LTC3335_15 Datasheet, PDF (21/28 Pages) Linear Technology – Nanopower Buck-Boost DC/DC with Integrated Coulomb Counter

LTC3335
Applications Information
0.0
T_A = 25°C
–0.2
V_OUT = 1.8V
V_OUT = 3.3V
V_OUT = 5V
–0.4
–0.6
–0.8
–1.0
–1.2
1.8 2.3 2.8 3.3 3.8 4.3 4.8 5.3
BAT VOLTAGE (V)
3335 F04
Figure 4. Typical Error Due to Input Quiescent
Current for 250mA I_PEAK Setting
2
1 ^{TA = 25°C}
0
–1
–2
–3
–4
–5
–6
–7
BAT = 1.8V
–8
BAT = 3.6V
–9
BAT = 4.2V
BAT = 5.5V
–10
0.01
0.10
1.00
10.00
LOAD CURRENT (mA)
3335 F06
Figure 6. Typical Error Due to Sleep Current
(250mA I_PEAK Setting, V_OUT = 3.3V)
0
T_A = 25°C
–5
–10
V_OUT = 1.8V
V_OUT = 3.3V
V_OUT = 5V
–15
–20
–25
–30
–35
–40
1.8 2.3 2.8 3.3 3.8 4.3 4.8 5.3
BAT VOLTAGE (V)
3335 G05
Figure 5. Typical Error Due to Input Quiescent
Current for 5mA I_PEAK Setting
Gate Charge Current Error
The gate charge current needed to turn on and off switches
A, B, and C is also a source of error for the coulomb coun-
ter. Gate charge current for switch D is provided from the
output and does not create an error. This error increases
at higher BAT voltages and is generally higher at low I_PEAK
settings as shown in Figures 7, 8, and 9.
Power Switches R_DS(ON) Error
The battery discharge coulombs are calculated assuming
that the inductor current rises to the I_PEAK setting value
linearly. However, finite R_DS(ON) of switches A and C cause
the actual inductor current to bow slightly which creates
an undercount in the coulomb counter (see Figure 10).
This error increases at lower BAT voltages and at higher
temperature.
Inductor DCR Error
An inductor with high DCR generates the same type of
error as the power switches R_DS(ON) error due to a simi-
lar nonlinear bowing of the inductor current waveform.
Recommended inductors from Table 10 assure that the
coulomb counter error due to the DCR is small.
Total Coulomb Counter Error
The total unadjusted coulomb counter error curves given
in the Typical Performance Characteristics section show
actual data taken from an actual circuit and include the
effects of all of the above mentioned error sources with
the exception of the sleep current error, as these curves
were taken with continuous switching. The errors present
during continuous switching are well-characterized for a
given set of operating conditions and can to first order be
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3335p
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