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LTC3810-5 Datasheet, PDF (16/36 Pages) Linear Technology – 60V Current Mode Synchronous Switching Regulator Controller | |||
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LTC3810-5
APPLICATIONS INFORMATION
The most important parameter in high voltage applications
is breakdown voltage BVDSS. Both the top and bottom
MOSFETs will see full input voltage plus any additional
ringing on the switch node across its drain-to-source dur-
ing its off-time and must be chosen with the appropriate
breakdown speciï¬cation. The LTC3810-5 is designed to
be used with a 4.5V to 14V gate drive supply (DRVCC pin)
for driving logic-level MOSFETs (VGS(MIN) ⥠4.5V).
For maximum efï¬ciency, on-resistance RDS(ON) and input
capacitance should be minimized. Low RDS(ON) minimizes
conduction losses and low input capacitance minimizes
transition losses. MOSFET input capacitance is a combi-
nation of several components but can be taken from the
typical âgate chargeâ curve included on most data sheets
(Figure 6).
VIN
MILLER EFFECT
VGS
a
b
QIN
CMILLER = (QB â QA)/VDS
V
+
VGS
â
+
VDS
â
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Figure 6. Gate Charge Characteristic
The curve is generated by forcing a constant input cur-
rent into the gate of a common source, current source
loaded stage and then plotting the gate voltage versus
time. The initial slope is the effect of the gate-to-source
and the gate-to-drain capacitance. The ï¬at portion of the
curve is the result of the Miller multiplication effect of the
drain-to-gate capacitance as the drain drops the voltage
across the current source load. The upper sloping line is
due to the drain-to-gate accumulation capacitance and
the gate-to-source capacitance. The Miller charge (the
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
16
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
Synchronous
Switch
Duty
Cycle =
VIN
â VOUT
VIN
The power dissipation for the main and synchronous
MOSFETs at maximum output current are given by:
( ) PTOP
=
VOUT
VIN
IMAX
2 (T )RDS(ON) +
VIN2
IMAX
2
(RDR
)(CMILLER
)
â¢
VCC
â
1
VTH(IL)
+
1
VTH(IL)
(f)
PBOT
=
VIN
â VOUT
VIN
(IMAX
)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), VIN is the drain potential and the change
in drain potential in the particular application. 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 topside N-chan-
nel equation incudes an additional term for transition
losses, which peak at the highest input voltage. For high
input voltage low duty cycle applications that are typical
for the LTC3810-5, transition losses are the dominate
loss term and therefore using higher RDS(ON) device with
lower CMILLER usually provides the highest efï¬ciency. The
synchronous MOSFET losses are greatest at high input
voltage when the top switch duty factor is low or during
a short-circuit when the synchronous switch is on close
to 100% of the period. Since there is no transition loss
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