アプリケーションノート 701

Using the DS32kHz with Maxim Real-Time Clocks


要約: This application note describes how to reduce current consumption when using the DS32kHz with Maxim real-time clocks (RTCs).

Overview

This application note is intended to answer some frequently asked questions about using the DS32kHz temperature-compensated crystal oscillator (TCXO) and Maxim 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 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

Maxim Integrated 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 3410
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 is drawn and the clock may 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.