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LM1085IS-ADJ データシート(PDF) 11 Page - Texas Instruments |
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LM1085IS-ADJ データシート(HTML) 11 Page - Texas Instruments |
11 / 29 page LM1085 www.ti.com SNVS038H – JULY 1999 – REVISED JANUARY 2015 Feature Description (continued) 7.3.3 Overload Recovery Overload recovery refers to regulator's ability to recover from a short circuited output. A key factor in the recovery process is the current limiting used to protect the output from drawing too much power. The current limiting circuit reduces the output current as the input to output differential increases. Refer to short circuit curve in the Typical Characteristics section. During normal start-up, the input to output differential is small since the output follows the input. But, if the output is shorted, then the recovery involves a large input to output differential. Sometimes during this condition the current limiting circuit is slow in recovering. If the limited current is too low to develop a voltage at the output, the voltage will stabilize at a lower level. Under these conditions it may be necessary to recycle the power of the regulator in order to get the smaller differential voltage and thus adequate start up conditions. Refer to Typical Characteristics section for the short circuit current vs. input differential voltage. 7.4 Device Functional Modes 7.4.1 Output Voltage The LM1085 adjustable version develops a 1.25V reference voltage, (VREF), between the output and the adjust terminal. As shown in Figure 13, this voltage is applied across resistor R1 to generate a constant current I1. This constant current then flows through R2. The resulting voltage drop across R2 adds to the reference voltage to sets the desired output voltage. The current IADJ from the adjustment terminal introduces an output error. But since it is small (120uA max), it becomes negligible when R1 is in the 100 Ω range. For fixed voltage devices, R1 and R2 are integrated inside the devices. Figure 13. Basic Adjustable Regulator 7.4.2 Stability Consideration Stability consideration primarily concerns the phase response of the feedback loop. In order for stable operation, the loop must maintain negative feedback. The LM1085 requires a certain amount series resistance with capacitive loads. This series resistance introduces a zero within the loop to increase phase margin and thus increase stability. The equivalent series resistance (ESR) of solid tantalum or aluminum electrolytic capacitors is used to provide the appropriate zero (approximately 500 kHz). Aluminum electrolytics are less expensive than tantalums, but their ESR varies exponentially at cold temperatures; therefore requiring close examination when choosing the desired transient response over temperature. Tantalums are a convenient choice because their ESR varies less than 2:1 over temperature. The recommended load/decoupling capacitance is a 10uF tantalum or a 50uF aluminum. These values will assure stability for the majority of applications. The adjustable versions allow an additional capacitor to be used at the ADJ pin to increase ripple rejection. If this is done the output capacitor should be increased to 22 uF for tantalum or to 150 uF for aluminum. Capacitors other than tantalum or aluminum can be used at the adjust pin and the input pin. A 10uF capacitor is a reasonable value at the input. See Ripple Rejection section regarding the value for the adjust pin capacitor. Copyright © 1999–2015, Texas Instruments Incorporated Submit Documentation Feedback 11 Product Folder Links: LM1085 |
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