Download TI - TI E2E Community

Risk for latchup of the CC1101
1
FINDINGS
It occurs that, during assembling, when touching the antenna , the device suddenly
consumes an extra-large current (estimated around 300mA)
 The battery voltage decreases within a few minutes from 3.72V to 3.11V (~25%
of the battery capacity ~55mAh) .
o The estimated current is around 300mA.
 During the time this large current is flowing, the communication with the device
was normal.
o Reading VBAT, Vpow and temperature was possible.
 No reset of device has been detected
o No oscillation error, nor any other error has been detected.
 After power down and recharging the device to 3.75 no abnormality has been
detected.
 This large current can only be stopped through switching off the battery
2
ANALYSIS
2.1.1
During manipulation
 The operator is grounded.
2.1.2
Except the large current the behavior of the device was normal
Following components are functioning properly.
 Main MSP and MSP powering
 Antenna CC1101
 Vdd rxtx was sufficient for communication ( minimal 1.8V)
 PICS for measuring VBAT and VPOW
 Oscillator and dividers
 The temp sensor
 The fuse (limit 500mA) is not broken
2.1.3
Feedthroughs
All feedthrough outputs are connected with capacitors to block the DC current. So no
current could flow through the pins.
2.1.4
In circuit analysis
No connections can be detected to be responsible for this large current
2.1.5
Battery connection
Accidental connection of battery pins is excluded:
 Manipulation at the antenna pin
 No continuous current flow but an extreme high current peak
2.1.6
VLDO LT3012 and gnd
 Current VLDO is limited to 250mA. The gnd current is 7mA
 VLDO on or off has no influence on the large current
2.1.7
Latchup problem
The reported events strongly indicates a latchup problem in the C1011 triggered by
touching the antenna. (the antenna input is connected to the ground with the inductor
L6 )
During latch up a current flows through parasitic pnp and npn junctions in the substrate,
(comparable with a thyristor ) which is triggered by a input signal. The current is
limited by internal structures and the maximum output current of the voltage supply.
Latch up: See Texas Instruments Application report SLYA014- may 2000.
2.1.8
The power supply for the C1101
The Vdd_RXTX is supplied by the TPS780300250DRV.
This voltage supply is current limited to minimal150mA, typical230 and max 400mA.
This limits the current in the substrate, preventing the chip to crash.
Power dissipation during latch up CC1101