RT9203 RICHTEK [Richtek Technology Corporation], RT9203 Datasheet - Page 14

• Manufacturer Part Number: RT9203
• Description: Dual Regulators - Synchronous Buck PWM DC-DC and Linear Controller
• Manufacturer: RICHTEK [Richtek Technology Corporation]

RT9203/A
Reference Voltage
Because RT9203/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
2.5V. In a fixed V
voltage vs. V
Feedback Divider
The reference of RT9203/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 k
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.
www.richtek.com
14
FB
0.82
0.81
0.80
0.79
0.78
10
+
-
20
OUT
IN
voltage can be calculated as Figure 11.
IN
and V
30
REP
0.8V
= 5V application, the FB reference
R1
1K
Figure 10
Figure 11
40
OUT
to avoid noise coupling issue.
1.75V
Duty (%)
settings. The FB reference
50
+
RAMP
-
EA
56K
R1
IN
60
= 5V and V
70
PWM
+
-
80
Preliminary
OUT
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 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 RT9203/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.
V
OUT
= V
L
FB
C
OUT
V
(1+
IN
Figure 12
R2
R1
R1
)
DS9203/A-10 March 2007
<1K
R2
C1
V
OUT
FB
RT9203/A