LTC3865/LTC3865-1
18
3865fb
APPLICATIONS INFORMATION
Lower ripple current reduces core losses in the inductor,
ESR losses in the output capacitors, and output voltage
ripple. Thus, highest effi ciency operation is obtained at
low frequency with a small ripple current. Achieving this,
however, requires a large inductor.
A reasonable starting point is to choose a ripple current
that is about 40% of I
OUT(MAX)
. Note that the largest ripple
current occurs at the highest input voltage. To guarantee
that ripple current does not exceed a specifi ed maximum,
the inductor should be chosen according to:
L
VV
fI
V
V
IN OUT
OSC RIPPLE
OUT
IN
≥
–
•
•
Inductor Core Selection
Once the inductance value is determined, the type of in-
ductor must be selected. Core loss is independent of core
size for a fi xed inductor value, but it is very dependent
on inductance selected. As inductance increases, core
losses go down. Unfortunately, increased inductance
requires more turns of wire and therefore copper losses
will increase.
Ferrite designs have very low core loss and are preferred
at high switching frequencies, so design goals can con-
centrate on copper loss and preventing saturation. Ferrite
core material saturates “hard,” which means that induc-
tance collapses abruptly when the peak design current is
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 each
controller in the LTC3865/LTC3865-1: 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 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
specifi cation 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
fl at divided by the specifi ed change in V
DS
. This result is
then multiplied by the ratio of the application applied V
DS
to the gate charge curve specifi ed 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
V
V
Synchronous Switc
OUT
IN
=
hh Duty Cycle
VV
V
IN OUT
IN
=
–
The MOSFET power dissipations at maximum output
current are given by:
P
V
V
IR
V
I
MAIN
OUT
IN
MAX DS ON
IN
MAX
=
()
+
()
+
()
2
2
1 δ
()
22
11
⎛
⎝
⎜
⎞
⎠
⎟
()( )
+
RC
VV V
DR MILLER
INTVCC TH MIN
•
–
() TTH MIN
OSC
SYNC
IN OUT
IN
MAX
f
P
VV
V
I
()
•
–
⎡
⎣
⎢
⎢
⎤
⎦
⎥
⎥
=
())
+
()
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
TH(MIN)
is the
typical MOSFET minimum threshold voltage.
Both MOSFETs have I
2
R losses while the topside N-channel
equation includes an additional term for transition losses,
