Ublox LISA-U1 Series System Integration Manual

Also see for LISA-U100: Integration manual

Contents

Ublox LISA-U1 Series System Integration Manual Manual pdf 30 page image

LISA-U1 series - System Integration Manual3G.G2-HW-10002-3 Preliminary System descriptionPage 30 of 125used to set the wake-up interval during idle-mode periods between network paging, but is able to provideprogrammable alarm functions by means of the internal 32.768 kHz clock.The RTC can be supplied from an external back-up battery through the V_BCKP, when the main voltage supplyis not provided to the module through VCC. This lets the time reference (date and time) run even when the mainsupply is not provided to the module. Consider that the module cannot switch on if a valid voltage is not presenton VCC even when the RTC is supplied through V_BCKP (meaning that VCC is mandatory to switch-on themodule).The RTC has very low power consumption, but is highly temperature dependent. For example at 25°C and aV_BCKP voltage of 2.3 V the power consumption is approximately 2 μA, whereas at 70°C and an equal voltagethe power consumption increases to 5-10 μA.The internal regulator for V_BCKP is optimized for low leakage current and very light loads. It is notrecommended to use V_BCKP to supply external loads.If V_BCKP is left unconnected and the module main voltage supply is removed from VCC, the RTC is suppliedfrom the 10 μF capacitor mounted inside the module. However, this capacitor is not able to provide a longbuffering time: within few milliseconds the voltage on V_BCKP will go below the valid range (1 V min). At thistime the internal RTC will stop counting and the date and time setting will be lost. This has no impact onwireless connectivity, as all the functionalities of the module do not rely on the date and time setting.Leave V_BCKP unconnected if the RTC is not required when the VCC supply is removed. The date andtime will not be updated when VCC is disconnected. If VCC is always supplied, then the internalregulator is supplied from the main supply and there is no need for an external component on V_BCKP.If RTC is required to run for a time interval of T [s] at 25°C when VCC supply is removed, place a capacitor with anominal capacitance of C [μF] at the V_BCKP pin. Choose the capacitor using the following formula:C [μF] = (Current_Consumption [μA] x T [s]) / Voltage_Drop [V] = 1.538 x T [s]The RTC current consumption is approximately 2 μA at 25°C, and the voltage drop is equal to 1.3 V (from theV_BCKP typical value of 2.3 V to the valid range minimum limit of 1.0 V).For example, a 100 μF capacitor (such as the Murata GRM43SR60J107M) can be placed at V_BCKP to provide along buffering time. This capacitor will hold V_BCKP voltage within its valid range for around 50 s at 25°C, afterthe VCC supply is removed. If a very long buffering time is required, a 70 mF super-capacitor (e.g. SeikoInstruments XH414H-IV01E) can be placed at V_BCKP, with a 4.7 k series resistor to hold the V_BCKP voltagewithin its valid range for approximately 10 hours at 25°C, after the VCC supply is removed. The purpose of theseries resistor is to limit the capacitor charging current due to the large capacitor specifications, and also to let afast rise time of the voltage value at the V_BCKP pin after VCC supply has been provided. These capacitors willallow the time reference to run during battery disconnection.LISA-U1 seriesC1(a)2 V_BCKPR2LISA-U1 seriesC2(superCap)(b)2 V_BCKPD3LISA-U1 series2.3 V(c)2 V_BCKPFigure 15: Real time clock supply (V_BCKP) application circuits : (a) using a 100 μF capacitor to let the RTC run for ~50 s at 25°C;(b) using a 70 mF capacitor to let the RTC run for ~10 hours at 25°C when the VCC supply is removed; (c) using non-rechargeablebattery