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LT1964 Datasheet(PDF) 11 Page - Linear Technology |
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LT1964 Datasheet(HTML) 11 Page - Linear Technology |
11 / 16 page LT1964 11 1964fb APPLICATIONS INFORMATION Figure 1. Adjustable Operation The LT1964 is a 200mA negative low dropout regulator with micropower quiescent current and shutdown. The device is capable of supplying 200mA at a dropout voltage of 340mV. Output voltage noise can be lowered to 30μVRMS over a 10Hz to 100kHz bandwidth with the addition of a 0.01μF reference bypass capacitor. Additionally, the refer- ence bypass capacitor will improve transient response of the regulator, lowering the settling time for transient load conditions. The low operating quiescent current (30μA) drops to 3μA in shutdown. In addition to the low quies- cent current, the LT1964 incorporates several protection features which make it ideal for use in battery-powered systems. In dual supply applications where the regulator load is returned to a positive supply, the output can be pulled above ground by as much as 20V and still allow the device to start and operate. Adjustable Operation The adjustable version of the LT1964 has an output volt- age range of –1.22V to –20V. The output voltage is set by the ratio of two external resistors as shown in Figure 1. The device servos the output to maintain the voltage at the ADJ pin at –1.22V referenced to ground. The current in R1 is then equal to –1.22V/R1 and the current in R2 is the current in R1 plus the ADJ pin bias current. The ADJ pin bias current, 30nA at 25°C, flows through R2 out of the ADJ pin. The output voltage can be calculated using the formula in Figure 1. The value of R1 should be less than 250k to minimize errors in the output voltage caused by the ADJ pin bias current. Note that in shutdown the output is turned off and the divider current will be zero. Curves of ADJ Pin Voltage vs Temperature and ADJ Pin Bias Current vs Temperature appear in the Typical Performance Characteristics section. The adjustable device is tested and specified with the ADJ pin tied to the OUT pin and a 5μA DC load (unless otherwise specified) for an output voltage of –1.22V. Specifications for output voltages greater than –1.22V will be propor- tional to the ratio of the desired output voltage to –1.22V; (VOUT/–1.22V). For example, load regulation for an output current change of 1mA to 200mA is 2mV typical at VOUT = –1.22V. At VOUT = –12V, load regulation is: (–12V/–1.22V) • (2mV) = 19.6mV Bypass Capacitance and Low Noise Performance The LT1964 may be used with the addition of a bypass capacitor from VOUT to the BYP pin to lower output voltage noise. A good quality low leakage capacitor is recom- mended. This capacitor will bypass the reference of the LT1964, providing a low frequency noise pole. The noise pole provided by this bypass capacitor will lower the out- put voltage noise to as low as 30μVRMS with the addition of a 0.01μF bypass capacitor. Using a bypass capacitor has the added benefit of improving transient response. With no bypass capacitor and a 10μF output capacitor, a –10mA to –200mA load step will settle to within 1% of its final value in less than 100μs. With the addition of a 0.01μF bypass capacitor, the output will stay within 1% for the same –10mA to –200mA load step (see LT1964-5 Transient Response in the Typical Characteristics section). However, regulator start-up time is proportional to the size of the bypass capacitor. Higher values of output voltage noise may be measured if care is not exercised with regard to circuit layout and testing. Crosstalk from nearby traces can induce unwanted noise onto the output of the LT1964-X. 1964 F01 GND ADJ IN OUT LT1964 VIN VOUT + R1 R2 VOUT = –1.22V(1 + ) – (IADJ)(R2) VADJ = –1.22V IADJ = 30nA AT 25°C OUTPUT RANGE = –1.22V TO –20V R2 R1 |
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