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PDF LM3424 Data sheet ( Hoja de datos )
| Número de pieza | LM3424 | |
| Descripción | Constant Current N-Channel Controller | |
| Fabricantes | National Semiconductor | |
| Logotipo |  |
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LM3424
August 22, 2009
Constant Current N-Channel Controller with Thermal
Foldback for Driving LEDs
General Description
The LM3424 is a versatile high voltage N-channel MosFET
controller for LED drivers . It can be easily configured in buck,
boost, buck-boost and SEPIC topologies. In addition, the
LM3424 includes a thermal foldback feature for temperature
management of the LEDs. This flexibility, along with an input
voltage rating of 75V, makes the LM3424 ideal for illuminating
LEDs in a very diverse, large family of applications.
Adjustable high-side current sense voltage allows for tight
regulation of the LED current with the highest efficiency pos-
sible. The LM3424 uses standard peak current-mode control
providing inherent input voltage feed-forward compensation
for better noise immunity. It is designed to provide accurate
thermal foldback with a programmable foldback breakpoint
and slope. In addition, a 2.45V reference is provided.
The LM3424 includes a high-voltage startup regulator that
operates over a wide input range of 4.5V to 75V. The internal
PWM controller is designed for adjustable switching frequen-
cies of up to 2.0 MHz and external synchronization is possible.
The controller is capable of high speed PWM dimming and
analog dimming. Additional features include slope compen-
sation, softstart, over-voltage and under-voltage lock-out, cy-
cle-by-cycle current limit, and thermal shutdown.
Features
■ VIN range from 4.5V to 75V
■ High-side adjustable current sense
■ 2Ω, 1A Peak MosFET gate driver
■ Input under-voltage and output over-voltage protection
■ PWM and analog dimming
■ Cycle-by-cycle current limit
■ Programmable slope compensation
■ Programmable, synchronizable switching frequency
■ Programmable thermal foldback
■ Programmable softstart
■ Precision voltage reference
■ Low power shutdown and thermal shutdown
Applications
■ LED Drivers - Buck, Boost, Buck-Boost, and SEPIC
■ Indoor and Outdoor Area SSL
■ Automotive
■ General Illumination
■ Constant-Current Regulators
Typical Application Circuit
300857k9
© 2009 National Semiconductor Corporation 300857
300857b6
www.national.com
1 page  
wwwS.DymatabSohl eet4U.com Parameter
THERMAL SHUTDOWN
TSD Thermal Shutdown
Threshold
THYS
Thermal Shutdown
Hysteresis
THERMAL RESISTANCE
θJA Junction to Ambient
Conditions
(Notes 3, 9)
(Notes 3, 9)
20L TSSOP EP (Note 4)
Min Typ Max
(Note 7) (Note 8) (Note 7)
Units
165
°C
25
34 °C/W
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur, including inoperability and degradation of device reliability
and/or performance. Functional operation of the device and/or non-degradation at the Absolute Maximum Ratings or other conditions beyond those indicated in
the Operating Ratings is not implied. The recommended Operating Ratings indicate conditions at which the device is functional and the device should not be
operated beyond such conditions.
Note 2: All voltages are with respect to the potential at the GND pin, unless otherwise specified.
Note 3: Internal thermal shutdown circuitry protects the device from permanent damage. Thermal shutdown engages at TJ=165°C (typical) and disengages at
TJ=140°C (typical).
Note 4: Junction-to-ambient thermal resistance is highly board-layout dependent. The numbers listed in the table are given for a reference layout wherein the
20L TSSOP EP package has its DAP pad populated with 9 vias. In applications where high maximum power dissipation exists, namely driving a large MosFET
at high switching frequency from a high input voltage, special care must be paid to thermal dissipation issues during board design. In high-power dissipation
applications, the maximum ambient temperature may have to be derated. Maximum ambient temperature (TA-MAX) is dependent on the maximum operating
junction temperature (TJ-MAX-OP = 125°C), the maximum power dissipation of the device in the application (PD-MAX), and the junction-to ambient thermal resistance
of the package in the application (θJA), as given by the following equation: TA-MAX = TJ-MAX-OP – (θJA × PD-MAX). In most applications there is little need for the full
power dissipation capability of this advanced package. Under these circumstances, no vias would be required and the thermal resistances would be 104 °C/W
for the 20L TSSOP EP. It is possible to conservatively interpolate between the full via count thermal resistance and the no via count thermal resistance with a
straight line to get a thermal resistance for any number of vias in between these two limits.
Note 5: Refer to National’s packaging website for more detailed information and mounting techniques. http://www.national.com/analog/packaging/
Note 6: Human Body Model, applicable std. JESD22-A114-C.
Note 7: All limits guaranteed at room temperature (standard typeface) and at temperature extremes (bold typeface). All room temperature limits are 100%
production tested. All limits at temperature extremes are guaranteed via correlation using standard Statistical Quality Control (SQC) methods. All limits are used
to calculate Average Outgoing Quality Level (AOQL).
Note 8: Typical numbers are at 25°C and represent the most likely norm.
Note 9: These electrical parameters are guaranteed by design, and are not verified by test.
Note 10: The measurements were made using the standard buck-boost evaluation board from AN-1967.
Note 11: The measurements were made using the standard boost evaluation board from AN-1969.
5 www.national.com
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FIGURE 2. Timing Circuitry
30085799
AVERAGE LED CURRENT
To first understand how the LM3424 regulates LED current,
the thermal foldback functionality will be ignored. Figure 3
shows the physical implementation of the LED current sense
circuitry assuming the thermal foldback circuitry is a simple
current source which, for now, will be set to zero (ITF = 0A).
The LM3424 uses an external current sense resistor (RSNS)
placed in series with the LED load to convert the LED current
(ILED) into a voltage (VSNS). The HSP and HSN pins are the
inputs to the high-side sense amplifier which are forced to be
equal potential (VHSP=VHSN) through negative feedback. Be-
cause of this, the VSNS voltage is forced across RHSP which
generates a current that is summed with the thermal foldback
current (ITF) to generate the signal current (ICSH) which flows
out of the CSH pin and through the RCSH resistor. The error
amplifier will regulate the CSH pin to 1.24V and assuming
ITF = 0A, ICSH can be calculated:
This means VSNS will be regulated as follows:
ILED can then be calculated:
The selection of the three resistors (RSNS, RCSH, and RHSP) is
not arbitrary. For matching and noise performance, the sug-
gested signal current ICSH is approximately 100 µA. This
current does not flow in the LEDs and will not affect either the
off-state LED current or the regulated LED current. ICSH can
be above or below this value, but the high-side amplifier offset
characteristics may be affected slightly. In addition, to mini-
mize the effect of the high-side amplifier voltage offset on LED
current accuracy, the minimum VSNS is suggested to be
50 mV. Finally, a resistor (RHSN = RHSP) should be placed in
series with the HSN pin to cancel out the effects of the input
bias current (~10 µA) of both inputs of the high-side sense
amplifier.
Note that he CSH pin can also be used as a low-side current
sense input regulated to 1.24V. The high-side sense amplifier
is disabled if HSP and HSN are tied to GND.
FIGURE 3. LED Current Sense Circuitry
11
30085757
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