R losses, and must be able to handle the peak
inductor current without saturating. To minimize radiated
noise, use a shielded inductor.
R
SENSE
Selection and Maximum Output Current
R
SENSE
is chosen based on the required output current. The
current comparator threshold sets the peak of the induc-
tor current in boost operation and the maximum inductor
valley current in buck operation. In boost operation, the
maximum average load current at V
IN(MIN)
is:
I
OUT(MAX _ BOOST)
=
51mV
R
SENSE
–
∆I
L
2






•
V
IN(MIN)
V
OUT
where ΔI
L
is peak-to-peak inductor ripple current. In buck
operation, the maximum average load current is:
I
OUT(MAX _ BUCK)
=
47.5mV
R
SENSE
+
∆I
L
2






The maximum current sensing R
SENSE
value for the boost
operation is:
R
SENSE(MAX)
=
2• 51mV• V
IN(MIN)
2•I
LED
• V
OUT
+ ∆I
L(BOOST)
• V
IN(MIN)
The maximum current sensing R
SENSE
value for the buck
operation is:
R
SENSE(MAX)
=
2• 47.5mV
2•I
LED
– ∆I
L(BUCK)
The final R
SENSE
value should be lower than the calculated
R
SENSE(MAX)
in both the boost and buck operation. A 20%
to 30% margin is usually recommended.
C
IN
and C
OUT
Selection
In boost operation, input current is continuous. In buck
operation, input current is discontinuous. In buck opera-
tion, the selection of input capacitor, C
IN
, is driven by the
need to filter the input square wave current. Use a low ESR
capacitor sized to handle the maximum RMS current. For
buck operation, the input RMS current is given by:
I
RMS
= I
LED
2
•D+
∆I
L
2
12
•D
BG1, BG2 DUTY CYCLE (%)
50
∆
I
L
/I
SENSE(MAX)
(%)
120
160
200
90
3791 F07
80
40
100
140
180
60
20
0
6055
7065
80 85 95
75
100
BOOST ∆I
L
/
I
SENSE(MAX)
LIMIT
BUCK ∆I
L
/
I
SENSE(MAX)
LIMIT
Figure 7. Maximum Peak-to-Peak Ripple vs Duty Cycle