LTC1871 Linear Technology, LTC1871 Datasheet - Page 15

• Manufacturer Part Number: LTC1871
• Description: Wide Input Range/ No RSENSE Current Mode Boost/ Flyback and SEPIC Controller
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
a lithium-ion battery or a 3.3V logic supply), then sublogic-
level threshold MOSFETs should be used.
Pay close attention to the BV
MOSFETs relative to the maximum actual switch voltage in
the application. Many logic-level devices are limited to 30V
or less, and the switch node can ring during the turn-off of
the MOSFET due to layout parasitics. Check the switching
waveforms of the MOSFET directly across the drain and
source terminals using the actual PC board layout (not just
on a lab breadboard!) for excessive ringing.
During the switch on-time, the control circuit limits the
maximum voltage drop across the power MOSFET to
about 150mV (at low duty cycle). The peak inductor
current is therefore limited to 150mV/R
tionship between the maximum load current, duty cycle
and the R
The V
cycle, and is reduced to about 100mV at a duty cycle of
92% due to slope compensation, as shown in Figure 10.
The
the R
Figure 11 illustrates the variation of normalized R
over temperature for a typical power MOSFET.
Figure 10. Maximum SENSE Threshold Voltage vs Duty Cycle
R
DS ON
DS(ON)
T
SENSE(MAX)
(
term accounts for the temperature coefficient of
DS(ON)
)
200
150
100
50
0
of the MOSFET, which is typically 0.4%/ C.
0
V
SENSE MAX
of the power MOSFET is:
term is typically 150mV at low duty
0.2
U
(
0.4
DUTY CYCLE
)
U
•
1
0.5
DSS
2
1
W
specifications for the
0.8
–
•
D
I
O MAX
MAX
1871 F10
(
DS(ON)
1.0
)
•
U
. The rela-
T
DS(ON)
Another method of choosing which power MOSFET to use
is to check what the maximum output current is for a given
R
discrete values.
It is worth noting that the 1 – D
I
wide input range to experience a dramatic range of maxi-
mum input and output current. This should be taken into
consideration in applications where it is important to limit
the maximum current drawn from the input supply.
Calculating Power MOSFET Switching and Conduction
Losses and Junction Temperatures
In order to calculate the junction temperature of the power
MOSFET, the power dissipated by the device must be
known. This power dissipation is a function of the duty
cycle, the load current and the junction temperature itself
(due to the positive temperature coefficient of its R
As a result, some iterative calculation is normally required
to determine a reasonably accurate value. Since the
controller is using the MOSFET as both a switching and a
sensing element, care should be taken to ensure that the
converter is capable of delivering the required load current
over all operating conditions (line voltage and tempera-
ture), and for the worst-case specifications for V
O(MAX)
DS(ON)
I
O MAX
(
, since MOSFET on-resistances are available in
Figure 11. Normalized R
and R
)
2.0
1.5
1.0
0.5
0
– 50
V
DS(ON)
SENSE MAX
JUNCTION TEMPERATURE ( C)
(
0
can cause boost converters with a
)
•
50
1
DS(ON)
MAX
2
1
100
vs Temperature
relationship between
–
•
R
D
DS ON
MAX
1871 F11
LTC1871
(
150
)
SENSE(MAX)
•
DS(ON)
T
15
).