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LT3508HUF-TRPBF 数据表(PDF) 15 Page - Linear Technology |
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LT3508HUF-TRPBF 数据表(HTML) 15 Page - Linear Technology |
15 / 24 page LT3508 15 3508fb Shutdown and Undervoltage Lockout Figure 7 shows how to add undervoltage lockout (UVLO) to the LT3508. Typically, UVLO is used in situations where the input supply is current limited, or has a relatively high source resistance. A switching regulator draws constant power from the source, so source current increases as source voltage drops. This looks like a negative resistance load to the source and can cause the source to current limit or latch low under low source voltage conditions. UVLO prevents the regulator from operating at source voltages where the problems might occur. An internal comparator will force the part into shutdown below the minimum VIN1 of 3.7V. This feature can be is much lower than the switching frequency. A phase-lead capacitor (CPL) across the feedback divider may improve the transient response. used to prevent excessive discharge of battery-operated systems. If an adjustable UVLO threshold is required, the SHDN pin can be used. The threshold voltage of the SHDN pin comparator is 2.63V. Current hysteresis is added above the SHDNthreshold.Thiscanbeusedtosetvoltagehysteresis of the UVLO using the following: R VV μA R V VV R μA HL H 3 75 4 263 263 3 8 = = – . . –. – Example: switching should not start until the input is above 4.75V and is to stop if the input falls below 4V. VV V V R VV μA k R HL == == = 475 4 0 3 475 4 75 100 4 2 ., . .– . .6 63 475 2 63 100 8 200 V VV k μA k .– . – = Keep the connection from the resistor to the SHDN pin short and make sure the interplane or surface capacitance to switching nodes is minimized. If high resistor values are used, the SHDN pin should be bypassed with a 1nF capacitor to prevent coupling problems from the switch node. Soft-Start The output of the LT3508 regulates to the lowest voltage present at either the TRACK/SS pin or an internal 0.8V reference. A capacitor from the TRACK/SS pin to ground is charged by an internal 1.2μA current source resulting in a linear output ramp from 0V to the regulated output whose duration is given by: t CV μA RAMP SS = •. . 08 12 At power up, internal open-collector ouputs discharge both TRACK/SS pins. The pins clamp at 1.3V. APPLICATIONS INFORMATION 0.8V VSW VC LT3508 GND 3508 F06 R1 OUTPUT ESR CF CC RC 2M ERROR AMPLIFIER FB R2 C1 C1 CURRENT MODE POWER STAGE gm = 2.5S gm = 300μS + POLYMER OR TANTALUM CERAMIC CPL Figure 6. Model for Loop Response Figure 7. Undervoltage Lockout 2.6V 7.5μA 8μA R3 R4 C1 SHDN LT3508 VIN TRACK/SS VC 3508 F07 |
类似零件编号 - LT3508HUF-TRPBF |
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类似说明 - LT3508HUF-TRPBF |
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