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ADP3338 bảng dữ liệu(PDF) 6 Page - Analog Devices |
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ADP3338 bảng dữ liệu(HTML) 6 Page - Analog Devices |
6 / 8 page REV. 0 ADP3338 –6– THEORY OF OPERATION The new anyCAP LDO ADP3338 uses a single control loop for regulation and reference functions. The output voltage is sensed by a resistive voltage divider consisting of R1 and R2 which is varied to provide the available output voltage option. Feedback is taken from this network by way of a series diode (D1) and a second resistor divider (R3 and R4) to the input of an amplifier. PTAT VOS gm NONINVERTING WIDEBAND DRIVER INPUT Q1 ADP3338 COMPENSATION CAPACITOR ATTENUATION (VBANDGAP/VOUT) R1 D1 R2 R3 R4 OUTPUT PTAT CURRENT (a) GND CLOAD RLOAD Figure 2. Functional Block Diagram A very high-gain error amplifier is used to control this loop. The amplifier is constructed in such a way that equilibrium pro- duces a large, temperature-proportional input, “offset voltage” that is repeatable and very well controlled. The temperature- proportional offset voltage is combined with the complementary diode voltage to form a “virtual bandgap” voltage, implicit in the network, although it never appears explicitly in the circuit. Ultimately, this patented design makes it possible to control the loop with only one amplifier. This technique also improves the noise characteristics of the amplifier by providing more flexibil- ity on the trade-off of noise sources that leads to a low noise design. The R1, R2 divider is chosen in the same ratio as the bandgap voltage to the output voltage. Although the R1, R2 resistor divider is loaded by the diode D1 and a second divider consisting of R3 and R4, the values can be chosen to produce a temperature-stable output. This unique arrangement specifically corrects for the load- ing of the divider, thus avoiding the error resulting from base current loading in conventional circuits. The patented amplifier controls a new and unique noninverting driver that drives the pass transistor, Q1. The use of this special noninverting driver enables the frequency compensation to include the load capacitor in a pole-splitting arrangement to achieve reduced sensitivity to the value, type, and ESR of the load capacitance. Most LDOs place very strict requirements on the range of ESR values for the output capacitor because they are difficult to stabilize due to the uncertainty of load capacitance and resistance. More- over, the ESR value, required to keep conventional LDOs stable, changes depending on load and temperature. These ESR limita- tions make designing with LDOs more difficult because of their unclear specifications and extreme variations over temperature. With the ADP3338 anyCAP LDO, this is no longer true. It can be used with virtually any good quality capacitor, with no constraint on the minimum ESR. This innovative design allows the circuit to be stable with just a small 1 µF capacitor on the output. Additional advantages of the pole-splitting scheme include superior line noise rejection and very high regulator gain, which leads to excellent line and load regulation. An impressive ±1.4% accuracy is guaranteed over line, load, and temperature. Additional features of the circuit include current limit and ther- mal shutdown. VIN OUT ADP3338 C1 1 F C2 1 F VOUT GND IN Figure 3. Typical Application Circuit A PPLICATION INFORMATION CAPACITOR SELECTION Output Capacitor The stability and transient response of the LDO is a function of the output capacitor. The ADP3338 is stable with a wide range of capacitor values, types, and ESR (anyCAP). A capacitor as low as 1 µF is all that is needed for stability. A higher capacitance may be necessary if high output current surges are anticipated or if the output capacitor cannot be located near the output and ground pins. The ADP3338 is stable with extremely low ESR capacitors (ESR ≈ 0), such as Multilayer Ceramic Capacitors (MLCC) or OSCON. Note that the effective capacitance of some capacitor types fall below the minimum over temperature or with dc voltage. Input Capacitor An input bypass capacitor is not strictly required but it is recom- mended in any application involving long input wires or high source impedance. Connecting a 1 µF capacitor from the input to ground reduces the circuit’s sensitivity to PC board layout and input transients. If a larger output capacitor is neces- sary, a larger value input capacitor is also recommended. OUTPUT CURRENT LIMIT The ADP3338 is short-circuit protected by limiting the pass transistor’s base drive current. The maximum output current is limited to about 2 A. See TPC 13. |
Số phần tương tự - ADP3338 |
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Mô tả tương tự - ADP3338 |
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