LTC3862 Datasheet, PDF (27/40 Pages) Linear Technology – Multi-Phase Current Mode Step-Up DC/DC Controller

LTC3862
APPLICATIONS INFORMATION
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:
( ) ^PFET
=
⎛
⎜
⎝
n
•
I_O(MAX)
1– D_MAX
⎞²
⎟
⎠
• R_DS(ON) • D_MAX • ρ_T
( ) ⁺
k
•
V_OUT2
•
n
•
I_O(MAX)
1– D_MAX
• C_RSS • f
The first term in the equation above represents the I²R
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 coefficient 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 significantly
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_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 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.
2.0
1.5
1.0
0.5
0
–50
0
50
100
150
JUNCTION TEMPERATURE (°C)
3862 F19
Figure 19. Normalized Power MOSFET R_DS(ON) vs Temperature
80
75
SLOPE = 0.625
70
SLOPE = 1
SLOPE = 1.66
65
60
55
50
0 10 20 30 40 50 60 70 80 90 100
DUTY CYCLE (%)
3862 F20
Figure 20. Maximum Sense Voltage Variation
with Duty Cycle and Slope Gain
3862fb
27