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PTB48520W Datasheet(PDF) 7 Page - Texas Instruments |
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PTB48520W Datasheet(HTML) 7 Page - Texas Instruments |
7 / 18 page www.ti.com Overtemperature Protection Undervoltage Lockout Primary-Secondary Isolation Output Voltage Adjustment Rset + 6.49 kW 1.225 V Vset * 1.805 V * 4.42 kW (1) PTB48520W SLTS233A–NOVEMBER 2004–REVISED APRIL 2005 APPLICATION INFORMATION (continued) higher. Depending on the load conditions on the VO Seq output, the voltage at VO Bus maybeupto100 mV higher than the converter set-point voltage. In addition, the Smart-Sense feature (incorporated into the converter) only engages sense compensation to the VO Seq output when that output voltage is close to the set point. During other conditions, such as power-up and power-down sequencing events, the sense circuit automatically defaults to sensing the VO Bus voltage, internal to the converter. Leaving the (+)Sense and (–)Sense pins open does not damage the converter or load circuitry. The converter includes default circuitry that keeps the output voltage in regulation. If the remote sense feature is not used, the (–)Sense pin should always be connected to VO COM. Note: The remote sense feature is not designed to compensate for the forward drop of nonlinear or frequency dependent components that may be placed in series with the converter output. Examples include OR-ing diodes, filter inductors, ferrite beads, and fuses. When these components are enclosed by the sense pin connections, they are effectively placed inside the regulation control loop, which can adversely affect the stability of the converter. Overtemperature protection is provided by an internal temperature sensor, which monitors the temperature of the converter PCB (close to pin 1). If the PCB temperature exceeds a nominal 115°C, the converter shuts down. The converter then automatically restarts when the sensed temperature falls to approximately 105°C. When operated outside its recommended thermal derating envelope (see data sheet derating curves), the converter typcially cycles on and off at intervals from a few seconds to one or two minutes. This is to ensure that the internal components are not permanently damaged from excessive thermal stress. The undervoltage lockout (UVLO) is designed to prevent the operation of the converter until the input voltage is close to the minimum operating voltage. The converter is held off when the input voltage is below the UVLO threshold, and turns on when the input voltage rises above the threshold. This prevents high start-up current during normal power up of the converter, and minimizes the current drain from the input source during low input voltage conditions. The converter meets full specifications when the minimum specified input voltage is reached. The UVLO circuitry also overrides the operation of the VO Enable control. Only when the input voltage is above the UVLO threshold is the VO Enable control functional. These converters incorporate electrical isolation between the input terminals (primary) and the output terminals (secondary). All converters are tested to a withstand voltage of 1500 VDC. This complies with UL/cUL 60950 and EN 60950 and the requirements for operational isolation. It allows the converter to be configured for either a positive or negative input voltage source. The data sheet Terminal Functions table provides guidance as to the correct reference that must be used for the external control signals. The VO Adjust control sets the output voltages to a value higher than 1.8 V. For output voltages other than 1.8 V a single external resistor, R(set), must be connected directly between VO Adjust (pin 7) and (–)Sense (pin 6) pins. A 0.05-W rated resistor can be used. The tolerance should be 1%, with a temperature stability of 100 ppm/°C (or better). Place the resistor close to the converter and connect it directly between pins 7 and 6 using dedicated PCB traces (see typical application). Table 1 gives the preferred value of the external resistor for a number of standard voltages, along with the actual output voltage that this resistance value provides. For other output voltages the value of the required adjust resistor may be calculated using Equation 1. 7 |
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