Datasheet Linear Technology LT1613 (Linear Technology) - 9
| Manufacturer | Linear Technology |
| Description | 1.4MHz, Single Cell DC/DC Converter in 5-Lead SOT-23 |
| Pages / Page | 12 / 9 — OPERATIO. START-UP/SOFT-START. Figure 18. Soft-Start Components in Figure … |
| Revision | S |
| File Format / Size | PDF / 302 Kb |
| Document Language | English |
OPERATIO. START-UP/SOFT-START. Figure 18. Soft-Start Components in Figure 17’s SEPIC
Model Line for this Datasheet
Text Version of Document
LT1613
U OPERATIO START-UP/SOFT-START
time required to reach final value increases to 1.7ms. In Figure 19, C When the LT1613 SHDN pin voltage goes high, the device S is increased to 33nF. Input current does not exceed the steady-state current the device uses to supply rapidly increases the switch current until internal current power to the 50Ω load. Start-up time increases to 4.3ms. limit is reached. Input current stays at this level until the C output capacitor is charged to final output voltage. Switch S can be increased further for an even slower ramp, if desired. current can exceed 1A. Figure 16’s oscillograph details start-up waveforms of Figure 17’s SEPIC into a 50Ω load without any soft-start. The output voltage reaches final VOUT value in approximately 200µs, while input current reaches 2V/DIV 400mA. Switch current in a SEPIC is 2x the input current, so the switch is conducting approximately 800mA peak. IIN Soft-start reduces the inrush current by taking more time 200mA/DIV to reach final output voltage. A soft-start circuit consisting VS of Q1, R 5V/DIV S1, RS2 and CS1 as shown in Figure 17 can be used 500µs/DIV 1613 F18 to limit inrush current to a lower value. Figure 18 pictures
Figure 18. Soft-Start Components in Figure 17’s SEPIC
VOUT and input current with RS2 of 33kΩ and CS of 10nF.
Reduces Inrush Current. CSS = 10nF, RLOAD = 50
Ω Input current is limited to a peak value of 200mA as the VOUT VOUT 2V/DIV 2V/DIV IIN IIN 200mA/DIV 200mA/DIV VSHDN VS 5V/DIV 5V/DIV 200µs/DIV 1613 F16 1ms/DIV 1613 F18
Figure 16. Start-Up Waveforms of Figure 19. Increasing CS to 33nF Further Figure 17’s SEPIC Into 50
Ω
Load Reduces Inrush Current. RLOAD = 50
Ω C3 L1 1µF 22µH VIN 4V + C1 L2 C D1 22µF V PL IN SW 22µH 330pF SOFT-START COMPONENTS R LT1613 R3 S1 33k 10k VOUT V SHDN FB S 5V Q1 GND R1 R2 R 37.4k LOAD 2N3904 C 12.1k S RS2 C2 10nF/ 33k 10µF 33nF 1613 F17 C1: AVX TAJB226M006 D1: MOTOROLA MBR0520 C2: TAIYO YUDEN LMK325BJ106MN L1, L2: MURATA LQH3C220 C3: TAIYO YUDEN LMK212BJ105MG
Figure 17. 5V SEPIC with Soft-Start Components
9
U OPERATIO START-UP/SOFT-START
time required to reach final value increases to 1.7ms. In Figure 19, C When the LT1613 SHDN pin voltage goes high, the device S is increased to 33nF. Input current does not exceed the steady-state current the device uses to supply rapidly increases the switch current until internal current power to the 50Ω load. Start-up time increases to 4.3ms. limit is reached. Input current stays at this level until the C output capacitor is charged to final output voltage. Switch S can be increased further for an even slower ramp, if desired. current can exceed 1A. Figure 16’s oscillograph details start-up waveforms of Figure 17’s SEPIC into a 50Ω load without any soft-start. The output voltage reaches final VOUT value in approximately 200µs, while input current reaches 2V/DIV 400mA. Switch current in a SEPIC is 2x the input current, so the switch is conducting approximately 800mA peak. IIN Soft-start reduces the inrush current by taking more time 200mA/DIV to reach final output voltage. A soft-start circuit consisting VS of Q1, R 5V/DIV S1, RS2 and CS1 as shown in Figure 17 can be used 500µs/DIV 1613 F18 to limit inrush current to a lower value. Figure 18 pictures
Figure 18. Soft-Start Components in Figure 17’s SEPIC
VOUT and input current with RS2 of 33kΩ and CS of 10nF.
Reduces Inrush Current. CSS = 10nF, RLOAD = 50
Ω Input current is limited to a peak value of 200mA as the VOUT VOUT 2V/DIV 2V/DIV IIN IIN 200mA/DIV 200mA/DIV VSHDN VS 5V/DIV 5V/DIV 200µs/DIV 1613 F16 1ms/DIV 1613 F18
Figure 16. Start-Up Waveforms of Figure 19. Increasing CS to 33nF Further Figure 17’s SEPIC Into 50
Ω
Load Reduces Inrush Current. RLOAD = 50
Ω C3 L1 1µF 22µH VIN 4V + C1 L2 C D1 22µF V PL IN SW 22µH 330pF SOFT-START COMPONENTS R LT1613 R3 S1 33k 10k VOUT V SHDN FB S 5V Q1 GND R1 R2 R 37.4k LOAD 2N3904 C 12.1k S RS2 C2 10nF/ 33k 10µF 33nF 1613 F17 C1: AVX TAJB226M006 D1: MOTOROLA MBR0520 C2: TAIYO YUDEN LMK325BJ106MN L1, L2: MURATA LQH3C220 C3: TAIYO YUDEN LMK212BJ105MG
Figure 17. 5V SEPIC with Soft-Start Components
9
