Abstract: This application note describes how to reduce current consumption when using the DS32kHz with Dallas Real-Time Clocks (RTCs).
Overview
This application note is intended to answer some frequently asked questions about using the DS32kHz TCXO (temperature-compensated crystal oscillator) and Dallas Semiconductor real-time clocks (RTCs).
Using the DS32kHz
The DS32kHz has four pins that are required for operation: VCC, VBAT, GND, and 32kHz_OUT. The VCC, VBAT, and GND are power supply connections and must either be connected to a positive supply or grounded. The 32kHz_OUT signal is intended to drive the X1 input of the RTC. The X2 pin of the RTC should be allowed to float when driving the X1 input with an oscillator.
The DS32kHz ICC and IBAT specifications are measured with no output load. The input characteristics of the oscillator on a RTC will determine how much additional current the DS32kHz will consume. The additional current can significantly increase the size of the battery needed for operation.
Choosing an RTC
Dallas Semiconductor has a wide selection of RTCs from which to choose. Some of these devices were designed using a P-WELL process while the more recent devices have been designed using the first-generation N-WELL process. The second-generation N-WELL RTCs incorporate improvements that reduce the current consumption of the TCXO/RTC combination. The following data shows the current consumed by typical devices from each process. The data were taken at a battery voltage of 3.5V at +25°C.
KEY:
Ind = Individual Currents; DS32kHz with output open, RTC with crystal attached and running.
Direct = DS32kHz driving the RTC X1 input directly.
RC = DS32kHz driving the RTC with a 1MΩ resistor in series with a 100pF capacitor to the X1 input.
Table 1. N-WELL (First Generation)
Mode
DS32kHz IBAT (µA)
DS1306 IBAT (nA)
Ind
1.60
518
Direct
11.9
386
RC
2.14
584
Table 2. N-WELL (Second Generation)
Mode
DS32kHz IBAT (µA)
DS1337 ICC (nA)
Ind
1.59
612
Direct
3.73
626
RC
2.11
622
Table 3. P-WELL
Mode
DS32kHz IBAT (µA)
DS1202 IBAT (nA)
Ind
1.63
625
Direct
87.1
3,410
RC
2.17
685
Note the RTC in each of the aforementioned tables are different RTCs. This accounts for the difference in the clock currents when comparing the tables.
If the RTC has an oscillator-enable bit, the oscillator must be enabled. If the bit is not enabled, additional current will be drawn and the clock might not operate.
Table 4. RTC LIST
P-WELL Devices
First Generation N-WELL Devices
Second Generation N-WELL Devices
DS1202
DS12885
DS1337
DS12885
DS1302
DS1338
DS1283
DS1305/DS1306
DS1339
DS1284
DS1307
DS1672
DS1384
DS1315
DS14285
DS1500/DS1501
DS1384
DS1315
DS1602
DS1670/DS1673/DS1677
DS1685
DS1688/DS1689
DS17285/DS17485/DS17885
Summary
When using P-WELL and first-generation N-WELL RTC devices, use an RC circuit to achieve the minimum
possible timekeeping current.
The second-generation N-WELL devices use only slightly more current with an RC circuit.
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