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PT4148A Datasheet(PDF) 4 Page - Texas Instruments |
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PT4148A Datasheet(HTML) 4 Page - Texas Instruments |
4 / 9 page For technical support and more information, see inside back cover or visit www.ti.com Using the Remote On/Off Function on the PT4120/ PT4140 Series of Isolated DC/DC Converters For applications requiring output voltage on/off control, the PT4120/4140 series of DC/DC converters incorporate a remote on/off function. This function may be used in applications that require battery conservation, power-up/ shutdown sequencing, and/or to co-ordinate the power- up of the regulator for active in-rush current control. (See the related application note, AN21). This function is provided by the Remote On/Off control, pin1. If pin 1 is left open-circuit, the converter provides a regulated output whenever a valid source voltage3 is ap- plied between +Vin(pin 3), and –Vin (pin 2). Applying a low-level ground signal 1 to pin 1 will disable the regula- tor output 5. Table 1 provides details of the threshold requirements for the Remote On/Off pin. Figure 1 shows how a discrete MOSFET (Q1) 4, may be referenced to the negative input voltage rail and used with this control input. Table 1 Inhibit Control Thresholds Parameter min max Enable (VIH) 2.5V (Open Circuit) 2, 4 Disable (VIL) –0.3V 0.8V Notes: 1. The on/off control uses -Vin (pin 2), the primary side of the converter as its ground reference. All voltages specified are with respect to -Vin. 2. The on/off control internal circuitry is a high impedance 10 µA current source. The open-circuit voltage may be as high as 8.3Vdc. 3. The PT4120/40 series incorporates an “Under Voltage Lockout” (UVLO) function. This function automatically inhibits the converter output until there is sufficient input voltage for the converter to produce a regulated output. Table 2 gives the applicable UVLO thresholds. Table 2 UVLO Thresholds Series UVLO Threshold V in Range PT4120 31V Typical 36 – 75V PT4140 15V Typical 18 – 40V 4. The Remote On/Off input of the PT4120/40 series regulators must be controlled with an open-collector (or open-drain) discrete transistor or MOSFET. Do not use a pull-up resistor. 5. When the converter output is disabled, the current drawn from the input supply is typically reduced to 8mA (16mA maximum). Figure 1 Turn-On Time: The converter typically produces a fully regulated output voltage within 50ms after the applica- tion of power, or the removal of the low voltage signal 6 from the Remote On/Off pin. The actual turn-on time will vary with the input voltage, output load, and the total amount of capacitance connected to the output. Using the circuit of Figure 1, Figure 2 shows the output voltage and input current waveforms of a PT4121 after Q1 is turned off. The turn off of Q1 corresponds to the drop in Q1 Vgs voltage. The waveforms were measured with a 48Vdc input voltage, and 2.75-A resistive load. Figure 2 +V in –V in +V o –V o PT4121 +Vout –V out +V in –Vin Remote On/Off V o(adj) 1 2 35 4 6 Q1 BSS138 Inh Note 2 Vout (2V/Div) Iin (0.2A/Div) Q1 Vgs (10V/Div) HORIZ. SCALE: 5ms/Div 6. Keep the on/off transition to less than 1ms. This prevents erratic operation of the ISR, whereby the output voltage may drift un-regulated between 0V and the rated output during power-up. Application Notes PT4120/4140 Series |
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