LTC1624
U
W U U
APPLICATIONS INFORMATION
identical voltages are applied to L1 and L2 throughout the
switching cycle. By making L1 = L2 and wound on the
same core the input ripple is reduced along with cost and
size. All SEPIC applications information that follows
assumes L1 = L2 = L.
highest at high total input plus output voltages. For
(VIN + VOUT) < 20V the high current efficiency generally
improves with larger MOSFETs, while for (VIN + VOUT) >
20V the transition losses rapidly increase to the point that
the use of a higher RDS(ON) device with lower CRSS actual
provideshigherefficiency.Foradditionalinformationrefer
to the Step-Down Converter: Power MOSFET Selection in
the Applications Information section.
SEPIC Converter: Power MOSFET Selection
One external N-channel power MOSFET must be selected
for use with the LTC1624 for the switch. As in boost
applications the source of the power MOSFET is grounded
along with the SW pin. The peak-to-peak gate drive levels
are set by the INTVCC voltage. This voltage is equal to
approximately 5V for VIN > 5.6V and a logic level MOSFET
can be used. At VIN voltages below 5V the INTVCC voltage
is equal to VIN – 0.6V and a sublogic level MOSFET should
be used.
SEPIC Converter: Inductor Selection
For most applications the equal inductor values will fall in
the range of 10µH to 100µH. Higher values reduce the
input ripple voltage and reduce core loss. Lower inductor
values are chosen to reduce physical size and improve
transient response.
Like the boost converter the input current of the SEPIC
converter is calculated at full load current. Peak inductor
current can be significantly higher than output current,
especially with smaller inductors and lighter loads. The
following formula assumes continuous mode operation
and calculates maximum peak inductor current at mini-
mum VIN:
Selection criteria for the power MOSFET include the “ON”
resistance RDS(ON), reverse transfer capacitance CRSS
,
input voltage and maximum output current. When the
LTC1624 is operating in continuous mode the duty cycle
for the MOSFET is given by:
V
+ V
D
OUT
Main Switch Duty Cycle =
V + V
+ V
D
IN
OUT
∆I
2
V
L1
OUT
I
=I
+
The MOSFET power dissipation and maximum switch
current at maximum output current are given by:
L1 PEAK
OUT MAX
(
)
(
)
V
IN MIN
(
IN MIN
)
V
+ V
D
P
=
∆I
(
)
MAIN
L2
I
=I
+
L2 PEAK
OUT MAX
(
)
(
)
2
2
V
IN MIN
V
+ V
D
OUT
I
1+ δ R
+
(
)
SW MAX
DS ON
(
)
(
)
V
+V
+ V
IN MIN
OUT D
(
)
The ripple current in the inductor (∆IL) is typically 20% to
1.85
30%ofthepeakcurrentoccuringatVIN(MIN) andIOUT(MAX)
,
k V
+V
I
C
( )(
RSS
200kHz
)
IN MIN
OUT
SW MAX
(
)
(
)
and ∆IL1 = ∆IL2. Maximum ∆IL occurs at maximum VIN.
V
V
+ V
(
)(
)
IN OUT
D
∆I
=
L P-P
(
)
V
+ V
D
OUT
200kHz L V + V
+ V
D
where I
=I
+1
IN
OUT
SW MAX
OUT MAX
(
)
(
)
V
IN MIN
(
)
By making L1 = L2 and wound on the same core the value
of inductance in all the above equations are replaced by
2L due to their mutual inductance. Doing this maintains
thesameripplecurrentandinductiveenergystorageinthe
inductors. For example a Coiltronix CTX10-4 is a 10µH
inductor with two windings. With the windings in parallel
δ is the temperature dependency of RDS(ON) and k is a
constant inversely related to the gate drive current. The
peak switch current is ISW(MAX) + ∆IL.
MOSFETs have I2R losses plus the PMAIN equation
includes an additional term for transition losses that are
16
Linear [ Linear ]
