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RT9259C View Datasheet(PDF) - Richtek Technology

Part Name
Description
MFG CO.
RT9259C Datasheet PDF : 15 Pages
RT9259C
2
4-Layers PCB
1.75
QFN-16L 4x4
1.5
1.25
SOP-14
1
0.75 SSOP-16
0.5
0.25
0
0
25
50
75
100
125
Ambient Temperature (°C)
Figure 11. Derating Curves for RT9259C 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 RT9259C. 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 12 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.
IQ1
IL
5V/12V
GND
Q1
IQ2
Q2
VOUT
LOAD
LGATE VCC GND
UGATE
RT9259C
FB
Figure 12. The connections of the critical components in
the converter
www.richtek.com
12
DS9259C-03 August 2007
 

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