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LTC3114-1 Datenblatt(PDF) 24 Page - Analog Devices |
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LTC3114-1 Datenblatt(HTML) 24 Page - Analog Devices |
24 / 34 page LTC3114-1 24 Rev. D For more information www.analog.com APPLICATIONS INFORMATION A simplified diagram of the average output current pro- gramming circuitry is shown in the Block Diagram. An internal sense resistor, RS, and low offset amplifier directly measure current in the VOUT path and produce a small fraction of this current out of the PROG pin. Accordingly, a resistor and filtering capacitor connected from PROG to ground produce a voltage proportional to average output current on PROG. An internal transconductance amplifier compares the PROG voltage to the fixed 1V internal refer- ence. If the PROG voltage tries to exceed the 1V reference level, this amplifier will pull down on VC and take com- mand of the PWM. As described earlier, VC is the current command voltage, so limiting VC in this manner will also limit output current. The resulting average output current is given by the following equation: IOUT(AVG) ≅ 25,000 • 1V RPROG where: RPROG = 24.9k to 100k. The largest recommended PROG pin resistor is 100k. Values of RPROG larger than 100k may latch-off the LTC3114-1 if VOUT is forced to less than 2V by an external load. This is generally not an issue for battery charging applications, but may prevent the charging of very large capacitors. In some general purpose power supply applications, this latch-off behavior may be desirable and in these cases, values of RPROG > 100k are acceptable to use. The gain of 25,000 is generated internal to the LTC3114-1 and is factory trimmed to provide the best accuracy at 500mA of output current. The accuracy of the pro- grammed output current is best at the high end of the range as the residual internal current sense amplifier off- set becomes a smaller percentage of the total current sense signal amplitude with increasing current. The pro- vided electrical specifications define the PROG pin current accuracy over a range of output currents. Selecting the capacitor, CPROG, to put in parallel with RPROG is a trade-off between response time, output cur- rent ripple and interaction with the normal output voltage control loop. In general, if speed is not a concern as is the case for most current sourcing applications, then CPROG should be made at least 3 times higher than the voltage error amplifier compensation capacitor, CP1, described in the Compensation section of this data sheet. This will ensure minimal to no interaction when the transition occurs between voltage regulation mode and output cur- rent regulation mode. In current sourcing applications, the maximum output compliance voltage of the LTC3114-1 is set by the voltage error amplifier dividers resistors as it is for standard voltage regulation applications. For LED drivIng appli- cations, select the VOUT divider resistors for a clamping level 1V to 2V higher than the expected forward voltage drop of the LED string. The average output current cir- cuitry can also be used to monitor, rather than control the output current. To do this, select an RPROG value that will limit the voltage on the PROG pin to 0.8V or less at the highest output current expected in the application. Connect a 20k resistor and 33nF capacitor from PROG to ground if the function is not going to be used to provide a higher level of protection against inadvertent short-circuit conditions on VOUT. Compensation of the Buck-Boost Converter The LTC3114-1 utilizes average current mode control to regulate the output voltage. Average current mode control has two loops that require frequency compensation, the inner average current loop and the outer voltage loop. The compensation for the inner average current loop is fixed within the LTC3114-1 in order to provide the highest possible bandwidth over the wide operating range of the LTC3114-1. Therefore, the only control loop that requires compensation design is the outer voltage loop. As will be shown, compensation design of the outer loop is similar to the techniques used in well known peak current mode control devices. The LTC3114-1 utilizing average current mode control can be conceptualized in its simplest form as a voltage-con- trolled current source (VCCS), driving the output load formed primarily by RLOAD and COUT, as shown in Figure 6. The error amplifier output (VC), provides the command input to the VCCS. The full-scale range of VC is 0.865V (135mV to 1V). With a full-scale command on VC, the LTC3114-1 buck-boost converter will generate an average 1.7A of inductor current (typical) from the converter for a |
Ähnliche Teilenummer - LTC3114-1 |
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Ähnliche Beschreibung - LTC3114-1 |
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