LTC3850-1_15 Datasheet, PDF (17/38 Pages) Linear Technology – Dual, 2-Phase Synchronous Step-Down Switching Controller

LTC3850/LTC3850-1
APPLICATIONS INFORMATION
voltage and maximum output current. Miller capacitance,
C_MILLER, can be approximated from the gate charge curve
usually provided on the MOSFET manufacturers’ data
sheet. C_MILLER is equal to the increase in gate charge
along the horizontal axis while the curve is approximately
flat divided by the specified change in V_DS. This result is
then multiplied by the ratio of the application applied V_DS
to the gate charge curve specified V_DS. When the IC is
operating in continuous mode the duty cycles for the top
and bottom MOSFETs are given by:
Main
Switch
Duty
Cycle
=
V_OUT
V_IN
Synchronous Switch Duty Cycle = ^{VIN – VOUT}
V_IN
The MOSFET power dissipations at maximum output cur-
rent 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_TH(MIN)
+
1
V_TH(MIN)



•
f_OSC
( ) ( ) P_SYNC
=
V_IN
– V_OUT
V_IN
I_MAX
2
1+ δ
R_DS(ON)
where d is the temperature dependency of R_DS(ON) and
R_DR (approximately 2Ω) is the effective driver resistance
at the MOSFET’s Miller threshold voltage. V_TH(MIN) is the
typical MOSFET minimum threshold voltage.
Both MOSFETs have I²R losses while the topside N-channel
equation includes an additional term for transition losses,
which are highest at high input voltages. For V_IN < 20V
the high current 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 + d) is generally given for a MOSFET in the
form of a normalized R_DS(ON) vs Temperature curve, but
d = 0.005/°C can be used as an approximation for low
voltage MOSFETs.
The optional Schottky diodes conduct during the dead
time between the conduction of the two power MOSFETs.
These prevent the body diodes of the bottom MOSFETs
from turning on, storing charge during the dead time and
requiring a reverse recovery period that could cost as much
as 3% in efficiency at high V_IN. A 1A to 3A Schottky is
generally a good compromise for both regions of opera-
tion due to the relatively small average current. Larger
diodes result in additional transition losses due to their
larger junction capacitance.
Soft-Start and Tracking
The LTC3850 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 1.2V. Its TK/SS pin is actively
pulled to ground in this shutdown state.
Once the RUN pin voltage is above 1.2V, the channel pow-
ers up. A soft-start current of 1.3µA 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
foldback 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 0.8V on the TK/
SS pin. The total soft-start time can be calculated as:
t_SOFTSTART
=
0.8
•
C_SS
1.3µA
Regardless of the mode selected by the MODE/PLLIN pin,
the regulator will always start in pulse-skipping mode up
to TK/SS = 0.64V. Between TK/SS = 0.64V and 0.74V, it
will operate in forced continuous mode and revert to the
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