STCMB1 datasheet(48/59 Pages) STMICROELECTRONICS

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Application information

STCMB1

48/59

DocID028958 Rev 4

As the overload is applied (1, in the left picture), the OCP1 is triggered: the output

power is limited by the periodic discharge of the soft-start capacitor and the internal

counting system starts.

In about 50 ms, the modulo 20 K counter reaches its EoC, triggering the soft-start

procedure (2, in the left picture): the CSS is completely discharged and the LLC

stopped after 128 cycles and VCC is naturally discharged.

The STCMB1 remains in the OFF state until VCC drops to VCCoff: here, the HV start-

up generator is activated and charges VCC. As VCCon is reached, the STCMB1 will

restart operations via a soft-start procedure (CSS has been previously discharged).

Because of the permanent overload, the OCP1 is triggered again and the above

sequence will be repeated: the soft-stop in about 50 ms (1, in the right picture), VCC

discharges (2, in the right picture), VCC recharges and the converter restarts (3, in the

right picture).

The hiccup operation will last for the time the overload condition is kept. As it is

removed, the converter will restart the normal operation at the first VCC recharge after

the removal of the overload condition.

7.24.2

Second level OCP protection

In case of particularly severe dead shorts the current in the resonant tank can rise very

quickly to very high levels. In such cases it would be dangerous for the converters to wait

the timing set up by the procedure described above, so the STCMB1 is equipped with an

additional comparator referenced at 1.5 V (typ. level, VISEN_HB). In case of dead shorts if the

signal on the ISEN_HB pin reaches the VISEN_HB the converter is stopped immediately after

terminating the current cycle and the internal CSS discharging switch is forced continuously

on.

The resonant HB will restart the operation after the VCC has dropped below the VVCCOff, and

then risen to the VVCCON by the activation of the HV start-up as during normal power-on.

The converter operation will result in a continuous intermittent operation (hiccup mode) until

the fault is removed.

In order to prevent the intervention of this protection at the start-up due to any initial

transient or spike, this second level OCP protection is inhibited until the voltage on Css is

higher than 0.3 V.

7.25

HB: high voltage bootstrap section and HB gate drivers (HVG

and LVG)

The supply of the floating high-side section is obtained by means of bootstrap circuitry. This

solution normally requires a high voltage fast recovery diode to charge the bootstrap

capacitor CBOOT, as shown in Figure 30a. In the STCMB1 a patented integrated structure

replaces this external diode. It is realized by means of a high voltage DMOS, working in the

third quadrant and driven synchronously with the low-side driver (LVG), with a diode in

series to the source, as shown in Figure 30b.

The diode prevents any current can flow from the VBOOT pin back to VCC in case that the

supply is quickly turned off when the internal capacitor of the pump is not fully discharged.

To drive the synchronous DMOS it is necessary a voltage higher than the supply voltage

VCC. This voltage is obtained by means of an internal charge pump.

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