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LT1725IGN 数据表(PDF) 17 Page - Linear Technology |
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LT1725IGN 数据表(HTML) 17 Page - Linear Technology |
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17 / 28 page  17 LT1725 1725f VIN (6a) “Standard” UVLO Divider Topology UVLO R1 R2 VIN (6b) Filter Capacitor Directly On UVLO Node UVLO R1 R2 C1 100pF VIN (6c) Recommended Topology to Filter High Frequency Ripple UVLO R1/2 R1/2 R2 1725 F06 C2 C1 100pF APPLICATIO S I FOR ATIO Figure 6 speed supply ripple, while leaving the UVLO pin node impedance relatively unchanged at high frequency. INTERNAL WIDE HYSTERESIS UNDERVOLTAGE LOCKOUT The LT1725 is designed to implement isolated DC/DC converters operating from input voltages of typically 48V or more. The standard operating topology utilizes a third transformer winding on the primary side that provides both feedback information and local power for the LT1725 via its VCC pin. However, this arrangement is not inherently self-starting. Start-up is effected by the use of an external “trickle-charge” resistor and the presence of an internal wide hysteresis undervoltage lockout circuit that monitors VCC pin voltage (see Figure 7). Operation is as follows: “Trickle charge” resistor R1 is connected to VIN and supplies a small current, typically on the order of a single mA, to charge C1. At first, the LT1725 is off and draws only its start-up current. After some time, the voltage on C1 (VCC) reaches the VCC turn-on threshold. The LT1725 then turns on abruptly and draws its normal supply current. Switching action commences at the GATE pin and the MOSFET begins to deliver power. The voltage on C1 begins to decline as the LT1725 draws its normal supply current, which greatly exceeds that delivered by R1. After some time, typically tens of milliseconds, the output voltage approaches its desired value. By this time, the third transformer winding is providing virtually all the supply current required by the LT1725. One potential design pitfall is undersizing the value of capacitor C1. In this case, the normal supply current + IVCC 1725 F07 R1 C1 VIN VIN IVCC VVCC VON THRESHOLD 0 VGATE VCC LT1725 GATE PGND SGND Figure 7 drawn by the LT1725 will discharge C1 too rapidly; before the third winding drive becomes effective, the VCC turn-off threshold will be reached. The LT1725 turns off, and the VCC node begins to charge via R1 back up to the turn-on threshold. Depending upon the particular situation, this may result in either several on-off cycles before proper operation is reached, or, permanent relaxation oscillation at the VCC node. Component selection is as follows: Resistor R1 should be selected to yield a worst-case minimum charging current greater than the maximum rated LT1725 start-up current, and a worst-case maxi- mum charging current less than the minimum rated LT1725 supply current. |
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