RT9202 Datasheet, PDF (13/16 Pages) Richtek Technology Corporation – SINGLE SYNCHRONOUS BUCK PWM DC-DC CONTROLLER

RT9202
VIN
L
+
C_OUT
V_OUT
R1
C1 _RT9202
FB
R2
< 1K
Fig. 9
PWM Layout Considerations
MOSFETs switch very fast and efficiently. The speed
with which the current transitions from one device to
another causes voltage spikes across the
interconnecting impedances and parasitic circuit
elements. The voltage spikes can degrade efficiency
and radiate noise, that results in ocer-voltage stress
on devices. Careful component placement layout and
printed circuit design can minimize the voltage spikes
induced in the converter. Consider, as an example,
the turn-off transition of the upper MOSFET prior to
turn-off, the upper MOSFET was carrying the full load
current. During turn-off, current stops flowing in the
upper MOSFET and is picked up by the low side
MOSFET or Schottky diode. Any inductance in the
switched current path generates a large voltage spike
during the switching interval. Careful component
selections, layout of the critical components, and use
shorter and wider PCB traces help in minimizing the
magnitude of voltage spikes.
There are two sets of critical components in a DC-DC
converter using the RT9202. The switching power
components are most critical because they switch
large amounts of energy, and as such, they tend to
generate equally large amounts of noise. The critical
small signal components are those connected to
sensitive nodes or those supplying critical bypass
current.
The power components and the PWM controller
should be placed firstly. Place the input capacitors,
especially the high-frequency ceramic decoupling
capacitors, close to the power switches. Place the
output inductor and output capacitors between the
MOSFETs and the load. Also locate the PWM
controller near by MOSFETs.
A multi-layer printed circuit board is recommended.
Fig.10 shows the connections of the critical
components in the converter. Note that the capacitors
CIN and COUT each of them represents numerous
physical capacitors. Use a dedicated grounding plane
and use vias to ground all critical components to this
layer. Apply another solid layer as a power plane and
cut this plane into smaller islands of common voltage
levels. The power plane should support the input
power and output power nodes. Use copper filled
polygons on the top and bottom circuit layers for the
PHASE node, but it is not necessary to oversize this
particular island. Since the PHASE node is subjected
to very high dV/dt voltages, the stray capacitance
formed between these island and the surrounding
circuitry will tend to couple switching noise. Use the
remaining printed circuit layers for small signal
routing. The PCB traces between the PWM controller
and the gate of MOSFET and also the traces
connecting source of MOSFETs should be sized to
carry 2A peak currents.
5V
GND
IQ1
IL
+
Q1
IQ2
Q2
V_OUT
+
+
LOAD
LGATE VCC GND
UGATE
RT9202
FB
Fig. 10
DS9202-02 August 2002
www.richtek.com
13