Datasheet LTC1778, LTC1778-1 (Analog Devices) - 10

LTC1778/LTC1778-1
U U W U APPLICATIO S I FOR ATIO
The basic LTC1778 application circuit is shown in resulting in nominal sense voltages of 50mV to 200mV. Figure 1. External component selection is primarily de- Additionally, the VRNG pin can be tied to SGND or INTVCC termined by the maximum load current and begins with in which case the nominal sense voltage defaults to 70mV the selection of the sense resistance and power MOSFET or 140mV, respectively. The maximum allowed sense switches. The LTC1778 uses the on-resistance of the voltage is about 1.33 times this nominal value. synchronous power MOSFET for determining the induc-
Power MOSFET Selection
tor current. The desired amount of ripple current and operating frequency largely determines the inductor value. The LTC1778 requires two external N-channel power Finally, C MOSFETs, one for the top (main) switch and one for the IN is selected for its ability to handle the large RMS current into the converter and C bottom (synchronous) switch. Important parameters for OUT is chosen with low enough ESR to meet the output voltage ripple and the power MOSFETs are the breakdown voltage V(BR)DSS, transient specification. threshold voltage V(GS)TH, on-resistance RDS(ON), reverse transfer capacitance CRSS and maximum current IDS(MAX).
Choosing the LTC1778 or LTC1778-1
The gate drive voltage is set by the 5V INTVCC supply. The LTC1778 has an open-drain PGOOD output that Consequently, logic-level threshold MOSFETs must be indicates when the output voltage is within ±7.5% of the used in LTC1778 applications. If the input voltage is regulation point. The LTC1778-1 trades the PGOOD pin for expected to drop below 5V, then sub-logic level threshold a VON pin that allows the on-time to be adjusted. Tying the MOSFETs should be considered. VON pin high results in lower values for RON which is useful When the bottom MOSFET is used as the current sense in high VOUT applications. The VON pin also provides a element, particular attention must be paid to its on- means to adjust the on-time to maintain constant fre- resistance. MOSFET on-resistance is typically specified quency operation in applications where VOUT changes and with a maximum value R to correct minor frequency shifts with changes in load DS(ON)(MAX) at 25°C. In this case, additional margin is required to accommodate the rise in current. Finally, the VON pin can be used to provide MOSFET on-resistance with temperature: additional current limiting in positive-to-negative convert- ers and as a control input to synchronize the switching R frequency with a phase locked loop. R SENSE DS ON ( ) MAX ( ) = ρT
Maximum Sense Voltage and VRNG Pin
The ρ Inductor current is determined by measuring the voltage T term is a normalization factor (unity at 25°C) accounting for the significant variation in on-resistance across a sense resistance that appears between the PGND and SW pins. The maximum sense voltage is set by the 2.0 voltage applied to the VRNG pin and is equal to approxi- mately (0.133)VRNG. The current mode control loop will 1.5 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- 1.0 nent values and a good guide for selecting the sense resistance is: 0.5 NORMALIZED ON-RESISTANCE T ρ V R RNG SENSE = 0 10 • IOUT MAX ( ) – 50 0 50 100 150 JUNCTION TEMPERATURE (°C) An external resistive divider from INTV 1778 F02 CC can be used to set the voltage of the VRNG pin between 0.5V and 2V
Figure 2. RDS(ON) vs. Temperature
1778fb 10