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| LTC3813 Datasheet, PDF (15/32 Pages) Linear Technology – 100V Current Mode Synchronous Step-Up Controller | |||
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LTC3813
APPLICATIONS INFORMATION
increase in coulombs on the horizontal axis from a to b
while the curve is ï¬at) is speciï¬ed for a given VDS drain
voltage, but can be adjusted for different VDS voltages by
multiplying by the ratio of the application VDS to the curve
speciï¬ed VDS values. A way to estimate the CMILLER term
is to take the change in gate charge from points a and b
on a manufacturers data sheet and divide by the stated
VDS voltage speciï¬ed. CMILLER is the most important se-
lection criteria for determining the transition loss term in
the top MOSFET but is not directly speciï¬ed on MOSFET
data sheets. CRSS and COS are speciï¬ed sometimes but
deï¬nitions of these parameters are not included.
When the controller is operating in continuous mode
the duty cycles for the top and bottom MOSFETs are
given by:
Main
Switch
Duty
Cycle
=
VOUT VIN
VOUT
Synchronous
Switch
Duty
Cycle
=
VIN
VOUT
The power dissipation for the main and synchronous
MOSFETs at maximum output current are given by:
PMAIN
=
DMAX
IO(MAX)
1 DMAX
2
(T
)RDS(ON)
+
1
2
VOUT2
IO(MAX)
1 DMAX
(RDR )(CMILLER )
â¢
DRVCC
1
â
VTH(IL)
+
1
VTH(IL)
(f)
PSYNC
=
1
1
DMAX
(IO(MAX))2(T
)
RDS(0N)
where ÏT is the temperature dependency of RDS(ON), RDR
is the effective top driver resistance (approximately 2Ω at
VGS = VMILLER). VTH(IL) is the data sheet speciï¬ed typical
gate threshold voltage speciï¬ed in the power MOSFET data
sheet at the speciï¬ed drain current. CMILLER is the calculated
capacitance using the gate charge curve from the MOSFET
data sheet and the technique described above.
Both MOSFETs have I2R losses while the bottom N-channel
equation includes an additional term for transition losses.
Both top and bottom MOSFET I2R losses are greatest at
lowest VIN, and the top MOSFET I2R losses also peak
during an overcurrent condition when it is on close to
100% of the period. For most LTC3813 applications,
the transition loss and I2R loss terms in the bottom
MOSFET are comparable, so best efï¬ciency is obtained
by choosing a MOSFET that optimizes both RDS(ON) and
CMILLER. Since there is no transition loss term in the syn-
chronous MOSFET, however, optimal efï¬ciency is obtained
by minimizing RDS(ON)âby using larger MOSFETs or
paralleling multiple MOSFETs.
Multiple MOSFETs can be used in parallel to lower RDS(ON)
and meet the current and thermal requirements if desired.
The LTC3813 contains large low impedance drivers capable
of driving large gate capacitances without signiï¬cantly
slowing transition times. In fact, when driving MOSFETs
with very low gate charge, it is sometimes helpful to
slow down the drivers by adding small gate resistors
(10Ω or less) to reduce noise and EMI caused by the
fast transitions.
3813fb
15
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