LTC3861-1 Datasheet, PDF (29/36 Pages) Linear Technology – Dual, Multiphase Step-Down Voltage Mode DC/DC Controller with Accurate Current Sharing

LTC3861-1
Applications Information
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 se-
lection 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_IN2
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 driving large gate capacitances without sig-
nificantly 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 LTC3861-1’s three-state
PWM outputs or the LTC3861-1’s PWM/PWMEN outputs
can be used.
38611f
29