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LTC1149CN-3.3 Datasheet(PDF) 11 Page - Linear Technology |
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LTC1149CN-3.3 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 20 page  11 LTC1149 LTC1149-3.3/LTC1149-5 APPLICATIO S I FOR ATIO Figure 5. High Efficiency Step-Down Regulator with VOUT > VCC regulator loop adapts to the current change and returns VOUT to its steady state value. During this recovery time VOUT can be monitored for overshoot or ringing which would indicate a stability problem. The Pin 7 external components shown in the Figure 1 circuit will prove adequate compensation for most applications. A second, more severe transient is caused by switching in loads with large (>1 µF) supply bypass capacitors. The discharged bypass capacitors are effectively put in parallel with COUT, causing a rapid drop in VOUT. No regulator can deliver enough current to prevent this problem if the load switch resistance is low and it is driven quickly. The only solution is to limit the rise time of the switch drive so that the load rise time is limited to approximately (25)(CLOAD). Thus a 10 µF capacitor would require a 250µs rise time, limiting the charging current to about 200mA. LTC1149 Adjustable Applications When an output voltage other than 3.3V or 5V is required, the LTC1149 adjustable version is used with an external resistive divider from VOUT to VFB Pin 10. The regulated voltage is determined: VOUT = 1.25 ) )1+ R2 R1 In applications where VOUT is greater than the LTC1149 internally regulated VCC voltage, RSENSE must be moved to output ripple at low frequencies will be large enough to trip the voltage comparator. This causes the Burst Mode operation to be activated when the LTC1149 series would normally be in continuous operation. The effect is most pronounced with low values of RSENSE and can be improved by operating at higher frequencies with lower values of L. The output remains in regulation at all times. Checking Transient Response Switching regulators take several cycles to respond to a step in DC (resistive) load current. When a load step occurs, VOUT shifts by an amount equal to (∆ILOAD)(ESR), where ESR is the effective series resistance of COUT. ∆ILOAD also begins to charge or discharge COUT until the 0.068 µF VIN CAP PDRIVE LTC1149 ITH CT PGATE VCC VCC SHDN2 SENSE– SENSE+ 100pF IRFZ34 0.047 µF 1N4148 1N4148 VIN IRF9Z34 1N5819 RSENSE 0.05 Ω 150 µF 50V 1 µF 0V = NORMAL >2V = SHUTDOWN 3300pF CT 200pF 1k 1149 F05 GNDS NGATE VFB 1000pF 100 µH R2 215k 1% R1 25k 1% 150 µF 16V OS-CON LOAD VOUT OUTPUT GROUND CONNECTION VOUT = 1.25 () 1 + R2 R1 VALUES SHOWN FOR VOUT = 12V + + + (VIN – VOUT) VOLTAGE (V) 0 600 800 1000 4 1149 F04 400 200 0 1 2 3 5 L = 50 µH RSENSE = 0.02Ω L = 25 µH RSENSE = 0.02Ω L = 50 µH RSENSE = 0.05Ω Figure 4. Minimum Suggested COUT |
Similar Part No. - LTC1149CN-3.3 |
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Similar Description - LTC1149CN-3.3 |
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