LTC3775 View Datasheet(PDF) - Linear Technology
Part Name
Description
Manufacturer
LTC3775 Datasheet PDF : 34 Pages
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LTC3775
APPLICATIONS INFORMATION
If topside MOSFET RDS(ON) sensing is used, the RILIMT
value is calculated from the following equation:
RILIMIT
=
T
• RDS(ON)(QT)(MAX)
•
IO(MAX) + 0.5 •
ILIMIT(MIN)
IL
RDS(ON)(QT)(MAX) is the maximum MOSFET on-resistance
typically specified at 25°C. The ρT term is a normalization
factor (unity at 25°C) accounting for the significant 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 ρT = 1.4 is reasonable.
The bottom side current limit can be programmed by
setting RILIMB as follows:
RILIMB
=
5
•
T
•
RDS(ON)(QB)(MAX)
•
IO(MAX) + 0.5 •
ILIMB(MIN)
IL
where ILIMB(MIN) = ILIMB pin minimum pull-up current of 9μA.
The resulting values of RILIMT and RILIMB should be checked
in an actual circuit to ensure that the current limit kicks
in as expected. Circuits that use MOSFETs with low value
RDS(ON) for current sensing should be checked carefully.
The PCB trace resistance and parasitic inductance can
significantly 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 flowing in the inductor during an overload
condition. The inductor saturation current rating needs to
be higher than the worst-case peak inductor current:
IL(SAT)
≥
ILIMT(MAX) • RILIMT
RSENSE(MIN)
or
IL(SAT)
≥
ILIMT(MAX) • RILIMT
RDSON(QT)(MIN)
or
( ) IL(SAT) ≥
0.2 •ILIMB(MAX) •RILIMB
RDSON(QB)(MIN)
ILIMT(MAX) = ILIMT pin maximum pull-down current of
110μA
ILIMB(MAX) = ILIMB pin maximum pull-up current of
11μA
RDS(ON)(QT)(MIN) and RDS(ON)(QB)(MIN) are the power
MOSFET minimum on-resistances. MOSFET data sheets
typically specify nominal and maximum values for RDS(ON),
but not a minimum. A reasonable assumption is that the
minimum RDS(ON) is the same percentage below the typical
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 final 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.
2.0
1.5
1.0
0.5
0
–50
0
50
100
150
JUNCTION TEMPERATURE (°C)
3775 F16
Figure 16. Typical MOSFET RDS(ON) vs Temperature
3775fa
23
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