
Preliminary
RT9263
DS9263-00 November 2001
www.richtek-ic.com.tw
7
Application Note
Output Voltage Setting
Referring to application circuits Fig.1 to Fig.4, the
output voltage of the switching regulator (V
OUT1
) can
be set with Eq.1.
Feedback Loop Design
Referring to application circuits Fig.1 to Fig.4, The
selection of R1 and R2 based on the trade-off
between
quiescent
current
interference immunity is stated below:
Follow Eq.1
Higher R reduces the quiescent current (Path
current = 1.25V/R2), however resistors beyond
5M
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 on Fig.1 to Fig.4 can improve the noise
immunity of the feedback loops, especially in an
improper layout. An empirical suggestion is around
100pF ~ 1nF for feedback resistors of M
, and
10nF ~ 0.1
μ
F for feedback resistors of tens to
hundreds K
.
consumption
and
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 “high impedance feedback
loops” are sensitive to any interference, which require
careful layout and avoid any interference, e.g.
probing to FB pins.
PRECAUTION 1
: Improper probing to FB pin will
cause fluctuation at V
OUT1
. It may damage RT9263
and system chips because V
OUT1
may drastically rise
to an over-rated level due to unexpected interference
or parasitics being added to FB pin.
PRECAUTION 2
: Disconnecting R1 or short circuit
across R2 may also cause similar IC damage as
described in precaution 1.
PRECAUTION 3
: When large R values were used in
feedback loops, any leakage in FB node may also
cause V
OUT1
voltage fluctuation, and IC damage. To
be especially highlight here is when the air moisture
frozen and re-melt on the circuit board may cause
several
μ
A leakage between IC or component pins.
So, when large R values are used in feedback loops,
post coating, or some other moisture-preventing
processes are recommended.
Layout Guide
A full GND plane without gap break.
V
OUT1
to GND noise bypass – Short and wide
connection for C2 to Pin2 and Pin5.
V
IN
to GND noise bypass – Add a 100
μ
F capacitor
close to L1 inductor, when VIN is not an idea
voltage source.
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.
V
25
.
×
)
2
R
1
R
1
V
1
OUT
+
=
Eq.1
Prober Parasitics
FB Pin
R1
R2
V
OUT1
_
Q
+