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Texas Instruments Device Behavior Under Slow VBAT Ramp-Down and Ramp-Up Application notes
Application Report
SLVA643 – January 2015
Device Behavior Under Slow VBAT Ramp-Down and
Ramp-Up
Samir Camdzic ..................................................................................... Automotive, Safety and Industrial
ABSTRACT
This application report explains device behavior under slow battery-supply voltage ramp-up and rampdown conditions, based on device supply-rail configuration. Laboratory test results and device simulations
results are included to support the analysis and device behavior.
1
Contents
Undervoltage Transients .................................................................................................... 2
List of Figures
1
Battery Slow Ramp-Down Bench Measurement Result ................................................................ 2
2
Battery Slow Ramp-Down Simulation Result
3
............................................................................
Battery Slow Ramp-Up Bench Measurement Result ...................................................................
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3
1
Undervoltage Transients
1
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Undervoltage Transients
Undervoltage transients can be observed on regulated supply outputs when one of the following occurs:
• The power-supply input (VBAT) of the device ramps down at a slow rate and reaches a voltage level of
6.3 V.
• The power-supply input (VBAT) of the device ramps up at a slow rate and reaches a voltage level of
6.6 V.
These transient impact the following regulated supplies:
• VDD5
• VDD3/5 when configured at 5 V
• VSOUT when configured as 5 V or higher
The VDD3/5 undervoltage transient causes a system-reset event (the NRES output is driven low) and
disables the EN_DRV output.
VDD3/5
VBATP
VDD6
NRES
VDD3/5 undervoltage
transient
VDD6 enters dropout mode as
VBAT ramps below 6.3 V
NRES driven low because of
VDD3/5 undervoltage transient
Figure 1. Battery Slow Ramp-Down Bench Measurement Result
As VDD6 enters dropout mode,the extended
VDD6 switch-off time is observed
VDD6 and VDD3/5 rails dip,
creating an undervoltage event
VBATP
VBAT UV Comparator
VDD6_EN
VDD3/5_EN
SDN6
VDD6 Inductor Current
VDD6
VDD3/5
Figure 2. Battery Slow Ramp-Down Simulation Result
2
Device Behavior Under Slow VBAT Ramp-Down and Ramp-Up
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Undervoltage Transients
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The VDD6 capacitor is discharged because of the extended VDD6 switch node off-time which causes a
strong decrease of inductor current. Small difference between the VDD6 and VBAT terminals result in a
slow increase of inductor current when the VDD6 switch node turns on.
VDD3/5
VBATP
VDD6
NRES
VDD3/5 undervoltage
transient
VDD6 transitions from dropout mode
to standard buck-operation as VBAT
ramps above 6.6 V
NRES driven low because of
VDD3/5 undervoltage transient
Figure 3. Battery Slow Ramp-Up Bench Measurement Result
The VDD6 terminal transitions from dropout mode to standard buck operation as the VBAT terminal
ramps-up and reaches 6.6 V. In the standard buck-operation mode, when the VDD6 terminal is at 6.6 V, it
is above the regulation target. Therefore the high-side (HS) switch turns off until the VDD6 terminal drops
to 6 V.
During the off-period, the inductor current reduces to 0 with a rate of 200 mA/µs (for a 33-µH inductor),
after which the load current on the VDD6 terminal is provided through VDD6 capacitor until the VDD6
terminal drops down to 6 V.
When turning on the VDD6 HS switch with Iind = 0, the current can only increase at a rate of 18mA/us (for
a 33-µH inductor and slow battery ramp-up). Until Iindavg reaches IVDD6, IVDD6 remains supplied through
CVDD6 and the voltage decreases.
The voltage drop on the VDD6 terminal (and thus on the VDD3/5 terminal when the VDD6 terminal is less
than 5 V) causes the undervoltage event on the VDD3/5 terminal which leads to NRES being driven low.
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