LTC3335_15 Datasheet, PDF (14/28 Pages) Linear Technology – Nanopower Buck-Boost DC/DC with Integrated Coulomb Counter

LTC3335
Operation
Buck-Boost Regulator
The buck-boost regulator consists of four internal switches,
labeled A, B, C, and D, as shown in Figure 2, and control
circuitry which together connect the input and output
voltages to the power inductor.
SW1 _100µH SW2
PBAT
A
D
PV_OUT
BAT
V_OUT
B
C
3335 F02
Figure 2. Power FETs
The buck-boost operates as an H-Bridge for all BAT and
V_OUT conditions when not in sleep. This means that switches
A and C are always on together, followed by switches B
and D always on together. A hysteretic voltage algorithm
is used to control the output through internal feedback
from the V_OUT sense pin.
The buck-boost regulator charges the output capacitor
through the inductor. Current is delivered first by ramping
the inductor current up to I_PEAK through switches A and
C, and then ramping it down to 0mA through switches B
and D. The I_PEAK level is programmable via the IPK[2:0]
pins and ranges from 5mA to 250mA (see Table 1).
Table 1. I_PEAK Selection
IPK2
IPK1
0
0
0
0
0
1
0
1
1
0
1
0
1
1
1
1
IPK0
I_PEAK
0
5mA
1
10mA
0
15mA
1
25mA
0
50mA
1
100mA
0
150mA
1
250mA
This cycle repeats until the output voltage rises to a value
slightly higher than the regulation point (sleep threshold)
after which the converter enters a low quiescent current
sleep state that monitors the output voltage with a sleep
comparator. During sleep, load current is provided by the
output capacitor.
When the output voltage falls to a value slightly lower than
the regulation point (wake-up threshold) the buck-boost
regulator wakes up and then the inductor current starts
ramping up again within 3µs (typical).
This hysteretic method of providing a regulated output
voltage reduces losses associated with FET switching
while regulating at light loads.
V_OUT can be set via the voltage select pins OUT[2:0] from
1.8V to 5V (see Table 2).
Table 2. Output Voltage Selection via Pins
OUT2
OUT1
OUT0
V_OUT
0
0
0
1.8V
0
0
1
2.5V
0
1
0
2.8V
0
1
1
3.0V
1
0
0
3.3V
1
0
1
3.6V
1
1
0
4.5V
1
1
1
5.0V
When the sleep comparator senses that the output voltage
has reached the sleep threshold, the buck-boost converter
may be in the middle of a cycle with current still flowing
through the inductor. The converter enters the low qui-
escent current sleep state only at the end of a full AC-BD
cycle after the inductor current reaches 0mA. This behavior
is necessary for counting coulombs accurately.
During start-up and until V_OUT rises to approximately
1.2V, switch D is held off and the body diode conducts.
This ensures proper I_PEAK/I_ZERO operation for coulomb
counter accuracy.
3335p
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