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LTM4676A Folha de dados(PDF) 54 Page - Linear Technology |
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LTM4676A Folha de dados(HTML) 54 Page - Linear Technology |
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54 / 136 page  LTM4676A 54 4676afa For more information www.linear.com/LTM4676A applicaTions inForMaTion MINIMUM ON-TIME CONSIDERATIONS Minimum on-time, tON(MIN), is the smallest time dura- tion that the LTM4676A is capable of turning on the top MOSFET. It is determined by internal timing delays and the gate charge required to turn on the top MOSFET. Low duty cycle applications may approach this minimum on-time limit and care should be taken to ensure that: tON(MIN) < VOUTn VINn • fOSC If the duty cycle falls below what can be accommodated by the minimum on-time, the controller will begin to skip cycles. The output voltage will continue to be regulated, but the ripple voltage and current will increase. Theminimumon-timefortheLTM4676Ais45ns,nominal, guardband to 90ns. VARIABLE DELAY TIME, SOFT-START AND OUTPUT VOLTAGE RAMPING The LTM4676A must enter its run state prior to soft-start. The RUNn pins are released after the part initializes and SVIN is greater than the VIN_ON threshold. If multiple LTM4676As are used in an application, they should be configured to share the same RUNn pins. They all hold their respective RUNn pins low until all devices initialize and SVIN exceeds the VIN_ON threshold for all devices. The SHARE_CLK pin assures all the devices connected to the signal use the same time base. After the RUNn pin releases, the controller waits for the user-specified turn-on delay (TON_DELAYn) prior to ini- tiating an output voltage ramp. Multiple LTM4676As and other LTC parts can be configured to start with variable delay times. To work correctly, all devices use the same timing clock (SHARE_CLK) and all devices must share the RUNn pin. This allows the relative delay of all parts to be synchronized. The actual variation in the delay will be dependent on the highest clock rate of the devices con- nected to the SHARE_CLK pin (all Linear Technology ICs are configured to allow the fastest SHARE_CLK signal to control the timing of all devices). The SHARE_CLK signal can be ±7.5% in frequency, thus the actual time delays will have some variance. Soft-start is performed by actively regulating the load voltage while digitally ramping the target voltage from 0V to the commanded voltage set point. The rise time of the voltage ramp can be programmed using the TON_RISEn command to minimize inrush currents associated with the start-up voltage ramp. The soft-start feature is disabled by setting TON_RISEn to any value less than 0.250ms. The LTM4676A performs the necessary math internally to assure the voltage ramp is controlled to the desired slope. However, the voltage slope can not be any faster than the fundamentallimitsofthepowerstage.Thenumberofsteps in the ramp is equal to TON_RISE/0.1ms. Therefore, the shorter the TON_RISEn time setting, the more jagged the soft-start ramp appears. The LTM4676A PWM always operates in discontinuous mode during the TON_RISEn operation. In discontinuous mode, the bottom MOSFET (MBn) is turned off as soon as reverse current is detected in the inductor. This allows the regulator to start up into a pre-biased load. There is no analog tracking feature in the LTM4676A; how- ever, two outputs can be given the same TON_RISEn and TON_DELAYn times to achieve ratiometric rail tracking. Because the RUNn pins are released at the same time and both units use the same time base (SHARE_CLK), the outputs track very closely. If the circuit is in a PolyPhase configuration, all timing parameters must be the same. Table 8. Recommended MFR_IIN_OFFSETn Setting vs Switching Frequency Setting SWITCHING FREQUENCY (kHz) FREQUENCY_ SWITCH COMMAND VALUE (HEX.) RECOMMENDED MFR_IIN_ OFFSETn SETTING (mA) RECOMMENDED MFR_IIN_ OFFSETn SETTING (HEX.) 250 0xF3E8 20.3 0x8A99 350 0xFABC 24.4 0x8B20 425 0xFB52 27.4 0x8B82 500 0xFBE8 30.5 0x8BE7 575 0x023F 33.6 0x9227 650 0x028A 36.7 0x9259 750 0x02EE 40.8 0x929C 1000 0x03E8 51.0 0x9344 Sync. to External Clock, fSYNC N/A 0.041 • fSYNC + 10.037 * *See Appendix C: PMBus Command Details, L11 data format. |
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