LTC3875_15 Datasheet, PDF (23/44 Pages) Linear Technology – Dual, 2-Phase, Synchronous Controller with Low Value DCR Sensing and Temperature Compensation

LTC3875
Applications Information
Main Switch Duty Cycle = ^VOUT
V_IN
Synchronous
Switch
Duty
Cycle
=


V_IN
– V_OUT
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 R_DS(ON) +
(
V_IN
)²


I_MAX
2


(R_DR
)(C_MILLER
)
•



V_INTVCC
1
–
V_MILLER
+
1
V_MILLER



•
f
( ) ( ) P_SYNC
=
V_IN
– V_OUT
V_IN
I_MAX
2
1+ δ
R_DS(ON)
where δ is the temperature dependency of R_DS(ON), R_DR
is the effective top driver resistance (approximately 2Ω at
V_GS = V_MILLER), V_IN is the drain potential and the change
in drain potential in the particular application. V_MILLER
is the data sheet specified typical gate threshold voltage
specified in the power MOSFET data sheet at the speci-
fied drain current. C_MILLER 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-channel equation includes
an additional term for transition losses, which peak at
the highest input voltage. For V_IN < 20V, the high cur-
rent efficiency 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.
The term (1 + δ ) is generally given for a MOSFET in the
form of a normalized R_DS(ON) vs temperature curve, but
δ = 0.005/°C can be used as an approximation for low
voltage MOSFETs.
An optional Schottky diode across the synchronous
MOSFET conducts during the dead time between the
conduction of the two large power MOSFETs. This pre-
vents the body diode of the bottom MOSFET from turning
on, storing charge during the dead time and requiring a
reverse-recovery period which could cost as much as sev-
eral percent in efficiency. A 2A to 8A Schottky is generally
a good compromise for both regions of operation due to
the relatively small average current. Larger diodes result
in additional transition loss due to their larger junction
capacitance.
Soft-Start and Tracking
The LTC3875 has the ability to either soft-start by itself
with a capacitor or track the output of another channel or
external supply. When one particular channel is configured
to soft-start by itself, a capacitor should be connected to
its TK/SS pin. This channel is in the shutdown state if its
RUN pin voltage is below 1.14V. Its TK/SS pin is actively
pulled to ground in this shutdown state.
Once the RUN pin voltage is above 1.22V, the channel pow-
ers up. A soft-start current of 1.25µA then starts to charge
its soft-start capacitor. Note that soft-start or tracking is
achieved not by limiting the maximum output current of
the controller but by controlling the output ramp voltage
according to the ramp rate on the TK/SS pin. Current
fold-back is disabled during this phase to ensure smooth
soft-start or tracking. The soft-start or tracking range is
defined to be the voltage range from 0V to 0.6V on the
TK/SS pin. The total soft-start time can be calculated as:
t_SOFTSTART
=
0.6 •
C_SS
1.25µA
Regardless of the mode selected by the MODE/PLLIN pin,
the regulator will always start in pulse-skipping mode
up to TK/SS = 0.5V. Between TK/SS = 0.5V and 0.56V, it
will operate in forced continuous mode and revert to the
selected mode once TK/SS > 0.56V. The output ripple
is minimized during the 60mV forced continuous mode
window ensuring a clean PGOOD signal.
When the channel is configured to track another supply,
the feedback voltage of the other supply is duplicated by
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