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LM2791 Datenblatt(PDF) 11 Page - Texas Instruments

Teilenummer LM2791
Bauteilbeschribung  LM2791 Current Regulated Switched Capacitor LED Driver with Analog Brightness Control
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LM2791
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SNVS156M – FEBRUARY 2001 – REVISED MAY 2013
PARALLEL Dx OUTPUTS FOR INCREASED CURRENT DRIVE
Outputs D1 and D2 may be connected together to drive a single LED. In such a configuration, two parallel
current sources of equal value drive the single LED. RSET and VBRGT should be chosen so that the current
through each of the outputs is programmed to 50% of the total desired LED current. For example, if 30mA is the
desired drive current for the single LED, RSET and VBRGT should be selected so that the current through each of
the outputs is 15mA. Connecting the outputs in parallel does not affect internal operation of the LM2791and has
no impact on the Electrical Characteristics and limits previously presented. The available Dx output current,
maximum Dx voltage, and all other specifications provided in the Electrical Characteristics table apply to this
parallel output configuration, just as they do to the standard 2-LED application circuit.
THERMAL PROTECTION
The LM2791 has internal thermal protection circuitry to disable the charge pump if the junction temperature
exceeds 150°C. This feature will protect the device from damage due to excessive power dissipation. The device
will recover and operate normally when the junction temperature falls below the maximum operating junction
temperature of 100°C. It is important to have good thermal conduction with a proper layout to reduce thermal
resistance.
POWER EFFICIENCY
An ideal power efficiency for a voltage doubler switched capacitor converter is given as the output voltage of the
doubler over twice the input voltage as follows:
Efficiency = (VDIODE* IDIODE) / ( VIN * IDIODE* Gain) = VDIODE / 2VIN
(2)
In the case of the LM2791, a more accurate efficiency calculation can be applied as the given formula below.
Efficiency = ((VD1* ID1) + (VD2* ID2)) / (ISUPPLY* VIN)
(3)
It is clear that the efficiency will depend on the supply voltage in the above equation. As such, the lower the
supply voltage, the higher the efficiency.
POWER DISSIPATION
The maximum allowable power dissipation that this package is capable of handling can be determined as
follows:
PDMax = (TJMax - TA) / θJA
(4)
where TJMax is the maximum junction temperature, TA is the ambient temperature, and θJA is the junction-to-
ambient thermal resistance of the specified package.
The actual power dissipation of the device can be calculated using this equation:
PDissipation = (2VIN -VDIODE)*ILOAD
(5)
As an example, if VIN in the target application is 4.2V, VDIODE = 3.0V and worse case current consumption is
32mA (16mA for each diode).
PDissipation = ((2*4.2) -3.0)*0.032 = 173mW
(6)
Power dissipation must be less than that allowed by the package. Please refer to the Absolute Maximum Rating
of the LM2791.
PCB LAYOUT CONSIDERATIONS
The WSON is a leadframe based Chip Scale Package (CSP) with very good thermal properties. This package
has an exposed DAP (die attach pad) at the center of the package measuring 2.0mm x 1.2mm. The main
advantage of this exposed DAP is to offer lower thermal resistance when it is soldered to the thermal land on the
PCB. For PCB layout, a 1:1 ratio between the package and the PCB thermal land is highly recommended. To
further enhance thermal conductivity, the PCB thermal land may include vias to a ground plane. For more
detailed instructions on mounting WSON packages, please refer to Application Note AN-1187. (SNOA401)
Copyright © 2001–2013, Texas Instruments Incorporated
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Product Folder Links: LM2791
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