RT9041A/B
9
DS9041A/B-04 November 2014 www.richtek.com
©
Copyright 2013 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.
Application Information
The RT9041A/B is a low voltage, low dropout linear
regulator with an external bias supply input, capable of
supporting an input voltage range from 1V to 5.5V with a
fixed output voltage from 1V to 2V in 0.1V increments.
Supply Voltage Setting
The bias supply voltage (V
DD
) operates from 3V to 5.5V.
For better efficiency, it is suggested to operate V
DD
at 5V
when the output voltage is higher than 1V. Figure 1 shows
the curves of the recommended V
DD
− V
OUT
range vs. the
dropout voltage (V
IN
− V
OUT
) values.
Figure 1. Dropout Voltage vs. V
DD
− V
OUT
Output Voltage Setting
The RT9041B output voltage is also adjustable from 0.8V
to 2.5V via the external resistive voltage divider. The voltage
divider resistors can have values up to 800kΩ because of
the very high impedance and low bias current of the sense
comparator. The output voltage is set according to the
following equation :
OUT REF
R1
V = V x 1 +
R2
where V
REF
is the reference voltage with a typical value of
0.8V.
Chip Enable Operation
The RT9041A/B goes into sleep mode when the EN pin is
in a logic low condition. In this condition, the pass transistor,
error amplifier, and band gap are all turned off, reducing
the supply current to 1μA (typ.). The EN pin can be directly
tied to VIN to keep the part on.
Current Limit
The RT9041A/B contains an independent current limit
circuitry, which monitors and controls the pass transistor’s
gate voltage, limiting the output current to 0.7A (typ.).
C
IN
and C
OUT
Selection
Like any low dropout regulator, the external capacitors of
the RT9041A/B must be carefully selected for regulator
stability and performance. Using a capacitor of at least
10μF is suitable. The input capacitor must be located at a
distance of not more than 0.5 inch from the input pin of
the IC. Any good quality ceramic capacitor can be used.
However, a capacitor with larger value and lower ESR
(Equivalent Series Resistance) is recommended since it
will provide better PSRR and line transient response.
The RT9041A/B is designed specifically to work with low
ESR ceramic output capacitor for space saving and
performance consideration. Using a ceramic capacitor with
value at least 10μF and ESR larger than 2mΩ on the
RT9041A/B output ensures stability. Nevertheless, the
RT9041A/B can still work well with other types of output
capacitors due to its wide range of stable ESR. Figure 2
shows the allowable ESR range as a function of load
current for various output capacitance. Output capacitors
with larger capacitance can reduce noise and improve load
transient response, stability, and PSRR. The output
capacitor should be located at a distance of not more than
0.5 inch from the output pin of the RT9041A/B.
Dropout Voltage vs. V
DD
- V
OUT
0
50
100
150
200
250
300
350
400
450
500
2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0
V
DD
- V
OUT
(V)
Dropout Voltage (mV)
I
O
= 500mA
I
O
= 300mA
