RT9202 Ver la hoja de datos (PDF) - Richtek Technology
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RT9202 Datasheet PDF : 14 Pages
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RT9202
VOUT = 3.3V
COUT = 1500µF(33mΩ)
L = 2µH
40 VOUT = 1.5V
VOUT = 2.5V
30 VOUT = 3.3V
PO = 2.9kHz
ZO = 3.2kHz
20
Loop Gain
10
100
1k
10k
100k
1M
Figure 6
Reference Voltage
Because RT9202 use a low 35dB gain error amplifier,
shown in Figure 7. The voltage regulation is dependent
on VIN & VOUT setting. The FB reference voltage of 0.8V
were trimmed at VIN = 5V & VOUT = 2.5V condition. In a
fixed VIN = 5V application, the FB reference voltage vs.
VOUT voltage can be calculated as Figure 8.
FB
+
-
R1
1K
REP
0.8V
R2
56K
-
EA
+
RAMP
1.75V
+
PWM
-
Figure 7
0.82
0.81
0.80
0.79
VIN = 5V
0.78
0.5 1 1.5 2
2.5 3
3.5 4 4.5
VOUT (V)
Figure 8
Feedback Divider
The reference of RT9202 is 0.8V. The output voltage can
be set using a resistor based divider as shown in Figure
9. Put the R1 and R2 as close as possible to FB pin and
R2 should less than 1 kΩ to avoid noise coupling. The C1
capacitor is a speed-up capacitor for reducing output ripple
to meet with the requirement of fast transient load.
Typically a 1nF to 0.1µF is enough for C1.
VIN
L
COUT
VOUT
R1
C1
R2
<1K
RT9202
FB
Figure 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 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.
www.richtek.com
12
DS9202-09 March 2007
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