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LM3401 Datenblatt(PDF) 10 Page - Texas Instruments

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Teilenummer LM3401
Bauteilbeschribung  LM3401 Hysteretic PFET Controller for High Power LED Drive
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ILED
50 mA /Div
IL
50 mA/Div
VCS
20V/Div
400 ns/ DIV
ILED_RIP = IL_RIP =
(VIN - VANODE) x 2 x delay
2 x SNSHYS
RSNS
L
+
ton =
D
fSW
LM3401
SNVS516C – AUGUST 2007 – REVISED MAY 2013
www.ti.com
If the input voltage falls below the forward voltage of the LED string, the LM3401 will operate at 100% duty cycle.
In this state, the anode voltage will be equal to the input voltage and LED current is determined by the v-i curve
of the LED. At 100% duty cycle, LED current will be continuous with a maximum value equal to the ILED_PK level
set at the HYS pin.
In some applications, maximum operating frequency will be limited by the minimum on-time as shown in the
following equation:
(10)
When the on-time reaches minimum (150 ns typical) due to increasing input voltage, the frequency will be
reduced in order to maintain the proper duty cycle.
INDUCTOR SELECTION
The most important parameters for the inductor are the inductance and the current rating. The LM3401 operates
over a wide frequency range and can use a wide range of inductance values.
Once an inductance value is determined from the frequency equation, the maximum operating current must be
verified.
Although peak-to-peak ripple current is controlled by the hysteresis value, there is some variation due to
propagation delay. This means that the inductance has a direct effect on LED current line regulation.
In general, a larger inductor will result in lower frequency and better line regulation. The actual peak-to-peak
inductor current can be calculated as follows:
(11)
Use the maximum value of Vin to determine the worst case ILED_RIP value. This value should be used to
determine the peak current, ILED_PK, shown in the Output Current Setting section.
Since the LM3401 can operate at 100% duty cycle, the inductor must be rated to handle ILED_PK continuously.
RIPPLE REDUCTION CAPACITOR
The typical application schematic shown on the front page is the simplest application of the LM3401. In this
schematic, inductor current is equal to LED current. Therefore, the equations for ripple current apply to both LED
ripple and inductor ripple.
However, LED ripple current can be reduced without altering the inductor current by using a bypass capacitor
placed in parallel with the LED string (shown as C2 in the Figure 24 schematic).
This allows larger hysteresis values to be used while significantly reducing ripple current in the LED string.
Figure 18 below shows this effect: inductor ripple current is unaffected while LED ripple current is dramatically
reduced.
Figure 18. LED Ripple Current Reduction with a 1 µF Ripple Reduction Capacitor
If a ripple reduction capacitor is used, the equation for SNSHYS_MAX only applies for 100% duty operation.
10
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Product Folder Links: LM3401


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