LTC3834 Datasheet, PDF (14/28 Pages) Linear Technology – 30μA IQ Synchronous Step-Down Controller

LTC3834
APPLICATIONS INFORMATION
exceeded. This results in an abrupt increase in inductor
ripple current and consequent output voltage ripple. Do
not allow the core to saturate!
Power MOSFET and Schottky Diode (Optional)
Selection
Two external power MOSFETs must be selected for the
LTC3834: one N-channel MOSFET for the top (main)
switch, and one N-channel MOSFET for the bottom (syn-
chronous) switch.
The peak-to-peak drive levels are set by the INTV_CC voltage.
This voltage is typically 5V during start-up (see EXTV_CC Pin
Connection). Consequently, logic-level threshold MOSFETs
must be used in most applications. The only exception is
if low input voltage is expected (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
14
The MOSFET power dissipations at maximum output
current are given by:
( ) ( ) P_MAIN
=
V_OUT
V_IN
I_MAX
2
1+ δΔT
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+ δΔT
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 + δΔT) 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 D1 shown in Figure 6 conducts
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.
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