Texas Instruments LM3401 Hysteretic PFET Controller for High Power LED Drive Evaluation Module LM3401EVAL/NOPB LM3401EVAL/NOPB 데이터 시트
제품 코드
LM3401EVAL/NOPB
Ripple Reduction Capacitor C3
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Ripple Reduction Capacitor C3
The component C3 is optional and not installed on the standard demo board. This capacitor is used to
reduce LED ripple current without changing average LED current or inductor current. Without C3, the LED
ripple current is equal to the inductor ripple current. The peak-to-peak ripple current values shown in
reduce LED ripple current without changing average LED current or inductor current. Without C3, the LED
ripple current is equal to the inductor ripple current. The peak-to-peak ripple current values shown in
are without C3 installed. A typical C3 value of 1 µF is recommended for most applications. Any
type of low ESR capacitor can be used. However, the working voltage must be rated higher than the
maximum input voltage. C3 can also be used to reduce switching noise which could affect the SNS and
DIM pins, especially at higher LED current. For noise reduction purposes, a C3 value of 220 pF is
recommended.
maximum input voltage. C3 can also be used to reduce switching noise which could affect the SNS and
DIM pins, especially at higher LED current. For noise reduction purposes, a C3 value of 220 pF is
recommended.
Table 2. Typical Operating Conditions
# of Series
LED Type
Average LED
Freq.
(1)
p-p Ripple
(1)
Minimum VIN
LEDs
Current
1
Osram golden dragon
340 mA
425 kHz
110 mA
4.5V
2
Osram golden dragon
340 mA
655 kHz
110 mA
7.0V
3
Luxeon Star
340 mA
800 kHz
110 mA
10.4V
4
Luxeon Star
340 mA
845 kHz
110 mA
13.8V
1
Luxeon V Star
700 mA
415 kHz
190 mA
7.0V
2
Luxeon V Star
700 mA
500 kHz
190 mA
14.5V
1
Luxeon K2 Emitter
1A
230 kHz
280 mA
4.5V
(1)
at 24Vin
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PWM Dimming
The DIM terminal on the PCB provides an input for a pulse width modulated (PWM) signal to control
dimming of the LED string. The PWM signal should have a minimum logic high level of 2.25V, and
minimum low and high periods of 100 ns. For the default demo board circuit, a dimming frequency less
than 1kHz is recommended to maintain an adequate contrast ratio. The maximum contrast ratio is shown
in
dimming of the LED string. The PWM signal should have a minimum logic high level of 2.25V, and
minimum low and high periods of 100 ns. For the default demo board circuit, a dimming frequency less
than 1kHz is recommended to maintain an adequate contrast ratio. The maximum contrast ratio is shown
in
, and assumes a minimum of 2 switching cycles. The maximum contrast ratio depends largely
on the rise time of the inductor current, and therefore is dependant on input voltage, inductor size, and
LED string forward voltage. Additionally, if C3 is installed, the maximum contrast ratio or DIM frequency
will be further reduced.
LED string forward voltage. Additionally, if C3 is installed, the maximum contrast ratio or DIM frequency
will be further reduced.
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Open Circuit/Short Circuit
If the LED array is disconnected or fails open, the LM3401 will operate at 100% duty cycle. The output
voltage (at LED+) and CS node will rise to equal the input voltage; the circuit is rated to 40V and will not
suffer damage. However, care should be taken not to connect an LED string when the demo board is
powered up and the output is high as excessive surge current may damage the LEDs.
voltage (at LED+) and CS node will rise to equal the input voltage; the circuit is rated to 40V and will not
suffer damage. However, care should be taken not to connect an LED string when the demo board is
powered up and the output is high as excessive surge current may damage the LEDs.
The default current limit threshold is typically 1.5A, set by R3. The current limit will protect the circuit from
a short at the LED anode or cathode. Current limit is sensed across the R
a short at the LED anode or cathode. Current limit is sensed across the R
DSon
of the PFET and will
therefore vary with temperature. R3 can be adjusted if a different current limit threshold is required. For
the current limit details, refer to LM3401 Hysteretic PFET Controller for High Power LED Drive
(
the current limit details, refer to LM3401 Hysteretic PFET Controller for High Power LED Drive
(
).
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SNVA254C – August 2007 – Revised April 2013
AN-1657 LM3401 Demo Board
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