XR71211 Datasheet, PDF (8/12 Pages) Exar Corporation – 1.5A Ultra Low Dropout Voltage Regulator

XR71211
Functional Description
Single Rail Input Voltage
The XR71211’s ultra low input voltage allows single rail
operation from 2.625V down to 1.4V without requiring any
extra biasing voltage. This single input voltage range pro-
vides easy conversions from industry standard 1.5V, 1.8V
and 2.5V power rails.
Dropout Voltage
The dropout voltage is the input-to-output differential volt-
age at which the XR71211 ceases to regulate against fur-
ther reductions in input voltage; this point occurs when the
input voltage approaches the output voltage. The XR71211
provides a maximum of 250mV dropout voltage over full
operating temperature range making it the perfect solution
conversions requiring a 300mV differential between Vin and
Vout. For example, 1.5Vin to 1.2Vout.
The maximum output current available for a given conver-
sion is dependent on the operating conditions and thermal
performance of DFN10/PCB. See note (a) on page 3.
Setting The Output Voltage
The XR71211 is an adjustable regulator that can be pro-
grammed to any value between 0.6V and 2.4V using 2
external resistors, R1 and R2. The relationship between the
resistors and the output voltage is:
R₁
=
R₂



V_{---0-O--.--U6---T-}
–
1_
where VOUT is the desired output voltage.
Sum of resistors R1 and R2 must meet the following
requirement:
R^---V-₁^---O-+---U--R^-T---₂^{-  20mA}
and should be placed as close as possible to the pins for
optimal performance.
Because of the low voltages and high currents supported
by this device, typical capacitance values of 10uF for input
and 22uF for output are recommended when powering high
speed digital circuits.
Enable Input and Shutdown
The enable pin (EN) can be used to shut down the
XR71211. Connecting this pin to ground signal or to a volt-
age less than typical 0.55V will completely turn off the regu-
lator. Pulling the EN above 0.67V enables the XR71211.
Where EN is derived from VIN it must be sequenced with
respect to VIN as shown in the application circuit. Where an
independent control signal is applied to EN, it must be
asserted high after VIN reaches its minimum operating
range.
The enable pin is a high impedance MOS input. In order to
allow the EN pin compatibility with 5V or 3.3V control sig-
nals, there is an internal clamp set to VIN+0.5V. When
using the EN pin with a control signal greater than
VIN+0.5V, a resistor must be placed in series with the EN
pin to limit the input current below 100uA. The resistor must
also be low enough to ensure at least 15uA can be sourced
to enable the part.
The precision feature enables simple sequencing of multi-
ple power supplies with a resistor divider from another
power supply or power good flag. The EN pin can also be
used as an external UVLO to disable the part when input
voltage falls below a lower boundary of operation.
Soft Start
The XR71211 features a soft-start which controls the output
voltage ramp and allows the regulator to gradually reach
the initial steady state operating point. This reduces current
spikes and surges at start up due to output capacitor inrush
current. Internal sequencing completes in 50us, and then
the reference voltage is linearly ramped over a period of
approximately 2000us.
The inrush current will be the lesser of the current limit
value or the current defined by the equation below:
Input and Output Capacitors
The XR71211 is designed to be stable for all available types
and values of output capacitors greater than 4.7μF.
The required capacitance on the VIN and VOUT pins
strongly depends on the input supply source impedance;
the minimum recommended capacitor for VIN is 4.7μF. Low
ESR ceramic X5R and X7R capacitors are recommended
I_INRUSH = C_OUT  _2---^V-0---0^O---0^U---_-^T---_s
© 2014 Exar Corporation
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exar.com/XR71211
Rev 1B