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LT1952IGN

Single Switch Synchronous Forward Controller

Linear Technology 

LT1952/LT1952-1

APPLICATIONS INFORMATION

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,

20

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 minimum

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/LT1952-1 are 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 LT1952-1 to provide

the simplest forward power converter circuit—using only

one primary MOSFET. The SOUT pin of the LT1952-1

provides a synchronous control signal for the LTC1698

located on the secondary. The LTC1698 drives secondary

side synchronous rectifier MOSFETs to achieve high

efficiency. 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

modules. Integrated soft-start, overcurrent 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

converter 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 temperatures. 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 LT1952-1-based solution in

Figure 14 is a powerful topology for replacement of a wide

range of power modules.

19521fe

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