rt9202 ETC-unknow, rt9202 Datasheet - Page 12

• Manufacturer Part Number: rt9202
• Description: Single Synchronous Buck Dc-dc Controller
• Manufacturer: ETC-unknow

40
30
20
10
RT9202
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,
requirement of fast load transient, high frequency
capacitors
recommended.
Another concern is high ESR induced ripple may
trigger UV or OV protections.
PWM Loop Stability
The RT9202 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 RT9202 Bode plot as shown Fig.6 is stable in
most of application conditions.
www.richtek.com
12
DC loop gain G
LC filter pole P
Error Amp pole P
ESR zero Z
V
V
V
V
C
L=2µH
OUT
OUT
OUT
100
OUT
OUT
= 1.5V
= 2.5V
= 3.3V
= 3.3V
= 1500µF(33mΩ)
with
it
O
=
O
is
1k
A
P
1
2
=
low
O
Loop Gain
= 35dB ×
A
× π × ESR × C
= 2.9kHz
required
1
2
= 300kHz
Fig. 6
× π ×
ESR/ESL
Z
O
10k
= 3.2kHz
1
LC
5
6 .
to
×
VOUT
meet
0
capacitors
8 .
100k
with
are
the
1M
Reference Voltage
Because RT9202 use a low 35dB gain error amplifier,
shown in Fig. 7. The voltage regulation is dependent
on V
0.8V were trimmed at V
condition. In a fixed V
reference voltage vs. V
as Fig. 8.
Feedback Divider
The reference of RT9202 is 0.8V. The output voltage
can be set using a resistor based divider as shown in
Fig.9. Put the R1 and R2 as close as possible to FB
pin and R2 should 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.
0.82
0.81
0.80
0.79
0.78
IN
FB
0.5
& V
_
OUT
1
setting. The FB reference voltage of
1.5
1K
REP
0.8V
I2
OUT
Fig. 7
Fig. 8
IN
2
= 5V application, the FB
1.75V
IN
_
voltage can be calculated
EA
V
56K
OUT
= 5V & V
I3
2.5
DS9202-02 August 2002
RAMP
(V)
3
_
PWM
OUT
3.5
= 2.5V
VIN = 5V
4
4.5