STCMB1 datasheet(36/59 Pages) STMICROELECTRONICS

36 / 59 page

Application information

STCMB1

36/59

DocID028958 Rev 4

During this frequency sweep phase the operating frequency will decay following the

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

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 the change very quickly as frequency approaches resonance

frequency (see Figure 20, right).

Figure 20. Soft-start circuit (left) and input impedance vs. frequency curve in an LLC resonant

half-bridge (right)

As a result, the dc input current will smoothly increase, without peaks that might occur

because of the resonant tank transfer function, and the output voltage will reach the

regulated value with almost no overshoot.

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

Equation 22

where fstart is recommended to be at least 4 times fmin. These criteria are quite empirical and

a workaround checking the waveforms and fine tuning of the RSS and CSS values might be

required.

It is recommended that the resonant current has no severe overshoots that might trigger the

OCP1 threshold (ISEN_HB

 0.8 V) in the first half bridge pulses, because the proper

charge of the CSS would be negatively prevented.

Please note also that an internal switch will discharge the CSS capacitor every time the chip

turns off (VCC < UVLO, FB < 1.75 V, ISEN_HB > 1.5 V) to make sure it will be soft-started at

the following restart. Additionally, the switch is activated when the voltage on the current

sense pin (ISEN) exceeds 0.8 V or when the converter is working in the capacitive mode

operation.

As long as the voltage on the pin is lower than 0.3 V, the burst mode operation and second

level OCP protection are inhibited.

start

min

;

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