Datasheet LTC1625 (Analog Devices) - 9

LTC1625
U U W U APPLICATIONS INFORMATION
The basic LTC1625 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 case tempera- the sense resistance and power MOSFETs. Because the ture TJC is around 10°C in most applications. For a LTC1625 uses MOSFET VDS sensing, the sense resistance maximum ambient temperature of 70°C, using ρ80°C ≅ 1.3 is the RDS(ON) of the MOSFETs. The operating frequency in the above equation is a reasonable choice. This equation and the inductor are chosen based largely on the desired is plotted in Figure 3 to illustrate the dependence of amount of ripple current. Finally, CIN is selected for its maximum output current on RDS(ON). Some popular ability to handle the large RMS current into the converter MOSFETs from Siliconix are shown as data points. and COUT is chosen with low enough ESR to meet the output voltage ripple specification. 2.0
Power MOSFET Selection
1.5 The LTC1625 requires two external N-channel power MOSFETs, one for the top (main) switch and one for the 1.0 bottom (synchronous) switch. Important parameters for the power MOSFETs are the breakdown voltage V(BR)DSS, threshold voltage V 0.5 GS(TH), on-resistance RDS(ON), reverse NORMALIZED ON RESISTANCE transfer capacitance C ρ T RSS and maximum current ID(MAX). The gate drive voltage is set by the 5.2V INTV 0 CC supply. – 50 0 50 100 150 Consequently, logic level threshold MOSFETs must be JUNCTION TEMPERATURE (°C) used in LTC1625 applications. If low input voltage opera- 1625 F02 tion is expected (VIN < 5V), then sub-logic level threshold
Figure 2. RDS(ON) vs Temperature
MOSFETs should be used. Pay close attention to the V(BR)DSS specification for the MOSFETs as well; many of 10 the logic level MOSFETs are limited to 30V or less. The MOSFET on-resistance is chosen based on the 8 Si4420 required load current. The maximum average output cur- 6 rent IO(MAX) is equal to the peak inductor current less half the peak-to-peak ripple current ∆I Si4410 L. The peak inductor 4 current is inherently limited in a current mode controller by the current threshold I Si4412 TH range. The corresponding 2 maximum V MAXIMUM OUTPUT CURRENT (A) Si9936 DS sense voltage is about 150mV under nor- mal conditions. The LTC1625 will not allow peak inductor 0 current to exceed 150mV/R 0 0.02 0.04 0.06 0.08 0.10 DS(ON)(TOP). The following RDS(ON) (Ω) equation is a good guide for determining the required 1625 F03 RDS(ON)(MAX) at 25°C (manufacturer’s specification), al-
Figure 3. Maximum Output Current vs R
lowing some margin for ripple current, current limit and
DS(ON) at VGS = 4.5V
variations in the LTC1625 and external component values: The power dissipated by the top and bottom MOSFETs strongly depends upon their respective duty cycles and mV 120 RDS ON ( ) MAX ( ) ≅ the load current. When the LTC1625 is operating in con- I ( )(ρ ) O MAX ( ) T tinuous mode, the duty cycles for the MOSFETs are: 9