PRELIMINARY TECHNICAL DATA ADP1715/ADP1716
Rev. PrA | Page 7 of 9
THEORY OF OPERATION
Introduction
The ADP1715/16 is a low-dropout linear regulator that uses
an advanced, proprietary architecture to provide high power
supply rejection ratio (PSRR) and excellent line and load
transient response with just a small 2.2µF ceramic output
capacitor. Supply current in shutdown mode is reduced to
1µA.
Three versions are available: one with fixed output voltage
and soft-start, one with fixed output voltage and output
voltage tracking, and one with adjustable output voltage. The
fixed output voltage is internally set to one of sixteen values
between 0.75V and 3.3V. The adjustable output voltage is set
between 0.8V and 5.0V by an external voltage divider from
OUT to ADJ. The soft-start uses an external capacitor at SS
to control the output voltage ramp. In the case of the
adjustable version, ADP1715-ADJ, the soft-start is controlled
by an internal 3pF capacitor, limiting the output voltage ramp-
up period to approximately 24µs. Tracking limits the output
voltage to the at-or-below the voltage at TRK.
The ADP1715/16 is available in a thermally-enhanced MSOP
package, making it a very compact solution for applications
requiring up to 500mA in a small, low-profile footprint.
Soft-start Function (ADP1715)
For applications that require a controlled startup, the ADP1715
provides a programmable soft-start function. Programmable
soft-start is useful for reducing inrush current upon startup and
for providing voltage sequencing. To implement soft-start,
connect a capacitor from SS to GND. Upon startup, a 1µA
current source charges the capacitor. The ADP1715 startup
output voltage is limited by the voltage at SS, providing a
smooth ramp up to the nominal output voltage. The soft-start
time is calculated by TSS= VREF x (CSS/ISS), where TSS is the
soft-start period, VREF is the 0.8V reference voltage, CSS is the
soft-start capacitance from SS to GND, and ISS is the current
sourced from SS (1µA). Note that when the ADP1715 is
disabled, the soft-start capacitor is discharged to GND through
an internal 100Ω resistor.
In the case of the adjustable output version, there is no pin for
soft-start, so the function is switched to an internal 3pF
capacitor. This sets the soft-start ramp-up period to
approximately 24µs. Using the suggested 2.2µF output
capacitor, the resulting input inrush current is approximately
460mA, which is less than the maximum 500mA load current.
Voltage Tracking Function (ADP1716)
For applications that require voltage tracking, the ADP1716
provides a TRK pin. The ADP1716’s output voltage is not
allowed to exceed the voltage on the TRK pin, allowing control
of the output voltage of the ADP1716 by another voltage rail in
the system. The ADP1716’s tracking function is always
enabled, hence if the voltage applied to TRK pin drops below
the output voltage at any time, the ADP1716’s output tracks it
down. This ensures that the ADP1716 provides proper voltage
tracking under startup and shutdown as well as under fault
conditions.
The ADP1716 is only available in the fixed output voltage
version. A voltage divider is present from TRK to the error
amplifier input whose divider ratio is equal to the divider from
OUT to the error amplifier. This sets the output voltage equal
to the tracking voltage. Both divider ratios are set by post-
package trim depending on the desired output voltage.
Output Capacitor Selection
The ADP1715/16 is optimized for operation with small ceramic
capacitors. For output voltages, VOUT ≥ 2V, the ADP1715/16
operates with just 2.2µF of output capacitance. For output
voltages below 1.8V, additional output capacitance may be
required. Ceramic capacitors are manufactured with a variety of
dielectrics, each with different behavior over temperature and
applied voltage. When choosing your capacitor be sure to
choose a dielectric adequate to ensure the minimum
capacitance over the necessary temperature range.
Thermal Protection and Current Limit
The ADP1715/16 contains internal current limiting to protect
itself and it’s loads during overload and short-circuit
conditions. In addition to current limiting, this device contains
automatic thermal overload protection circuitry to provide
additional protection under continuous short-cir cuit conditions.
Adjustable Output Voltage
The ADP1715-ADJ’s output voltage is adjustable over a 0.8V
to 5.0V range. The output voltage is set by connecting a
resistive voltage divider from OUT to ADJ. The output voltage
is calculated using the equation VOUT=0.8V(1+RTOP/RBOTTOM)
where RTOP is the resistor from OUT to ADJ and RBOTTOM is the
resistor from ADJ to GND. The maximum bias current into
ADJ is 100nA, so for less than 0.1% error due to the bias
current, use values less than 12kΩ for RBOTTOM.
Shutdown
The ADP1715/16 includes a low power shutdown state entered
by driving EN to GND. In shutdown, power consumption is
1µA. When the ADP1715-XX is disabled, the soft-start
capacitor CSS is discharged to GND with a 100Ω resistance.
Package and Power Dissipation
The ADP1715/16 use a thermally-enhanced MSOP package,
which achieves a respectable TBD°C/W for such a small
footprint.