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LTC1625CGN Datasheet(PDF) 9 Page - Linear Technology |
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LTC1625CGN Datasheet(HTML) 9 Page - Linear Technology |
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9 / 24 page  9 LTC1625 APPLICATIONS INFORMATION The basic LTC1625 application circuit is shown in Figure 1. External component selection is primarily determined by the maximum load current and begins with the selection of the sense resistance and power MOSFETs. Because the LTC1625 uses MOSFET VDS sensing, the sense resistance is the RDS(ON) of the MOSFETs. The operating frequency and the inductor are chosen based largely on the desired amount of ripple current. Finally, CIN is selected for its ability to handle the large RMS current into the converter and COUT is chosen with low enough ESR to meet the output voltage ripple specification. Power MOSFET Selection The LTC1625 requires two external N-channel power MOSFETs, one for the top (main) switch and one for the bottom (synchronous) switch. Important parameters for the power MOSFETs are the breakdown voltage V(BR)DSS, threshold voltage VGS(TH), on-resistance RDS(ON), reverse transfer capacitance CRSS and maximum current ID(MAX). The gate drive voltage is set by the 5.2V INTVCC supply. Consequently, logic level threshold MOSFETs must be used in LTC1625 applications. If low input voltage opera- tion is expected (VIN < 5V), then sub-logic level threshold MOSFETs should be used. Pay close attention to the V(BR)DSS specification for the MOSFETs as well; many of the logic level MOSFETs are limited to 30V or less. The MOSFET on-resistance is chosen based on the required load current. The maximum average output cur- rent IO(MAX) is equal to the peak inductor current less half the peak-to-peak ripple current ∆IL. The peak inductor current is inherently limited in a current mode controller by the current threshold ITH range. The corresponding maximum VDS sense voltage is about 150mV under nor- mal conditions. The LTC1625 will not allow peak inductor current to exceed 150mV/RDS(ON)(TOP). The following equation is a good guide for determining the required RDS(ON)(MAX) at 25°C (manufacturer’s specification), al- lowing some margin for ripple current, current limit and variations in the LTC1625 and external component values: R mV I DS ON MAX O MAX T ()( ) () ≅ ()( ) 120 ρ The ρT is a normalized term accounting for the significant variation in RDS(ON) with temperature, typically about 0.4%/ °C as shown in Figure 2. Junction to case tempera- ture TJC is around 10°C in most applications. For a maximum ambient temperature of 70 °C, using ρ80°C≅1.3 in the above equation is a reasonable choice. This equation is plotted in Figure 3 to illustrate the dependence of maximum output current on RDS(ON). Some popular MOSFETs from Siliconix are shown as data points. JUNCTION TEMPERATURE ( °C) –50 1.0 1.5 150 1625 F02 0.5 0 0 50 100 2.0 Figure 2. RDS(ON) vs Temperature RDS(ON) (Ω) 0 6 8 10 0.08 1625 F03 4 2 0 0.02 0.04 0.06 0.10 Si4420 Si4410 Si4412 Si9936 Figure 3. Maximum Output Current vs RDS(ON) at VGS = 4.5V The power dissipated by the top and bottom MOSFETs strongly depends upon their respective duty cycles and the load current. When the LTC1625 is operating in con- tinuous mode, the duty cycles for the MOSFETs are: |
Similar Part No. - LTC1625CGN |
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Similar Description - LTC1625CGN |
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