MIC2182_04 Datasheet, PDF (20/28 Pages) Micrel Semiconductor – High-Efficiency Synchronous Buck Controller

MIC2182
pins and routed with a wide trace. The capacitor should be a
good quality tantalum. An additional 1µF ceramic capacitor
may be necessary when driving large MOSFETs with high
gate capacitance. Incorrect placement of the V_DD decoupling
capacitor will cause jitter or oscillations in the switching
waveform and large variations in the overcurrent limit.
A 0.1µF ceramic capacitor is required to decouple the VIN.
The capacitor should be placed near the IC and connected
directly to between pin 10 (Vcc) and pin 12 (PGND).
PCB Layout and Checklist
PCB layout is critical to achieve reliable, stable and efficient
performance. A ground plane is required to control EMI and
minimize the inductance in power, signal and return paths.
The following guidelines should be followed to insure proper
operation of the circuit.
• Signal and power grounds should be kept
separate and connected at only one location.
Large currents or high di/dt signals that occur
when the MOSFETs turn on and off must be
kept away from the small signal connections.
• The connection between the current-sense
resistor and the MIC2182 current-sense inputs
(pin 8 and 9) should have separate traces,
routed from the terminals directly to the IC pins.
The traces should be routed as closely as
possible to each other and their length should be
minimized. Avoid running the traces under the
inductor and other switching components. A 1nF
to 0.1µF capacitor placed between pins 8 and 9
will help attenuate switching noise on the current
sense traces. This capacitor should be placed
close to pins 8 and 9.
Micrel
• When the high-side MOSFET is switched on, the
critical flow of current is from the input capacitor
through the MOSFET, inductor, sense resistor,
output capacitor, and back to the input capacitor.
These paths must be made with short, wide
pieces of trace. It is good practice to locate the
ground terminals of the input and output capaci-
tors close to each.
• When the low-side MOSFET is switched on,
current flows through the inductor, sense
resistor, output capacitor, and MOSFET. The
source of the low-side MOSFET should be
located close to the output capacitor.
• The freewheeling diode, D1 in Figure 2, con-
ducts current during the dead time, when both
MOSFETs are off. The anode of the diode
should be located close to the output capacitor
ground terminal and the cathode should be
located close to the input side of the inductor.
• The 4.7µF capacitor, which connects to the VDD
terminal (pin 11) must be located right at the IC.
The VDD terminal is very noise sensitive and
placement of this capacitor is very critical.
Connections must be made with wide trace. The
capacitor may be located on the bottom layer of
the board and connected to the IC with multiple
vias.
• The V_IN bypass capacitor should be located
close to the IC and connected between pins 10
and 12. Connections should be made with a
ground and power plane or with short, wide
trace.
M9999-042204
20
April 22, 2004