LTC3882_15 Datasheet, PDF (42/104 Pages) Linear Technology – Dual Output PolyPhase Step-Down DC/DC Voltage Mode Controller with Digital Power System Management

LTC3882
APPLICATIONS INFORMATION
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_IN
2 ^IMAX
2
R_DR
C_MILLER •
( ) ^


V_GG
–
1
V_TH(IL)
+
1
V_TH(IL)



f_PWM
( ) P_SYNC
=
V_IN
– V_OUT
V_IN
I_MAX
2 _{(1+ δ)RDS(ON)}
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_GG is the applied gate voltage, V_TH(IL) is
the typical gate threshold voltage specified in the power
MOSFET data sheet at the specified drain current, and
C_MILLER is the capacitance calculated using the technique
previously described.
The term (1 + δ) is generally given for a MOSFET in the
form of a normalized R_DS(ON) versus temperature curve.
Typical values for δ range from 0.005/°C to 0.01/°C de-
pending on the particular MOSFET used.
Both MOSFETs have I²R losses while the topside N-channel
losses also include transition losses, which are highest
at high input voltages. For V_IN < 20V the high current ef-
ficiency generally improves with larger MOSFETs, while
for V_IN > 20V the transition losses rapidly increase to
the point that the use of a higher R_DS(ON) device with
lower C_MILLER actually provides higher efficiency. 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.
Multiple MOSFETs can be used in parallel to lower R_DS(ON)
and meet the current and thermal requirements if desired.
If using discrete drivers and MOSFETs, check the stress
on the MOSFETs by independently measuring the drain-
to-source voltages directly across the device terminals.
Beware of inductive ringing that could exceed the maximum
voltage rating of the MOSFET. If this ringing cannot be
avoided and exceeds the maximum rating of the device,
choose a higher voltage rated MOSFET.
MOSFET Driver Selection
Gate driver ICs, DrMOS devices and power blocks with an
interface compatible with the LTC3882 3.3V PWM control
output(s) can be used. An external resistor divider may be
needed to set three-state control voltage outputs to mid-
rail while in the high impedance state, depending on the
driver selected. In some cases an external pull-up resistor
may be necessary to connect an open-drain enable control
to a particular FET driver. These external driver/power
circuits do not typically present a heavy capacitive load to
the LTC3882 PWM outputs. Suitable drivers such as the
LTC4449 are capable of driving large gate capacitances
at high transition rates. 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 fast transitions.
Using PWM Protocols
For successful utilization of the driver selected, the appro-
priate LT3882 PWM control protocol must be programmed.
The LTC3882 supports a wide range of PWM control pro-
tocols. See bits[2:1] of the MFR_PWM_MODE_LTC3882
PMBus command.
When the LTC3882 first initializes, a basic 1-wire, 3-state
control is selected. This places the PWM pin in high imped-
ance and loads EN with a light pull-down test current. In
this protocol (bits[2:1] = 0x0), EN functions as an input
to select an additional sub-protocol feature. If EN is left
floating or tied to ground, DCM operation is allowed. If EN
is wired to V_DD33 (a 3.3k pull-up is recommended), then
all DCM operation is disabled, even during soft-start and
regardless of the state of MFR_PWM_MODE_LTC3882 bit 0.
The first of these sub-protocols is for drivers controlled
by a single 3-state input that have sufficiently short delay
to the diode emulation state (both top and bottom power
MOSFETs disabled in a fraction of a PWM cycle), such as
the LTC4449. The second sub-protocol handles all other
3.3V compatible drivers with a single 3-state control input.
3882f
42
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