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поискавой системы для электроныых деталей |
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поискавой системы для электроныых деталей |
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LTC2970CUFD датащи(PDF) 28 Page - Linear Technology |
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LTC2970CUFD датащи(HTML) 28 Page - Linear Technology |
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28 / 36 page  LTC2970/LTC2970-1 28 29701fc APPLICATIO S I FOR ATIO 4-Step Resistor Selection Procedure for DC/DC Converters with External Feedback Resistors The following 4-step procedure should be used to quickly calculate the resistor values shown for the Typical Applica- tion Circuit shown in Figure 1. 1. Assume values for feedback resistor R20 and the nominal DC/DC converter output voltage VDC0,NOM, and solve for R10. VDC0,NOM is the desired output voltage of the DC/DC con- verter when the LTC2970’s VOUT0 pin is in a high impedance state. VFB0 is the voltage at the converter’s feedback node when the loop is in regulation, and IFB0 is the feedback node’s input current. R RV VI R V FB DC NOM FB FB 10 20 20 0 00 = • −• − , (1) 2. Solve for the maximum value of R30 that yields the maximum required DC/DC converter output voltage VDC0,max. When VOUT0 is at 0V, the output of the DC/DC converter is at its maximum voltage. Note that the 10mV term cor- responds to the maximum offset voltage of the IDAC 1X voltage buffer. R RV mV VV FB DC MAX DC NOM 30 20 10 ≤ •− () − ,, (2) 3. Solve for the minimum value of R40 that’s needed to yield the minimum required DC/DC converter output voltage VDC0,MIN. The DC/DC converter output voltage will be a minimum when IDAC0 is at its full-scale current. In order to guarantee that R40 is large enough, assume that IDAC0’s full-scale current is at the datasheet minimum of 236μA. R VV R R VmV A DC NOM DC MIN FB 40 30 20 10 236 ≥ − ()•+ + ,, μ (3) 4. Re-calculate the minimum, nominal, and maximum DC/DC converter output voltages and the resulting mar- gining resolution. VV R R IR DC NOM FB FB 0 1 20 10 20 , =• + ⎛ ⎝⎜ ⎞ ⎠⎟ +• (4) VV R R RA V m DC MIN DC NOM FB 00 0 20 30 40 236 10 ,, ≤− • •− − μ V V () (5) VV R R VmV DC MAX DC NOM FB 00 0 20 30 10 ,, ≥+ • − () (6) The margining resolution is bounded by: V R R RA RES ≤ •• 20 30 40 276 256 μ volts/DAC LSB (7) Margining DC/DC Converters with a TRIM Pin Figure 2 illustrates a typical application circuit for margining the output voltage of a DC/DC converter with a TRIM Pin. The LTC2970’s VOUT0 pin connects directly to the TRIM pin through resistor R30 and the IOUT0 pin is terminated at the converter's point-of-load ground throught R40. Resistors R30 and R40 give this application circuit two degrees of freedom so that the margin-up and margin- down percentages can be specified independently. Following power-up, the LTC2970's VOUT0 pin defaults to a high-impedance state. If the soft-connect feature is used, Figure 2. LTC2970 Application Circuit for DC/DC Converters with a TRIM Pin VO+ VSENSE– VSENSE+ R40 0.1 μF 0.1 μF TRIM 1/2 LTC2970 VOUT0 VIN0_AP VDC0 + – IOUT0 VIN0_AM ALERT SCL I2C BUS SDA GPIO_0 REF 12VIN GND ASEL0 ASEL1 LOAD VDD GPIO_CFG DC/DC CONVERTER VO– R30 29701 F02 ON/OFF VIN 8V TO 15V 0.1 μF |
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