LTC3330_15 Datasheet, PDF (17/32 Pages) Linear Technology – Nanopower Buck-Boost DC/DC with Energy Harvesting Battery Life Extender

LTC3330
OPERATION
PMOS switch and then ramping it down to 0mA through
an internal NMOS switch. This efficiently delivers energy
to the output capacitor. The ramp rate is determined by
V_IN, 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
voltage 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 the output voltage select
bits, OUT[2:0] (see Table 1).
When the sleep comparator senses 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 I_{PEAK_BB}, which is programmable
via the IPK[2:0] bits 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 I_{PEAK_BB} 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 I_{PEAK_BB} when M3 is on and is ramped down 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 I_{PEAK_BB}
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
average current delivered to V_OUT. For this reason the
lower current threshold is set to I_{PEAK_BB}/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
current still ramps up to I_{PEAK_BB} and down to I_{PEAK_BB}/2.
It can still ramp down if BAT is greater than V_OUT because
the final value of the current in the inductor would be
(V_IN – V_OUT)/ (R_ON1 + R_ON4). If BAT is exactly I_{PEAK_BB}/
2•(R_ON1 + R_ON4) above V_OUT the inductor current will
For more information www.linear.com/LTC3330
3330fb
17