Datasheet LT1765, LT1765-1.8, LT1765-2.5, LT1765-3.3, LT1765-5 (Analog Devices) - 10

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