 2012 Microchip Technology Inc.

DS01427A-page 9

AN1427

LAYOUT TIPS

To design a low noise, high-efficiency converter, it is

necessary to observe the current paths. Everything

should be placed as tightly as possible and connected

by generous copper areas. Only use traces instead of

copper pours where the current ripple is small (e.g.,

after the output capacitors). Even in that case, traces

should be thick enough to accommodate the passing

current.

Because current limiting is done on the current peak in

a pulse-by-pulse fashion, it is needed to have a proper

inductor current waveform. The inductor and the output

capacitors filter the output current nicely and make the

waveform unsuitable for the PIC MCU comparator.

Normally, a high side current sensor would do the trick,

but that would complicate the layout and add a lot of

extra components. A quick solution is to connect the

output capacitors to the ground through the current

shunt. There is a very small loss of efficiency because

of the higher ESR, but the current waveform has the

correct shape. Even with this trick measured, efficiency

goes above 90% at a 3A output current.

Closing ground loops properly means the layout needs

to have a short (and clear of obstruction) path between

the ground connections of important components: input

capacitors, low-side switch and output capacitors (cur-

rent shunt in this case, because of the trick mentioned

earlier). It is ideal to use solid ground connections for

these components but, if it really makes it impossible to

solder the components by hand, use a thermal relief

with very thick spokes (40 mils or more).

Use ground planes on both sides, if possible, and a

generous number of vias to decrease the impedance.

One side should have unobstructed, short paths

between component ground connections. If it is not

possible to use ground planes on both sides, use cop-

per pours (on the side you cannot use a plane on), and

pack it with vias to minimize the impedance to the other

side.

Other things that need to be laid out carefully:

- Connections from the MCP14628 driver to

the MOSFET gates should be thick and

short, because peak current is 2A. The mea-

sured positive and negative edges on the

transistor gates are < 10 ns, resulting in a sig-

nal bandwidth close to 1 GHz.

- MCP14628 should be decoupled properly

with a capacitor placed very close to the

power pin. When driving the transistor gates,

high current pulses are needed.

- MCP14628 BOOT and PHASE pins should

have short and thick connections.

- Capacitor C9 (See Figure 9) from the current

sense RC filter should be placed close to the

microcontroller comparator pin to properly

attenuate high-frequency switching noise.

CONCLUSIONS

The new generation of microcontroller peripherals like

the COG allows easy implementation of cost-effective,

high-performance switching converters, while allowing

a lot of flexibility. Current mode converters have many

important applications, especially in LED lighting.

Bringing together the flexibility of a PIC microcontroller

and a synchronous topology will result in a high-

efficiency power supply design (>90%) with a lot of

intelligence and room for customization.

REFERENCES

•“MCP14628 – 2A Synchronous Buck Power

•“PIC12F/LF1822/PIC16F/LF1823 8/14-Pin Flash

MCUs w/ nanoWatt XLP Technology“ (DS41413),

•“PIC12F752/HV752 Data Sheet“ (DS41576),