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PT7673A Datasheet(PDF) 7 Page - Texas Instruments |
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PT7673A Datasheet(HTML) 7 Page - Texas Instruments |
7 / 9 page Application Notes For technical support and more information, see inside back cover or visit www.ti.com PT7670 Series Using the Standby Function on the PT7670 Series of 30-A Switching Regulators For applications requiring On/Off control of the output voltage, the 30-A rated PT7670 series of Excalibur ISRs incorporate an on/off “Standby” function. This feature may be used for power-up/shutdown sequencing, or to change the output voltage while input power is applied. See related notes: “Pin-Coded Output Voltage Adjustment of the PT7670 Series of 30-A ISRs.” The standby function is provided by the STBY* control, pin 5. If pin 5 is left open-circuit the regulator operates normally, providing a regulated output whenever a valid supply voltage is applied to Vin (pins 7-11) with respect to GND (pins 13-19). Connecting pin 5 to ground 1 will disable the regulator output 2. This places the regulator in standby mode, and reduces the input current to typically 6mA. If a ground signal is applied to pin 5 prior to power- up, the regulator output will remain inactive during the period that input power is applied. The standby input must be controlled with an open- collector (or open-drain) discrete transistor (See Figure 3-1). Table 3-1 gives the input requirements. Table 3-1 Standby Control Input Requirements Parameter Min Typ Max Enable 2 V Open Cct. 1 Disable –0.2 V 0.8 V Istby (low) 0.5 mA Vstby (o/c) Vin Notes: 1. The Standby input of the PT7670 series of regulators should be controlled using an open-collector (or open- drain) discrete transistor. Do Not use a pull-up resistor. The control input has an open-circuit voltage equal to Vin. To set the regulator output to zero, the control pin must be “pulled” to less than 0.8Vdc with a 0.5mA sink to ground. Figure 3-1 Turn-On Time: Turning Q1 in Fig. 3-1 off, removes the low-voltage signal at pin 5 and enables the output 3. Af- ter a delay of less than 5 ms, the output voltage rises to full regulation within 30 ms 4. Fig. 3-2 shows the typical output voltage waveform of a PT7671 following the turn-off of Q1 at time t =0 secs. In the circuit of Fig. 3-1, the output voltage is set to 3.3 V. The waveform was measured with a +5-V input source voltage, and 15-A output load. Figure 3-2 C out + C in + 5 V COM Inhibit L O A D PT7671 43 1 26 20–25 12 513–19 7–11 Vo Vin GND Senes(+) STBY VID3 - VID0 Senes(–) V o =3.3 V COM Q 1 BSS138 2 Iin (5A/Div) Vo (2V/Div) Vstby (5V/Div) HORIZ SCALE: 5ms/Div 2. In the standby mode the output of the regulator is tri- state, and the output voltage falls at the rate that the load circuit discharges the output filter capacitors. 3. When the ground signal to the Standby pin is removed, the regulator output initiates a soft-start cycle by first asserting a low impedance to ground. If an external voltage is applied to the output bus, it will sink current and possibly over-stress the part. 4. The turn-off time of Q1, or rise time of the standby input is not critical. Turning Q1 off slowly will not affect regulator operation. However, a slow turn-off time will increase both the initial delay and rise-time of the output voltage. |
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