STM1404CSOCQ6F datasheet(13/35 Pages) STMICROELECTRONICS

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STM1404

Pin descriptions

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2.9

VREF, reference voltage output (1.237, typ)

This is valid only when VCC is between 2.4 V and 3.6 V. When VCC falls below 2.4 V (VSW),

VREF is pulled to ground with an internal 100 kΩ resistor. This is an optional feature

available on the STM1404A. On the STM1404B/C, this pin is VTPU (internally switched VCC

or VBAT). If unused, this pin should float.

2.10

VOUT

This is the supply voltage output. When VCC rises above VSO (battery backup switchover

voltage), VOUT is supplied from VCC. In this condition, VOUT may be connected externally to

VCC through a p-channel MOSFET switch. When VCC falls below the lower value of VSW

(~2.4 V), or VBAT, VOUT is supplied from VBAT. It is recommended that the VOUT pin be

connected externally to a capacitor that will retain a charge for a period of time, in case an

intruder forces VCC or VBAT to ground. The rectifying diode connected from the positive

terminal of the battery to the VBAT pin of the STM1404 will prevent discharge of the

capacitor. Three variations of parts will be offered with the following options:

STM1404A: VOUT remains ON when SAL is active-low; Vccsw pin will continue to

operate in normal mode (see Section 2.3.1: Vccsw, VCC switch output on page 11);

STM1404B: VOUT is taken to High-Z when SAL is active-low; Vccsw pin will be set to

high when this occurs; and

STM1404C: VOUT is driven to ground when SAL is active-low; Vccsw pin will be set to

high when this occurs.

2.11

VTPU

For STM1404B and STM1404C, this pin provides pull-up voltage for the physical tamper

pins (TP1-4). This pin is not to be used as voltage supply source for any other purpose.

Note:

VTPU is the internally switched supply voltage from either the VCC pin or the VBAT pin.

2.12

VCC

This is the supply voltage (2.2 V to 3.6 V).

2.13

VBAT

This is the secondary (backup battery) supply voltage. The pin is connected to the positive

terminal of the battery with a rectifying diode like the BAT54J from STMicroelectronics for

reverse charge protection. Voltage at this pin, after diode rectification, will be approximately

0.2 V less than the battery voltage, and will depend on the type of battery used as well as

the IBAT being drawn. (A capacitor of at least 1.0 µF connected between the VBAT pin and

VSS is required.) If no battery is used, connect the VBAT pin to the VCC pin.

2.14

VSS

Ground, VSS, is the reference for the power supply. It must be connected to system ground.

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