SH3100 Datasheet, PDF (55/80 Pages) Semtech Corporation – Supervisory IC with I2C Interface and PWM

SH3100
POWER MANAGEMENT
Functional Descriptions
32.768 kHz Crystal Oscillator
The crystal oscillator uses a current-starved design to
minimize current consumption. It includes a 5-bit adjust-
able padding capacitance to GND on both XIN and XOUT
to allow optimization of the crystal load capacitance un-
der varying conditions.
The 5-bit control selects one of 32 padding capacitances
from 10pF to 44pF in a geometric progression. This non-
linear capacitance curve compensates for the non linear-
ity of the crystal and results in a net linear progression of
frequency with adjustment code.
Note: If no crystal is present, XIN should be tied to ground to
avoid noise pickup on XIN being misinterpreted as crystal os-
cillations.
Note: There is no failsafe function on the crystal oscillator, so
if the crystal fails during normal operation, the SH3100 hangs
up and needs to be power-cycled to allow a fresh start-up. If
the crystal does not start up within ten seconds of the next
power-up, then it is ignored, and the internal 32kHz oscillator
is used as the system clock.
This allows the crystal frequency to be maintained within
±2ppm of 32.768 kHz over temperature, by measuring
temperature using the on chip temperature sensor, and
then adjusting the padding capacitance accordingly, us-
ing a look-up table for that particular device/crystal/PCB
combination.
Parameter
Crystal operating frequency
Duty cycle
Start-up time
Min padding capacitance
Max padding capacitance
ppm adjustment resolution
XIN input switching threshold
XOUT pull-up current
Symbol
Min
Fx
DC
25
Tst
Cmin
9
Cmax
32
Rppm
Vth
0.3
Ixout
450
Typ
32.768
50
10
36
5
0.5
Max
Units
kHz
75
%
3
seconds
11
pF
40
pF
ppm
0.7
V
750
nA
© 2006 Semtech Corp.
55
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