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LED5000

Application notes - buck conversion

Doc ID 023951 Rev 1

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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 RDSON, 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 VOUT (nLED∗VLED + 200 mV) and VIN, but in practice it is substantially

AM13501v1

RDSON_HS

OUT

()

⋅

LED5000 数据表(PDF) 27 Page - STMicroelectronics