LTC3860_15 Datasheet, PDF (25/36 Pages) Linear Technology – Dual, Multiphase Step-Down Voltage Mode DC/DC Controller with Current Sharing

LTC3860
APPLICATIONS INFORMATION
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 flat 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 flat) is specified for a given
V_DS drain voltage, but can be adjusted for different V_DS
voltages by multiplying by the ratio of the application
V_DS to the curve specified V_DS values. A way to estimate
the C_MILLER term is to take the change in gate charge
from points a and b on a manufacturers data sheet and
divide by the stated V_DS voltage specified. C_MILLER is
the most important selection criteria for determining
the transition loss term in the top MOSFET but is not
directly specified on MOSFET data sheets. C_RSS and
C_OS are specified sometimes but definitions 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
V_IN
Synchronous Switch Duty Cycle = ^{VIN – VOUT}
V_IN
The power dissipation for the main and synchronous
MOSFETs at maximum output current are given by:
( ) P_MAIN
=
V_OUT
V_IN
I_MAX
2 _{(1+ δ)RDS(ON) +}
V_I2N
I_MAX
2
(R_DR
)(C_MILLER
)
•
⎡
⎢
⎣⎢
V_CC
1
– V_TH(IL)
+
1
V_TH(IL)
⎤
⎥(f)
⎦⎥
P_SYNC
=
V_IN
− V_OUT
V_IN
(I_MAX )²(1+
δ)R_DS(0N)
where δ is the temperature dependency of R_DS(ON), R_DR
is the effective top driver resistance, V_IN is the drain po-
tential and the change in drain potential in the particular
application. V_TH(IL) is the data sheet specified typical gate
threshold voltage specified in the power MOSFET data sheet
at the specified drain current. C_MILLER is the calculated
capacitance using the gate charge curve from the MOSFET
data sheet and the technique previously described.
The term (1 + δ) is generally given for a MOSFET in the
form of a normalized R_DS(ON) vs temperature curve. Typical
values for δ range from 0.005/°C to 0.01/°C depending
on the particular MOSFET used.
Multiple MOSFETs can be used in parallel to lower R_DS(ON)
and meet the current and thermal requirements if desired.
Suitable drivers such as the LTC4449 are capable of driv-
ing large gate capacitances without significantly 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 (5Ω or less) to
reduce noise and EMI caused by the fast transitions
MOSFET Driver Selection
Gate driver ICs, DRMOSs and power blocks with an interface
compatible with the LTC3860’s three-state PWM outputs
or the LTC3860’s PWM/PWMEN outputs can be used.
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