ltc1624 Linear Technology Corporation, ltc1624 Datasheet - Page 16

• Manufacturer Part Number: ltc1624
• Description: High Efficiency So-8 N-channel Switching Regulator Controller
• Manufacturer: Linear Technology Corporation

APPLICATIONS
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.
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 INTV
approximately 5V for V
can be used. At V
is equal to V
be used.
Selection criteria for the power MOSFET include the “ON”
resistance R
input voltage and maximum output current. When the
LTC1624 is operating in continuous mode the duty cycle
for the MOSFET is given by:
The MOSFET power dissipation and maximum switch
current at maximum output current are given by:
LTC1624
constant inversely related to the gate drive current. The
peak switch current is I
MOSFETs have I
includes an additional term for transition losses that are
16
is the temperature dependency of R
P
Main Switch Duty Cycle =
where I
MAIN
k V
I
SW MAX
IN MIN
IN
SW MAX
DS(ON)
– 0.6V and a sublogic level MOSFET should
IN
2
V
2
, reverse transfer capacitance C
U
voltages below 5V the INTV
OUT
R losses plus the P
V
CC
IN MIN
IN
I
voltage. This voltage is equal to
SW(MAX)
OUT MAX
1 85
INFORMATION
> 5.6V and a logic level MOSFET
V
U
OUT
I
SW MAX
V
V
OUT
+ I
V
IN
D
V
V
W
OUT
V
OUT
L
IN MIN
V
V
.
OUT
D
DS(ON)
C
RSS
V
V
MAIN
1
D
D
V
D
and k is a
U
CC
1
200
R
equation
DS ON
voltage
kHz
RSS
,
highest at high total input plus output voltages. For
(V
improves with larger MOSFETs, while for (V
20V the transition losses rapidly increase to the point that
the use of a higher R
provides higher efficiency. For additional information refer
to the Step-Down Converter: Power MOSFET Selection in
the Applications Information section.
SEPIC Converter: Inductor Selection
For most applications the equal inductor values will fall in
the range of 10 H to 100 H. 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 V
The ripple current in the inductor ( I
30% of the peak current occuring at V
and I
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
the same ripple current and inductive energy storage in the
inductors. For example a Coiltronix CTX10-4 is a 10 H
inductor with two windings. With the windings in parallel
IN
I
I
L1 PEAK
L2 PEAK
+ V
I
L
L1
IN
P-P
OUT
:
= I
) < 20V the high current efficiency generally
L2
I
I
OUT MAX
200
OUT MAX
. Maximum I
kHz L V
DS(ON)
V
IN
V
V
V
device with lower C
IN MIN
OUT
IN MIN
V
IN
V
OUT
L
IN MIN
occurs at maximum V
V
V
OUT
D
IN(MIN)
L
V
) is typically 20% to
D
2
I
V
L
D
1
and I
IN
I
2
L
RSS
+ V
OUT(MAX)
2
OUT
actual
IN
) >
.
,