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LT1765-2.5 Datasheet(PDF) 10 Page - Linear Technology |
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LT1765-2.5 Datasheet(HTML) 10 Page - Linear Technology |
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10 / 20 page  LT1765/LT1765-1.8/LT1765-2.5/ LT1765-3.3/LT1765-5 10 1765fd APPLICATIONS INFORMATION APPLICATIONS INFORMATION Table 3 PART NUMBER VALUE (μH) IRMS (Amps) DCR (Ω) HEIGHT (mm) Coiltcraft DO1608C-222 2.2 2.4 0.07 2.9 Sumida CDRH3D16-1R5 1.5 1.6 0.043 1.8 CDRH4D18-1R0 1.0 1.7 0.035 2.0 CDC5D23-2R2 2.2 2.2 0.03 2.5 CR43-1R4 1.4 2.5 0.056 3.5 CDRH5D28-2R6 2.6 2.6 0.013 3.0 Toko (D62F)847FY-2R4M 2.4 2.5 0.037 2.7 (D73LF)817FY-2R2M 2.2 2.7 0.03 3.0 CATCH DIODE The diode D1 conducts current only during switch off time. Peak reverse voltage is equal to regulator input voltage. Average forward current in normal operation can be calculated from: I IV V V D AVG OUT IN OUT IN () = − () The only reason to consider a larger than 3A diode is the worst-case condition of a high input voltage and shorted output.Withashortedcondition,diodecurrentwillincreaseto a typical value of 4A, determined by peak switch current limit of the LT1765. A higher forward voltage will also limit switch current. This is safe for short periods of time, but it would be prudent to check with the diode manufacturer if continuous operation under these conditions must be tolerated. BOOST PIN For most applications, the boost components are a 0.18μF capacitor and a CMDSH-3 diode. The anode is typically connected to the regulated output voltage to generate a voltage approximately VOUT above VIN to drive the output stage. The output driver requires at least 2.7V of headroom throughout the on period to keep the switch fully saturated. However, the output stage discharges the boost capacitor during this on time. If the output voltage is less than 3.3V, it is recommended that an alternate boost supply is used. The boost diode can be connected to the input, although, care must be taken to prevent the 2x VIN boost voltage from exceeding the BOOST pin absolute maximum rating. The additional voltage across the switch driver also increases power loss, reducing efficiency. If available, an independent supply can be used with a local bypass capacitor. A 0.18μF boost capacitor is recommended for most ap- plications. Almost any type of film or ceramic capacitor is suitable, but the ESR should be <1Ω to ensure it can be fully recharged during the off time of the switch. The capacitor value is derived from worst-case conditions of 700ns on-time, 90mA boost current, and 0.7V discharge ripple. This value is then guard banded by 2x for secondary factors such as capacitor tolerance, ESR and temperature effects. The boost capacitor value could be reduced under less demanding conditions, but this will not improve cir- cuit operation or efficiency. Under low input voltage and low load conditions, a higher value capacitor will reduce discharge ripple and improve start up operation. SHUTDOWN AND UNDERVOLTAGE LOCKOUT Figure 4 shows how to add undervoltage lockout (UVLO) to the LT1765. 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 these problems might occur. Figure 4. Undervoltage Lockout 1.33V GND VSW INPUT R1 1765 F04 OUTPUT SHDN VCC IN LT1765 3μA R2 C1 + 7μA |
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