Datasheet

LTC3775
23
3775fa
If topside MOSFET R
DS(ON)
sensing is used, the R
ILIMT
value is calculated from the following equation:
R
ILIMIT
=
T
•R
DS(ON)(QT)(MAX)
•
I
O(MAX)
+ 0.5 • I
L
I
LIMIT(MIN)
R
DS(ON)(QT)(MAX)
is the maximum MOSFET on-resistance
typically specifi ed at 25°C. The ρ
T
term is a normalization
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
T
= 1.4 is reasonable.
The bottom side current limit can be programmed by
setting R
ILIMB
as follows:
R
ILIMB
= 5•
T
•R
DS(ON)(QB)(MAX)
•
I
O(MAX)
+ 0.5 • I
L
I
LIMB(MIN)
where I
LIMB(MIN)
= I
LIMB
pin minimum pull-up current of 9µA.
The resulting values of R
ILIMT
and R
ILIMB
should be checked
in an actual circuit to ensure that the current limit kicks
in as expected. Circuits that use MOSFETs with low value
R
DS(ON)
for current sensing should be checked carefully.
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:
I
IR
R
L SAT
LIMT MAX ILIMT
SENSE MIN
()
()
()
•
≥
or
I
IR
R
L SAT
LIMT MAX ILIMT
DSON QT MIN
()
()
()( )
•
≥
or
I
IR
R
L SAT
LIMB MAX ILIMB
DSON QB MIN
()
()
()( )
.• •
≥
()
02
I
LIMT(MAX)
= I
LIMT
pin maximum pull-down current of
110µA
I
LIMB(MAX)
= I
LIMB
pin maximum pull-up current of
11µA
R
DS(ON)(QT)(MIN)
and R
DS(ON)(QB)(MIN)
are the power
MOSFET minimum on-resistances. MOSFET data sheets
typically specify nominal and maximum values for R
DS(ON)
,
but not a minimum. A reasonable assumption is that the
minimum R
DS(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 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.
APPLICATIONS INFORMATION
JUNCTION TEMPERATURE (°C)
–50
R
T
NORMALIZED ON-RESISTANCE
1.0
1.5
150
0.5
0
0
50
100
2.0
3775 F16
Figure 16. Typical MOSFET R
DS(ON)
vs Temperature