LTC3890-3 Datasheet, PDF (18/40 Pages) Linear Technology – 60V Low IQ, Dual, 2-Phase Synchronous Step-Down DC/DC Controller

LTC3890-3
Applications Information
Power MOSFET and Schottky Diode
(Optional) Selection
Two external power MOSFETs must be selected for each
controller in the LTC3890-3: one N-channel MOSFET for
the top (main) switch, and one N-channel MOSFET for the
bottom (synchronous) switch.
The peak-to-peak drive levels are set by the INTV_CC voltage.
This voltage is typically 5.1V 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.
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_M2AX


(^R_DR
)
(^C_MILLER
)
•



V_INTVCC
1
–
V_THMIN
+
1
V_THMIN

(

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_THMIN 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+ 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.
18
For more information www.linear.com/3890-3
38903f