rt9202b Richtek Technology Corporation, rt9202b Datasheet - Page 12

• Manufacturer Part Number: rt9202b
• Description: Single Synchronous Buck Dc-dc Controller
• Manufacturer: Richtek Technology Corporation

RT9202B
30
10
Reference Voltage
Because RT9202B use a low 35dB gain error amplifier,
shown in Figure 7. The voltage regulation is dependent
on V
were trimmed at V
fixed V
V
www.richtek.com
12
40
20
OUT
FB
V
C
L = 2 H
V
V
V
IN
OUT
OUT
OUT
OUT
voltage can be calculated as Figure 8.
0.81
0.80
OUT
0.82
0.79
0.78
100
& V
IN
0.5
= 3.3V
= 1.5V
= 2.5V
= 3.3V
= 1500 F(33m )
= 5V application, the FB reference voltage vs.
OUT
+
-
setting. The FB reference voltage of 0.8V
1
1k
Loop Gain
IN
1.5
P
O
= 5V & V
REP
0.8V
1K
R1
= 2.9kHz
Figure 7
Figure 6
Figure 8
2
Z
10k
V
O
1.75V
OUT
= 3.2kHz
2.5
OUT
RAMP
-
+
EA
56K
(V)
R2
= 2.5V condition. In a
3
100k
3.5
VIN = 5V
+
PWM
-
4
1M
4.5
Feedback Divider
The reference of RT9202B 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 k 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 ~ 0.1 F is enough for C1.
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.
L
C
OUT
V
IN
Figure 9
R1
DS9202B-03 March 2007
<1K
R2
C1
V
OUT
RT9202B
FB