RT2517A
Application Information
The RT2517A is a low voltage, low dropout linear regulator
with an external bias supply input capable of supporting
an input voltage range from 2.2V to 6V and adjustable
output voltage from 1.2V to (V
IN
−
V
DROP
).
Output Voltage Setting
The RT2517A output voltage is adjustable via the external
resistive voltage divider. The output voltage is set according
to the following equation :
V
OUT
= V
ADJ
1+ R1
R2
For ADJ pin noise immunity, the resistive divider total value
of R1 and R2 are suggested not over 100kΩ, where V
ADJ
is the reference voltage with a typical value of 1.2V.
Chip Enable Operation
Feed-Forward Capacitor (C
FF
)
The RT2517A is designed to be stable without the external
feed-forward capacitor (C
FF
). However, an external
feedforward capacitor between VOUT and ADJ pin is often
adopted to optimizes the transient, noise, and PSRR
performance. Regarding to the resistance value of the
voltage divider, the recommended C
FF
values are as below :
C
FF
= 1nF, for both R1 and R2 are larger than 1kΩ
C
FF
= 10nF, for both R1 and R2 are smaller than 1kΩ
RT2517A
V
IN
C
IN
R1
EN
GND
ADJ
R2
VIN
VOUT
*C
FF
V
OUT
C
OUT
Dropout Voltage
The dropout voltage refers to the voltage difference between
the VIN and VOUT pins while operating at specific output
current. The dropout voltage V
DROP
also can be expressed
as the voltage drop on the pass-FET at specific output
current (I
RATED
) while the pass-FET is fully operating at
ohmic region and the pass-FET can be characterized as
an resistance R
DS(ON)
. Thus the dropout voltage can be
defined as (V
DROP
= V
IN
−
V
OUT
= R
DS(ON)
x I
RATED
). For
normal operation, the suggested LDO operating range is
(V
IN
> V
OUT
+ V
DROP
) for good transient response and
PSRR ability. Vice versa, while operating at the ohmic
region will degrade these performance severely.
The RT2517A 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 only 10μA (max.). The EN pin can
be directly tied to VIN to keep the part on.
Current Limit
The RT2517A contains an independent current limit
circuitry, which controls the pass transistor's gate voltage,
limiting the output current to 1.6A (typ.).
C
IN
and C
OUT
Selection
The RT2517A is designed specifically to work with low
ESR ceramic output capacitor for space saving and
performance consideration. Using a ceramic capacitor with
capacitance range from 10μF to 47μF on the RT2517A
output ensures stability.
Input capacitance is selected to minimize transient input
droop during load current steps. For general application,
the requirement of input capacitor with a 10μF is
recommended to minimize input impedance and provide
the desired effect and do not affect stability.
Thermal Considerations
For continuous operation, do not exceed absolute
maximum junction temperature. The maximum power
dissipation depends on the thermal resistance of the IC
package, PCB layout, rate of surrounding airflow, and
Figure 2. Application Circuit with C
FF
Copyright
©
2018 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS2517A-02 August 2018
www.richtek.com
9
RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
