Datasheet LTC1775 (Analog Devices) - 9

LTC1775
U U W U APPLICATIO S I FOR ATIO
The basic LTC1775 application circuit is shown in Figure 1. The ρT is a normalized term accounting for the significant External component selection is primarily determined by variation in RDS(ON) with temperature, typically about the maximum load current and begins with the selection of 0.4%/°C as shown in Figure 2. Junction to ambient tem- the sense resistance for the desired current level. Since the perature TJA is around 20°C in most applications. For a LTC1775 senses current using the on-resistance of the maximum ambient temperature of 70°C, using ρ90°C ≅ 1.3 power MOSFET, the maximum application current prima- in the above equation is a reasonable choice. This equation rily determines the choice of MOSFET. The operating is plotted in Figure 3 to illustrate the dependence of frequency and the inductor are chosen based largely on maximum output current on RDS(ON). Some popular the desired amount of ripple current. Finally, CIN is se- MOSFETs are shown as data points. lected for its ability to handle the RMS current into the converter and COUT is chosen with low enough ESR to 2.0 meet the output voltage ripple specification. 1.5
Power MOSFET Selection
The LTC1775 requires two external N-channel power 1.0 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 0.5 (BR)DSS, NORMALIZED ON RESISTANCE ρ T threshold voltage VGS(TH), on-resistance RDS(ON), reverse transfer capacitance CRSS and maximum current ID(MAX). 0–50 0 50 100 150 The gate drive voltage is set by the 5.2V INTVCC supply. JUNCTION TEMPERATURE (°C) Consequently, logic level threshold MOSFETs must be 1775 F02 used in LTC1775 applications. If low input voltage opera-
Figure 2. RDS(ON) vs Temperature
tion is expected (VIN < 5V), then sub-logic level threshold MOSFETs should be used. Pay close attention to the 25 V(BR)DSS specification for the MOSFETs as well; many of IRL3803 the logic level MOSFETs are limited to 30V or less. 20 The MOSFET on-resistance is chosen based on the 15 required load current. The maximum average output cur- rent I SUD50N03-10 O(MAX) is equal to the peak inductor current less half 10 the peak-to-peak ripple current ∆IL. The peak inductor current is inherently limited in a current mode controller FDS8936A 5 MAXIMUM OUTPUT CURRENT (A) Si9936 by the current threshold ITH range. The corresponding maximum VDS sense voltage is about 300mV under nor- 0 0 0.02 0.04 0.06 0.08 0.10 mal conditions. The LTC1775 will not allow peak inductor RDS(ON) (Ω) current to exceed 300mV/RDS(ON)(TOP). The following 1775 F03 equation is a good guide for determining the required
Figure 3. Maximum Output Current vs RDS(ON) at VGS = 4.5V
RDS(ON)(MAX) at 25°C (manufacturer’s specification), al- lowing some margin for ripple current, current limit and The 300mV maximum sense voltage of the LTC1775 variations in the LTC1775 and external component values: allows a large current to be obtained from power MOSFET switches. It also causes a significant amount of power mV 240 RDS ON ( ) MAX ( ) ≅ dissipation in those switches and careful attention must be I ( )(ρ ) O MAX ( ) T 9