LM3463_12 Datasheet, PDF (22/40 Pages) Texas Instruments – LM3463 Dynamic Headroom Controller with Thermal Control Interface and Individual Channel Dimming Control

LM3463
SNVS807 – MAY 2012
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PROCEDURES
Begin design
REMARKS
Identify V_LED-MAX-COLD
Identify V_LED-MIN-HOT
V_LED-MAX-COLD, the highest forward
voltage of LED strings under low
temperature
V_LED-MIN-HOT, the lowest forward voltage
of LED strings under high temperature
Adjust the nominal output voltage of the
primary power supply,
V_RAIL(nom) = V_LED-MIN-HOT - 5V
Set the peak output voltage of the primary
power supply (at V_OutP = 0.15V),
V_RAIL(peak) = V_LED-MAX-COLD + 10V
Set the LED turn on voltage,
V_{DHC_READY} = V_LED-MAX-COLD + 5V
End of design
V_RAIL(nom), the nominal output voltage
of the primary power supply.
e.g. Assume V_FB = 2.5V,
V_RAIL(nom) = 2.5V
R₁ + R₂
R2
V_RAIL(peak), the output voltage of the
primary power supply when the OutP
pin is pulling to its minimum.
V_RAIL(peak) = R₁
V_FB V_FB - 0.15 - 0.6
R₂
R_DHC
V_FB
V_{DHC_READY}, the LED turn on voltage is
defined by R_FB1 and R_FB2 connected to
the VLedFB pin.
V_{DHC_READY} = 2.5V
R_FB1 + R_FB2
R_FB2
Figure 9. Procedure of defining startup parameters
Setting the R_DHC and V_RAIL
Prior to defining the parameters for the operations in steady state, the value of the R_DHC and different levels of
the supply rail voltage (V_RAIL) during system startup must be determined. Figure 9 illustrates the procedures of
determining the value of the R_DHC and voltage levels of the V_RAIL(nom), V_RAIL(peak) and V_{DHC_READY}.
In Figure 9, the V_LED-MAX-COLD and V_LED-MIN-HOT are the maximum and minimum forward voltages of the LED
strings under the required lowest and highest operation temperatures respectively. In order to ensure all the LED
string are supplied with adequate forward current when turning on the LEDs, the V_{DHC_READY} must be set higher
than the V_LED-MAX-COLD. For most applications, the V_{DHC_READY} can be set 5 V higher than the V_LED-MAX-COLD.
In order to reserve voltage headroom to perform DHC under high operation temperature, the nominal output
voltage of the primary power supply must be set lower than the V_LED-MIN-HOT. For most applications, the V_RAIL(nom)
can be set 5 V lower than the V_LED-MIN-HOT. Figure 10 shows an example connection diagram of interfacing the
LM3463 to a power supply of 2.5V feedback reference voltage.
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