require_once($_SERVER["DOCUMENT_ROOT"] . "/bitrix/modules/main/classes/general/csv_data.php");
CModule::IncludeModule("sale");
CModule::IncludeModule("catalog");
$filePath = $_SERVER['DOCUMENT_ROOT'] . '/upload/coupons.csv';
$fp = fopen($filePath, 'w+');
@fclose($fp);

$fields_type = 'R'; //дописываем строки в файл
$delimiter = ";";   //разделитель для csv-файла
$csvFile = new \CCSVData($fields_type, false);
$csvFile->SetFieldsType($fields_type);
$csvFile->SetDelimiter($delimiter);
$csvFile->SetFirstHeader(true);
$csvFile->SaveFile($filePath, array("ID", "COUPON"));

$rsCoupons = CCatalogDiscountCoupon::GetList([],['DISCOUNT_ID' => 20, "<ID" => 3000, ">ID" => 1000]);
$arCoupons = [];
while($arCoupon = $rsCoupons->Fetch()) {
    $arCoupons[] = [$arCoupon['ID'], $arCoupon['COUPON']];
}

foreach($arCoupons as $line)
{
    $csvFile->SaveFile($filePath, $line);
}