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Performance Data and Typical Characteristic Curves
9
SLUUBM0 – May 2017
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TPS92518EVM Dual Buck Controller Evaluation Module
3.4.2
Diode, Inductor, and Boot Capacitor Current at High Output Current
Showing inductor current into the LED and current flow through the catch diode and boot capacitor used for supplying
gate drive power. V
IN
= 65 V, V
LED
= 23 V, T
OFF
register setting 50, peak threshold register setting = 1
Figure 11. Switch Node Current
3.4.3
Shunt FET Dimming
Shunt FET dimming is simple with the TPS92518. Short leads between the evaluation board and the LED
load boards are important to prevent V
LED
overshoot. Locating the shunt FET on or near the LED load
board also helps to reduce V
LED
overshoot. Adding an appropriately rated diode from the LED+ line that
conducts back to the positive V
IN
input will clamp voltage overshoot.
NOTE:
There is no provision for mounting such a diode on the board: it must be soldered into the
wiring used to connect the shunt FET into the circuit.
Similarly, repopulating R17 and C21 with different values will also protect the V
LED
pin from overshoots.
Figure 12
illustrates the circuitry modifications for shunt FET dimming: increasing the size of R17, adding
a diode to V
IN
, and adding an appropriately-rated FET with a gate pulldown resistor. Either adding a diode
or increasing R17 should be adequate. While the modified schematic shows both output capacitors
removed, replacing one with a 220-pF capacitor rather than just removing both may give better V
LED
overshoot performance.
