Datasheet LT1931, LT1931A (Analog Devices) - 7
| Hersteller | Analog Devices |
| Beschreibung | 1.2MHz/2.2MHz Inverting DC/DC Converters in ThinSOT |
| Seiten / Seite | 12 / 7 — APPLICATIO S I FOR ATIO. Figure 4. Transient Response of Inverting … |
| Dateiformat / Größe | PDF / 459 Kb |
| Dokumentensprache | Englisch |
APPLICATIO S I FOR ATIO. Figure 4. Transient Response of Inverting Converter
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LT1931/LT1931A
U U W U APPLICATIO S I FOR ATIO
VOUT V 20mV/DIV OUT 2V/DIV AC COUPLED IL1A + IL1B IIN 0.5A/DIV 0.5A/DIV AC COUPLED AC COUPLED LOAD 200mA 5V CURRENT 100mA VSHDN 0V 100µs/DIV 1931 F04 500µs/DIV 1931 F06
Figure 4. Transient Response of Inverting Converter Figure 6. Start-Up Waveforms for 5V to – 5V Application with 220pF Phase Lead Capacitor (Figure 1). No Soft-Start Circuit. VOUT Reaches – 5V in 500
µ
s; Input Current Peaks at 800mA
VOUT 0.1V/DIV regulator tries to charge up the output capacitor as quickly AC COUPLED as possible, which results in a large inrush current. Fig- ure 6 shows a typical oscillograph of the start-up wave- IL1A + IL1B 0.5A/DIV form for the application of Figure 1 starting into a load of AC COUPLED 33Ω. The lower waveform shows SHDN being pulsed LOAD 200mA from 0V to 5V. The middle waveform shows the input CURRENT 100mA 50µs/DIV 1931 F05 current, which reaches as high as 0.8A. The total time required for the output to reach its final value is approxi-
Figure 5. Transient Response of Inverting Converter with 22
µ
F Tantalum Output Capacitor and No Phase Lead Capacitor
mately 500µs. 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 R
START-UP/SOFT-START
SS and CSS, as shown in Figure 7, can be used to limit inrush current For most LT1931/LT1931A applications, the start-up in- to a lower value. Figure 8 shows the relevant waveforms rush current can be high. This is an inherent feature of with RSS = 15k and CSS = 33nF. Input current, measured switching regulators in general since the feedback loop is at VIN, is limited to a peak value of 0.5A as the time required saturated due to VOUT being far from its final value. The to reach final value increases to 1ms. In Figure 9, CSS is CURRENT C2 L1A L1B PROBE 1µF 10µH 10µH VIN 5V D1 + V C1 IN SW VOUT 4.7µF –5V R1 R C4 SS LT1931 29.4k 15k 220pF V C3 SS SHDN NFB 22µF GND R2 10k D2 1N4148 CSS 33nF/68nF C1: TAIYO YUDEN X5R JMK212BJ475MG 1931 F07 C2: TAIYO YUDEN X5R LMK212BJ105MG VOUT 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
1931fa 7
U U W U APPLICATIO S I FOR ATIO
VOUT V 20mV/DIV OUT 2V/DIV AC COUPLED IL1A + IL1B IIN 0.5A/DIV 0.5A/DIV AC COUPLED AC COUPLED LOAD 200mA 5V CURRENT 100mA VSHDN 0V 100µs/DIV 1931 F04 500µs/DIV 1931 F06
Figure 4. Transient Response of Inverting Converter Figure 6. Start-Up Waveforms for 5V to – 5V Application with 220pF Phase Lead Capacitor (Figure 1). No Soft-Start Circuit. VOUT Reaches – 5V in 500
µ
s; Input Current Peaks at 800mA
VOUT 0.1V/DIV regulator tries to charge up the output capacitor as quickly AC COUPLED as possible, which results in a large inrush current. Fig- ure 6 shows a typical oscillograph of the start-up wave- IL1A + IL1B 0.5A/DIV form for the application of Figure 1 starting into a load of AC COUPLED 33Ω. The lower waveform shows SHDN being pulsed LOAD 200mA from 0V to 5V. The middle waveform shows the input CURRENT 100mA 50µs/DIV 1931 F05 current, which reaches as high as 0.8A. The total time required for the output to reach its final value is approxi-
Figure 5. Transient Response of Inverting Converter with 22
µ
F Tantalum Output Capacitor and No Phase Lead Capacitor
mately 500µs. 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 R
START-UP/SOFT-START
SS and CSS, as shown in Figure 7, can be used to limit inrush current For most LT1931/LT1931A applications, the start-up in- to a lower value. Figure 8 shows the relevant waveforms rush current can be high. This is an inherent feature of with RSS = 15k and CSS = 33nF. Input current, measured switching regulators in general since the feedback loop is at VIN, is limited to a peak value of 0.5A as the time required saturated due to VOUT being far from its final value. The to reach final value increases to 1ms. In Figure 9, CSS is CURRENT C2 L1A L1B PROBE 1µF 10µH 10µH VIN 5V D1 + V C1 IN SW VOUT 4.7µF –5V R1 R C4 SS LT1931 29.4k 15k 220pF V C3 SS SHDN NFB 22µF GND R2 10k D2 1N4148 CSS 33nF/68nF C1: TAIYO YUDEN X5R JMK212BJ475MG 1931 F07 C2: TAIYO YUDEN X5R LMK212BJ105MG VOUT 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
1931fa 7