LTC3335_15 Datasheet, PDF (18/28 Pages) Linear Technology – Nanopower Buck-Boost DC/DC with Integrated Coulomb Counter

LTC3335
Applications Information
Input/Output Capacitor Selection
The input capacitor for the buck-boost on the BAT pin
should be bypassed with at least 4.7μF to GND. In cases
where the series resistance of the battery is high, a larger
capacitor may be desired to handle transients.
A larger capacitor may also be necessary when operating
close to 1.8V at higher I_PEAK settings to prevent the bat-
tery voltage from falling below 1.8V when the buck-boost
is switching.
The duration for which the buck-boost regulator sleeps
depends on the load current and the size of the V_OUT
capacitor. The sleep time decreases as the load current
increases and/or as the output capacitor decreases. The
DC sleep hysteresis window is ±6mV for the 1.8V output
setting and scales linearly with the output voltage setting
(±12mV for the 3.6V setting, etc.). Ideally this means that
the sleep time is determined by the following equation:
t_SLEEP
=
C_OUT
•
V_{DC _ HYS}
I_LOAD
(4)
This is true for output capacitance on the order of 100μF
or larger, but as the output capacitance decreases towards
10μF, delays in the internal sleep comparator along with the
load current itself may result in the V_OUT voltage slewing
past the DC thresholds. This will lengthen the sleep time
and increase V_OUT ripple. An output capacitance less than
22μF is not recommended as V_OUT ripple could increase
to an undesirable level.
If transient load currents higher than the maximum deliver-
able are required then a larger capacitor should be used at
the output. This capacitor will be continuously discharged
during a load condition and the capacitor can be sized for
an acceptable drop in V_OUT:
C_OUT
=
t_LOAD
•
I_LOAD –I_DC/DC
V_OUT ⁺ – V_OUT ^–
(5)
Here V_OUT+ is the value of V_OUT when PGOOD goes high
and V_OUT– is the desired lower limit of V_OUT. I_DC/DC is
the average current being delivered from the buck-boost
converter, and t_LOAD is the duration of the transient load.
The LTC3335 always operates as an H-bridge, even at
start-up. The start-up duration is dependent on the load
current and the output capacitor; a larger output capacitor
makes the start-up time longer.
A standard surface mount ceramic capacitor can be used
for C_OUT, though some applications may be better suited
to a low leakage aluminum electrolytic capacitor or a
supercapacitor. These capacitors can be obtained from
manufacturers such as Vishay, Illinois Capacitor, AVX,
or CAP-XX.
Inductor Selection
The AC(ON) and BD(ON) times are determined to first
order by BAT, V_OUT, the inductor value, and the I_PEAK
current setting.
AC(ON)
=
I_PEAK •L
PBAT
(6)
BD(ON)
=
I_PEAK •L
PV_OUT
(7)
The buck-boost is designed to work with a 100μH inductor
for typical applications using the 100mA peak current setting.
For the other seven I_PEAK settings the inductor value should
scale so as to keep the I_PEAK • L product approximately
constant. This maintains on-times required for accurate
coulomb counter operation. The nominal recommended
inductor values for the buck-boost for each I_PEAK setting
are listed in Table 8.
Table 8. Recommended Inductor Value L_REC vs I_PEAK
I_PEAK SETTING (mA)
L_REC (µH)
5
2200
10
1000
15
680
25
470
50
220
100
100
150
68
250
47
3335p
18
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