MIC2168A_15 Datasheet, PDF (16/18 Pages) Micrel Semiconductor – 1MHz PWM Synchronous Buck-Control IC

Micrel, Inc.
MIC2168A
Total Open-Loop Response
The open-loop response for the MIC2168A controller is
easily obtained by adding the power path and the error
amplifier gains together, since they already are in Log
scale. It is desirable to have the gain curve intersect
zero dB at tens of kilohertz, this is commonly called
crossover frequency; the phase margin at crossover
frequency should be at least 45°. Phase margins of 30°
or less cause the power supply to have substantial
ringing when subjected to transients, and have little
tolerance for component or environmental variations.
Figure 9 and Figure 10 show the open-loop gain and
phase margin. It can be seen from Figure 9 that the gain
curve intersects the 0dB at approximately 50kHz, and
from Figure 10 that at 50kHz, the phase shows
approximately 50° of margin.
Figure 9. Open-Loop Gain Margin
Figure 10. Open-Loop Phase Margin
Design Example
Layout and Checklist:
1. Connect the current limiting (CS) resistor directly to
the drain of top MOSFET Q1.
2. Use a 10Ω resistor from the input supply to the V_IN
pin on the MIC2168A. Also, place a 1µF ceramic
capacitor from this pin to GND not through via.
3. The feedback resistors R1 and R2 should be placed
close to the FB pin. The top side of R1 should
connect directly to the output node. Run this trace
away from the switch node (junction of Q1, Q2, and
L1). The bottom side of R1 should connect to the
GND pin on the MIC2168A.
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
MIC2168A rather than going to the plane.
5. Add a snubber circuit (resistor and a capacitor) from
the switch node to GND. A good starting point is
1000pF and 1.4Ω.
6. Add a place holder for a gate resistor on the top
MOSFET gate drive. A gate resistors of 10Ω or less
should be used. No gate resistor should be used on
the low side MOSFET.
7. Low gate charge MOSFETs should be used to
maximize efficiency, such as Si4800, Si4804BDY,
IRF7821, IRF8910, FDS6680A, and FDS6912A to
mention a few.
8. Add a 1Ω to 4Ω resistor from the SW pin on the
MIC2168A to the switch node on the circuit (junction
of MOSFETs and inductor).
9. Compensation component GND, feedback resistor
ground, chip ground, 1µF V_IN ceramic capacitor
ground, and 10µF V_DD capacitor ground should all
run together and connect to the output capacitor
ground. See demo board layout, top layer.
10. The 10µF ceramic input capacitor should be placed
between the drain of top MOSFET and source of
bottom MOSFET.
11. The 10µF ceramic capacitor should be placed right
on the V_DD pin without any vias.
12. 12. 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.
13. 13. Place a 0.1µF ceramic capacitor in parallel with
the CS resistor to filter any switching noise.
April 22, 2015
16
Revision 3.0