LTC3335_15 Datasheet, PDF (19/28 Pages) Linear Technology – Nanopower Buck-Boost DC/DC with Integrated Coulomb Counter

LTC3335
Applications Information
Higher value inductors provide the benefit of lower switch-
ing losses by increasing both AC(ON) and BD(ON). How-
ever, care must be taken so that AC(ON) never exceeds
the max full-scale time t_FS (11.74µs).
Recommended inductor values from Table 8 assure that
for PBAT from 1.8V to 5.5V, PV_OUT from 1.8V to 5V, and
±20% inductor variation, the AC(ON) time is always below
11.74µs.
If in the application the minimum PBAT voltage is higher
than 1.8V, the inductor value can be increased using the
formula below:
L_MAX
=
PBAT_MIN •L_REC
1.8
(8)
where L_MAX is the maximum inductor value (included
production tolerance), L_REC is the inductor value from
Table 8 and PBAT_MIN is the minimum PBAT voltage used
in the application. Inductors typically have production
tolerances of ±20%.
The DCR of the inductor can have an impact on efficiency
as it is a source of loss. In addition it is a source of error for
the coulomb counter because it increases the nonlinearity
of the inductor current during the AC(ON) time.
Choose an inductor with an I_SAT rating at least 50% greater
than the selected I_PEAK value. Table 9 lists several inductors
that work well. Trade-offs between price, size, and DCR
should be evaluated.
Load Current Capability
The maximum load current the buck-boost can support
depends on the I_PEAK setting, the BAT voltage, and the
V_OUT voltage and is ideally given by:
I_{LOAD(MAX )}
= ^IPEAK
2
• ^BAT
BAT + V_OUT
(9)
However, due to finite R_DS(ON) of power FETs A, B, C, and
D, as well as inductor DCR, the maximum deliverable
current is actually lower. Refer to the curves given in the
Typical Performance Characteristics section for actual load
current capability under various conditions.
Coulomb Counter Errors
The battery discharge coulombs is calculated by count-
ing the number of AC(ON) cycles and multiplying by the
number of coulombs per AC(ON) time given by the fol-
lowing formula:
q_AC(ON)
=
I_PEAK •
2
t_AC
(10)
This formula assumes that the LTC3335 input quiescent
current, gate charge current, R_DS(ON) of the power switches,
and the inductor DCR have negligible effect. It also as-
sumes that every pulse starts from an inductor current
equal to 0 and ends at I_PEAK. The contribution of each of
these errors will be discussed in the following sections.
Input Quiescent Current Error
The control circuit of the buck-boost consumes DC quies-
cent current when not in sleep. This current is dependent
on BAT voltage and temperature as shown in the Typical
Performance Characteristics section. This current, (typi-
cally 360µA) generates a small error at the 250mA peak
current setting, but can be significant for lower peak cur-
rent settings as shown in Figures 4 and 5.
When the buck-boost is sleeping, the DC quiescent current
is typically 680nA. This equates to an error of 5.96mA • hr
per year of cumulative sleep time. For a battery capacity of
18.3A • hr, the error is only 0.033% per year.
As shown in Figure 6, for load currents smaller than
approximately 100µA, the sleep current can result in a
significant error.
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3335p
19