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LTC3717 Datasheet(PDF) 9 Page - Linear Technology |
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LTC3717 Datasheet(HTML) 9 Page - Linear Technology |
9 / 20 page 9 LTC3717 sn3717 3717fs APPLICATIO S I FOR ATIO the load current. During LTC3717’s normal operation, the duty cycles for the MOSFETs are: D V V D VV V TOP OUT IN BOT IN OUT IN = = – The resulting power dissipation in the MOSFETs at maxi- mum output current are: PTOP = DTOP IOUT(MAX)2 ρT(TOP) RDS(ON)(MAX) + k VIN2 IOUT(MAX) CRSS f PBOT = DBOT IOUT(MAX)2 ρT(BOT) RDS(ON)(MAX) Both MOSFETs have I2R losses and the top MOSFET includes an additional term for transition losses, which are largest at high input voltages. The constant k = 1.7A–1 can be used to estimate the amount of transition loss. The bottom MOSFET losses are greatest when the bottom duty cycle is near 100%, during a short-circuit or at high input voltage. JUNCTION TEMPERATURE ( °C) –50 1.0 1.5 150 3717 F02 0.5 0 0 50 100 2.0 Figure 2. RDS(ON) vs. Temperature Operating Frequency The choice of operating frequency is a tradeoff between efficiency and component size. Low frequency operation improves efficiency by reducing MOSFET switching losses but requires larger inductance and/or capacitance in order to maintain low output ripple voltage. The operating frequency of LTC3717 applications is deter- mined implicitly by the one-shot timer that controls the the LTC3717 and external component values and a good guide for selecting the sense resistance is: R V I SENSE RNG OUT MAX = 10 • () An external resistive divider from INTVCC can be used to set the voltage of the VRNG pin between 0.5V and 2V resulting in nominal sense voltages of 50mV to 200mV. Additionally, the VRNG pin can be tied to SGND or INTVCC in which case the nominal sense voltage defaults to 70mV or 140mV, respectively. The maximum allowed sense voltage is about 1.3 times this nominal value for positive output current and 1.7 times the nominal value for negative output current. Power MOSFET Selection The LTC3717 requires two external N-channel power MOS- FETs, 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 V(GS)TH, on-resistance RDS(ON), reverse transfer capacitance CRSS and maximum current IDS(MAX). The gate drive voltage is set by the 5V INTVCC supply. Consequently, logic-level threshold MOSFETs must be used in LTC3717 applications. If the input voltage is expected to drop below 5V, then sub-logic level threshold MOSFETs should be considered. When the bottom MOSFET is used as the current sense element, particular attention must be paid to its on- resistance. MOSFET on-resistance is typically specified with a maximum value RDS(ON)(MAX) at 25°C. In this case, additional margin is required to accommodate the rise in MOSFET on-resistance with temperature: R R DS ON MAX SENSE T ()( ) = ρ The ρT term is a normalization factor (unity at 25°C) accounting for the significant variation in on-resistance with temperature, typically about 0.4%/ °C as shown in Figure 2. For a maximum junction temperature of 100 °C, using a value ρT = 1.3 is reasonable. The power dissipated by the top and bottom MOSFETs strongly depends upon their respective duty cycles and |
Similar Part No. - LTC3717_15 |
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Similar Description - LTC3717_15 |
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