LT1952 datasheet(19/24 Pages) LINER

19 / 24 page

LT1952

1952f

APPLICATIO S I FOR ATIO

0.6 = k • 0.522(SS_MAXDC(DC)/SD_VSEC) –

(tDELAY • fOSC)

For SD_VSEC = 1.32V, fOSC = 200kHz and RDELAY = 40k

This gives k = 1 and tDELAY = 40ns.

Re-arranging the above equation to solve for SS_MAXDC

= VSS(REG)

= [0.6 + (tDELAY • fOSC)(SD_VSEC)]/(k • 0.522)

= [0.6 + (40ns • 200kHz)(1.32V)]/(1 • 0.522)

= (0.608)(1.32)/0.522 = 1.537V

Step 3: Calculate t(VSS(REG)) – t(VSS(ACTIVE))

Recall the time for SS_MAXDC to charge to a given voltage

VSS is given by,

t = RCHARGE • CSS • (–1) • ln(1 – VSS/SS_MAXDC(DC))

(Figure 11 gives the model for SS_MAXDC charging)

For RT = 35.7k, RB = 100k, RCHARGE = 26.3k

For CSS = 0.1µF, this gives t(VSS(ACTIVE))

= t(VSS(0.8V)) = 2.63e4 • 1e–7 • (–1) • ln(1 – 0.8/1.84)

= 2.63e–3 • (–1) • ln(0.565) = 1.5e–3 s

t(VSS(REG)) = t(VSS(1.537V)) = 26.3k • 0.1µF • –1 •

ln(1 – 1.66/1.84) = 2.63e–3 • (–1) • ln(0.146)

= 5e–3 s

The rise time for the converter output

= t(VSS(REG)) – t(VSS(ACTIVE)) = (5 – 1.5)e–3 s

= 3.5e–3 s

Example (3) Time For Maximum Duty Cycle Clamp to

Reach Within X% of Target Value

A maximum duty cycle clamp of 72% was calculated

previously in the section ‘Programming Maximum Duty

Cycle Clamp’. The programmed value used for

SS_MAXDC(DC) was 1.84V.

The time for SS_MAXDC to charge from its minimum

value VSS(MIN) to within X% of SS_MAXDC(DC) is given

by,

t(SS_MAXDC charge time within X% of target)

= t[(1 – (X/100) • SS_MAXDC(DC)] – t(VSS(MIN))

For X = 2 and VSS(MIN) = 0.45V, t(0.98 • 1.84) –

t(0.45) = t(1.803) – t(0.45)

From previous calculations, t(0.45) = 7.3e – 4 s.

Using previous values for RT, RB, and CSS,

t(1.803) = 2.63e–4 • 1e–7 • (–1) • ln(1 – 1.803/1.84)

= 2.63e–3 • (–1) • ln(0.02) = 1.03e–2 s

Hence the time for SS_MAXDC to charge from its mini-

mum reset threshold of 0.45V to within 2% of its target

value is given by,

t(1.803) – t(0.45) =

1.03e–2 – 7.3e–4 = 9.57e–3

Forward Converter Applications

The following section covers applications where the LT1952

is used in conjunction with other LTC parts to provide

highly efficient power converters using the single switch

forward converter topology.

95% Efficient, 5V, Synchronous Forward Converter

The circuit in Figure 14 is based on the LT1952 to provide

the simplest forward power converter circuit — using only

one primary MOSFET. The SOUT pin of the LT1952 pro-

vides a synchronous control signal for the LTC1698 lo-

cated on the secondary. The LTC1698 drives secondary

side synchronous rectifier MOSFETs to achieve high effi-

ciency. The LTC1698 also serves as an error amplifier and

optocoupler driver.

Efficiency and transient response are shown in Figures 12

and 13. Peak efficiencies of 95% and ultra-fast transient

response are superior to presently available power mod-

ules. Integrated soft-start, over-current detection and

short circuit hiccup mode provide low stress, reliable

protection. In addition, the circuit in Figure 14 is an all-

ceramic capacitor solution providing low output ripple

voltage and improved reliability. The LT1952-based con-

verter can be used to replace power module converters at

a much lower cost. The LT1952 solution benefits from

thermal conduction of the system board resulting in

higher efficiencies and lower rise in component tempera-

tures. The 7mm height allows dense packaging and the

circuit can easily be adjusted to provide an output voltage

from 1.23V to 26V. Higher currents are achievable by

simple scaling of power components. The LT1952-based

solution in Figure 14 is a powerful topology for replace-

ment of a wide range of power modules.

LT1952 Datasheet(PDF) 19 Page - Linear Technology

LT1952 Datasheet(HTML) 19 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Datasheet Download

Link URL