LTC3811_15 Datasheet, PDF (29/48 Pages) Linear Technology – High Speed Dual, Multiphase Step-Down DC/DC Controller

LTC3811
APPLICATIONS INFORMATION
+
C_IN
V_IN
TG1
LTC3811
EXTV_CC
SW1
BG1
PGND
V_IN
+
BAT85
0.22μF
BAT85
VN2222LL
BAT85
R_SENSE
L1
V_OUT
+
D1
C_OUT
3811 F16
Figure 16. Capacitive Charge Pump for EXTV_CC
The peak-to-peak drive levels are set by the INTV_CC
voltage. This voltage is typically 6V during start-up
(see EXTV_CC Pin Connection). Consequently, logic-level
threshold MOSFETs should 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
=
V_IN
– V_OUT
V_IN
The MOSFET power dissipations at maximum output
current are given by:
( ) ( ) P_MAIN =
V_OUT
V_IN
I_MAX
2 _RDS(ON)
1+ δ
+
( ) ( )( ) ^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 top gate 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.
The optional Schottky diode, D1, shown in Figure 16 con-
ducts 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 1% 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.
3811f
29