IS43LD16640C datasheet(57/142 Pages) ISSI

57 / 142 page

IS43/46LD16640C

IS43/46LD32320C

Integrated Silicon Solution, Inc. — www.issi.com

Rev. C

07/02/2021

® Long-term Support

World Class Quality

The LPDDR2 devices provide significant flexibility in scheduling REFRESH commands as long as

the required boundary conditions are met (see figure of tSRF Definition).

In the most straightforward implementations, a REFRESH command should be scheduled every

tREFI. In this case, self refresh can be entered at any time.

Users may choose to deviate from this regular refresh pattern, for instance, to enable a period in

which no refresh is required. As an example, using a 1Gb LPDDR2 device, the user can choose

to issue a refresh burst of 4096 REFRESH commands at the maximum supported rate (limited

by tREFBW), followed by an extended period without issuing any REFRESH commands, until the

refresh window is complete. The maximum supported time without REFRESH commands is calcu-

lated as follows: tREFW - (R/8) × tREFBW= tREFW - R × 4 × tRFCab.

For example, a 1Gb device at TC

≤ 85˚C can be operated without a refresh for up to 32ms - 4096 ×

4 × 130ns

≈ 30ms.

Both the regular and the burst/pause patterns can satisfy refresh requirements if they are repeated

in every 32ms window. It is critical to satisfy the refresh requirement in every rolling refresh window

during refresh pattern transitions. The supported transition from a burst pattern to a regular distrib-

uted pattern is shown in figure of Supported Transition from Repetitive REFRESH Burst .

If this transition occurs immediately after the burst refresh phase, all rolling tREFW intervals will

meet the minimum required number of REFRESH commands.

A nonsupported transition is shown in Figure of Nonsupported Transition from Repetitive RE-

FRESH Burst . In this example, the regular refresh pattern starts after the completion of the pause

phase of the burst/pause refresh pattern. For several rolling tREFW intervals, the minimum number

of REFRESH commands is not satisfied.

Understanding this pattern transition is extremely important, even when only one pattern is em-

ployed. In self refresh mode, a regular distributed refresh pattern must be assumed.

ISSI recommends entering self refresh mode immediately following the burst phase of a burst/

pause refresh pattern; upon exiting self refresh, begin with the burst phase (see Figure of Recom-

mended Self Refresh Entry and Exit ).

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