LTC1775 LINER [Linear Technology], LTC1775 Datasheet - Page 9

• Manufacturer Part Number: LTC1775
• Description: High Power No RSENSE TM Current Mode Synchronous Step-Down Switching Regulator
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
The basic LTC1775 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 for the desired current level. Since the
LTC1775 senses current using the on-resistance of the
power MOSFET, the maximum application current prima-
rily determines the choice of MOSFET. The operating
frequency and the inductor are chosen based largely on
the desired amount of ripple current. Finally, C
lected for its ability to handle the RMS current into the
converter and C
meet the output voltage ripple specification.
Power MOSFET Selection
The LTC1775 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
threshold voltage V
transfer capacitance C
The gate drive voltage is set by the 5.2V INTV
Consequently, logic level threshold MOSFETs must be
used in LTC1775 applications. If low input voltage opera-
tion is expected (V
MOSFETs should be used. Pay close attention to the
V
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 I
the peak-to-peak ripple current I
current is inherently limited in a current mode controller
by the current threshold I
maximum V
mal conditions. The LTC1775 will not allow peak inductor
current to exceed 300mV/R
equation is a good guide for determining the required
R
lowing some margin for ripple current, current limit and
variations in the LTC1775 and external component values:
(BR)DSS
DS(ON)(MAX)
R
DS ON MAX
O(MAX)
(
specification for the MOSFETs as well; many of
)(
is equal to the peak inductor current less half
DS
at 25 C (manufacturer’s specification), al-
sense voltage is about 300mV under nor-
)
OUT
IN
GS(TH)
U
I
< 5V), then sub-logic level threshold
O MAX
is chosen with low enough ESR to
(
RSS
240
, on-resistance R
U
and maximum current I
mV
TH
)
DS(ON)(TOP)
range. The corresponding
T
W
L
. The peak inductor
. The following
DS(ON)
U
CC
, reverse
IN
(BR)DSS
supply.
D(MAX)
is se-
,
.
The
variation in R
0.4%/ C as shown in Figure 2. Junction to ambient tem-
perature T
maximum ambient temperature of 70 C, using
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
MOSFETs are shown as data points.
The 300mV maximum sense voltage of the LTC1775
allows a large current to be obtained from power MOSFET
switches. It also causes a significant amount of power
dissipation in those switches and careful attention must be
Figure 3. Maximum Output Current vs R
T
is a normalized term accounting for the significant
JA
2.0
1.5
1.0
0.5
25
20
10
15
0
5
0
– 50
0
is around 20 C in most applications. For a
Figure 2. R
DS(ON)
IRL3803
0.02
JUNCTION TEMPERATURE ( C)
SUD50N03-10
0
with temperature, typically about
DS(ON)
0.04
R
DS(ON)
FDS8936A
50
vs Temperature
0.06
( )
DS(ON)
100
0.08
DS(ON)
Si9936
. Some popular
1775 F02
1775 F03
LTC1775
0.10
150
at V
GS
90 C
= 4.5V
9
1.3