diff --git a/include/mbedtls/psa_util.h b/include/mbedtls/psa_util.h
index 06732d8..132c73f 100644
--- a/include/mbedtls/psa_util.h
+++ b/include/mbedtls/psa_util.h
@@ -191,7 +191,12 @@
  * \param       raw_len     Length of \p raw in bytes.
  * \param[out]  der         Buffer that will be filled with the converted DER
  *                          output. It can overlap with raw buffer.
- * \param       der_size    Size of \p der in bytes.
+ * \param       der_size    Size of \p der in bytes. Given \p bits parameter:
+ *                          * #MBEDTLS_ECDSA_MAX_SIG_LEN(\p bits) can be used
+ *                            to determine a large enough buffer for any
+ *                            \p raw input vector.
+ *                          * The minimum size might be smaller in case
+ *                            \p raw input vector contains padding zeros.
  * \param[out]  der_len     On success it contains the amount of valid data
  *                          (in bytes) written to \p der. It's undefined
  *                          in case of failure.