Datasheet

LED5000 Application notes - buck conversion
Doc ID 023951 Rev 1 27/51
The input and output loops are minimized to avoid radiation and high frequency resonance
problems. The feedback pin to the sensing resistor path must be designed as short as
possible to avoid pick-up noise. Another important issue is the ground plane of the board.
Since the package has an exposed pad, it is very important to connect it to an extended
ground plane in order to reduce the thermal resistance junction-to-ambient and increase the
noise immunity during the switching operation.
In addition, to increase the design noise immunity, different signal and power grounds
should be designed in the layout (see
Chapter 5.13: Application circuit ). The signal ground
serves the small signal components, the device analog ground pin, the exposed pad and a
small filtering capacitor connected to the VCC pin. The power ground serves the device
ground pin and the input filter. The different grounds are connected underneath the output
capacitor. Neglecting the current ripple contribution, the current flowing through this
component is constant during the switching activity and so this is the cleanest ground point
of the buck application circuit.
Figure 17. Layout example
5.11 Thermal considerations
The dissipated power of the device is tied to three different sources:
● Conduction losses due to the R
DSON
, which are equal to:
Equation 37
Where D is the duty cycle of the application. Note that the duty cycle is theoretically given by
the ratio between V
OUT
(n
LED
∗V
LED
+ 200 mV) and V
IN
, but in practice it is substantially
AM13501v1
P
ON
R
RDSON_HS
I
OUT
()⋅
2
D ⋅=