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LTC1775 Datasheet(PDF) 9 Page - Linear Technology |
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LTC1775 Datasheet(HTML) 9 Page - Linear Technology |
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9 / 24 page  9 LTC1775 The basic LTC1775 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 for the desired current level. Since the LTC1775 senses current using the on-resistance of the power MOSFET, the maximum application current prima- rily determines the choice of MOSFET. The operating frequency and the inductor are chosen based largely on the desired amount of ripple current. Finally, CIN is se- lected for its ability to handle the RMS current into the converter and COUT is chosen with low enough ESR to meet the output voltage ripple specification. Power MOSFET Selection The LTC1775 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 LTC1775 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 300mV under nor- mal conditions. The LTC1775 will not allow peak inductor current to exceed 300mV/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 LTC1775 and external component values: R mV I DS ON MAX O MAX T ()( ) () ≅ ()() 240 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 ambient tem- perature TJA is around 20°C in most applications. For a maximum ambient temperature of 70 °C, using ρ90°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 are shown as data points. Figure 2. RDS(ON) vs Temperature Figure 3. Maximum Output Current vs RDS(ON) at VGS = 4.5V The 300mV maximum sense voltage of the LTC1775 allows a large current to be obtained from power MOSFET switches. It also causes a significant amount of power dissipation in those switches and careful attention must be JUNCTION TEMPERATURE ( °C) –50 1.0 1.5 150 1775 F02 0.5 0 0 50 100 2.0 RDS(ON) (Ω) 0 15 20 25 0.08 1775 F03 10 5 0 0.02 0.04 0.06 0.10 IRL3803 SUD50N03-10 FDS8936A Si9936 APPLICATIO S I FOR ATIO |
Similar Part No. - LTC1775_15 |
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Similar Description - LTC1775_15 |
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