SH3100IMTR SEMTECH [Semtech Corporation], SH3100IMTR Datasheet - Page 10

• Manufacturer Part Number: SH3100IMTR
• Description: Supervisory IC with I2C Interface and PWM
• Manufacturer: SEMTECH [Semtech Corporation]

To avoid degradation of Lithium cells connected to VBAK,
charging current over the operating lifetime needs to be
limited to less than 3% of the cell capacity. It is therefore
necessary to limit any current fl owing from VDD to VBAK.
On the SH3100, VDD and VBAK have diode protection
structures, so VDD and VBAK can independently have
any DC voltage between 0 V and 5.5V, and no DC current
fl ows between VDD and VBAK in either direction.
However, since CLK0 and CLK1 may be powered either
from VDD or VBAK, there is capacitance between CLK0 or
CLK1 and VBAK, so an AC signal on CLK0 or CLK1 pow-
ered by VDD would couple some net current through to
VBAK. This current coupling is proportional to frequency
and to the voltage differential between VDD and VBAK.
The following graph shows the variation in reverse cur-
rent from VDD to VBAK versus CLK frequency for different
settings of VBAK. VDD is fi xed at 5.5V, which is the worst-
© 2006 Semtech Corp.
POWER MANAGEMENT
VBAK Protection
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SH3100 R1: VBAK Current versus CLK0 Frequency for different VBAK values
VDD = 5.5V (Worst case condition)
Negative current indicates flow out of VBAK pin
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CLK0 Frequency (MHz)
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case condition. Note that as VBAK approaches VDD, the
net current changes direction so that it fl ows into VBAK.
As additional protection, if the battery is connected in re-
verse, then a large protection diode between VBAK and
GND means that VBAK limits at approximately 0.7V below
GND depending on the current fl owing through the diode.
A safe maximum current through this protection diode is
10mA. The SH3100 continues to function normally (apart
from battery backup) while VBAK is negative.
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Note: If the SH3100 is operated at clock frequencies and sup-
ply levels that mean that current fl ow out of VBAK into the bat-
tery would be a problem, then one solution would be to use
a series diode between the battery and VBAK to inhibit any
reverse current. This lowers the battery voltage by 0.7V on to
VBAK; however, since VBAK can go as low as 0.9V, this is un-
likely to be a problem for lithium cells.
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VBAK = 2.5V
VBAK = 5.5V
VBAK = 3V
VBAK = 2.0V
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SH3100
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