RT8101 Просмотр технического описания (PDF) - Richtek Technology
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RT8101 Datasheet PDF : 15 Pages
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Preliminary
RT8101/A
1.4
PSOP-8
1.2
1
0.8
SOP-8
0.6
0.4
0.2
0
0
20 40 60 80 100 120 140
Ambient Temperature (°C)
Figure 10. Derating Curves for RT8101/A Packages
PCB 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 over-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 RT8101/A. 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.
Figure 11 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 islands 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/12V
GND
IQ1
IL
Q1
IQ2
Q2
VOUT
LOAD
LGATE VCC GND
UGATE
RT8101/A
FB
Figure 11. The connections of the critical components
in the converter
DS8101/A-01 March 2007
www.richtek.com
13
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