MIC2159 Datasheet, PDF (15/17 Pages) Micrel Semiconductor – SYNCHRONOUS-itty™ Step-Down Converter IC

Micrel
Design Example
Layout and Checklist:
1. Connect the current limiting resistor directly to the
drain of top MOSFET.
2. Use a resistor from the input supply to the VIN pin on
the MIC2159. Also, place a 1µF ceramic
Capacitor from this pin to GND, preferably not thru a via.
3. The feedback resistors should be placed close to the
FB pin. The top side of the resistor should connect
directly to the output node. Run this trace away from the
switch node. The bottom side of the lower resistor
should connect to the GND pin on the MIC2159.
4. The compensation resistor and capacitors should be
placed right next to the COMP pin and the other side
should connect directly to the GND pin on the MIC2159
rather than going to the plane.
5. Add a 1.4Ω resistor and a 1000pF capacitor from the
switch node to ground pin. See page 7, Current Limiting
section for more detail.
6. Add place holders for gate resistors on the top and
MIC2159
bottom MOSFET gate drives. If necessary, gate resistors
of 5Ω or less should be used.
7. Low gate charge MOSFETs should be used to
maximize efficiency.
8. Compensation component GND, feedback resistor
ground, chip ground should all run together and connect
to the output capacitor ground. See demo board layout,
bottom layer.
9. The 10µF ceramic capacitor should be placed
between the drain of the top MOSFET and the source of
the bottom MOSFET.
10. The 10µF ceramic capacitor should be placed right
on the VDD pin without any vias.
11. The source of the bottom MOSFET should connect
directly to the input capacitor GND with a thick trace.
The output capacitor and the input capacitor should
connect directly to the GND plane.
12. Place a 0.01µF to 0.1µF Ceramic capacitor in
parallel with the CS resistor to filter any switching noise.
October 2006
15
M9999-101206