LTC3872 Linear Technology, LTC3872 Datasheet - Page 9

• Manufacturer Part Number: LTC3872
• Description: No RSENSE Current Mode Boost DC/DC Controller
• Manufacturer: Linear Technology

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the maximum drain current (I
thermal resistances (R
V
voltage is high, otherwise if low input voltage operation
is expected (e.g., supplying power from a lithium-ion
battery or a 3.3V logic supply), then sublogic-level (2.5V
V
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 270mV, 100mV and 170mV at low duty cycle with
IPRG tied to V
peak inductor current is therefore limited to (270mV,
170mV and 100mV)/R
the IPRG pin.
The relationship between the maximum load current, duty
cycle and the R
APPLICATIO S I FOR ATIO
GS-RATED
GS-RATED
Figure 3. Maximum SENSE Threshold Voltage vs Duty Cycle
R
DS ON
(
) threshold MOSFETs should be used when input
) threshold MOSFETs should be used.
)
≤
300
250
200
150
100
50
V
0
IN
SENSE MAX
DS(ON)
1
, GND, or left fl oating respectively. The
U
(
20
TH(JC)
of the power MOSFET is:
DS(ON)
IPRG = FLOAT
IPRG = HIGH
IPRG = LOW
DUTY CYCLE (%)
)
U
40
•
and R
⎛
⎜
⎝
depending on the status of
1
D(MAX)
DSS
+
60
TH(JA)
χ
2
1
specifi cations for the
⎞
⎟
⎠
W
–
) and the MOSFET’s
•
D
I
). Logic-level (4.5V
80
O MAX
MAX
(
3872 G03
100
)
•
U
ρ
T
V
power MOSFET. V
and 100mV. It is reduced with increasing duty cycle as
shown in Figure 3. The ρ
coeffi cient of the R
0.4%/°C. Figure 4 illustrates the variation of normalized
R
Another method of choosing which power MOSFET to
use is to check what the maximum output current is for a
given R
in 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 coeffi cient of its R
O(MAX)
SENSE(MAX)
DS(ON)
I
O MAX
(
DS(ON)
and R
over temperature for a typical power MOSFET.
Figure 4. Normalized R
)
=
2.0
1.5
1.0
0.5
V
is the maximum voltage drop across the
, since MOSFET on-resistances are available
0
DS(ON)
SENSE MAX
– 50
DS(ON)
SENSE(MAX)
(
JUNCTION TEMPERATURE (°C)
can cause boost converters with a
0
T
term accounts for the temperature
of the MOSFET, which is typically
)
•
⎛
⎜
⎝
1
DS(ON)
is typically 270mV, 170mV
50
+
MAX
χ
2
1
⎞
⎟
⎠
vs Temperature
relationship between
–
•
100
R
D
DS ON
MAX
LTC3872
(
3872 F04
DS(ON)
150
)
•
ρ
T
). As a
3872fa
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