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 2005 Semtech Corp.

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SC1470

POWER MANAGEMENT

Figure 9: Power Component Placement and Copper Pours

Key points for the power section:

1) there should be a very small input loop, well decoupled.

2) the phase node should be a large copper pour, but compact since this is the noisiest node.

3) input power ground and output power ground should not connect directly, but through the ground planes instead.

4) Notice in Figure 9 above placement of the 0

Ω resistor at the bottom of the output capacitor to connect to VSSA.

5) The current limit resistor should be placed as close as possible to the ILIM and LX pins.

Connecting the control and power sections should be accomplished as follows (see Figure 10 below):

1) Route VSSA and VOUT as a differential pair routed in a “quiet” layer away from noise sources.

2) Route DL, DH and LX (low side FET gate drive, high side FET gate drive and phase node) to chip using wide traces

with multiple vias if using more than one layer. These connections to be as short as possible for loop minimization,

with a length to width ratio less than 20:1 to minimize impedance. DL is the most critical gate drive, with power

ground as its return path. LX is the noisiest node in the circuit, switching between V

thus should be kept as short as practical. DH has LX as its return path.

3) BST is also a noisy node and should be kept as short as possible.

4) Connect PGND pin on the chip directly to the VDDP decoupling capacitor and then drop vias directly to the ground

plane.