LTC3775 Linear Technology, LTC3775 Datasheet - Page 23

• Manufacturer Part Number: LTC3775
• Description: High Frequency Synchronous Step-Down Voltage Mode DC/DC Controller
• Manufacturer: Linear Technology

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If topside MOSFET R
value is calculated from the following equation:
R
typically specifi ed at 25°C. The ρ
factor (unity at 25°C) accounting for the signifi cant variation
in on-resistance with temperature, typically about 0.5%/°C
as shown in Figure 16. For a maximum junction temperature
of 100°C, using a value ρ
The bottom side current limit can be programmed by
setting R
where I
The resulting values of R
in an actual circuit to ensure that the current limit kicks
in as expected. Circuits that use MOSFETs with low value
R
The PCB trace resistance and parasitic inductance can
signifi cantly change the actual current limit threshold. Care
should be taken to shorten the PCB trace at the SENSE,
SW and PGND connections.
The current limit setting also determines the worst-case
peak current fl owing in the inductor during an overload
condition. The inductor saturation current rating needs to
be higher than the worst-case peak inductor current:
or
or
APPLICATIONS INFORMATION
DS(ON)(QT)(MAX)
DS(ON)
R
R
I
I
I
L SAT
L SAT
L SAT
ILIMIT
ILIMB
(
(
(
LIMB(MIN)
for current sensing should be checked carefully.
)
)
)
ILIMB
≥
≥
≥
=
=
5
I
I
(
ρ •
LIMT MAX
LIMT MAX
0 2
•
R
T
. •
as follows:
ρ
R
DSON QT MIN
= I
T
(
SENSE MIN
(
R
is the maximum MOSFET on-resistance
R
I
•
LIMB
DS ON QT MAX
LIMB MAX
DSON QB MIN
R
(
(
DS ON QB MAX
)
)
DS(ON)
(
pin minimum pull-up current of 9μA.
(
•
•
ILIMT
(
)(
(
)(
R
R
T
ILIMT
ILIMT
)(
)
)(
= 1.4 is reasonable.
)
)(
)
and R
)
sensing is used, the R
•
)(
R
)
T
ILIMB
)
ILIMB
term is a normalization
•
I
)
O MAX
•
(
I
O MAX
should be checked
I
(
LIM
)
I
LI
+
I I T MIN
M M B MIN
)
(
0 5 Δ
+
. •
(
0 5
)
. •
)
ILIMT
I
L
Δ
I
L
R
MOSFET minimum on-resistances. MOSFET data sheets
typically specify nominal and maximum values for R
but not a minimum. A reasonable assumption is that the
minimum R
value as the maximum lies above it. Consult the MOSFET
manufacturer for further guidelines.
The saturation current rating for the inductor should be
determined at the maximum input voltage, maximum output
current and the maximum expected core temperature. The
saturation current ratings for most commercially available
inductors drop at high temperature. To verify safe operation,
it is a good idea to characterize the inductor’s core/winding
temperature under the following conditions: 1) worst-case
operating conditions, 2) maximum allowable ambient
temperature and 3) with the power supply mounted in
the fi nal enclosure. Thermal characterization can be done
by placing a thermocouple in intimate contact with the
winding/core structure, or by burying the thermocouple
within the windings themselves.
DS(ON)(QT)(MIN)
I
110μA
I
11μA
LIMT(MAX)
LIMB(MAX)
Figure 16. Typical MOSFET R
1.0
0.5
2.0
1.5
DS(ON)
0
–50
= I
= I
LIMT
LIMB
and R
is the same percentage below the typical
JUNCTION TEMPERATURE (°C)
0
pin maximum pull-down current of
pin maximum pull-up current of
DS(ON)(QB)(MIN)
50
DS(ON)
100
vs Temperature
LTC3775
3775 F16
are the power
150
23
DS(ON)
3775f
,