LED2001PHR Datasheet, PDF (26/42 Pages) STMicroelectronics – 4 A monolithic step-down current source with synchronous rectification

Application information
LED2001
7.2
Layout considerations
The layout of switching DC-DC converters is very important to minimize noise and
interference. Power-generating portions of the layout are the main cause of noise and so
high switching current loop areas should be kept as small as possible and lead lengths as
short as possible.
High impedance paths (in particular the feedback connections) are susceptible to
interference, so they should be as far as possible from the high current paths. A layout
example is provided in Figure 16.
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.
As 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.
To increase the design noise immunity, different signal and power ground should be
implemented in the layout (see Section 7.5: 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 16. Layout example
26/42
DocID024346 Rev 1