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LTC3713 Datasheet(PDF) 10 Page - Linear Technology |
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LTC3713 Datasheet(HTML) 10 Page - Linear Technology |
10 / 24 page 10 LTC1778/LTC1778-1 1778fb APPLICATIO S I FOR ATIO The basic LTC1778 application circuit is shown in Figure 1. External component selection is primarily de- termined by the maximum load current and begins with the selection of the sense resistance and power MOSFET switches. The LTC1778 uses the on-resistance of the synchronous power MOSFET for determining the induc- tor current. The desired amount of ripple current and operating frequency largely determines the inductor value. 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 and transient specification. Choosing the LTC1778 or LTC1778-1 The LTC1778 has an open-drain PGOOD output that indicates when the output voltage is within ±7.5% of the regulation point. The LTC1778-1 trades the PGOOD pin for a VON pin that allows the on-time to be adjusted. Tying the VON pin high results in lower values for RON which is useful in high VOUT applications. The VON pin also provides a means to adjust the on-time to maintain constant fre- quency operation in applications where VOUT changes and to correct minor frequency shifts with changes in load current. Finally, the VON pin can be used to provide additional current limiting in positive-to-negative convert- ers and as a control input to synchronize the switching frequency with a phase locked loop. Maximum Sense Voltage and VRNG Pin Inductor current is determined by measuring the voltage across a sense resistance that appears between the PGND and SW pins. The maximum sense voltage is set by the voltage applied to the VRNG pin and is equal to approxi- mately (0.133)VRNG. The current mode control loop will not allow the inductor current valleys to exceed (0.133)VRNG/RSENSE. In practice, one should allow some margin for variations in the LTC1778 and external compo- nent 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 1.33 times this nominal value. Power MOSFET Selection The LTC1778 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 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 LTC1778 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 Figure 2. RDS(ON) vs. Temperature JUNCTION TEMPERATURE (°C) –50 1.0 1.5 150 1778 F02 0.5 0 0 50 100 2.0 |
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