10
FN9042.8
June 8, 2006
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, radiate noise into the circuit, and lead to
device overvoltage stress. Careful component layout and
printed circuit design minimizes the voltage spikes in the
converter. Consider, as an example, the turn-off transition of
one of the upper PWM MOSFETs. Prior to turn-off, the upper
MOSFET is carrying the full load current. During the turn-off,
current stops flowing in the upper MOSFET and is picked up
by the lower MOSFET. Any inductance in the switched
current path generates a voltage spike during the switching
interval. Careful component selection, tight layout of the
critical components, and short, wide circuit traces minimize
the magnitude of voltage spikes. See the Application Note
AN9983 for the evaluation board component placement and
the printed circuit board layout details.
There are two sets of critical components in a DC/DC
converter using an ISL6224 controller. The switching power
components are the 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 bias currents.
Power Components Layout Considerations
The power components and the controller IC should be
placed first. Locate the input capacitors, especially the high-
frequency ceramic decoupling capacitors, close to the power
MOSFETs. Locate the output inductor and output capacitors
between the MOSFETs and the load. Locate the PWM
controller close to the MOSFETs.
Insure the current paths from the input capacitors to the
MOSFETs, to the output inductors and output capacitors are
as short as possible with maximum allowable trace widths.
A multi-layer printed circuit board is recommended. Dedicate
one solid layer for a ground plane and make all critical
component ground connections with vias to this layer.
Dedicate another solid layer as a power plane and break 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 nodes, but do not
unnecessarily oversize these particular islands. Since the
phase nodes are subjected to very high dV/dt voltages, the
stray capacitor formed between these islands and the
surrounding circuitry will tend to couple switching noise. Use
the remaining printed circuit layers for small signal wiring.
The wiring traces from the control IC to the MOSFET gate
and source should be sized to carry 2A peak currents.
Small Components Signal Layout Considerations
The Vin pin 1 input should be bypassed with a 1.0µF
capacitor. The bypass capacitors for Vin and the soft-start
capacitor, should be located close to their connecting pins on
the control IC.
Refer to the Application Note AN9983 for a recommended
component placement and interconnections.
Figures 5, 6 and 7 show application circuits for the three modes
of operation. Mode 1 is operating from battery voltage and
operating at 300kHz switching frequency. Mode 2 is operating
off of 5V and operating at 300kHz switching frequency. Mode 3
is operating off of 5V and operating at 600kHz switching
frequency.
ISL6224
