LTC3335_15 Datasheet, PDF (16/28 Pages) Linear Technology – Nanopower Buck-Boost DC/DC with Integrated Coulomb Counter

LTC3335
Operation
Table 4. Write Registers A, B, and E
BIT
NAME OPERATION
A[3:0]
Prescaler Bits Set coulomb counter
prescaling factor M from
0 to 15
A[7:4]
B[7:0]
V_OUT Selection
Alarm Level
Selection of output voltage
Coulomb count alarm level
threshold calculated by the
user based on battery capacity
and I_PEAK current
E[0]
Clear_Int Clear interrupt (Alarm reset)
E[1] Counter Test Counter check using IRQ pin
E[7:2] Not Used
DEFAULT
0000
0000
11111111
0
0
Table 5. Read/Write Register C
BIT
NAME
OPERATION
C[7:0] Accumulated Read back 8 MSBs of counter
Charge
data
DEFAULT
00000000
Table 6. Read Register D
BIT
NAME OPERATION
D[0] AC(ON) Time AC(ON) time operating fault
Overflow
(t_AC > t_FS) due to improperly
chosen inductor value timing
out the AC(ON) measurement
D[1]
Coulomb Coulomb counter operating
Counter fault due to an improperly
Overflow chosen prescalar causing the
ripple counter to overflow
D[2] Alarm Trip Accumulator Register C value
has met or exceeded the Alarm
threshold set in Register B
D[7:3] Not Used
DEFAULT
0
0
0
00000
Output Voltage Setting Via I²C
Selection of the output voltage can be done not only via
pins OUT[2:0], but also via I²C (see Table 7). The user
can change the V_OUT settings dynamically by writing to
register A. Note that bit A[7] must be set to 1 for bits
A[6:4] to take effect.
Table 7. V_OUT Selection Bits
BIT
SET DESCRIPTION
A[4]
0/1 Overwrite OUT0 Pin
A[5]
0/1 Overwrite OUT1 Pin
A[6]
0/1 Overwrite OUT2 Pin
A[7]
1 Must be 1 to set output voltage via I²C
Choosing Coulomb Counter Prescaler M
To preserve digital resolution for a wide range of battery
capacities and peak current values, the LTC3335 includes
a programmable prescaler. The user can set the prescaler
value from 0 to 15 by writing bits A[3:0]. The default value
for the prescaler is 0.
To use the majority of the range of the Accumulated Charge
Register C, the prescaler factor (M) should be chosen for
a given battery capacity Q_BAT based on Formula (2):
M
=
log₂


q_LSB • 255
Q_BAT


(2)
where Q_BAT is the battery size in A • hr and q_LSB is the
typical value (for M=0) from the Electrical Characteristics
table for the selected I_PEAK. M must be an integer, so the
result of Formula (2) must be rounded down to the next
integer value. M has a maximum value of 15.
A smaller capacity battery will require a higher prescaler
factor M than a larger capacity battery for the same I_PEAK.
Likewise, a lower I_PEAK will require a higher prescaler fac-
tor M than a higher I_PEAK for the same capacity battery.
The amount of charge represented by the least significant
bit (q_{LSB_M}) of the accumulated charge register is given by:
q_LSB
_
M
=
q_LSB
2^M
(3)
where q_LSB is the typical value in Electrical Characteristics
table for the selected I_PEAK.
Counter Check Test
Setting the bit E[1] = 1 allows the user to verify that the
coulomb counter is operating correctly without having to
wait for the accumulated charge register to increment from
00000000. In this mode the input clock of the ripple counter
is output to the IRQ pin, and the frequency of switching
seen at the IRQ pin will increase with output load.
3335p
16
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