Design Guide
Refer to Basic Topology Schematics section.
SPECIFICATIONS
Number of series LEDs: N
Single LED forward voltage: V
LED
Single LED dynamic resistance: r
LED
Nominal input voltage: V
IN
Input voltage range: V
IN-MAX
, V
IN-MIN
Switching frequency: f
SW
Current sense voltage: V
SNS
Average LED current: I
LED
Inductor current ripple:
Δ
i
L-PP
LED current ripple:
Δ
i
LED-PP
Peak current limit: I
LIM
Input voltage ripple:
Δ
v
IN-PP
Output OVLO characteristics: V
TURN-OFF
, V
HYSO
Input UVLO characteristics: V
TURN-ON
, V
HYS
Thermal foldback characteristics: T
BK
, T
END
Total start-up time: t
TSU
1. OPERATING POINT
Given the number of series LEDs (N), the forward voltage
(V
LED
) and dynamic resistance (r
LED
) for a single LED, solve
for the nominal output voltage (V
O
) and the nominal LED
string dynamic resistance (r
D
):
Solve for the ideal nominal duty cycle (D):
Buck
Boost
Buck-boost
Using the same equations, find the minimum duty cycle
(D
MIN
) using maximum input voltage (V
IN-MAX
) and the maxi-
mum duty cycle (D
MAX
) using the minimum input voltage (V
IN-
MIN
). Also, remember that D' = 1 - D.
2. SWITCHING FREQUENCY
Set the switching frequency (f
SW
) by solving for R
T
:
3. AVERAGE LED CURRENT
For all topologies, set the average LED current (I
LED
) knowing
the desired current sense voltage (V
SNS
) and solving for
R
SNS
:
If the calculated R
SNS
is too far from a desired standard value,
then V
SNS
will have to be adjusted to obtain a standard value.
Setup the suggested signal current of 100 µA by assuming
R
CSH
= 12.4 k
Ω
and solving for R
HSP
:
If the calculated R
HSP
is too far from a desired standard value,
then R
CSH
can be adjusted to obtain a standard value.
4. THERMAL FOLDBACK
For all topologies, set the thermal foldback breakpoint (T
BK
)
by finding corresponding R
NTC-BK
from manufacturer's
datasheet and solving for R
BIAS
:
The easiest approach is to set R
REF1
= R
REF2
, therefore set-
ting R
BIAS
= R
NTC-BK
will properly set T
BK
. Remember, capac-
itance is recommended at the TSENSE and TREF pins, so
ensure C
REF
> C
NTC
to prevent start-up in foldback.
Then set the thermal foldback endpoint (T
END
) by finding the
corresponding R
NTC-END
from manufacturer's datasheet and
solving for R
GAIN
:
5. INDUCTOR RIPPLE CURRENT
Set the nominal inductor ripple current (
Δ
i
L-PP
) by solving for
the appropriate inductor (L1):
Buck
Boost and Buck-boost
To set the worst case inductor ripple current, use V
IN-MAX
and
D
MIN
when solving for L1.
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LM3424