LTC1625 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1625' PDF : 24 Pages View PDF
LTC1625
APPLICATIONS INFORMATION
The basic LTC1625 application circuit is shown in Figure 1.
External component selection is primarily determined by
the maximum load current and begins with the selection of
the sense resistance and power MOSFETs. Because the
LTC1625 uses MOSFET VDS sensing, the sense resistance
is the RDS(ON) of the MOSFETs. The operating frequency
and the inductor are chosen based largely on the desired
amount of ripple current. 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 specification.
The ρT is a normalized term accounting for the significant
variation in RDS(ON) with temperature, typically about
0.4%/°C as shown in Figure 2. Junction to case tempera-
ture TJC is around 10°C in most applications. For a
maximum ambient temperature of 70°C, using ρ80°C ≅ 1.3
in the above equation is a reasonable choice. This equation
is plotted in Figure 3 to illustrate the dependence of
maximum output current on RDS(ON). Some popular
MOSFETs from Siliconix are shown as data points.
2.0
Power MOSFET Selection
The LTC1625 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 VGS(TH), on-resistance RDS(ON), reverse
transfer capacitance CRSS and maximum current ID(MAX).
The gate drive voltage is set by the 5.2V INTVCC supply.
Consequently, logic level threshold MOSFETs must be
used in LTC1625 applications. If low input voltage opera-
tion is expected (VIN < 5V), then sub-logic level threshold
MOSFETs should be used. Pay close attention to the
V(BR)DSS specification for the MOSFETs as well; many of
the logic level MOSFETs are limited to 30V or less.
The MOSFET on-resistance is chosen based on the
required load current. The maximum average output cur-
rent IO(MAX) is equal to the peak inductor current less half
the peak-to-peak ripple current ∆IL. The peak inductor
current is inherently limited in a current mode controller
by the current threshold ITH range. The corresponding
maximum VDS sense voltage is about 150mV under nor-
mal conditions. The LTC1625 will not allow peak inductor
current to exceed 150mV/RDS(ON)(TOP). The following
equation is a good guide for determining the required
RDS(ON)(MAX) at 25°C (manufacturer’s specification), al-
lowing some margin for ripple current, current limit and
variations in the LTC1625 and external component values:
( )( ) RDS(ON)(MAX) ≅
120mV
IO(MAX) ρT
1.5
1.0
0.5
0
– 50
0
50
100
150
JUNCTION TEMPERATURE (°C)
1625 F02
Figure 2. RDS(ON) vs Temperature
10
8
Si4420
6
Si4410
4
Si4412
2
Si9936
0
0
0.02 0.04 0.06 0.08 0.10
RDS(ON) (Ω)
1625 F03
Figure 3. Maximum Output Current vs RDS(ON) at VGS = 4.5V
The power dissipated by the top and bottom MOSFETs
strongly depends upon their respective duty cycles and
the load current. When the LTC1625 is operating in con-
tinuous mode, the duty cycles for the MOSFETs are:
9
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