SH3100 Datasheet, PDF (10/80 Pages) Semtech Corporation – Supervisory IC with I2C Interface and PWM

SH3100
POWER MANAGEMENT
VBAK Protection
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 flowing 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
flows between VDD and VBAK in either direction.
case condition. Note that as VBAK approaches VDD, the
net current changes direction so that it flows into VBAK.
Note: If the SH3100 is operated at clock frequencies and sup-
ply levels that mean that current flow 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.
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 fixed at 5.5V, which is the worst-
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 flowing 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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0
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SH3100 R1: VBAK Current versus CLK0 Frequency for different VBAK values
VBAK = 5.5V
5
10
15
20
25
30
35
40
VBAK = 3V
VDD = 5.5V (Worst case condition)
Negative current indicates flow out of VBAK pin
CLK0 Frequency (MHz)
VBAK = 2.5V
VBAK = 2.0V
© 2006 Semtech Corp.
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