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LT3695

3695f

APPLICATIONS INFORMATION

A good choice of switching frequency should allow an

adequate input voltage range (see Input Voltage Range sec-

tion) and keep the inductor and capacitor values small.

Input Voltage Range

The minimum input voltage is determined by either the

LT3695’s minimum operating voltage of ~3.6V (VBD > 3V)

or by its maximum duty cycle (see equation in Operating

Frequency Trade-Offs section). The minimum input voltage

due to duty cycle is:

IN MIN

OUT

SW OFF MIN

DSW

()

−

−+

where VIN(MIN) is the minimum input voltage, and tOFF(MIN)

is the minimum switch off time. Note that a higher switch-

ing frequency will increase the minimum input voltage.

If a lower dropout voltage is desired, a lower switching

frequency should be used.

The maximum input voltage for LT3695 applications

depends on switching frequency, the Absolute Maximum

Ratings of the VIN and BOOST pins and the operating mode.

The LT3695 can operate from continuous input voltages

up to 36V. Input voltage transients of up to 60V are also

safely withstood. However, note that while VIN > VOVLO

(overvoltage lockout, 38V typical), the LT3695 will stop

switching, allowing the output to fall out of regulation.

For a given application where the switching frequency

and the output voltage are already ﬁxed, the maximum

input voltage that guarantees optimum output voltage

ripple for that application can be found by applying the

following expression:

IN MAX

OUT

SW ON MIN

DSW

()

−+

where VIN(MAX) is the maximum operating input voltage,

VOUT is the output voltage, VD is the catch diode drop

(~0.5V), VSWistheinternalswitchdrop(~0.5Vatmaxload),

fSW is the switching frequency (set by RT) and tON(MIN) is

the minimum switch on time (~150ns). Note that a higher

switching frequency will reduce the maximum operating

input voltage. Conversely, a lower switching frequency

will be necessary to achieve optimum operation at high

input voltages.

Special attention must be paid when the output is in start-

up, short-circuit or other overload conditions. During these

events, the inductor peak current might easily reach and

even exceed the maximum current limit of the LT3695,

especially in those cases where the switch already operates

at minimum on-time. The circuitry monitoring the current

through the catch diode via the DA pin prevents the switch

from turning on again if the inductor valley current is above

1.6A nominal. In these cases, the inductor peak current is

therefore the maximum current limit of the LT3695 plus

the additional current overshoot during the turn off delay

due to minimum on time:

LPEAK

IN MAX

OUT OL

ON MIN

()

(

)

()

•

−

where IL(PEAK) is the peak inductor current, VIN(MAX) is

the maximum expected input voltage, L is the inductor

value, tON(MIN) is the minimum on time and VOUT(OL) is the

output voltage under the overload condition. The part is

robust enough to survive prolonged operation under these

conditions as long as the peak inductor current does not

exceed 3.5A. Inductor current saturation and excessive

junction temperature may further limit performance.

Input voltage transients of up to VOVLO are acceptable

regardless of the switching frequency. In this case, the

LT3695 may enter pulse-skipping operation where some

switching pulses are skipped to maintain output regulation.

In this mode the output voltage ripple and inductor current

ripple will be higher than in normal operation.

Input voltage transients above VOVLO and up to 60V can

be tolerated. However, since the part will stop switching

during these transients, the output will fall out of regulation

and the output capacitor may eventually be completely

discharged. This case must be treated then as a start-up

condition as soon as VIN returns to values below VOVLO

and the part starts switching again.

LT3695 Datasheet(PDF) 13 Page - Linear Technology

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