RT9204_07 Datasheet, PDF (12/14 Pages) Richtek Technology Corporation – Dual Regulators - Standard Buck PWM DC-DC and Linear Controller

RT9204/A
Preliminary
Reference Voltage
Because RT9204/A use a low 35dB gain error amplifier,
shown in Figure 10. The voltage regulation is dependent
on V_IN & V_OUT setting. The FB reference voltage of 0.8V
were trimmed at V_IN = 5V & V_OUT = 2.5V condition. In a
fixed V_IN = 5V application, the FB reference voltage vs.
V_OUT voltage can be calculated as Figure 11.
FB
+
-
I2
1K
REP
0.8V
I3
56K
-
EA
+
RAMP
1.75V
+
PWM
-
Figure 10
0.82
0.81
0.80
VIN = 5V
0.79
0.78
0.5 1 1.5 2 2.5 3 3.5 4 4.5
V_OUT (V)
Figure 11
Feedback Divider
The reference of RT9204/A is 0.8V. Both the PWM and
LDO output voltages can be set using a resistor based
divider as shown in Figure 12. Put the R1&R2 and R3&R4
as close as possible to FB pin and R2&R4 should be 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 ~
0.1µF is enough for C1.
V_IN
L
V_OUT1
C_OUT
R1
C1
V_OUT1 = 0.8V
(1+
R1
R2
)
R2
<1K
V_OUT2 = 0.8V (1+ _RR43)
DRV
RT9204/A
FB FBL
V_OUT2
R3
R4
R4<1K
Figure 12
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 ocer-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 RT9204/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.
www.richtek.com
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DS9204/A-08 March 2007