LT3470 datasheet(9/20 Pages) AD

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LT3470

Rev. E

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APPLICATIONS INFORMATION

Input Voltage Range

The minimum input voltage required to generate a par-

ticular output voltage in an LT3470 application is limited

by either its 4V undervoltage lockout or by its maximum

duty cycle. The duty cycle is the fraction of time that the

internal switch is on and is determined by the input and

output voltages:

DC =

VOUT + VD

VIN – VSW + VD

where VD is the forward voltage drop of the catch diode

(~0.6V) and VSW is the voltage drop of the internal switch

at maximum load (~0.4V). Given DCMAX = 0.90, this leads

to a minimum input voltage of:

VIN(MIN) =

VOUT + VD

DCMAX

⎛

⎝⎜

⎞

⎠⎟

+ VSW – VD

Thisanalysisassumestheparthasstartedupsuchthatthe

capacitor tied between the BOOST and SW pins is charged

to more than 2V. For proper start-up, the minimum input

voltage is limited by the boost circuit as detailed in the

section BOOST Pin Considerations.

The maximum input voltage is limited by the absolute

maximum VIN rating of 40V, provided an inductor of suf-

ficient value is used.

Inductor Selection

The switching action of the LT3470 during continuous

operation produces a square wave at the SW pin that

results in a triangle wave of current in the inductor. The

hysteretic mode control regulates the top and bottom

current limits (see Electrical Characteristics) such that

the average inductor current equals the load current. For

safe operation, it must be noted that the LT3470 cannot

turn the switch on for less than ~150ns. If the inductor is

small and the input voltage is high, the current through the

switch may exceed safe operating limit before the LT3470

is able to turn off. To prevent this from happening, the

following equation provides a minimum inductor value:

LMIN =

VIN(MAX) • tON-TIME(MIN)

IMAX

where VIN(MAX) is the maximum input voltage for the ap-

plication,tON-TIME(MIN)is~150nsandIMAXisthemaximum

allowable increase in switch current during a minimum

switch on-time (150mA). While this equation provides a

safe inductor value, the resulting application circuit may

switch at too high a frequency to yield good efficiency.

It is advised that switching frequency be below 1.2MHz

during normal operation:

f =

1– DC

(

) VD + VOUT

(

)

L • ∆IL

where f is the switching frequency, ∆IListheripplecurrent

in the inductor (~150mA), VD is the forward voltage drop

of the catch diode, and VOUT is the desired output voltage.

If the application circuit is intended to operate at high duty

cycles (VIN close to VOUT), it is important to look at the

calculated value of the switch off-time:

tOFF-TIME =

1– DC

The calculated tOFF-TIME should be more than LT3470’s

minimum tOFF-TIME (See Electrical Characteristics), so

the application circuit is capable of delivering full rated

output current. If the full output current of 200mA is not

required, the calculated tOFF-TIME can be made less than

minimum tOFF-TIME possibly allowing the use of a smaller

inductor. See Table 1 for an inductor value selection guide.

Table 1. Recommended Inductors for Loads up to 200mA

VOUT

VIN UP TO 16V

VIN UP TO 40V

2.5V

10µH

33µH

3.3V

10µH

33µH

15µH

33µH

12V

33µH

47µH

Chooseaninductorthatisintendedforpowerapplications.

Table 2 lists several manufacturers and inductor series.

For robust output short-circuit protection at high VIN (up

to 40V) use at least a 33µH inductor with a minimum

450mA saturation current. If short-circuit performance is

not required, inductors with ISAT of 300mA or more may

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