Datasheet LT3027 (Analog Devices) - 9

LT3027
U U W U APPLICATIO S I FOR ATIO
The LT3027 is a dual 100mA low dropout regulator with OUT VOUT independent inputs, micropower quiescent current and IN + ⎛ R2⎞ V = 1.22V 1+ I R OUT ADJ 2 shutdown. The device is capable of supplying 100mA per R2 ⎝⎜ R ⎠⎟ + ( )( ) 1 LT3027 VIN = channel at a dropout voltage of 300mV. Output voltage V 1.22V ADJ ADJ I = n 30 A AT 25 C ° noise can be lowered to 20µV GND ADJ RMS over a 10Hz to 100kHz R1 OUTPUT RANGE 1 22V TO = . 20V bandwidth with the addition of a 0.01µF reference bypass 3027 F01 capacitor. Additionally, the reference bypass capacitor will improve transient response of the regulator, lowering the
Figure 1. Adjustable Operation
settling time for transient load conditions. The low oper- ating quiescent current (25µA per channel) drops to less The device is tested and specified with the ADJ pin tied to than 1µA in shutdown. In addition to the low quiescent the corresponding OUT pin for an output voltage of 1.22V. current, the LT3027 regulator incorporates several pro- Specifications for output voltages greater than 1.22V will tection features which make it ideal for use in battery- be proportional to the ratio of the desired output voltage to powered systems. The device is protected against reverse 1.22V: VOUT/1.22V. For example, load regulation for an input voltages. Additionally, in dual supply applications output current change of 1mA to 100mA is –1mV typical where the regulator load is returned to a negative supply, at VOUT = 1.22V. At VOUT = 12V, load regulation is: the output can be pulled below ground by as much as 20V (12V/1.22V)(–1mV) = – 9.8mV and still allow the device to start and operate.
Bypass Capacitance and Low Noise Performance Adjustable Operation
The LT3027 regulator may be used with the addition of a The LT3027 has an output voltage range of 1.22V to 20V. bypass capacitor from V The output voltage is set by the ratio of two external resis- OUT to the corresponding BYP pin to lower output voltage noise. A good quality low leakage tors as shown in Figure 1. The device servos the output to capacitor is recommended. This capacitor will bypass the maintain the corresponding ADJ pin voltage at 1.22V ref- reference of the regulator, providing a low frequency noise erenced to ground. The current in R1 is then equal to 1.22V/ pole. The noise pole provided by this bypass capacitor will R1 and the current in R2 is the current in R1 plus the ADJ lower the output voltage noise to as low as 20µV pin bias current. The ADJ pin bias current, 30nA at 25°C, RMS with the addition of a 0.01µF bypass capacitor. Using a bypass flows through R2 into the ADJ pin. The output voltage can capacitor has the added benefit of improving transient be calculated using the formula in Figure 1. The value of R1 response. With no bypass capacitor and a 10µF output should be no greater than 250k to minimize errors in the capacitor, a 10mA to 100mA load step will settle to within output voltage caused by the ADJ pin bias current. Note that 1% of its final value in less than 100µs. With the addition in shutdown the output is turned off and the divider current of a 0.01µF bypass capacitor, the output will stay within will be zero. Curves of ADJ Pin Voltage vs Temperature and 1% for a 10mA to 100mA load step (see Transient Reponse ADJ Pin Bias Current vs Temperature appear in the Typical in Typical Performance Characteristics section). However, Performance Characteristics. regulator start-up time is inversely proportional to the size of the bypass capacitor, slowing to 15ms with a 0.01µF bypass capacitor and 10µF output capacitor. 3027fa 9