Datasheet

LTC3862-1
27
38621f
result, some iterative calculation is normally required to
determine a reasonably accurate value.
The power dissipated by the MOSFET in a multi-phase
boost converter with n phases is:
P
FET
=
I
O(MAX)
n• 1–D
MAX
( )






2
•R
DS(ON)
•D
MAX
• 
T
+ k•V
OUT
2
•
I
O(MAX)
n• 1–D
MAX
( )
•C
RSS
•f
The fi rst term in the equation above represents the I
2
R
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
T
term accounts for the temperature coeffi cient of
the R
DS(ON)
of the MOSFET, which is typically 0.4%/ºC.
Figure 19 illustrates the variation of normalized R
DS(ON)
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:
T
J
= T
A
+ P
FET
• R
TH(JA)
The R
TH(JA)
to be used in this equation normally includes
the R
TH(JC)
for the device plus the thermal resistance from
the case to the ambient temperature (R
TH(CA)
). This value
of T
J
can then be compared to the original, assumed value
used in the iterative calculation process.
It is tempting to choose a power MOSFET with a very low
R
DS(ON)
in order to reduce conduction losses. In doing
so, however, the gate charge Q
G
is usually signifi cantly
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 effi ciency is usually obtained by using a MOSFET
with a higher R
DS(ON)
and lower Q
G
. The equation above
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 LTC3862-1 is
75mV at low duty cycle and with a normalized slope gain of
1.00, and is measured from SENSE
+
to SENSE
–
. Figure 20
illustrates the change in the sense threshold with varying
duty cycle and slope gain.
APPLICATIONS INFORMATION
Figure 19. Normalized Power MOSFET R
DS(ON)
vs Temperature
JUNCTION TEMPERATURE (°C)
–50
R
T
NORMALIZED ON RESISTANCE
1.0
1.5
150
38621 F19
0.5
0
0
50
100
2.0
DUTY CYCLE (%)
30
MAXIMUM CURRENT SENSE THRESHOLD (mV)
60
70
80
55
50
45
40
35
65
75
20 40 60 80
38621 F20
10010030507090
SLOPE = 0.625
SLOPE = 1
SLOPE = 1.66
Figure 20. Maximum Sense Voltage Variation
with Duty Cycle and Slope Gain