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LTC1624

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.

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.

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:

Main Switch Duty Cycle =

OUT

The MOSFET power dissipation and maximum switch

current at maximum output current are given by:

k V

kHz

where

MAIN

SW MAX

OUT

IN MIN

OUT

DS ON

IN MIN

OUT

SW MAX

RSS

SW MAX

OUT MAX

OUT

IN MIN





















()















 ()(

)











()

( )

()

185

200





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

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

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

µH 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:

L1 PEAK

L2 PEAK

()

( ) ()

()

( )

()

























OUT MAX

OUT

IN MIN

OUT MAX

IN MIN

∆

The ripple current in the inductor (

∆IL) is typically 20% to

30% of the peak current occuring at VIN(MIN) and IOUT(MAX),

and

∆IL1 = ∆IL2. Maximum ∆IL occurs at maximum VIN.

∆I

kHz L V

OUT

P-P

()=

() +

()

()( ) ++

()

200

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

µH

inductor with two windings. With the windings in parallel

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