LTC3335_15 Datasheet, PDF (22/28 Pages) Linear Technology – Nanopower Buck-Boost DC/DC with Integrated Coulomb Counter

LTC3335
Applications Information
0
–1
IDEAL Q_BAT
REAL
ERROR
INDUCTOR
–2
CURRENT
I_L
–3
–4
–5
–6
1.8
250mA
100mA
25mA
10mA
150mA
50mA
15mA
5mA
2.3 2.8 3.3 3.8 4.3 4.8 5.3
BAT (V)
3335 F07
Figure 7. Typical Error Due to Gate Charge Current
V_OUT = 1.8V, Ambient Temperature
0
–1
–2
–3
AC
BD
t_AC
TIME
Figure 10.
3335 F10
compensated for by applying a multiplicative scale factor
to the raw coulomb count reported by the LTC3335. The
error due to the sleep current can then be compensated
for by adding an offset term equal to the sleep current
multiplied by the cumulative sleep time (or if not known,
the battery service time). The error adjustment is given
by the following formula:
Adjusted Coulomb Count = (Raw Coulomb Count) • q_{LSB_M}
• [1/(1+Error)] + (5.96mA • hr) • Years
–4
–5
–6
1.8
250mA
100mA
25mA
10mA
150mA
50mA
15mA
5mA
2.3 2.8 3.3 3.8 4.3 4.8 5.3
BAT (V)
3335 F08
Figure 8. Typical Error Due to Gate Charge Current
V_OUT = 3.3V, Ambient Temperature
0
–1
–2
–3
–4
–5
250mA
150mA
–6
100mA
25mA
50mA
15mA
10mA
5mA
–7
1.8 2.3 2.8 3.3 3.8 4.3 4.8 5.3
BAT (V)
3335 F09
Figure 9. Typical Error Due to Gate Charge Current
V_OUT = 5V, Ambient Temperature
where
Error is the error in % from Figures G37-G44 in the
Typical Performance Characteristics, q_{LSB_M} is the least
significant bit of the accumulated charge register for the
chosen Prescaler M in A • hr, and Years is the number of
years of cumulative battery service.
The following two examples further illustrate how to
compensate for the raw coulomb count error.
Example 1: A Tadiran TL4903 primary cell (3.6V nominal,
2.4A • hr) is powering a 3.3V output and the I_PEAK setting
is 100mA. The appropriate pre-scaler is M=8. From curve
G39 in the Typical Performance Characteristics, the nominal
error for continuous switching under these conditions is
only –0.5%. The raw coulomb count q_{LSB_8} • C[7:0] read
from the LTC3335 can be adjusted by multiplying by 1/
(1-.005). To this result, the error due to the sleep current
(5.96mA • hr for each year of use) can be added, but this
additional term corresponds to less than 0.25% of the
battery's capacity per year of service. In this example,
the difference between the raw and adjusted coulomb
count is minimal.
3335p
22
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