Textbox binding - how to do validation on PropertyChanged but only UpdateSource on LostFocus?

I have a textbox that is setup for binding like below

<TextBox FontWeight="Normal"><TextBox.Text><Binding Path="Students" Mode="TwoWay" NotifyOnValidationError="True" ValidatesOnExceptions="True" UpdateSourceTrigger="PropertyChanged" Converter="{StaticResource StudentConverter}"><Binding.ValidationRules><StaticResource ResourceKey="StudentsCountValidator" /></Binding.ValidationRules></Binding></TextBox.Text></TextBox>

It seems to me that the trigger for the validation is dependent on whichever set for UpdateSourceTrigger. Is there a way to make it independent from UpdateSourceTrigger without using code behind?

What I want is to do validation whenever the text is changed, but I only want to update source when the textbox is lost focus. Seems mission impossible for me now without code behind or nasty custom textbox control.

Thanks.