LTC3330

14

3330p

For more information www.linear.com/LTC3330

operaTion

output voltage into regulation the converter enters a low

quiescentcurrentsleepstatethatmonitorstheoutputvolt-

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

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.Normallybothsynchronousswitcheswouldturn

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

Thebuck-boostusesthesamehystereticvoltagealgorithm

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

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).

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

boost mode is very similar to boost mode in that M1 is

the inductor current is ramped up to IPEAK through M3.

When M4 turns on the current in the inductor will start to

M1 and M4 have finite on-resistance the current ramp

will exhibit a slow exponential decay, lowering the aver-

current threshold is set to IPEAK/2 in buck-boost mode

to maintain high average current to the load. If BAT is

rent still ramps up to IPEAK and down to IPEAK/2. It can

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

Figure 2: Buck-Boost Power Switches

3330 F02

SWA

SWB

M1

BAT

M4

M3

M2