LM3401MMX National Semiconductor Corporation, LM3401MMX Datasheet - Page 13

• Manufacturer Part Number: LM3401MMX
• Description: Hysteretic Pfet Controller For High Power Led Drive
• Manufacturer: National Semiconductor Corporation

Design Example
The following design example is intended to illustrate the
step-by-step design process described in the previous sec-
tions. The example refers to the circuit in Figure 11, and the
results are summaized in Table 1. The resulting circuit will
drive a string of 2 Luxeon V Star LEDs at 700 mA from an
input voltage between 18V and 35V.
The example LEDs have a maximum DC current rating of 700
mA, a forward voltage of 5.4V to 8.3V, and a maximum peak
current rating of 1.0A.
First, set the LED DC current with R1:
And the required wattage is:
Select a standard value of 290 mΩ, 1/4W resistor, which will
result in a 690 mA LED DC current.
To keep the peak LED current below I
hysteresis is determined by:
Which gives a maximum R2 value of:
Next, a preferred switching frequency of 1 MHz is selected for
this example.
Since this is a relatively high switching frequency, a low start-
ing point of 25 mV is selected for the comparator hysteresis
to maintain good line regulation. This will allow a larger in-
ductor at the same operating frequency and is well below the
calculated maximum. Set a preliminary hysteresis value with
R2:
SNS
HYS_MAX
W
RSNS
= 700 mA
= (1.0A - .690A) x 0.29Ω = 90 mV
2
x 0.286 = 140 mW
LED_MAX
, the maximum
FIGURE 11. Example Circuit
13
For 1 MHz switching frequency and 25 mV hysteresis, induc-
tance can be calculated. Because frequency varies with input
voltage and LED forward voltage, for this calculation, assume
typical values of 24V and 13.6V respectively, and a PFET
delay time of 15 ns.
Select a value of 33 µH and the hysteresis can be adjusted
downward by re-arranging the same frequency equation:
This gives a new R2 value of 5.6k. This will result in a typical
operating frequency of 1 MHz at 24V
ified that the peak LED current is within the maximum allowed.
For now, the design is created without using a ripple reduction
capacitor. Therefore, LED ripple current is equal to inductor
ripple current. Maximum LED ripple current is calculated as:
Note that the maximum input voltage and minimum anode
voltage were used for this worst case calculation.
Now peak LED current can be determined:
IN
. Next, it must be ver-
30021441
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