Edit in JSFiddle

function go() {
    var osmLyr = new ol.layer.Tile({
        source: new ol.source.OSM()
    });
    var map = new ol.Map({
        target: 'map',
        layers: [
            osmLyr,
            new ol.layer.GeoportalWMTS({
                layer: "GEOGRAPHICALGRIDSYSTEMS.PLANIGNV2",
                olParams: {
                    minResolution: 1222.9924523925781
                }
            }),
            new ol.layer.GeoportalWMTS({
                layer: "ORTHOIMAGERY.ORTHOPHOTOS",
                olParams: {
                    opacity: 0.7
                }
            })
        ],
        view: new ol.View({
            center: [288074.8449901076, 6247982.515792289],
            zoom: 12
        })
    });    
    var lsControl = new ol.control.LayerSwitcher({
        layers : [{
            layer: osmLyr,
            config: {
                title: "OSM",
                description: "Couche OpenStreet Map",
            }
        }], 
        options : {
            collapsed: true
        }
    });
    map.addControl(lsControl);
    
    var layers = map.getLayers().getArray();
    var layersNb = layers.length;
    for (var i=0;i<layersNb;i++) {
        var minRes = layers[i].getMinResolution();
        var clientMinRes = minRes/Math.pow(2,5) // Client zoom up to 3 more zoom levels
        if (clientMinRes < 0.074) {
            clientMinRes = 0.074;
        }
        layers[i].setMinResolution(clientMinRes);
    }
    //console.log(map.getLayers().getArray());
}

Gp.Services.getConfig({
    apiKey: "essentiels",
    onSuccess: go
});

var infoDiv = document.getElementById("info");
infoDiv.innerHTML = "<p> Extension OL version " + Gp.olExtVersion + " (" + Gp.olExtDate + ")</p>";

<div id="map"></div>
<div id="info"></div>

#map {
    padding: 5px;
    width: 100%;
    height: 300px;
    box-shadow: 0 0 10px #999;
}

#info {
    padding: 5px;
    width: 100%;
    height: 20px;
    font-family: "monospace";
    font-size: 10px;
}