CDK declaration for free tier RDS instance

Hi guys,

I'm trying to reproduce through CDK a 'free tier' RDS configuration, the same than i successfully did via the console and by selecting the 'free tier configuration'.

However, i can't understand why the following code does not create a 'free tier' ressource:

    const rdsInstance = new rds.DatabaseInstance(this, 'PostGis', {
        engine: rds.DatabaseInstanceEngine.postgres({
            version: rds.PostgresEngineVersion.VER_16_1
        }),
        instanceType: ec2.InstanceType.of(ec2.InstanceClass.T3, ec2.InstanceSize.MICRO),
        vpc,
        vpcSubnets: {
            subnetType: ec2.SubnetType.PUBLIC
        },
        securityGroups: [securityGroup],
        multiAz: false,
        allocatedStorage: 20,
        maxAllocatedStorage: 20,
        backupRetention: cdk.Duration.days(7),
        deletionProtection: false,
        databaseName: dataBaseName,
        credentials: rds.Credentials.fromUsername('pgadmin', {
        password : cdk.SecretValue.unsafePlainText('####') 
        }) 
    });

i've been comparing in console the configuration between the 'free tier' console created, and the CDK created, without being able to point out any difference. Still, i can see in billing i'm been charged for a regular t3.micro for the CDK created one.

Any idea will be appreciated,

Thanks !

Nico