LTC3786 LINER [Linear Technology], LTC3786 Datasheet - Page 28

• Manufacturer Part Number: LTC3786
• Description: Multi-Phase Current Mode Step-Up DC/DC Controller
• Manufacturer: LINER [Linear Technology]

applicaTions inForMaTion
LTC3862-2
The power dissipated by the MOSFET in a multi-phase
boost converter with n phases is:
The first term in the equation above represents the I
losses in the device, and the second term, the switching
losses. The constant, k = 1.7, is an empirical factor inversely
related to the gate drive current and has the dimension
of 1/current.
The ρ
the R
Figure 19 illustrates the variation of normalized R
over temperature for a typical power MOSFET.
From a known power dissipated in the power MOSFET, its
junction temperature can be obtained using the following
formula:
28
Figure 19. Normalized Power MOSFET R
T
P
J
FET
DS(ON)
= T
T
term accounts for the temperature coefficient of
=
A



+ P
0.5
+ k • V
2.0
1.5
1.0
n • 1– D
0
–50
of the MOSFET, which is typically 0.4%/ºC.
FET
I
(
O(MAX)
OUT
• R
2
JUNCTION TEMPERATURE (°C)
•
MAX
TH(JA)
0
n • 1– D
(
I
)
O(MAX)



2
50
• R
MAX
DS(ON)
)
100
• C
DS(ON)
RSS
• D
38622 F19
MAX
• f
150
vs Temperature
• ρ
T
DS(ON)
2
R
The R
the R
the case to the ambient temperature (R
of T
used in the iterative calculation process.
It is tempting to choose a power MOSFET with a very low
R
so, however, the gate charge Q
higher, which increases switching and gate drive losses.
Since the switching losses increase with the square of
the output voltage, applications with a low output voltage
generally have higher MOSFET conduction losses, and
high output voltage applications generally have higher
MOSFET switching losses. At high output voltages, the
highest efficiency is usually obtained by using a MOSFET
with a higher R
can easily be split into two components (conduction and
switching) and entered into a spreadsheet, in order to
compare the performance of different MOSFETs.
Programming the Current Limit
The peak sense voltage threshold for the LTC3862-2 is
75mV at low duty cycle and with a normalized slope gain of
1.00, and is measured from SENSE
illustrates the change in the sense threshold with varying
duty cycle and slope gain.
DS(ON)
J
can then be compared to the original, assumed value
TH(JC)
TH(JA)
in order to reduce conduction losses. In doing
Figure 20. Maximum Sense Voltage Variation
with Duty Cycle and Slope Gain
80
75
70
65
60
55
50
45
40
35
30
for the device plus the thermal resistance from
to be used in this equation normally includes
0
DS(ON)
10
20
30
and lower Q
DUTY CYCLE (%)
40
SLOPE = 1
50
60
SLOPE = 1.66
G
SLOPE = 0.625
70
is usually significantly
G
+
. The equation above
to SENSE
80
TH(CA)
38622 F20
90
100
). This value
–
. Figure 20
38622f