LM3424BSTEVAL National Semiconductor, LM3424BSTEVAL Datasheet - Page 25

• Manufacturer Part Number: LM3424BSTEVAL
• Description: LM3424 Constant Current N-Channel Controller with Thermal Foldback for Driving LEDs; Qty per Container: 1/
• Manufacturer: National Semiconductor

Design Guide
Refer to Basic Topology Schematics section.
SPECIFICATIONS
Number of series LEDs: N
Single LED forward voltage: V
Single LED dynamic resistance: r
Nominal input voltage: V
Input voltage range: V
Switching frequency: f
Current sense voltage: V
Average LED current: I
Inductor current ripple: Δi
LED current ripple: Δi
Peak current limit: I
Input voltage ripple: Δv
Output OVLO characteristics: V
Input UVLO characteristics: V
Thermal foldback characteristics: T
Total start-up time: t
1. OPERATING POINT
Given the number of series LEDs (N), the forward voltage
(V
for the nominal output voltage (V
string dynamic resistance (r
Solve for the ideal nominal duty cycle (D):
Buck
Boost
Buck-boost
Using the same equations, find the minimum duty cycle
(D
mum duty cycle (D
MIN
2. SWITCHING FREQUENCY
Set the switching frequency (f
LED
MIN
). Also, remember that D' = 1 - D.
) and dynamic resistance (r
) using maximum input voltage (V
MAX
LIM
TSU
LED-PP
) using the minimum input voltage (V
IN-MAX
SW
LED
IN-PP
IN
SNS
L-PP
D
, V
):
TURN-ON
SW
LED
IN-MIN
TURN-OFF
) by solving for R
LED
LED
O
BK
) for a single LED, solve
) and the nominal LED
, V
, T
IN-MAX
, V
END
HYS
HYSO
) and the maxi-
T
:
IN-
25
3. AVERAGE LED CURRENT
For all topologies, set the average LED current (I
the desired current sense voltage (V
R
If the calculated R
then V
Setup the suggested signal current of 100 µA by assuming
R
If the calculated R
then R
4. THERMAL FOLDBACK
For all topologies, set the thermal foldback breakpoint (T
by finding corresponding R
datasheet and solving for R
The easiest approach is to set R
ting R
itance is recommended at the TSENSE and TREF pins, so
ensure C
Then set the thermal foldback endpoint (T
corresponding R
solving for R
5. INDUCTOR RIPPLE CURRENT
Set the nominal inductor ripple current (Δi
the appropriate inductor (L1):
Buck
Boost and Buck-boost
To set the worst case inductor ripple current, use V
D
SNS
CSH
MIN
:
when solving for L1.
= 12.4 kΩ and solving for R
BIAS
SNS
CSH
REF
= R
will have to be adjusted to obtain a standard value.
can be adjusted to obtain a standard value.
GAIN
> C
NTC-BK
NTC
:
NTC-END
SNS
HSP
will properly set T
to prevent start-up in foldback.
is too far from a desired standard value,
is too far from a desired standard value,
from manufacturer's datasheet and
BIAS
NTC-BK
:
REF1
HSP
:
= R
BK
from manufacturer's
SNS
. Remember, capac-
REF2
END
L-PP
) and solving for
) by finding the
, therefore set-
) by solving for
LED
www.national.com
IN-MAX
) knowing
and
BK
)