ltc3713 Linear Technology Corporation, ltc3713 Datasheet - Page 11

• Manufacturer Part Number: ltc3713
• Description: Low Input Voltage, High Power, No Rsense Synchronous Buck Dc/dc Controller
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
range with respect to zero current. This can be accom-
plished by introducing an offset into the sense voltage as
shown in Figure 2.
The first step in calculating the amount of required offset
voltage is to determine the maximum sense voltage.
A good rule of thumb is to set the maximum sense voltage
for a current limit that is 30% greater than the maximum
source current.
The voltage on pin V
of V
V
The offset voltage is added as shown in Figure 2 and can
be set by choosing the values of R
The offset voltage must be scaled to V
fering with the internal current limit foldback.
Power MOSFET Selection
The LTC3713 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
OS
V
V
V
V
SENSE
SENSE
RNG
OS
OS
can be calculated using the following formula:
= 0.6V
= V
.
R
= I
V
OS
OUT
SENSE
OUT(MAX)
WITHOUT
SENSE
1
SENSE
SENSE
SENSE
Figure 2. Sense Voltage Offset
R
•
SENSE
V
R
R
/0.133
OUT
OS
OS
(GS)TH
+
+
–
U
RNG
1
2
RSS
R
• R
OS2
should be set based on the value
, on-resistance R
SENSE
and maximum current I
U
+
R
V
OS1
OS
OS1
W
–
R
3713 F02
OUT
and R
SENSE
DS(ON)
to avoid inter-
OS2
U
:
, reverse
DS(MAX)
(BR)DSS
,
.
The gate drive voltage is set by the 5V INTV
Consequently, logic-level threshold MOSFETs must be
used in LTC3713 applications.
When the bottom MOSFET is used as the current sense
element, particular attention must be paid to its
on-resistance. MOSFET on-resistance is typically speci-
fied with a maximum value R
case, additional margin is required to accommodate the
rise in MOSFET on-resistance with temperature:
The
accounting for the significant variation in on-resistance
with temperature, typically about 0.4%/ C as shown in
Figure 3. For a maximum junction temperature of 100 C,
using a value
The power dissipated by the top and bottom MOSFETs
strongly depends upon their respective duty cycles and
the load current. When the LTC3713 is operating in
continuous mode, the duty cycles for the MOSFETs are:
R
D
D
DS ON MAX
TOP
BOT
T
(
term is a normalization factor (unity at 25 C)
)(
2.0
1.0
0.5
1.5
V
V
0
– 50
V
OUT
IN
IN
Figure 3. R
–
V
T
)
IN
= 1.3 is reasonable.
V
OUT
JUNCTION TEMPERATURE ( C)
R
0
SENSE
T
DS(ON)
50
DS(ON)(MAX)
vs Temperature
100
3713 F03
LTC3713
at 25 C. In this
150
CC
11
supply.
3713fa