LM3401EVAL National Semiconductor, LM3401EVAL Datasheet - Page 14

• Manufacturer Part Number: LM3401EVAL
• Description: BOARD EVALUATION FOR LM3401
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM3401EVAL

• Current - Output / Channel: 1A
• Outputs And Type: 1, Non-Isolated
• Features: Dimmable
• Voltage - Input: 4.5 ~ 35V
• Utilized Ic / Part: LM3401
• Kit Contents: Board
• Svhc: No SVHC (15-Dec-2010)
• Kit Features: Hysteretic Control For Speed And
• Lead Free Status / RoHS Status: Not applicable / Not applicable
• Voltage - Output: -
• Lead Free Status / Rohs Status: Supplier Unconfirmed

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This confirms that the component selections will keep LED
peak current below the maximum LED rating. Notice if a ripple
reduction capacitor is chosen, the peak inductor current is still
804mA, but the LED peak current is reduced. Therefore, the
inductor must be rated for a DC current greater than 804 mA.
Now that the inductor value has been selected and verified,
the operating frequency range can be determined. Lowest
operating frequency occurs at minimum input voltage and
maximum anode voltage. For this example the values are 18V
minimum input and 16.8V maximum anode voltage (200 mV
SNS voltage plus the maximum LED forward voltages) and
calculate:
At duty cycles close to 100% (96% in this case) the frequency
equation becomes less accurate. Actual switching frequency
will typically be lower than the calculated value.
To estimate maximum operating frequency, calculate using a
Vin which corresponds to a duty cycle of 25%. In this example,
25% duty cycle would occur above 35V
frequency will occur at the maximum input voltage:
Using the equation in the Switching Frequency section, it can
be verified that this maximum frequency is within the minimum
on-time limited frequency (and below the maximum operating
frequency).
The maximum frequency calculation is only an estimate, the
actual maximum should be verified on the bench.
The next step is to select a PFET. The critical PFET param-
eters must meet the minimum circuit requirements of 35V
input, 804 mA DC current, and adequate gate drive voltage
rating.
Therefore, select a PFET with the following ratings:
40V maximum V
-20V maximum V
1.8A continuous Id
130mohm maximum R
Typically the PFET may be only sourcing 690 mA for about
50% duty cycle. However, at minimum input voltage the duty
cycle will increase close to 100%. Therefore, the PFET Id rat-
ing should be based on its continuous, not pulsed, current
capability.
Now the power dissipation should be verified. Assume the
selected PFET has a gate capacitance of 200 pF, which is
within recommendation, and a gate charge of 15 nC. Maxi-
mum frequency and input voltage are used for a worst case
calculation:
PD = (1.05 mA x 35V) + (16.5 mA x 4.7V) = 0.114W
T
a_max
= 125°C - (151°C/W x 0.114W) = 108°C
I
G
DS
= 15 nC x 1.1 MHz = 16.5 mA
GS
DS(on)
IN
, therefore maximum
14
With the selected components, the maximum ambient tem-
perature is above 100°C, sufficient for most applications. Note
that this limit applies to the IC only and depends on the pcb
type and size. Lower ambient temperature limits may apply to
the PFET and other components.
Now the current limit threshold is set with R3 at 0.95A, which
is 120% of the maximum peak current. The worst case R
(on)
worst case ILIM pin sink current.
The typical current limit threshold will be higher than 1A and
can be determined by using typical values for R
I
be able to handle this current for short periods of time.
The next component is the input capacitor, C1. A low ESR
ceramic capacitor must be used and properly located on the
PCB. For this design, the capacitor working voltage must be
rated to at least 40V, and 50V is recommended. A 2.2 µF input
capacitor should be sufficient, assuming a good PCB layout.
The worst case input RMS current is calculated below 50% at
duty cycle:
It must be verified that the selected input capacitor can toler-
ate this current. An additional bulk capacitor placed at the
input voltage connection is also recommended.
Next, select D1, the catch diode. A Schottky diode should be
used. The reverse voltage rating must be greater than 35V
and the average forward current rating must be greater than:
This calculation assumes the minimum duty cycle, which is
maximum input voltage and minimum anode voltage. The
diode must also be able to handle peak currents as high as
the current limit threshold for short periods. We select a 1A
diode to ensure adequate capability over temperature.
If desired, a ripple reduction capacitor can be added at C2 to
reduce the LED ripple current. A minimum starting value of
100 nF is recommended for C2, and a value of 1 µF will work
well in most applications. In case of an open LED failure, the
ripple reduction capacitor must be rated to the maximum input
voltage of 35V. If C2 is used, LED ripple current is reduced
and the calculated maximum R2 value no longer applies as a
limit.
Finally, check the accuracy. The static accuracy is calculated
below using a 1% sense resistor.
To estimate line regulation, maximum input voltage and 60%
duty cycle input voltage is used. For this example 60% duty
cycle occurs at (13.6V + 0.2V)/0.60 or 23V input.
LIM
value at 125°C is used, which is 150% of nominal, and the
sink current. The PFET, inductor, and catch diode must
I
DIODE
= 690 mA x (1 - 0.31) = 480 mA
DS(on)
and
DS