LTC3330 Datasheet, PDF (14/20 Pages) Linear Technology – Energy Harvesting DC/DC with Battery Backup Low Noise LDO Post Regulator

LTC3330
Operation
V_OUT, and the inductor value. When the buck brings the
output voltage into regulation the converter enters a low
quiescent current sleep state that monitors the output volt-
age with a sleep comparator. During this operating mode
load current is provided by the output capacitor. When the
output voltage falls below the regulation point the buck
regulator wakes up and the cycle repeats. This hysteretic
method of providing a regulated output reduces losses
associated with FET switching and maintains an output
at light loads. The buck delivers a minimum of 100mA of
average load current when it is switching. V_OUT can be set
from 1.8V to 5V via OUT[2:0] (see Table 1).
When the sleep comparator signals that the output has
reached the sleep threshold the buck converter may be in
the middle of a cycle with current still flowing through the
inductor. Normally both synchronous switches would turn
off and the current in the inductor would freewheel to zero
through the NMOS body diode, but the NMOS switch is
kept on to prevent the conduction loss that would occur in
the diode if the NMOS were off. If the PMOS is on when the
sleep comparator trips the NMOS will turn on immediately
in order to ramp down the current. If the NMOS is on it
will be kept on until the current reaches zero.
Though the quiescent current when the buck is switching
is much greater than the sleep quiescent current, it is still
a small percentage of the average inductor current which
results in high efficiency over most load conditions. The
buck operates only when sufficient energy has been ac-
cumulated in the input capacitor and the length of time the
converter needs to transfer energy to the output is much
less than the time it takes to accumulate energy. Thus, the
buck operating quiescent current is averaged over a long
period of time so that the total average quiescent current
is low. This feature accommodates sources that harvest
small amounts of ambient energy.
BUCK-BOOST Converter
The buck-boost uses the same hysteretic voltage algorithm
as the buck to control the output, V_OUT, with the same
sleep comparator. The buck-boost has three modes of
operation: buck, buck-boost, and boost. An internal mode
comparator determines the mode of operation based on
BAT and V_OUT. Figure 2 shows the four internal switches
of the buck-boost converter. In each mode the inductor
current is ramped up to IPEAK. This IPEAK value is pro-
grammable via IPK[2:0] and ranges from 5mA to 250mA
(see Table 3).
M1
BAT
SWA
M2
SWB
M3
M4
V_OUT
3330 F02
Figure 2: Buck-Boost Power Switches
In BUCK mode M4 is always on and M3 is always off. The
inductor current is ramped up through M1 to IPEAK and
down to 0mA through M2. In boost mode M1 is always on
and M2 is always off. The inductor current is ramped up
to IPEAK when M3 is on and is ramped to 0mA when M4
is on as V_OUT is greater than BAT in boost mode. Buck-
boost mode is very similar to boost mode in that M1 is
always on and M2 is always off. If BAT is less than V_OUT
the inductor current is ramped up to IPEAK through M3.
When M4 turns on the current in the inductor will start to
ramp down. However, because BAT is close to V_OUT and
M1 and M4 have finite on-resistance the current ramp
will exhibit a slow exponential decay, lowering the aver-
age current delivered to V_OUT. For this reason the lower
current threshold is set to IPEAK/2 in buck-boost mode
to maintain high average current to the load. If BAT is
greater than V_OUT in buck-boost mode the inductor cur-
rent still ramps up to IPEAK and down to IPEAK/2. It can
still ramp down If BAT is greater than V_OUT because the
final value of the current in the inductor is (V_IN – V_OUT)/
(R_ON1 + R_ON4). If BAT is exactly IPEAK/2•(R_ON1 + R_ON4)
above V_OUT the inductor current will not reach the IPEAK/2
threshold and switches M1 and M4 will stay on all the time.
For higher BAT voltages the mode comparator will switch
the converter to buck mode. M1 and M4 will remain on
for BAT voltages up to V_OUT + IPEAK•(R_ON1 + R_ON4). At
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