LTC3807_15 Datasheet, PDF (18/32 Pages) Linear Technology – Low IQ, Synchronous Step-Down Controller with 24V Output Voltage Capability

LTC3807
APPLICATIONS INFORMATION
Power MOSFET and Schottky Diode (Optional)
Selection
Two external power MOSFETs must be selected for the
LTC3807 controller: 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’
datasheet. 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_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.
A Schottky diode can be inserted in parallel with the bot-
tom MOSFET to conduct during the dead-time between
the conduction of the two power MOSFETs. This prevents
the body diode of the bottom MOSFET 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 operation due to
the relatively small average current. Larger diodes result
in additional transition losses due to their larger junction
capacitance.
C_IN and C_OUT Selection
The selection of C_IN is usually based off the worst-case RMS
input current. The highest (V_OUT)(I_OUT) product needs to
be used in the formula shown in Equation 1 to determine
the maximum RMS capacitor current requirement.
In continuous mode, the source current of the top MOSFET
is a square wave of duty cycle (V_OUT)/(V_IN). To prevent
large voltage transients, a low ESR capacitor sized for the
3807f
18
For more information www.linear.com/LTC3807