RT9164A
Application Information
Output voltage adjustment
Like most regulators, the RT9164A regulates the output
by comparing the output voltage to an internally generated
reference voltage. On the adjustable version as shown in
Figure 4, the V
REF
is available externally as 1.25V between
V
OUT
and ADJ. The voltage ratio formed by R1 and R2
should be set to conduct 10mA (minimum output load).
The output voltage is given by the following equation:
V
OUT
= V
REF
(1+
internally.
R
2
R
1
Output Capacitor
RT9164A requires a capacitor from V
OUT
to GND to provide
compensation feedback to the internal gain stage. This is
to ensure stability at the output terminal. Typically, 10μF
tantalum or 50μF aluminum electrolytic
with 30mΩ to 2Ω
range capacitor is sufficient.
The output capacitor does not have
a theoretical upper
limit and increasing its value will increase stability.
C
OUT
= 100μF or more is typical for high current regulator
design.
) + I
ADJ
R
2
On fixed versions of RT9164A, the voltage divider is provided
Region of Stable C
OUT
ESR vs. Load Current
10
V
IN
C
IN
10uF
+
RT9164A
VIN
VOUT
ADJ
I
ADJ
65uF
C
OUT
= 100uF
V
OUT
Instable
10uF
+
C
OUT
ESR (Ω)
V
REF
R1
C
OUT
1
C
OUT
= 10uF
Stable
+
C
ADJ
R2
0.1
Figure 4. Basic Adjustable Regulator
Input Bypass Capacitor
An input capacitor is recommended. A 10μF tantalum on
the input is a suitable input bypassing for almost all
applications.
Adjust Terminal Bypass Capacitor
The adjust terminal can be bypassed to ground with a
bypass capacitor (C
ADJ
) to improve ripple rejection. This
bypass capacitor prevents ripple from being amplified as
the output voltage is increased. At any ripple frequency,
the impedance of the C
ADJ
should be less than R1 to prevent
the ripple from being amplified:
(2π x f
RIPPLE
x C
ADJ
) < R1
The R1 is the resistor between the output and the adjust
pin. Its value is normally in the range of 100-200Ω. For
example, with R1 = 124Ω and f
RIPPLE
= 120Hz, the C
ADJ
should be > 11μF.
Load Regulation
0.01
0
0.2
0.4
Instable
0.6
0.8
1
Load Current (A)
Figure 5
When the adjustable regulator is
used (Figure 6), the
best
load regulation is accomplished when the top of the resistor
divider (R1) is connected directly to the output pin of the
RT9164A. When so connected, R
P
is not multiplied by
the divider ratio. For Fixed output version, the top of R1 is
internally connected to the output and ground pins can be
connected to low side of the load.
www.richtek.com
10
DS9164A-13 August 2007
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
