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FN6288.5
October 7, 2008
components that connect to sensitive nodes or supply critical
bypassing current and signal coupling.
Equally important are the connections of the internal gate
drives (UGATE, LGATE, PHASE, PGND, BOOT): since they
drive the power train MOSFETs using short, high current
pulses, it is important to size them accordingly and reduce
their overall impedance. While not always esthetically
pleasing, straightest connections encircling the least area
result in the lowest parasitic inductance build-up, and,
consequentially, are the better choice.
The power train components should be placed first. Locate
the input capacitors close to the power switches. Minimize
the length of the connections between the input capacitors,
C
IN
, especially the high frequency decoupling, and the
power switches. Locate the output inductor and output
capacitors between the MOSFETs and the load. Locate all
the high-frequency decoupling capacitors (ceramics) as
close as practicable to their decoupling target, making use of
the shortest connection paths to any internal planes, such as
vias to GND immediately next, or even onto the capacitor’s
grounded solder pad.
The critical small signal components include the bypass
capacitors for VIN, VCC and PVCC. Locate the bypass
capacitors, C
BP
, close to the device. It is especially
important to locate the components associated with the
feedback circuit close to their respective controller pins,
since they belong to a high-impedance circuit loop, sensitive
to EMI pick-up. Place all the other highlighted components
close to the respective pins of the ISL6540A.
A multi-layer printed circuit board is recommended. Figure 8
shows the connections of the critical components of the
converter. Note that capacitors C
xxIN
and C
xxOUT
could each
represent numerous physical capacitors. Dedicate one solid
layer, usually the one underneath the component side of the
board, to a ground plane and make all critical component
ground connections with vias to this layer. Dedicate another
solid layer as a power plane and break this plane into smaller
islands of common voltage levels. Keep the PHASE island as
small as practicable, while still allowing for proper heat-sinking
of the lower MOSFET. The power plane should support the
input power and output power nodes. Use copper-filled
polygons on the top and bottom circuit layers for large
current-carrying circuit nodes. Use the remaining printed
circuit layers for small signal wiring.
Size the trace interconnects commensurate with the signals
they are carrying. Use narrow (0.004” to 0.008”) and short
traces for the high-impedance, small-signal connections, such
as the feedback, compensation, soft-start, frequency set,
reference input, offset, etc. The wiring traces from the IC to
the MOSFETs’ gates and sources should be wide (0.02” to
0.05”) and short, encircling the smallest area possible.
The metal pad of the ISL6540A’s package should be
connected to the ground plane via 6 to 9 small vias evenly
placed in the bottom pad’s footprint. The GND and PGND
pins should be connected to this bottom pad to find a
convenient, low inductance path to the rest of the circuitry.
This recommended connection provides not only an
electrically low impedance path, but a low thermal path as
well, helping with the heat dissipation taking place in the
part.
Compensating the Converter
The ISL6540A single-phase converter is a voltage-mode
controller. This section highlights the design considerations for
a voltage-mode controller requiring external compensation. To
address a broad range of applications, a type-3 feedback
network is recommended (see Figure 7).
FIGURE 7. COMPENSATION CONFIGURATION FOR
ISL6540A WHEN USING DIFFERENTIAL REMOT
SENSE
ISL6540A
COMP
C
1
R
2
R
1
FB
VMON
C
2
R
3
C
3
ISL6540A
