rt9205 Richtek Technology Corporation, rt9205 Datasheet - Page 13

• Manufacturer Part Number: rt9205
• Description: Dual Regulators - Synchronous Buck Pwm Dc-dc And Linear Controller
• Manufacturer: Richtek Technology Corporation

Reference Voltage
Because RT9205/A uses a low 35dB gain error amplifier,
as shown in Figure 10. The voltage regulation is
dependent on V
voltage of 0.8V were trimmed at V
In a fixed V
vs. V
Feedback Divider
The reference of RT9205/A is 0.8V. The output voltage can
be set using a resistor-divider as shown in Figure 12. Put
the R1 and R2 as close as possible to FB pin. R2 value
should be less than 1 kΩ to avoid noise coupling issue.
The C1 capacitor is a speed-up capacitor for reducing output
ripple to meet with the requirement of fast transient load.
Typically, value between 1nF and 0.1μF is enough for C1.
DS9205/A-09 October 2008
FB
OUT
0.79
0.82
0.81
0.80
0.78
voltage can be calculated as Figure 11.
+
-
10
V
IN
FB
= 5V application, the FB reference voltage
20
= 0.8 - (
IN
and V
30
REP
0.8V
1K
I 2
Duty - 50
Figure 10
Figure 11
100
OUT
40
1.75V
settings. The FB reference
) 6.25mV
Duty (%)
×
50
RAMP
-
+
EA
56K
I 3
IN
= 5V and V
60
70
VIN = 5V
+
PWM
-
OUT
80
= 2.5V.
90
PWM Layout Considerations
MOSFETs switch very fast in efficiency. 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 the
layout for 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 lower MOSFET or Schottky diode. Any inductance in
the switched current path generates a large voltage spike
during the switching interval. Care with component
selections, layout of the critical components, and use
shorter and wider PCB traces that help in minimizing the
magnitude of voltage spikes.
There are two sets of critical components in a DC-DC
converter using the RT9205/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.
V
OUT
L
= V
FB
C
OUT
V
×
IN
(1 +
Figure 12
R2
R1
R1
)
<1K
R2
RT9205/A
C1
V
OUT
www.richtek.com
FB
RT9205/A
13