rt9275 Richtek Technology Corporation, rt9275 Datasheet - Page 13

• Manufacturer Part Number: rt9275
• Description: Tiny Package, High Efficiency, Step-up Dc/dc Converter
• Manufacturer: Richtek Technology Corporation

Application Information
Output Voltage Setting
Referring to Typical Application Circuits, the output voltage
of the switching regulator (V
(1).
Feedback Loop Design
Referring to Typical Application Circuits. The selection of
R1 and R2 based on the trade-off between quiescent current
consumption and interference immunity is stated below:
For applications without standby or suspend modes, lower
values of R1 and R2 are preferred. For applications
concerning the current consumption in standby or suspend
modes, the higher values of R1 and R2 are needed. Such
“
interference, which require careful layout and avoid any
interference, e.g. probing to FB pin.
DS9275-06 March 2007
high impedance feedback loops
Follow Equation (1)
Higher R reduces the quiescent current (Path current =
1.25V/R2), however resistors beyond 5MΩ are not
recommended.
Lower R gives better noise immunity, and is less sensitive
to interference, layout parasitics, FB node leakage, and
improper probing to FB pins.
A proper value of feed forward capacitor parallel with R1
can improve the noise immunity of the feedback loops,
especially in an improper layout. An empirical suggestion
is
2
π
R1C4
1
Q
+
_
V
Prober Parasitics
OUT
=
10
= ( 1+
to
20kHz
R2
R1
OUT
) 1.25V
×
) can be set with Equation
”
V
are sensitive to any
OUT
R1
R2
FB Pin
Preliminary
(1)
Layout Guide
Board Layout Example (2-Layer Board)
(Refer to Typical Application Circuit for the board)
A full GND plane without gap break.
V
for the 1μF MLCC capacitor between Pin5 and Pin3.
V
inductor, when V
Minimized FB node copper area and keep far away from
noise sources.
Minimized parasitic capacitance connecting to LX and
EXT nodes, which may cause additional switching loss.
DD
IN
to GND noise bypass
to GND noise bypass
IN
- Bottom Layer -
is not an idea voltage source.
- Top Layer -
−
−
Add a capacitor close to L1
Short and wide connection
RT9275
www.richtek.com
13