RT9068
11
DS9068-06 March 2017 www.richtek.com
©
Copyright 2017 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.
Applications Information
The RT9068 is a high input-voltage linear regulator
specifically designed to minimize external components.
The input voltage range is from 4.5V to 60V. The device
supplies 50mA of output current with a maximum dropout
voltage of 300mV.
Fixed V
OUT
Applications
The fixed output versions are equipped with internal
resistive dividers to set the output voltage to 2.5V, 3.3V,
or 5V, respectively. The total resistance of the each internal
feedback divider is about 1MΩ The internal dividers connect
between SENSE and GND, with the midpoint connecting
to the input of the error amplifier. These versions are all
internally compensated.
Adjustable Output Voltage and Compensation
RT9068
The adjustable output versions may be set to provide from
1.25V to 60V, using external feedback voltage divider
resistors (Figure 1). These included an internal feed-
forward compensation capacitor, connected between
SENSE and FB, which may be used when regulating at
output voltages up to 25V. To achieve the correct
compensation (together with your external FB divider, use
an upper divider resistor (R1) value between 500kΩ and
1MΩ. Calculate R2 according to the following formula :
R2 = R1 / (V
OUT
/ 1.23V − 1).
For output voltages greater than 25V an external feed-
forward capacitor (Figure 2) should be used and SENSE
should be left unconnected. Choose a lower divider resistor
(R2) at least 10kΩ and calculate R1 according to the
following formula : R1 = R2 x (V
OUT
/ 1.23V − 1), Then,
calculate the compensation capacitor (C
COMP
) value
according to the following formula : C
COMP
= 25μs / R1.
Figure 1. RT9068 Adjustable Output Up to 25V
Figure 2. RT9068 Adjustable Output Above 25V
VIN
GND
VOUT
RT9068
V
IN
V
OUT
4.5V to 60V
C
IN
0.18µF
SHDN
C
OUT
1µF
SENSE
1.25V to 25V
R1
R2
FB
Remote-Sense Applications
Since VOUT and SENSE (and FB) are separate pins, it is
possible to connect SENSE to the output voltage remotely
at the load (as long as the output voltage is 25V or less).
FB can also be connected remotely, with SENSE
unconnected if the output exceeds 25V. Remote sensing
eliminates output voltage errors due to any resistive voltage
drop between VOUT and the load. (Voltage drops in the
ground connection cannot be compensated.)
Added External NPN for High-Current Applications
Higher output currents and/or increased power dissipation
are possible using an external NPN output transistor. VOUT
drives the base of the transistor and SENSE and/or FB
monitors the actual output voltage, as in normal
applications. The fixed-output versions (Figure 3) or
adjustable-output version (Figure 4) can be used.
Figure 3 and Figure 4 have no current limit and should be
used with care. If a crude current limit is desired, add the
two diodes and resistor shown in Figure 5. The approximate
current limit is 0.7V/R3. Any small switching diodes can
be used and size R3 for the power dissipated in R3, which
is typically 0.7V x 0.7V / R3.
Figure 3. RT9068-25/33/50 External Transistor
Application
VIN
GND
VOUT
RT9068
V
IN
V
OUT
4.5V to 60V
C
IN
0.18µF
SHDN
C
OUT
10µF
SENSE
2.5V/3.3V/5V
100
MJD31C
VIN
GND
VOUT
RT9068
V
IN
4.5V to 60V
C
IN
0.18µF
SHDN
C
OUT
1µF
SENSE
R1
R2
FB
C
COMP
V
OUT
1.25V to 60V
