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LT3668

3668fa

For more information www.linear.com/LT3668

APPLICATIONS INFORMATION

Input Voltage Range

The minimum input voltage is determined by either the

LT3668’s minimum operating voltage of 4.3V or by its

maximum duty cycle (as discussed in the previous sec-

tion). The minimum input voltage due to duty cycle is:

VIN1(MIN) =

VOUT1+ VD

1– fSW • tOFF(MIN)

– VD + VSW

where VIN(MIN) is the minimum input voltage, VOUT1 is the

output voltage, VD is the catch diode drop (~0.5V), VSW

is the internal switch drop (~0.5V at maximum load), fSW

is the switching frequency, and tOFF(MIN) is the minimum

switch off-time (~190ns). Note that a higher switching

frequency will increase the minimum input voltage. If a

lower dropout voltage is desired, a lower switching fre-

quency should be used.

The highest allowed VIN1 during normal operation

(VIN1(OP-MAX)) is limited by minimum duty cycle and is

given by:

VIN1(OP-MAX) =

VOUT1+ VD

fSW • tON(MIN)

– VD + VSW

where VOUT1 is the output voltage, VD is the catch diode

drop (~0.5V), VSW is the internal switch drop (~0.5V at

maximum load), fSW is the switching frequency, and

tON(MIN) is the minimum switch on-time (~150ns).

However,theLT3668willtolerateinputsuptotheabsolute

maximum ratings of the VIN1 and BOOST pins, regardless

of the chosen switching frequency. During such transients

where VIN1 is higher than VIN1(OP-MAX), the part will skip

pulses to maintain output regulation. The output voltage

ripple and inductor current ripple will be higher than in

normal operation. Input voltage transients of up to 60V are

also safely withstood, though the LT3668 stops switching

while VIN1 > VOVLO (overvoltage lockout, 42V typical), al-

lowing the output to fall out of regulation.

Duringstart-up,short-circuit,orotheroverloadconditions

the inductor peak current might reach and even exceed the

maximum current limit of the LT3668, especially in those

cases where the switch already operates at minimum on-

time. The catch diode current limit circuitry prevents the

switch from turning on again if the inductor valley current

is above 500mA nominal.

Inductor Selection and Maximum Output Current

For a given input and output voltage, the inductor value

and switching frequency will determine the ripple current,

which increases with higher VIN1 or VOUT1 and decreases

with higher inductance and higher switching frequency.

A good first choice for the inductor value is:

L = VOUT1+ VD

(

)• 2.4

fSW

where fSW is the switching frequency in MHz, VOUT1 is the

output voltage, VD is the catch diode drop (~0.5V) and L is

theinductorvalueinμH.Theinductor’sRMScurrentrating

must be greater than the maximum load current and its

saturation current should be about 30% higher. For robust

operation in fault conditions (start-up or short-circuit) and

high input voltage (>30V), the saturation current should

be above 900mA. To keep the efficiency high, the series

resistance (DCR) should be less than 0.3Ω, and the core

material should be intended for high frequency applica-

tions. Table 2 lists several vendors.

Table 2. Inductor Vendors

VENDOR

URL

Coilcraft

www.coilcraft.com

Sumida

www.sumida.com

Toko

www.tokoam.com

Würth Elektronik

www.we-online.com

Coiltronics

www.cooperet.com

Murata

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