LM3409MY National Semiconductor, LM3409MY Datasheet - Page 16

• Manufacturer Part Number: LM3409MY
• Description: LM3409 LM3409HV PFET Buck Controller for High Power LED Drives; Package: MINI SOIC EXP PAD; No of Pins: 10; Qty per Container: 1000/Reel
• Manufacturer: National Semiconductor

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Design Guide
TYPICAL APPLICATION
SPECIFICATIONS
Nominal input voltage: V
Maximum input voltage: V
Nominal output voltage (# of LEDs x forward voltage): V
LED string dynamic resistance: r
Switching frequency (at nominal V
Average LED current: I
Inductor current ripple: Δi
LED current ripple: Δi
Input voltage ripple: Δv
UVLO characteristics: V
Expected efficiency: η
1. NOMINAL SWITCHING FREQUENCY
Calculate switching frequency (f
point (V
1nF) and a system efficiency (η). Solve for R
2. INDUCTOR RIPPLE CURRENT
Set the inductor ripple current (Δi
propriate inductor (L1):
IN
and V
O
). Assume a C
LED-PP
LED
IN-PP
TURN-ON
IN
L-PP
IN-MAX
OFF
SW
and V
D
L-PP
value (between 470pF and
IN
) at the nominal operating
, V
) by solving for the ap-
HYS
O
): f
SW
OFF
:
O
16
3. AVERAGE LED CURRENT
Set the average LED current (I
inductor current (I
Peak inductor current is detected across the sense resistor
(R
1.24V) at the IADJ pin and solve for R
If the calculated R
ning of the process can be iterated to choose a new R
and R
proach the iterative process is to change the nominal f
target knowing that the switching frequency varies with oper-
ating conditions anyways.
Another method for finding a standard R
the V
age source or a resistor from the IADJ pin to GND as ex-
plained in the Theory of Operation section of this datasheet.
SNS
ADJ
). In most cases, assume the maximum value (V
SNS
value. However, this would require an external volt-
value that is a closer fit. The easiest way to ap-
L-MAX
SNS
is far from a standard value, the begin-
):
LED
) by first solving for the peak
30085601
SNS
SNS
:
value is to change
OFF
ADJ
, L1,
SW
=