LTC3780_12 Datasheet, PDF (18/28 Pages) Linear Technology – High Effi ciency, Synchronous, 4-Switch Buck-Boost Controller

LTC3780
APPLICATIONS INFORMATION
Power MOSFET Selection and
Efficiency Considerations
The LTC3780 requires four external N-channel power
MOSFETs, two for the top switches (switch A and D, shown
in Figure 1) and two for the bottom switches (switch B and C
shown in Figure 1). Important parameters for the power
MOSFETs are the breakdown voltage V_BR,DSS, threshold
voltage V_GS,TH, on-resistance R_DS(ON), reverse transfer
capacitance C_RSS and maximum current I_DS(MAX).
The drive voltage is set by the 6V INTV_CC supply. Con-
sequently, logic-level threshold MOSFETs must be used
in LTC3780 applications. If the input voltage is expected
to drop below 5V, then the sub-logic threshold MOSFETs
should be considered.
In order to select the power MOSFETs, the power dis-
sipated by the device must be known. For switch A, the
maximum power dissipation happens in boost mode, when
it remains on all the time. Its maximum power dissipation
at maximum output current is given by:
P_A,BOOST
=
⎛
⎝⎜
V_OUT
V_IN
s I_OUT(MAX)^{⎞⎠⎟ 2}
s ρ_T
s R_DS(ON)
where ρ_T is a normalization factor (unity at 25°C) ac-
counting for the significant variation in on-resistance with
temperature, typically about 0.4%/°C as shown in Figure 9.
For a maximum junction temperature of 125°C, using a
value ρ_T = 1.5 is reasonable.
2.0
1.5
1.0
0.5
0
–50
0
50
100
150
JUNCTION TEMPERATURE (°C)
3780 F09
Figure 9. Normalized R_DS(ON) vs Temperature
Switch B operates in buck mode as the synchronous
rectifier. Its power dissipation at maximum output current
is given by:
P_B,BUCK
=
V_IN
– V_OUT
V_IN
s I_OUT(MAX)2
s ρ_T
s R_DS(ON)
Switch C operates in boost mode as the control switch. Its
power dissipation at maximum current is given by:
( ) _{PC,BOOST =}
V_OUT
– V_IN
V_IN2
V_OUT
s I_OUT(MAX)2
s ρ_T
s R_DS(ON)
+
k
s
V_OUT3
s
I_OUT(MAX)
V_IN
s C_RSS
s
f
where C_RSS is usually specified by the MOSFET manufactur-
ers. The constant k, which accounts for the loss caused
by reverse recovery current, is inversely proportional to
the gate drive current and has an empirical value of 1.7.
For switch D, the maximum power dissipation happens in
boost mode, when its duty cycle is higher than 50%. Its
maximum power dissipation at maximum output current
is given by:
P_D,BOOST
=
V_IN
V_OUT
⎛
s _⎝⎜
V_OUT
V_IN
⎞²
s I_{OUT(MAX) ⎠⎟}
s ρ_T
s R_DS(ON)
For the same output voltage and current, switch A has the
highest power dissipation and switch B has the lowest
power dissipation unless a short occurs at the output.
From a known power dissipated in the power MOSFET, its
junction temperature can be obtained using the following
formula:
T_J = T_A + P • R_TH(JA)
The R_TH(JA) to be used in the equation normally includes
the R_TH(JC) for the device plus the thermal resistance from
the case to the ambient temperature (R_TH(JC)). This value
of T_J can then be compared to the original, assumed value
used in the iterative calculation process.
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