Datasheet ADP1715, ADP1716 (Analog Devices) - 10

ADP1715/ADP1716 THEORY OF OPERATION
The ADP1715/ADP1716 are low dropout, CMOS linear connect a small ceramic capacitor from SS to GND. Upon regulators that use an advanced, proprietary architecture to startup, a 1.2 μA current source charges this capacitor. The provide high power supply rejection ratio (PSRR) and excellent ADP1715 start-up output voltage is limited by the voltage at SS, line and load transient response with just a small 2.2 μF ceramic providing a smooth ramp up to the nominal output voltage. The output capacitor. Both devices operate from a 2.5 V to 5.5 V soft-start time is calculated by input rail and provide up to 500 mA of output current. Supply TSS = VREF × (CSS/ISS) (1) current in shutdown mode is typically 100 nA. where: TSS is the soft-start period.
IN OUT
VREF is the 0.8 V reference voltage. CSS is the soft-start capacitance from SS to GND. ISS is the current sourced from SS (1.2 μA).
CURRENT LIMIT THERMAL PROTECT
When the ADP1715 is disabled (using EN), the soft-start capacitor is discharged to GND through an internal 100 Ω resistor.
SHUTDOWN SS/ ADJ/ TRK EN SOFT EN REFERENCE START
1 -02
GND
0 11 06 Figure 26. Internal Block Diagram
IV D 1
Internally, the ADP1715/ADP1716 consist of a reference, an
2V/
error amplifier, a feedback voltage divider, and a PMOS pass
OUT
transistor. Output current is delivered via the PMOS pass
VIN = 5V V
device, which is controlled by the error amplifier. The error
OUT = 3.3V IV C /D 2 OUT = 2.2µF
amplifier compares the reference voltage with the feedback
C
41
SS = 22nF
0
1V I
0-
LOAD = 500mA
11 voltage from the output and amplifies the difference. If the 06
TIME (4ms/DIV)
feedback voltage is lower than the reference voltage, the gate of Figure 27. OUT Ramp-Up with External Soft-Start Capacitor the PMOS device is pulled lower, allowing more current to pass and increasing the output voltage. If the feedback voltage is The ADP1715 adjustable version and the ADP1716 have no higher than the reference voltage, the gate of the PMOS device pins for soft start, so the function is switched to an internal soft- is pulled higher, allowing less current to pass and decreasing the start capacitor. This sets the soft-start ramp-up period to output voltage. approximately 24 μs. For the worst-case output voltage of 5 V, using the suggested 2.2 μF output capacitor, the resulting input The ADP1715 is available in two versions, one with fixed output inrush current is approximately 460 mA, which is less than the voltage options and one with an adjustable output voltage. The maximum 500 mA load current. fixed output voltage options are set internally to one of sixteen values between 0.75 V and 3.3 V, using an internal feedback network. The adjustable output voltage can be set to between 0.8 V and 5.0 V by an external voltage divider connected from
EN
OUT to ADJ. The fixed output version of ADP1715 allows for connection of an external soft-start capacitor, which controls the output voltage ramp during startup. The ADP1716 features
IV
a track pin and is available with fixed output voltage options. All
/D 1 2V
devices are controlled by an enable pin (EN).
SOFT-START FUNCTION (ADP1715) V
For applications that require a controlled startup, the ADP1715
IN = 5V IV OUT VOUT = 1.6V /D 2
42 provides a programmable soft-start function. Programmable
COUT = 2.2µF
0
1V I
0-
LOAD = 10mA
11 soft start is useful for reducing inrush current upon startup and 06
TIME (20µs/DIV)
for providing voltage sequencing. To implement soft start, Figure 28. OUT Ramp-Up with Internal Soft-Start Rev. 0 | Page 10 of 20 Document Outline FEATURES APPLICATIONS TYPICAL APPLICATION CIRCUITS GENERAL DESCRIPTION TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS THERMAL RESISTANCE ESD CAUTION PIN CONFIGURATIONS AND FUNCTION DESCRIPTIONS TYPICAL PERFORMANCE CHARACTERISTICS THEORY OF OPERATION SOFT-START FUNCTION (ADP1715) ADJUSTABLE OUTPUT VOLTAGE (ADP1715 ADJUSTABLE) TRACK MODE (ADP1716) ENABLE FEATURE APPLICATION INFORMATION CAPACITOR SELECTION CURRENT LIMIT AND THERMAL OVERLOAD PROTECTION THERMAL CONSIDERATIONS PRINTED CIRCUIT BOARD LAYOUT CONSIDERATIONS OUTLINE DIMENSIONS ORDERING GUIDE