LTC3851-1_15 Datasheet, PDF (14/28 Pages) Linear Technology – Synchronous Step-Down Switching Regulator Controller

LTC3851-1
APPLICATIONS INFORMATION
The peak-to-peak drive levels are set by the INTV_CC voltage.
This voltage is typically 5V during start-up. Consequently,
logic-level threshold MOSFETs must be used in most ap-
plications. The only exception is if low input voltage is ex-
pected (V_IN < 5V); then, sub-logic level threshold MOSFETs
(V_GS(TH) < 3V) should be used. Pay close attention to the
BV_DSS specification for the MOSFETs as well; most of the
logic-level MOSFETs are limited to 30V or less.
Selection criteria for the power MOSFETs include the on-
resistance, R_DS(ON), Miller capacitance, C_MILLER, input
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 = ^VOUT
V_IN
Synchronous Switch Duty Cycle = ^{VIN – VOUT}
V_IN
The MOSFET power dissipations 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_TH(MIN)
+
1
V_TH(MIN)
⎤
⎥(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) 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.
14
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
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.
The optional Schottky diode conducts during the dead time
between the conduction of the two power MOSFETs. This
prevents the body diode of the bottom MOSFET from turn-
ing 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 size 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 LTC3851-1 has the ability to either soft-start by itself
with a capacitor or track the output of another channel
or external supply. When the LTC3851-1 is configured
to soft-start by itself, a capacitor should be connected to
the TK/SS pin. The LTC3851-1 is in the shutdown state if
the RUN pin voltage is below 1.25V. TK/SS pin is actively
pulled to ground in this shutdown state.
Once the RUN pin voltage is above 1.25V, the LTC3851-1
powers up. A soft-start current of 1μA 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
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