Datasheet

Vcathode
2V/Div
ILED
100 mA/Div
IL
500 mA/Div
1 Ps/DIV
L
(MIN)
=
V
IN(MAX)
x D
(MIN)
'i
L
x f
SW
I
L(AVE)
=
'¶
I
OUT
400 ns/DIV
Vout
200 mV/Div
IL
500 mA/Div
Vsw
10V/Div
LM3431
www.ti.com SNVS547G –NOVEMBER 2007–REVISED MAY 2013
Figure 17. Inductor Current, SW Voltage, and V
OUT
The important quantities in determining a proper inductance value are I
L(AVE)
(the average inductor current) and
Δi
L
(the peak to peak inductor current ripple). If Δi
L
is larger than 2 x I
L(AVE)
, the inductor current will drop to zero
for a portion of the cycle and the converter will operate in discontinuous conduction mode. If Δi
L
is smaller than 2
x I
L
, the inductor current will stay above zero and the converter will operate in continuous conduction mode.
To determine the minimum L, first calculate the I
L(AVE)
at both minimum and maximum input voltage:
Where
• I
OUT
is the sum of all LED string currents at 100% dimming (8)
I
L(AVE)
will be highest at the minimum input voltage. Then determine the minimum L based on Δi
L
with the
following equation:
(9)
A good starting point is to set Δi
L
to 150% of the minimum I
L(AVE)
and calculate using that value. The maximum
recommended Δi
L
is 200% of I
L(AVE)
to maintain continuous current in normal operation. In general a smaller
inductor (higher ripple current) will give a better dimming response due to the higher dI/dt. This is shown
graphically below.
Figure 18. Inductor Current During Dimming
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