LM3421MH/NOPB National Semiconductor, LM3421MH/NOPB Datasheet - Page 24

• Manufacturer Part Number: LM3421MH/NOPB
• Description: IC LED DRVR HP CONS CURR 16TSSOP
• Manufacturer: National Semiconductor
• Series: PowerWise®r
• Type: High Power, Constant Currentr

Specifications of LM3421MH/NOPB

• Constant Current: Yes
• Topology: PWM, SEPIC, Step-Down (Buck), Step-Up (Boost)
• Number Of Outputs: 1
• Internal Driver: No
• Type - Primary: Automotive
• Type - Secondary: High Brightness LED (HBLED)
• Frequency: 2MHz
• Voltage - Supply: 4.5 V ~ 75 V
• Voltage - Output: 3 V ~ 72 V
• Mounting Type: Surface Mount
• Package / Case: 16-TSSOP Exposed Pad, 16-eTSSOP, 16-HTSSOP
• Operating Temperature: -40°C ~ 125°C
• Current - Output / Channel: 1A
• Internal Switch(s): No
• Efficiency: 95%
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: *LM3421MH/NOPB; LM3421MH

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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
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
1 nF and solving for R
Buck (Constant Ripple vs. V
LED
MIN
). Also, remember that D' = 1 - D.
) using maximum input voltage (V
) and dynamic resistance (r
MAX
LIM
LED-PP
) using the minimum input voltage (V
IN-MAX
SW
T
LED
IN-PP
:
IN
SNS
L-PP
D
, V
):
TURN-ON
SW
LED
IN
IN-MIN
TURN-OFF
)
) by assuming a C
LED
LED
O
) for a single LED, solve
) and the nominal LED
, V
IN-MAX
, V
HYS
HYSO
) and the maxi-
T
value of
IN-
24
Buck (Constant Ripple vs. V
Boost and Buck-boost
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. 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
The minimum allowable inductor RMS current rating (I
can be calculated as:
Buck
Boost and Buck-boost
SNS
CSH
MIN
:
when solving for L1.
= 12.4 kΩ and solving for R
SNS
CSH
will have to be adjusted to obtain a standard value.
can be adjusted to obtain a standard value.
SNS
HSP
is too far from a desired standard value,
is too far from a desired standard value,
O
)
HSP
:
SNS
L-PP
) and solving for
) by solving for
LED
IN-MAX
) knowing
L-RMS
and
)