link to page 8 LT8309 OPERATION Undervoltage LockoutSetting the DRAIN Pin Resistor The part features a INTVCC undervoltage lockout (UVLO) The DRAIN pin resistor sets when the LT8309 turns off to prevent switching until the INTVCC voltage is above 4V. the MOSFET. The trip point, VOFFSET, is set with the fol- lowing equation: INTVCC LDO VOFFSET = 20mV – 10µA • RDRAIN An internal LDO regulator provides a regulated 7V output R from the V DRAIN is the resistor connected between the drain of IN pin to the INTVCC pin. An output capacitor is the MOSFET and the DRAIN pin of the part. RDRAIN needed to provide the current needed for the gate driver. needs to be at least 800Ω to operate correctly. For A 4.7µF capacitor is recommended and must be placed most applications, VOFFSET should be set at –5mV. High as close as possible to the INTVCC pin. The current limit RDS(ON) MOSFETs may require a more negative VOFFSET for the LDO is 42mA. voltage to keep the drain-to-source current from reversing. If the current is reversing, decrease VOFFSET in 5mV steps MOSFET Selection to eliminate the cross-conduction. A MOSFET’s RDS(ON) is important to the operation of the Short-Circuit Operation LT8309. The drain-source voltage is used to determine when to turn off the MOSFET. The peak current through In the Typical Application diagram on Page 1, the VCC pin the MOSFET times the MOSFET’s R is connected to the output of the flyback converter. During DS(ON) should be above 75mV. When this voltage is too low, the high speed an output short-circuit condition, the LT8309 is off and the body diode of the MOSFET must handle the short-circuit comparator may trip early due to ringing on the DRAIN condition. This puts additional thermal requirements on pin. When this voltage is too high, the MOSFET dissipates the MOSFET. The drain voltage of the MOSFET is equal a large amount of power which causes efficiency to go to VIN/N in short-circuit and capable of powering the down and may cause thermal issues with the MOSFET. LT8309 with the circuit in Figure 2. This allows the LT8309 VIN VOUT • • VCC DRAIN LT8309 GATE INTVCC GND 8309 F02 Figure 2. Short-Circuit Application Circuit Rev. B 8 For more information www.analog.com Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Order Information Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Typical Applications Package Description Revision History Typical Application Related Parts
