They do have obligation to buy if the minority shareholders demand it. If they don't want to trigger the requirement they need to keep their ownership under 90%

They do have obligation to buy if the minority shareholders demand it. If they don't want to trigger the requirement they need to keep their ownership under 90%. There's also requirement (with some exceptions) to make offer to buy publicly listed company's shares when the shareholder's control over the votes increases above 30% and 50%. Normally the price in that case would be the highest price that shareholder paid for the shares during previous 6 months.