RT9209 RICHTEK [Richtek Technology Corporation], RT9209 Datasheet - Page 12

• Manufacturer Part Number: RT9209
• Description: Synchronous Buck PWM DC-DC with Enable & PGOOD
• Manufacturer: RICHTEK [Richtek Technology Corporation]

RT9209/A
Input / Output Capacitor
High frequency/long life decoupling capacitors should be
placed as close to the power pins of the load as physically
possible. Be careful not to add inductance to the PCB
trace, as it could eliminate the performance from utilizing
these low inductance components. Consult with the
manufacturer of the load on specific decoupling
requirements.
The output capacitors are necessary for filtering output
and stabilizing the close loop (see the PWM loop stability).
For powering advanced, high-speed processors, it is
required to meet with the requirement of fast load transient,
high frequency capacitors with low ESR/ESL capacitors
are recommended.
Another concern is high ESR induced ripple may trigger
UV or OV protections
PWM Loop Stability
The RT9209/A is a voltage mode buck controller designed
for 5V step-down applications. The gain of error amplifier
is fixed at 35dB for simplified design.
The output amplitude of ramp oscillator is 1.6V, the loop
gain and loop pole/zero are calculated as follows :
The RT9209/A Bode plot as shown Figure 5 is stable in
most of application conditions.
www.richtek.com
12
DC loop gain G = 35 dB
LC filter pole P =
Error Amp pole P = 300kHz
ESR zero Z =
40
30
20
10
V
C
L = 2uH
V
V
V
OUT
OUT
OUT
OUT
100
OUT
= 3.3V
= 1.5V
= 2.5V
= 3.3V
= 1500uF(33mΩ)
O
O
A
2
1
A
× ×
1k
2
1
Loop Gain
π
× ×
P
Figure 5
π
ESR C
O
= 2.9kHz
×
10k
Z
1.6
O
LC
5
×
= 3.2kHz
×
V
0.8
OUT
100k
1M
Reference Voltage
Because RT9209/A use a low 35dB gain error amplifier,
shown in Figure 6. The voltage regulation is dependent
on V
were trimmed at V
fixed V
V
Feedback Divider
The reference of RT9209/A is 0.8V. The output voltage can
be set using a resistor based divider as shown in Figure 8.
Put the R1 and R2 as close as possible to FB pin and R2
should less than 1 kΩ to avoid noise coupling.
OUT
FB
IN
0.79
0.82
0.81
0.80
0.78
voltage can be calculated as Figure 7.
& V
IN
0.5
= 5V application, the FB reference voltage vs.
+
-
OUT
A
setting. The FB reference voltage of 0.8V
1
VIN
L
IN
1.5
REF
0.8V
R1
1K
= 5V & V
Figure 6
Figure 7
Figure 8
2
C
OUT
1.75V
V
2.5
OUT
OUT
DS9209/A-08 March 2007
+
RAMP
-
EA
56K
R2
R1
R2
(V)
= 2.5V condition. In a
V
OUT
3
3.5
FB
RT9209/A
VIN = 5V
+
PWM
-
4
4.5