LT1956IGN datasheet(13/28 Pages) LINER

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LT1956/LT1956-5

1956f

APPLICATIO S I FOR ATIO

where:

f = switching frequency

tON = switch minimum on time

VF = diode forward voltage

VIN = input voltage

I • R = inductor I • R voltage drop

If this condition is not observed, the current will not be

limited at IPK, but will cycle-by-cycle ratchet up to some

higher value. Using the nominal LT1956 clock frequency

of 500KHz, a VIN of 12V and a (VF + I • R) of say 0.7V, the

maximum tON to maintain control would be approximately

116ns, an unacceptably short time.

The solution to this dilemma is to slow down the oscillator

when the FB pin voltage is abnormally low thereby indicat-

ing some sort of short-circuit condition. Oscillator fre-

quency is unaffected until FB voltage drops to about 2/3 of

its normal value. Below this point the oscillator frequency

decreases roughly linearly down to a limit of about 100kHz.

This lower oscillator frequency during short-circuit condi-

tions can then maintain control with the effective mini-

mum on time. Even with frequency foldback, however, the

LT1956 will not survive a permanent output short at the

absolute maximum voltage rating of VIN = 60V; this is

defined solely by internal semiconductor junction break-

down effects.

For the maximum input voltage allowed during an output

short to ground, the previous equation defining minimum

on-time can be used. Assuming VF (D1 catch diode) =

0.63V at 1A (short-circuit current is folded back to typical

switch current limit • 0.5), I (inductor) • DCR = 1A • 0.128

= 0.128V (L = CDRH6D28-22), typical f = 100kHz (folded

back) and typical minimum on-time = 300ns, the maxi-

mum allowable input voltage during an output short to

ground is typically:

VIN = (0.63V + 0.128V)/(100kHz • 300ns)

VIN(MAX) = 25V

Increasing the DCR of the inductor will increase the maxi-

mum VIN allowed during an output short to ground but will

also drop overall efficiency during normal operation.

Every time the converter wakes up from shutdown or

undervoltage lockout to begin switching, the output

capacitor may potentially be starting from 0V. This re-

quires that the part obey the overall duty cycle demanded

by the loop, related to VIN and VOUT, as the output voltage

rises to its target value. It is recommended that for [VIN/

(VOUT + VF)] ratios > 4, a soft-start circuit should be used

to control the output capacitor charge rate during start-up

or during recovery from an output short circuit, thereby

adding additional control over peak inductor current. See

Buck Converter with Adjustable Soft-Start later in this

data sheet.

OUTPUT CAPACITOR

The LT1956 will operate with either ceramic or tantalum

output capacitors. The output capacitor is normally cho-

sen by its effective series resistance (ESR), because this

is what determines output ripple voltage. The ESR range

for typical LT1956 applications using a tantalum output

capacitor is 0.05

Ω to 0.2Ω. A typical output capacitor is an

AVX type TPS, 100

µF at 10V, with a guaranteed ESR less

than 0.1

Ω. This is a “D” size surface mount solid tantalum

capacitor. TPS capacitors are specially constructed and

tested for low ESR, so they give the lowest ESR for a given

volume. The value in microfarads is not particularly criti-

cal, and values from 22

µF to greater than 500µF work well,

but you cannot cheat mother nature on ESR. If you find a

tiny 22

µF solid tantalum capacitor, it will have high ESR,

and output ripple voltage will be terrible. Table 3 shows

some typical solid tantalum surface mount capacitors.

Table 3. Surface Mount Solid Tantalum Capacitor ESR

and Ripple Current

E CASE SIZE

ESR (MAX,

Ω)

RIPPLE CURRENT (A)

AVX TPS, Sprague 593D

0.1 to 0.3

0.7 to 1.1

D CASE SIZE

AVX TPS, Sprague 593D

0.1 to 0.3

0.7 to 1.1

C CASE SIZE

AVX TPS

0.2 (typ)

0.5 (typ)

Unlike the input capacitor, RMS ripple current in the

output capacitor is normally low enough that ripple cur-

rent rating is not an issue. The current waveform is

triangular with a typical value of 125mARMS. The formula

to calculate this is:

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