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LT3021ES8-1.2 Datasheet(PDF) 10 Page - Linear Dimensions Semiconductor |
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LT3021ES8-1.2 Datasheet(HTML) 10 Page - Linear Dimensions Semiconductor |
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10 / 16 page  LT3021/LT3021-1.2/ LT3021-1.5/LT3021-1.8 10 3021fc APPLICATIONS INFORMATION The LT3021 is a very low dropout linear regulator capable of 1V input supply operation. Devices supply 500mA of output current and dropout voltage is typically 155mV. Quiescent current is typically 120μA and drops to 3μA in shutdown. The LT3021 incorporates several protection features, making it ideal for use in battery-powered sys- tems. The device protects itself against reverse-input and reverse-output voltages. In battery backup applications where the output is held up by a backup battery when the input is pulled to ground, the LT3021 acts as if a diode is in series with its output which prevents reverse current flow. In dual supply applications where the regulator load is returned to a negative supply, the output can be pulled below ground by as much as 10V without affecting start-up or normal operation. Adjustable Operation The LT3021’s output voltage range is 0.2V to 9.5V. Figure 1 shows that the output voltage is set by the ratio of two external resistors. The device regulates the output to main- tain the ADJ pin voltage at 200mV referenced to ground. The current in R1 equals 200mV/R1 and the current in R2 is the current in R1 minus the ADJ pin bias current. The ADJ pin bias current of 20nA flows out of the pin. Use the formula in Figure 1 to calculate output voltage. An R1 value of 20k sets the resistor divider current to 10μA. Note that in shutdown the output is turned off and the divider current is zero. Curves of ADJ Pin Voltage vs Temperature and ADJ Pin Bias Current vs Temperature appear in the Typical Performance Characteristics section. Specifications for output voltages greater than 200mV are proportional to the ratio of desired output voltage to 200mV; (VOUT/200mV). For example, load regulation for an output current change of 1mA to 500mA is typically 0.4mV at VADJ = 200mV. At VOUT = 1.5V, load regulation is: (1.5V/200mV) • (0.4mV) = 3mV Output Capacitance and Transient Response The LT3021’s design is stable with a wide range of output capacitors, but is optimized for low ESR ceramic capacitors. The output capacitor’s ESR affects stability, most notably with small value capacitors. Use a minimum output ca- pacitor of 3.3μF with an ESR of 0.2Ω or less to prevent oscillations. The LT3021 is a low voltage device, and output load transient response is a function of output capacitance. Larger values of output capacitance decrease the peak deviations and provide improved transient response for larger load current changes. For output capacitor values greater than 22μF a small feedforward capacitor with a value of 300pF across the upper divider resistor (R2 in Figure 1) is required. Under extremely low output current conditions (ILOAD < 30μA) a low frequency small signal oscillation (200Hz/8mVP-P at 1.2V output) can occur. A minimum load of 100μA is recommended to prevent this instability. Give extra consideration to the use of ceramic capacitors. Manufacturers make ceramic capacitors with a variety of dielectrics, each with a different behavior across tempera- ture and applied voltage. The most common dielectrics are Z5U, Y5V, X5R and X7R. The Z5U and Y5V dielectrics provide high C-V products in a small package at low cost, but exhibit strong voltage and temperature coefficients. The X5R and X7R dielectrics yield highly stable character- isitics and are more suitable for use as the output capacitor at fractionally increased cost. The X5R and X7R dielectrics both exhibit excellent voltage coefficient characteristics. The X7R type works over a larger temperature range and exhibits better temperature stability whereas X5R is less expensive and is available in higher values. Figures 2 and 3 show voltage coefficient and temperature coefficient comparisons between Y5V and X5R material. Voltage and temperature coefficients are not the only sources of problems. Some ceramic capacitors have a piezoelectric response. A piezoelectric device generates voltage across its terminals due to mechanical stress, similar to the way a piezoelectric accelerometer or micro- phone works. For a ceramic capacitor, the stress can be induced by vibrations in the system or thermal transients. The resulting voltages produced can cause appreciable Figure 1. Adjustable Operation IN SHDN R2 R1 3021 F01 OUT VIN ADJ GND LT3021 VOUT + R2 R1 VOUT = 200mV VADJ = 200mV IADJ = 20nA AT 25°C OUTPUT RANGE = 0.2V TO 9.5V 1 + – IADJ (R2) () |
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