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PT7756A Datasheet(PDF) 3 Page - Texas Instruments |
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PT7756A Datasheet(HTML) 3 Page - Texas Instruments |
3 / 3 page For assistance or to order, call (800) 531-5782 3 Power Trends, Inc. 27715 Diehl Road, Warrenville, IL 60555 (800) 531-5782 Fax: (630) 393-6902 http://www.powertrends.com Applicatio n Note s PT7750 Series More Application Notes Pin-Coded Output Voltage Adjustment on the “Big Hammer III” Series ISRs Power Trends PT7750 series ISRs incorporating pin-coded voltage control, use pins 1, 2, 3, 4, & 6 to adjust the ouput volt- age. The control pins are identified VID0 - VID4 respectively. When the control pins are left open-circuit, the ISR output will regulate at its factory trimmed output voltage. Each control pin is internally connected to a precision resistor, and when grounded increases the output voltage by a set amount. The internal resistors are binary code weighted, allowing the output voltage of the ISR to be programmed as a function of a binary code. VID0 represents the LSB, and VID4 the MSB (or range change bit). The output voltage ranges offered by these regula- tors are compatible with some microprocessors, and provide a convenient method of output voltage selection for many other applications. Refer to Figure 1 below for the connection sche- matic, and the PT7750 Data Sheet for the programming code information. Notes: 1. The programming convention is as follows:- Logic 0: Connect to pin12 (Remote Sense Ground). Logic 1: Open circuit/open drain (See notes 2, & 4) 2. Do not connect pull-up resistors to the voltage program- ming pins. 3. To minimize output voltage error, always use pin 12 (Re- mote Sense Ground) as the logic “0” reference. While the regular ground (pins 13-19) can also be used for program- ming, doing so will degrade the load reglation of the product. Figure 1 4. If active devices are used to ground the voltage control pins, low-level open drain MOSFET devices should be used over bipolar transistors. The inherent Vce(sat) in bipolar devices introduces errors in the device’s internal divider network. Discrete transistors such as the BSS138, 2N7002, IRLML2402, or the 74C906 hex open-drain buffer are examples of appropriate devices. Active Voltage Programming: Special precautions should be taken when making changes to the voltage control progam code while the unit is powered. It is highly recommended that the ISR be either powered down or in standby. Changes made to the program code while Vout is en- abled induces high current transients through the device. This is the result of the electrolytic output capacitors being either charged or discharged to the new output voltage set-point. The transient current can be minimized by making only incremental changes to the binary code, i.e. one LSB at a time. A minimum of 100µs settling time between each program state is also recom- mended. Making non-incremental changes to VID3 and VID4 with the output enabled is discouraged. If they are changed, the transients induced can overstress the device resulting in a perma- nent drop in efficiency. If the use of active devices prevents the program code being asserted prior to power-up, pull pin 5 (STBY) to the device GND during the period that the input voltage is applied to Vin. Releasing pin 5 will then allow the device output to execute a soft-start power-up to the pro- grammed voltage. Cout + Cin + 1 µH (Optional) +24V COM STBY L O A D PT7750 6 4 3 2 1 26 20-25 12 5 13-19 27 7-11 Vo Vin GND SNS(+) SNS(-) Synch Out STBY VID4 - VID0 |
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