LTC3859AL_15 Datasheet, PDF (23/44 Pages) Linear Technology – Triple Output, Buck/Buck/Boost Synchronous Controller with 28A Burst Mode IQ

LTC3859AL
Applications Information
Power MOSFET and Schottky Diode
(Optional) Selection
Two external power MOSFETs must be selected for each
controller in the LTC3859AL: one N-channel MOSFET for
the top switch (main switch for the buck, synchronous
for the boost), and one N-channel MOSFET for the bot-
tom switch (main switch for the boost, synchronous for
the buck).
The peak-to-peak drive levels are set by the INTV_CC voltage.
This voltage is typically 5.4V during start-up (see EXTV_CC
Pin Connection). Consequently, logic-level threshold
MOSFETs must be used in most applications. Pay close
attention to the BV_DSS specification for the MOSFETs as
well; many 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:
Buck Main Switch Duty Cycle = ^VOUT
V_IN
Buck Sync Switch Duty Cycle = ^{VIN − VOUT}
V_IN
Boost Main Switch Duty Cycle = ^{VOUT − VIN}
V_OUT
Boost Sync Switch Duty Cycle = ^VIN
V_OUT
The MOSFET power dissipations at maximum output
current are given by:
( ) ( ) ^PMAIN _BUCK
=
V_OUT
V_IN
I_OUT(MAX) ²
1+ δ R_DS(ON) +
(V_IN
)²
I_OUT(2MAX
)

(R_DR

)(C_MILLER
)
•



V_INTVCC
1
−
V_THMIN
+
1
V_THMIN

^(f)

( ) ( ) ^PSYNC _BUCK
=
V_IN
− V_OUT
V_IN
I_{OUT(MAX )}
2
1+ δ R_DS(ON)
( ) _{( ) PMAIN_BOOST =}
V_OUT − V_IN
V_IN2
V_OUT
2
I_OUT(MAX) •
(1+
δ )R_DS(ON)
+


V_OUT2
V_IN




I_{OUT(MAX )}
2


•
( )( ) R_DR
C_MILLER
•



V_INTVCC
1
−
V_THMIN
+
1
V_THMIN
_(f)

( ) ^{( ) P}_{SYNC _BOOST}
=
V_IN
V_OUT
I_{OUT(MAX )}
2
1+ δ
R_DS(ON)
where z is the temperature dependency of R_DS(ON) and
RDR (approximately 2Ω) is the effective driver resistance
at the MOSFET’s Miller threshold voltage. V_THMIN is the
typical MOSFET minimum threshold voltage.
Both MOSFETs have I²R losses while the main N-channel
equations for the buck and boost controllers include an
additional term for transition losses, which are highest at
high input voltages for the bucks and low input voltages for
the boost. For V_IN < 20V (high V_IN for the boost) the high
current efficiency generally improves with larger MOSFETs,
while for V_IN > 20V (low V_IN for the boost) 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
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3859alf
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