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 V
DS
sensing, the sense resistance
is the R
DS(ON)
of the MOSFETs. The operating frequency
and the inductor are chosen based largely on the desired
amount of ripple current. Finally, C
IN
is selected for its
ability to handle the large RMS current into the converter
and C
OUT
is chosen with low enough ESR to meet the
output voltage ripple specification.
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 V
GS(TH)
, on-resistance R
DS(ON)
, reverse
transfer capacitance C
RSS
and maximum current I
D(MAX)
.
The gate drive voltage is set by the 5.2V INTV
CC
supply.
Consequently, logic level threshold MOSFETs must be
used in LTC1625 applications. If low input voltage opera-
tion is expected (V
IN
< 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.
MAXIMUM OUTPUT CURRENT (A)
ρ
T
NORMALIZED ON RESISTANCE
The MOSFET on-resistance is chosen based on the
required load current. The maximum average output cur-
rent I
O(MAX)
is equal to the peak inductor current less half
the peak-to-peak ripple current
∆I
L
. The peak inductor
current is inherently limited in a current mode controller
by the current threshold I
TH
range. The corresponding
maximum V
DS
sense voltage is about 150mV under nor-
mal conditions. The LTC1625 will not allow peak inductor
current to exceed 150mV/R
DS(ON)(TOP)
. The following
equation is a good guide for determining the required
R
DS(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:
R
DS(ON)(MAX)
≅
(
I
O(MAX)
)(
ρ
T
)
120mV
U
W
U
U
The
ρ
T
is a normalized term accounting for the significant
variation in R
DS(ON)
with temperature, typically about
0.4%/°C as shown in Figure 2. Junction to case tempera-
ture T
JC
is around 10°C in most applications. For a
maximum ambient temperature of 70°C, using
ρ
80°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 R
DS(ON)
. Some popular
MOSFETs from Siliconix are shown as data points.
2.0
1.5
1.0
0.5
0
– 50
50
100
0
JUNCTION TEMPERATURE (°C)
150
1625 F02
Figure 2. R
DS(ON)
vs Temperature
10
8
Si4420
6
Si4410
4
Si4412
2
Si9936
0
0
0.02
0.06
0.04
R
DS(ON)
(Ω)
0.08
0.10
1625 F03
Figure 3. Maximum Output Current vs R
DS(ON)
at V
GS
= 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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