1/5 february 2002 AN922 application note using a super cap to back-up the m41t56, m41t00, m41t11, m41t81, m41t94, and m41st84 (16-pin) introduction the m41t56, m41t00, m41t11, m41t81, m41t94, and m41st84 (16-pin) tim ekeeper ? devices from stmicroelectronics are used by applications designers who need a single chip device that offers fast sram storage and an integrated real time clock (the m41t00 and m41t81 provide the real time clock only). many of their designs switch in a battery to maintain the data and keep the clock running when the external power supply falls below specification (or is completely absent). when the battery is depleted, though, the designer or user can be faced with the issues of replacement and disposal (see the application note an1011, battery technology used in nvram products from st). this document describes a more maintenance-free way to sustain the data and clock in systems that only experience short breaks in the power supply (on the order of days). a super cap can be used as a type of secondary cell (a rechargeable battery) and can therefore provide an alternative solution to using a pri- mary cell. figure 1, page 2 (for the m41t56) and figure 2, page 4 (for the m41t00, m41t11, m41t81, and m41t94 as well as for the 16-pin m41st84) show two typical circuit arrangements. since the super cap is limited to a certain maximum charging current, a series-limiting resistor may also be required (please consult the data sheet for the super cap.) in this document, the reliability, leakage current, and charging cycle limitations of the super cap have not been taken into account. please consult the data sheet of the super cap for details. calculating the values of the circuit components for the m41t56 the minimum battery voltage for this device is 2.5v, while the maximum battery supply voltage is 3.5v. this gives the maximum delta voltage swing across the capacitor (1.0v). note: charging the capacitor above 3.5v will result in a higher power-fail deselect voltage (v pfd ) trip point, and may cause inadvertent deselection of the device at nominal v cc values. the voltage divider provides a bias on the transistor; the resistor divider is calculated according to the ratio of v cc to v base . limit the maximum voltage charge on the capacitor using the formula, derive the maximum voltage as follows: maximum supply voltage is 3.5v; v be is typically 0.6v, so the typical value of v base is 4.1v. v pfd 1.25 v bat typ () = v bat v base v C be = v base maximumsupplyvoltage =v + be
AN922 - application note 2/5 recommended starting values for r1 and r2 are r1=22k w and r2=100k w (with v cc = 5v). since the battery current, i bat , is limited to a maximum value of 550na, the capacitance and the duration of pow- er-out time can be calculated using the formula: where i = 550na, d v = 1.0v, c = capacitance is in farads, and d t = power out time is in seconds. using a 100,000 m f capacitor, for example, the equation would be: solving for d t, the maximum power down time is about 181,818 seconds. this is just over two days. figure 1. external connections to the m41t56 icvt d d = 550na 0.1f 1.0 v t d = v cc ai02481 m41t56 v bat v base r1 r2 v ss c +5v
3/5 AN922 - application note calculating the values of the circuit components for the m41t00, m41t11, and m41t81 the minimum operating voltage for these devices is 2.0v, with a typical v bat voltage of v cc C v f (diode). therefore, the typical delta voltage swing across the capacitor is: where v f is approximately 0.5v. therefore: since the battery current (i bat ) is limited to maximum value of 1.0 m a, the capacitance and the duration of power-out time can be calculated using the formula: where i=1.0 m a, d v=2.5v, c = capacitance in farads, and d t = power-out time in seconds. using a 100,000 m f capacitor, for example, the equation would be: solving for d t, the maximum power down time is about 250,000 seconds. this is 69.4 hours, or 2.9 days. calculating the values of the circuit components for the m41st84 and m41t94 the minimum operating voltage for these devices is 2.5v, with a typical v bat voltage of v cc C v f (diode). therefore, the typical delta voltage swing across the capacitor is: where v f is approximately 0.5v. therefore: vv cc v f C v cc C = d min v 5.0v0.5v2.0v C C = d v2.5v = d icvt d d = 1.0 m a0.1f2.5vt d = vv cc v f C v cc C = d min v 5.0v0.5v2.5v C C = d v2.0v = d
AN922 - application note 4/5 since the battery current (i bat ) is limited to maximum value of 500na, the capacitance and the duration of power-out time can be calculated using the formula: where i=500na, d v=2.0v, c = capacitance in farads, and d t = power-out time in seconds. using a 100,000 m f capacitor, for example, the equation would be: solving for d t, the maximum power down time is about 400,000 seconds. this is 111.1 hours, or 4.63 days. figure 2. external connections to the m41t00, m41t11, m41t81, m41t94, and m41st84 (16-pin) note: the same configuration is possible using the m41st84 (16-pin) icvt d d = 500na 0.1f 2.0v t d = v cc ai02854 v bat v ss c +5v v f ~0.5v
5/5 AN922 - application note contact information if you have any questions or suggestions concerning the matters raised in this document, please send them to the following electronic mail addresses: please remember to include your name, company, location, telephone number, and fax number. apps.nvram@st.com (for application support) ask.memory@st.com (for general inquiries) information furnished is believed to be accurate and reliable. however, stmicroelectronics assumes no responsibility for the co nsequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. no license is granted by implication or otherwise under any patent or patent rights of stmicroelectronics. specifications mentioned in this publicati on are subject to change without notice. this publication supersedes and replaces all information previously supplied. stmicroelectronics prod ucts are not authorized for use as critical components in life support devices or systems without express written approval of stmicroelectro nics. the st logo is registered trademark of stmicroelectronics all other names are the property of their respective owners. ? 2002 stmicroelectronics - all rights reserved stmicroelectronics group of companies australia - brazil - canada - china - finland - france - germany - hong kong - india - israel - italy - japan - malaysia - malta - morocco - singapore - spain - sweden - switzerland - united kingdom - u.s.a. www.st.com
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