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LT1931 数据表(PDF) 7 Page - Linear Technology |
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LT1931 数据表(HTML) 7 Page - Linear Technology |
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7 / 12 page  7 LT1931/LT1931A 1931fa START-UP/SOFT-START For most LT1931/LT1931A applications, the start-up in- rush current can be high. This is an inherent feature of switching regulators in general since the feedback loop is saturated due to VOUT being far from its final value. The APPLICATIO S I FOR ATIO regulator tries to charge up the output capacitor as quickly as possible, which results in a large inrush current. Fig- ure 6 shows a typical oscillograph of the start-up wave- form for the application of Figure 1 starting into a load of 33 Ω. The lower waveform shows SHDN being pulsed from 0V to 5V. The middle waveform shows the input current, which reaches as high as 0.8A. The total time required for the output to reach its final value is approxi- mately 500 µs. For some applications, this initial inrush current may not be acceptable. If a longer start-up time is acceptable, a soft-start circuit consisting of RSS and CSS, as shown in Figure 7, can be used to limit inrush current to a lower value. Figure 8 shows the relevant waveforms with RSS = 15k and CSS = 33nF. Input current, measured at VIN, is limited to a peak value of 0.5A as the time required to reach final value increases to 1ms. In Figure 9, CSS is VOUT 20mV/DIV AC COUPLED IL1A + IL1B 0.5A/DIV AC COUPLED 200mA 100mA LOAD CURRENT 100 µs/DIV 1931 F04 Figure 4. Transient Response of Inverting Converter with 220pF Phase Lead Capacitor VOUT 0.1V/DIV AC COUPLED IL1A + IL1B 0.5A/DIV AC COUPLED 200mA 100mA LOAD CURRENT 50 µs/DIV 1931 F05 Figure 5. Transient Response of Inverting Converter with 22 µF Tantalum Output Capacitor and No Phase Lead Capacitor VIN VIN 5V VSS VOUT VOUT –5V 1931 F07 SW L1A 10 µH CURRENT PROBE L1B 10 µH D1 GND LT1931 C1 4.7 µF RSS 15k D2 1N4148 C3 22 µF C2 1 µF R2 10k R1 29.4k C4 220pF NFB SHDN + CSS 33nF/68nF C1: TAIYO YUDEN X5R JMK212BJ475MG C2: TAIYO YUDEN X5R LMK212BJ105MG C3: TAIYO YUDEN XR5 JMK325BJ226MM D1: ON SEMICONDUCTOR MBR0520 L1: SUMIDA CLS62-100 Figure 7. RSS and CSS at SHDN Pin Provide Soft-Start to LT1931 Inverting Converter VOUT 2V/DIV IIN 0.5A/DIV AC COUPLED 5V 0V VSHDN 500 µs/DIV 1931 F06 Figure 6. Start-Up Waveforms for 5V to – 5V Application (Figure 1). No Soft-Start Circuit. VOUT Reaches – 5V in 500 µs; Input Current Peaks at 800mA |
类似零件编号 - LT1931 |
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类似说明 - LT1931 |
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