L6699D datasheet(20/38 Pages) STMICROELECTRONICS

20 / 38 page

Application information

L6699

20/38

Doc ID 022835 Rev 2

the operating frequency is essentially determined by the current sunk by the optocoupler's

phototransistor.

During this frequency sweep phase the operating frequency decays following the

exponential charge of CSS: then, it initially changes relatively quickly but the rate of change

gets slower and slower. This counteracts the non-linear frequency dependence of the tank

circuit that makes the converter's input impedance change little as frequency is away from

resonance and change very quickly as frequency approaches resonance frequency (see

Figure 13).

As a result, the DC input current smoothly increases, without the peaking that occurs with

linear frequency sweep, and the output voltage reaches the regulated value with almost no

overshoot.

Typically, RSS and CSS is selected based on the following relationships:

Equation 5

where fstart is recommended to not be over 300 kHz. Please refer to the timing diagram of

Figure 20 to see some significant signals during the soft-start phase.

Figure 12.

Soft-start circuit

Figure 13.

Input impedance vs.

frequency curve in an LLC

resonant half bridge

min

start

min

;

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