LTC3331_15 Datasheet, PDF(18/34 Page) Linear Technology – Nanopower Buck-Boost DC/DC with Energy Harvesting Battery Charger

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LTC3331_15 Datasheet, PDF (18/34 Pages) Linear Technology – Nanopower Buck-Boost DC/DC with Energy Harvesting Battery Charger

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LTC3331

OPERATION

capacitor through an inductor to a value slightly higher

than the regulation point. It does this by ramping the

inductor current up to IPEAK_BUCK through an internal

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

VIN, VOUT, 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 out-

put voltage with a sleep comparator. During sleep 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 the output

at light loads. The buck delivers a minimum of 100mA

of average load current when it is switching. VOUT 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. Instead, 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, VOUT, 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 BB_IN and

VOUT. Figure 2 shows the four internal switches of the

buck-boost converter. In each mode the inductor current

is ramped up to IPEAK_BB, which is programmable via the

IPK[2:0] bits and ranges from 5mA to 250mA (see Table 3).

SWA

BB_IN

SWB

VOUT

3331 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_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 IPEAK_BB when M3 is on and is ramped down to 0mA

when M4 is on as VOUT is greater than BB_IN in boost

mode. Buck-boost mode is very similar to boost mode in

that M1 is always on and M2 is always off. If BB_IN is less

than VOUT the inductor current is ramped up to IPEAK_BB

through M3. When M4 turns on the current in the inductor

will start to ramp down. However, because BB_IN is close to

VOUT and M1 and M4 have finite on-resistance the current

ramp will exhibit a slow exponential decay, potentially

lowering the average current delivered to VOUT. For this

reason the lower current threshold is set to IPEAK_BB/2 in

buck-boost mode to maintain high average current to the

load. If BB_IN is greater than VOUT in buck-boost mode

the inductor current still ramps up to IPEAK_BB and down

to IPEAK_BB/2. It can still ramp down if BB_IN is greater

than VOUT because the final value of the current in the

inductor would be (VIN â VOUT)/(RON1 + RON4). If BB_IN

is exactly IPEAK_BB/2 â¢ (RON1 + RON4) above VOUT the

inductor current will not reach the IPEAK_BB/2 threshold

and switches M1 and M4 will stay on all the time. For

higher BB_IN voltages the mode comparator will switch

the converter to buck mode. M1 and M4 will remain on for

BB_IN voltages up to VOUT + IPEAK_BB â¢ (RON1 + RON4). At

3331fb

For more information www.linear.com/LTC3331

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