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LTM4676A Datasheet(PDF) 55 Page - Linear Technology |
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LTM4676A Datasheet(HTML) 55 Page - Linear Technology |
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55 / 136 page  LTM4676A 55 4676afa For more information www.linear.com/LTM4676A DAC VOLTAGE ERROR (NOT TO SCALE) TIME DELAY OF MANY SECONDS DIGITAL SERVO MODE ENABLED FINAL OUTPUT VOLTAGE REACHED TON_MAX_FAULT_LIMITn TON_RISEn TIME 4676A F03 TON_DELAYn VOUTn RUNn VOUT_UV_FAULT_LIMITn Figure 3. Timing Controlled VOUT Rise applicaTions inForMaTion Coincident rail tracking can be achieved by setting two outputs to have the same turn-on/off slew rates, identical turn-on delays, and appropriately chosen turn-off delays: VOUT _COMMANDRAIL1 TON_RISERAIL1 = VOUT _COMMANDRAIL2 TON_RISERAIL2 and VOUT _COMMANDRAIL1 TOFF _FALLRAIL1 = VOUT _COMMANDRAIL2 TOFF _FALLRAIL2 and TON_DELAYRAIL1 = TON_DELAYRAIL2 and(ifVOUT_COMMANDRAIL2≥VOUT_COMMANDRAIL1) TOFF _DELAYRAIL1 = TOFF _DELAYRAIL2 + 1– VOUT _COMMANDRAIL1 VOUT _COMMANDRAIL2 ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ •TOFF _FALLRAIL2 orelse(VOUT_COMMANDRAIL2<VOUT_COMMANDRAIL1) TOFF _DELAYRAIL2 = TOFF _DELAYRAIL1+ 1– VOUT _COMMANDRAIL2 VOUT _COMMANDRAIL1 ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ •TOFF _FALLRAIL1 Thedescribedmethodofstart-upsequencingistimebased. For concatenated events it is possible to control the RUN pin based on the GPIOn pin of a different controller (see Figure 2). The GPIOn pin can be configured to release when the output voltage of the converter is greater than the VOUT_UV_FAULT_LIMITn. It is recommended to use the unfiltered VOUT UV fault limit because there is little ap- preciabletimedelaybetweentheconvertercrossingtheUV thresholdandtheGPIOnpinreleasing.Theunfilteredoutput can be enabled by the MFR_GPIO_PROPAGATEn[12] set- ting. (Refer to the MFR section of the PMBus commands in Appendix C: PMBus Command Details). The unfiltered signal may have some glitching as the VOUT signal transi- tions through the comparator threshold. A small digital filter of 250µs internally deglitches the GPIOn pins. If the TON_RISE time is greater than 100ms, the deglitch filter should be complimented with an externally applied capacitor between GPIOn and ground—to further filter the waveform. The RC time-constant of the filter should be set sufficiently fast to assure no appreciable delay is incurred. For most applications, a value of 300µs to 500µs will provide sufficient filtering without significantly delay- ing the trigger event. DIGITAL SERVO MODE For maximum accuracy in the regulated output voltage, enable the digital servo loop by asserting bit 6 of the MFR_PWM_MODEn command. In digital servo mode, the LTM4676A adjusts the regulated output voltage based on the ADC voltage reading. Every 90ms the digital servo loop steps the LSB of the DAC (nominally 1.375mV or 0.6875mV depending on the voltage range bit, MFR_PWM_MODEn[1]) until the output is at the cor- rect ADC reading. At power-up this mode engages after TON_MAX_FAULT_LIMITnunlessthelimitissetto 0(infi- nite). If the TON_MAX_FAULT_LIMITnissetto0(infinite), the servo begins after TON_RISEn is complete and VOUTn has exceeded VOUT_UV_FAULT_LIMITn and IOUT_OCn is not present. This same point in time is when the output changes from discontinuous to the mode commanded by MFR_PWM_MODEn[0]. Refer to Figure 3 for details on the VOUTn waveform under time based sequencing. |
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