VTM48EH020 x 040A00  
					7.0 USING THE CONTROL SIGNALS VC, PC, TM, IM  
					Current Monitor (IM) pin provides a voltage proportional to  
					the output current of the VTM module. The nominal voltage  
					will vary between 0.3 V and 1.64 V over the output current  
					range of the VTM module (See Figures 8–10). The accuracy of  
					the IM pin will be within 25% under all line and temperature  
					conditions between 50% and 100% load.  
					The VTM Control (VC) pin is an input pin which powers the  
					internal VCC circuitry when within the specified voltage range  
					of 12 V to 16.5 V. This voltage is required in order for the  
					VTM™ current multiplier to start, and must be applied as long  
					as the input is below 26 V. In order to ensure a proper start,  
					the slew rate of the applied voltage must be within the  
					specified range.  
					8.0 STARTUP BEHAVIOR  
					Some additional notes on the using the VC pin:  
					Depending on the sequencing of the VC with respect to the  
					input voltage, the behavior during startup will vary as follows:  
					• In most applications, the VTM module will be powered by  
					an upstream PRM™ regulator which provides a 10 ms VC  
					pulse during startup. In these applications the VC pins of the  
					PRM and VTM modules should be tied together.  
					• Normal Operation (VC applied prior to VIN): In this case the  
					controller is active prior to ramping the input. When the  
					input voltage is applied, the VTM module output voltage will  
					track the input (See Figure 13). The inrush current is  
					determined by the input voltage rate of rise and output  
					capacitance. If the VC voltage is removed prior to the input  
					reaching 26 V, the module may shut down.  
					• The VC voltage can be applied indefinitely allowing for  
					continuous operation down to 0 VIN.  
					• The fault response of the VTM module is latching. A positive  
					edge on VC is required in order to restart the unit. If VC is  
					continuously applied the PC pin may be toggled to restart  
					the module.  
					• Stand Alone Operation (VC applied after VIN): In this case the  
					module output will begin to rise upon the application of  
					the VC voltage (See Figure 14). The Adaptive Soft Start  
					circuit (See Section 10) may vary the ouput rate of rise in  
					order to limit the inrush current to it’s maximum level. When  
					starting into high capacitance, or a short, the output current  
					will be limited for a maximum of 900 µsec. After this period,  
					the adaptive soft start circuit will time out and the VTM  
					module may shut down. No restart will be attempted until  
					VC is re-applied, or PC is toggled. The maximum output  
					capacitance is limited to 1500 µF in this mode of operation  
					to ensure a sucessful start.  
					Primary Control (PC) pin can be used to accomplish the  
					following functions:  
					• Delayed start: Upon the application of VC, the PC pin will  
					source a constant 100 µA current to the internal RC  
					network. Adding an external capacitor will allow further  
					delay in reaching the 2.5 V threshold for module start.  
					• Auxiliary voltage source: Once enabled in regular  
					operational conditions (no fault), each VTM module PC  
					provides a regulated 5 V, 2 mA voltage source.  
					• Output disable: PC pin can be actively pulled down in order  
					to disable the module. Pull down impedance shall be lower  
					than 400 Ω.  
					9.0 THERMAL CONSIDERATIONS  
					•
					V I Chip™ products are multi-chip modules whose  
					• Fault detection flag: The PC 5 V voltage source is internally  
					turned off as soon as a fault is detected. It is important to  
					notice that PC doesn’t have current sink capability. Therefore,  
					in an array, PC line will not be capable of disabling  
					neighboring modules if a fault is detected.  
					temperature distribution varies greatly for each part number as  
					well as with the input / output conditions, thermal  
					management and environmental conditions. Maintaining the  
					top of the VTM48EH020T040A00 case to less than 100ºC will  
					•
					keep all junctions within the V I Chip module below 125ºC for  
					most applications.  
					• Fault reset: PC may be toggled to restart the unit if VC  
					is continuously applied.  
					The percent of total heat dissipated through the top surface  
					versus through the J-lead is entirely dependent on the  
					particular mechanical and thermal environment. The heat  
					dissipated through the top surface is typically 60%. The heat  
					dissipated through the J-lead onto the PCB board surface is  
					typically 40%. Use 100% top surface dissipation when  
					designing for a conservative cooling solution.  
					Temperature Monitor (TM) pin provides a voltage  
					proportional to the absolute temperature of the converter  
					control IC.  
					It can be used to accomplish the following functions:  
					• Monitor the control IC temperature: The temperature in  
					Kelvin is equal to the voltage on the TM pin scaled  
					by 100. (i.e. 3.0 V = 300 K = 27ºC). If a heat sink is applied,  
					TM can be used to thermally protect the system.  
					•
					It is not recommended to use a V I Chip module for an  
					extended period of time at full load without proper heat sinking.  
					• Fault detection flag: The TM voltage source is internally  
					turned off as soon as a fault is detected. For system  
					monitoring purposes (microcontroller interface) faults are  
					detected on falling edges of TM signal.  
					Rev. 2.0  
					5/2011  
					Page 9 of 16  
					V•I CHIP CORP. (A VICOR COMPANY) 25 FRONTAGE RD. ANDOVER, MA 01810 800-735-6200  
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