LT1766_1 datasheet(16/30 Pages) LINER

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LT1766/LT1766-5

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INPUT CAPACITOR

Step-down regulators draw current from the input supply in

pulses. The rise and fall times of these pulses are very fast.

The input capacitor is required to reduce the voltage ripple

this causes at the input of LT1766 and force the switching

current into a tight local loop, thereby minimizing EMI.

The RMS ripple current can be calculated from:

RIPPLE RMS

OUT

() =

()

–/

Ceramic capacitors are ideal for input bypassing. At 200kHz

switching frequency, the energy storage requirement of the

input capacitor suggests that values in the range of 2.2μF

to 20μF are suitable for most applications. If operation is

required close to the minimum input required by the output

of the LT1766, a larger value may be required. This is to

prevent excessive ripple causing dips below the minimum

operating voltage resulting in erratic operation.

Depending on how the LT1766 circuit is powered up you

may need to check for input voltage transients.

The input voltage transients may be caused by input volt-

age steps or by connecting the LT1766 converter to an

already powered up source such as a wall adapter. The

sudden application of input voltage will cause a large surge

of current in the input leads that will store energy in the

parasitic inductance of the leads. This energy will cause the

input voltage to swing above the DC level of input power

source and it may exceed the maximum voltage rating of

input capacitor and LT1766.

The easiest way to suppress input voltage transients is

to add a small aluminum electrolytic capacitor in parallel

with the low ESR input capacitor. The selected capacitor

needs to have the right amount of ESR in order to criti-

cally dampen the resonant circuit formed by the input lead

inductance and the input capacitor. The typical values of

ESR will fall in the range of 0.5Ω to 2Ω and capacitance

will fall in the range of 5μF to 50μF.

If tantalum capacitors are used, values in the 22μF to 470μF

range are generally needed to minimize ESR and meet

ripple current and surge ratings. Care should be taken to

ensure the ripple and surge ratings are not exceeded. The

AVX TPS and Kemet T495 series are surge rated. AVX

recommends derating capacitor operating voltage by 2:1

for high surge applications.

CATCH DIODE

Highest efﬁciency operation requires the use of a Schottky

type diode. DC switching losses are minimized due to its

low forward voltage drop, and AC behavior is benign due

to its lack of a signiﬁcant reverse-recovery time. Schottky

diodes are generally available with reverse-voltage ratings

of up to 60V and even 100V, and are price competitive

with other types.

The use of so-called ultrafast recovery diodes is generally

not recommended. When operating in continuous mode,

the reverse-recovery time exhibited by ultrafast diodes will

result in a slingshot type effect. The power internal switch

will ramp up VIN current into the diode in an attempt to

get it to recover. Then, when the diode has ﬁnally turned

off, some tens of nanoseconds later, the VSW node volt-

age ramps up at an extremely high dV/dt, perhaps 5 to

even 10V/ns ! With real world lead inductances, the VSW

node can easily overshoot the VIN rail. This can result in

poor RFI behavior and if the overshoot is severe enough,

damage the IC itself.

The suggested catch diode (D1) is an International Rectiﬁer

10MQ060N Schottky. It is rated at 1.5A average forward

current and 60V reverse voltage. Typical forward voltage

is 0.63V at 1A. The diode conducts current only during

switch off time. Peak reverse voltage is equal to regulator

input voltage. Average forward current in normal operation

can be calculated from:

DAVG

OUT

()

–

()

This formula will not yield values higher than 1.5A with

maximum load current of 1.5A. The only reason to

consider a larger diode is the worst-case condition of a

high input voltage and shorted output. With a shorted

condition, diode current will increase to a typical value

of 2A, determined by peak switch current limit. This is

safe for short periods of time, but it would be prudent to

check with the diode manufacturer if continuous operation

under these conditions must be tolerated.

APPLICATIONS INFORMATION

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