Datasheet LT3023 (Analog Devices) - 9

LT3023
APPLICATIONS INFORMATION
The LT3023 is a dual 100mA low dropout regulator with IN OUT1/OUT2 VOUT micropower quiescent current and shutdown. The device ⎛ 2⎞ + R V V = 1 2 . 2V 1+ I R2 IN OUT ADJ is capable of supplying 100mA per channel at a dropout LT3023 R2 ⎝⎜ R ⎠⎟ + 1 ( )( ) V = 1 2 . V 2 voltage of 300mV. Output voltage noise can be lowered ADJ ADJ1/ADJ2 GND I = n 30 A ° AT 25 C to 20μV ADJ RMS over a 10Hz to 100kHz bandwidth with the R1 OUTPUT RANGE = 1.22V TO 20V addition of a 0.01μF reference bypass capacitor. Addition- 3023 F01 ally, the reference bypass capacitor will improve transient response of the regulator, lowering the settling time for
Figure 1. Adjustable Operation
transient load conditions. The low operating quiescent The device is tested and specifi ed with the ADJ1/ADJ2 current (20μA per channel) drops to less than 1μA in pin tied to the corresponding OUT1/OUT2 pin for an out- shutdown. In addition to the low quiescent current, the put voltage of 1.22V. Specifi cations for output voltages LT3023 regulator incorporates several protection features greater than 1.22V will be proportional to the ratio of the which make it ideal for use in battery-powered systems. desired output voltage to 1.22V: VOUT/1.22V. For example, The device is protected against both reverse input and load regulation for an output current change of 1mA to reverse output voltages. In battery backup applications 100mA is –1mV typical at VOUT = 1.22V. At VOUT = 12V, where the output can be held up by a backup battery when load regulation is: the input is pulled to ground, the LT3023 acts like it has a diode in series with its output and prevents reverse current (12V/1.22V)(–1mV) = – 9.8mV fl ow. Additionally, in dual supply applications where the regulator load isreturned to a negative supply, the output
Bypass Capacitance and Low Noise Performance
can be pulled below ground by as much as 20V and still The LT3023 regulator may be used with the addition of a allow the device to start and operate. bypass capacitor from VOUT to the corresponding BYP1/ BYP2 pin to lower output voltage noise. A good quality
Adjustable Operation
low leakage capacitor is recommended. This capacitor The LT3023 has an output voltage range of 1.22V to 20V. will bypass the reference of the regulator, providing a The output voltage is set by the ratio of two external resis- low frequency noise pole. The noise pole provided by this tors as shown in Figure 1. The device servos the output bypass capacitor will lower the output voltage noise to to maintain the corresponding ADJ1/ADJ2 pin voltage as low as 20μVRMS with the addition of a 0.01μF bypass at 1.22V referenced to ground. The current in R1 is then capacitor. Using a bypass capacitor has the added benefi t equal to 1.22V/R1 and the current in R2 is the current in of improving transient response. With no bypass capacitor R1 plus the ADJ1/ADJ2 pin bias current. The ADJ1/ADJ2 and a 10μF output capacitor, a 10mA to 100mA load step pin bias current, 30nA at 25°C, fl ows through R2 into the will settle to within 1% of its fi nal value in less than 100μs. ADJ1/ADJ2 pin. The output voltage can be calculated us- With the addition of a 0.01μF bypass capacitor, the output ing the formula in Figure 1. The value of R1 should be no will stay within 1% for a 10mA to 100mA load step (see greater than 250k to minimize errors in the output voltage Transient Reponse in Typical Performance Characteristics caused by the ADJ1/ADJ2 pin bias current. Note that in section). However, regulator start-up time is proportional shutdown the output is turned off and the divider current will to the size of the bypass capacitor, slowing to 15ms with be zero. Curves of ADJ1/ADJ2 Pin Voltage vs Temperature a 0.01μF bypass capacitor and 10μF output capacitor. and ADJ1/ADJ2 Pin Bias Current vs Temperature appear in the Typical Performance Characteristics. 3023fa 9