BACKGROUND

1. Field

Embodiments of the present disclosure relate to a scheme for determining an optimal read threshold voltage in a memory system.

2. Description of the Related Art

The computer environment paradigm has shifted to ubiquitous computing systems that can be used anytime and anywhere. As a result, the use of portable electronic devices such as mobile phones, digital cameras, and notebook computers has rapidly increased. These portable electronic devices generally use a memory system having memory device(s), that is, data storage device(s). The data storage device is used as a main memory device or an auxiliary memory device of the portable electronic devices.

Memory systems using memory devices provide excellent stability, durability, high information access speed, and low power consumption, since they have no moving parts. Examples of memory systems having such advantages include universal serial bus (USB) memory devices, memory cards having various interfaces such as a universal flash storage (UFS), and solid state drives (SSDs). Memory systems may determine an optimal read threshold voltage among read threshold voltages according to various schemes.

SUMMARY

Aspects of the present invention include a system and a method for parametric PV-level modeling and an optimal read threshold voltage estimation based on the parametric PV-level modeling.

In one aspect, a memory system includes a memory device including a plurality of cells and a controller. The controller performs read operations on the cells using a plurality of read threshold voltages; generate cumulative mass function (CMF) samples based on the results of the read operations; receives first and second cumulative distribution function (CDF) values, selected from among a plurality of CDF values, which correspond to the CMF samples, each of the first and second CDF values representing a skew normal distribution; estimates first and second probability distribution parameter sets corresponding to the first and second CDF values, respectively; determine first and second probability density function (PDF) values corresponding to the first and second CDF values using the first and second probability distribution parameter sets, respectively; and estimate, as an optimal read threshold voltage, a read threshold voltage corresponding to a cross-point of the first and second PDF values.

In another aspect, a method for operating a memory system, which includes a memory device including a plurality of cells and a controller, includes: performing read operations on the cells using a plurality of read threshold voltages; generating cumulative mass function (CMF) samples based on the results of the read operations; receiving first and second cumulative distribution function (CDF) values, selected from among a plurality of CDF values, which correspond to the CMF samples, each of the first and second CDF values representing a skew normal distribution; estimating first and second probability distribution parameter sets corresponding to the first and second CDF values, respectively; determine first and second probability density function (PDF) values corresponding to the first and second CDF values using the first and second probability distribution parameter sets, respectively; and estimating, as an optimal read threshold voltage, a read threshold voltage corresponding to a cross-point of the first and second PDF values.

Additional aspects of the present invention will become apparent from the following description.

