Calculating the current consumption of an SDRAM

APPLICATIONS NOTE
Calculating the current consumption of an SDRAM
Part Type:
Manufacturer:
SDRAM
Maxwell Technologies
Document No. :
Revision :
1009219
1
Date :
0202-0808-06
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Table of Contents
Purpose
Abstract
Example Calculation
Conclusion
Document 1009219 Rev 1
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PURPOSE: This application note describes the method used to calculate the current
consumption of an SDRAM.
ABSTRACT: The average current consumption of an SDRAM is the sum of the operating
currents divided by the total time.
(Operating current + Standby current + Burst Current + Refresh current) / Total Time
Current Consumption Formula:
Operating Current
[{ICC1 - iRASmin * (tRASmin - tRCmin) – iRPmin * (tRPmin / tRCmin)} * AN * (tRASmin + tRPmin)
Standby Current
+ iRAS * ΣtRAS + iRP + ΣtRP
Burst Operating Current
+ (iBST – iRAS) * (BN – AN) * tCK
Refresh Current
+ ICC5 * RN * tRC1min
Total Time
/T]
Note: Definitions of the terms used in the above formula can be found in Table 2.
Example Calculation: This example uses Maxwell Technologies 256Mbit SDRAM,
running at 100 MHZ. The operation used is a read-modify-write. The timing diagram in
Figure 1. shows the SDRAM commands and their timing relationship used to perform the
read-modify-write operation.
Figure 1. Timing Diagram
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Table 2. Terms Required for current consumption calculations
Symble
T
AN
BN
RN
tCK
ΣtRAS
ΣRP
tRASmin
tRPmin
tRCmin
tRC1min
tCKmin
ICC1
ICC2N
ICC3N
ICC4
ICC5
tCKi2N
tCKi3N
iRASmin
iRPmin
iRAS
iRP
iBST
Definition
Total Time (ns)
Number of times an ACTIVE command is performed
Number of times read/write is performed
Number of times a CBR is issued
System Clock Cycle
Time period between an ACTIVE command and a PRECHARGE command
T - ΣtRAS – tRC1min * RN = (230ns – 120ns -67.5ns) * 1 time
ACTIVE to PRECHARGE command period
PRECHARGE to ACTIVE command period
ACTIVE to ACTIVE command period
REFRESH to REFRESH command period
System clock cycle time
Operating Current ( CAS* latency = 3 )
Standby Current in non-power down
Active standby current in non-power down
Burst Operating Current
Refresh Current
tCK when ICC2 is measured
tCK when ICC3 is measured
ICC3 * (tCKi3N / tCKmin) = 30 mA * (15 ns / 7.5 ns)
ICC2N * (tCKi2N / tCKmin) = 2f0 mA * ( 15 ns / 7.5 ns)
ICC3N * (tCKi3N / tCK) = 30 mA * (15 ns / 10 ns)
ICC2N * (tCKi2N / tCK) = 20 mA * (15 ns / 10 ns)
ICC4 * (tCKmin / tCK) = 145 mA * (7.5 ns / 10 ns)
Value
230 ns
2 times
8 times
1 time
10 ns
120 ns
42.5 ns
50 ns
20 ns
70 ns
70 ns
7.5 ns
115 mA
20 mA
30 mA
145 mA
220 mA
15 ns
15 ns
60 mA
40 mA
45 mA
30 mA
108.75 mA
Note: The values presented in Table 1 are taken from the Maxwell data sheet, 48SD1616, 256mbit SDRAM running at a clock frequency
of 100MHz.
Current Consumption Calculations:
Average Operating Current Consumption Calculation
{ICC1 – iRASmin * (tRASmijn / tRCmin) – iRPmin *(tRPmin / tRCmin)} * AN (tRASmin + tRPmin)
= {115 mA – 60mA * (45 ns / 67.5 ns) – 40mA * (20 ns / 67.5 ns)} * 2 times * ( 45 ns + 20 ns)
= (115 mA – 60 mA*0.67 – 40 mA) * 2 times * 65 ns
= 115 mA – 40 mA – 12 mA * 130ns
= 63 mA * 130 ns
= 8190 mA –ns
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Average Standby Current Consumption Calculation
iRAS * ΣtRAS + iRP * ΣtRP
= 45 mA * 120 ns + 30 mA * 42.5 ns
= 6675 mA-ns
Average Burst Operating Current Consumption Calculation
(iBST –iRAS) * ( BN – AN) * tCK
= (108.75 mA – 45mA) * (8-2) times * 10 ns
= 3825 mA
Average Refresh Current Consumption Calculation
ICC5 * RN * tRC1min
= 220 mA * 1 time * 67.5 ns
= 14850 mA-ns
Average Current Consumption Calculation
(Operating Current + Standby Current + Burst Current + Refresh Current) / Total Time
(8190 mA-ns + 6675 mA-ns + 3825 mA-ns + 14850 mA-ns ) / 230 ns
= 167.7 mA
Conclusion: The average current consumption of an SDRAM is the sum of the operating
currents divided by the total time. The average current consumption is dependent on the
system clock frequency.
When analyzing the current consumption of an SDRAM, the designer must take into
account the varies operations performed and calculate the average current consumption for
each operation. Once this is complete the worst case current consumption can be
determined.
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