Datasheet

LT3791-1
15
37911f
applicaTions inForMaTion
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
The formula has a maximum at V
IN
= 2V
OUT
. Note that
ripple current ratings from capacitor manufacturers are
often based on only 2000 hours of life which makes it
advisable to derate the capacitor.
In boost operation, the discontinuous current shifts
from the input to the output, so C
OUT
must be capable
of reducing the output voltage ripple. The effects of ESR
(equivalent series resistance) and the bulk capacitance
must be considered when choosing the right capacitor
for a given output ripple voltage. The steady ripple due to
charging and discharging the bulk capacitance is given by:
∆V
RIPPLE BOOST _CAP
( )
=
I
LED
• V
OUT
– V
IN(MIN)
( )
C
OUT
• V
OUT
• f
∆V
RIPPLE BUCK _ CAP
( )
≈
∆I
L
8 • f • C
OUT
where C
OUT
is the output filter capacitor.
The steady ripple due to the voltage drop across the ESR
is given by:
ΔV
BOOST(ESR)
= I
LED
• ESR
ΔV
BUCK(ESR)
= I
LED
• ESR
Multiple capacitors placed in parallel may be needed to meet
the ESR and RMS current handling requirements. Output
capacitors are also used for stability for the LT3791-1. A
good starting point for output capacitors is seen in the
Typical Applications circuits. Ceramic capacitors have
excellent low ESR characteristics but can have a high
voltage coefficient and are recommended for applications
less than 100W. Capacitors available with low ESR and
high ripple current ratings, such as OS-CON and POSCAP
may be needed for applications greater than 100W.
Programming V
IN
UVLO and OVLO
The falling UVLO value can be accurately set by the resistor
divider R1 and R2. A small 3µA pull-down current is active
when the EN/UVLO is below the threshold. The purpose
of this current is to allow the user to program the rising
hysteresis. The following equations should be used to
determine the resistor values:
V
IN(UVLO
–
)
=1.2•
R1
+
R2
R2
V
IN(UVLO
+
)
= 3µA •R1+1.215 •
R1+R2
R2