Image sensors are used extensively in many fields, including the medical field, where they are used by doctors and hospital staff for medical diagnostics. The medical instruments are often subjected to a harsh environment that includes constantly being turned on and off, and constantly being plugged in and unplugged. Since the image sensor is the most sensitive component in the medical imager, medical device manufacturers take precautions to minimize situations that stress the circuitry or may cause the imager to malfunction. One such event is an incomplete power-up or power-down cycle.
When the instrument is turned-off, power to the image sensor ideally should be turned off as quickly as possible so the imager can quickly be restarted under ideal conditions, namely with no voltages lingering in the circuit. If the sensor is powered from a typical linear regulator, even though power to the regulator may turn off, the regulator's output is often held up by the output capacitor, allowing the image sensor to remain powered up and potentially disrupting the desired turn-off and turn-on performance of the instrument.
To ensure proper power sequencing, Analog Devices provides the LT3063 - a micropower, 1.6V to 45V input, 200mA output rugged linear regulator with an active output discharge feature that rapidly discharges the output capacitor when power is removed or the device is put into shutdown mode.
Output discharge is realized with an internal low resistance NMOS device from VOUT to ground. When the shutdown pin (SHDN) is driven low, control circuitry drives the gate of the NMOS high. Given a 2.9V output and a 10μF decoupling capacitor, the NMOS discharges the output to 290mV in 750μs. When the input pin (IN) is driven to ground, the NMOS rapidly discharges the output pin (OUT) to the threshold voltage of the NMOS, approximately 800mV. From 800mV, the external load discharges the OUT pin at a reduced rate. The control circuitry implements protection features which allow the OUT pin to be driven from –1V to 20V without adversely affecting the LT3063. Current limit foldback for output voltages greater than 6V protects the NMOS pull-down, but increases discharge times for higher output voltages.
The following graphs show the output discharge characteristic with various input voltages and as a function of temperature.
Other protective features of the LT3063 include reverse-battery protection to ±50V, reverse-current protection, current limit with foldback and thermal shutdown. The device is ideally suited for low noise portable applications with its 30µVRMS output voltage noise (10Hz to 100kHz), 45µA quiescent current and less than 3µA shutdown current. 300mV typical dropout voltage at the rated 200mA load current can also extend battery life.
In addition to medical applications, the LT3063 also targets audio systems (i.e. the audio circuitry for MP3 players, where a voltage at the regulator output may result in unwanted audible noise emanating from a speaker when power is removed), portable RF applications, modems and other non-medical portable imaging applications.