LTM4607 datasheet(14/30 Pages) LINER

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LTC3780

3780ff

For more information www.linear.com/LTC3780

operaTion

on, switches B and D are then turned on. Finally, switches

A and D are turned on for the remainder of the time.

Boost Region (VIN < VOUT)

Switch A is always on and synchronous switch B is always

off in boost mode. Every cycle, switch C is turned on first.

Inductor current is sensed when switch C is turned on.

After the sensed inductor current exceeds the reference

voltage which is proportional to VITH,switchCisturnedoff

and synchronous switch D is turned on for the remainder

SWITCH A

CLOCK

SWITCH B

SWITCH C

SWITCH D

HIGH

3780 F03

Figure 3. Buck Mode (VIN > VOUT)

SWITCH A

CLOCK

SWITCH B

SWITCH C

SWITCH D

3780 F04a

(4a) Buck-Boost Mode (VIN ≥ VOUT)

SWITCH A

CLOCK

SWITCH B

SWITCH C

SWITCH D

3780 F04b

(4b) Buck-Boost Mode (VIN ≤ VOUT)

Figure 4. Buck-Boost Mode

of the cycle. switches C and D will alternate, behaving like

a typical synchronous boost regulator.

ThedutycycleofswitchCdecreasesuntiltheminimumduty

cycle of the converter in boost mode reaches DMIN_BOOST,

given by:

DMIN_BOOST = DBUCK-BOOST

where DBUCK-BOOST is the duty cycle of the buck-boost

switch range:

DBUCK-BOOST = (200ns • f) • 100%

and f is the operating frequency in Hz.

Figure 5 shows typical boost mode waveforms. If VIN ap-

proaches VOUT, the buck-boost region is reached.

SWITCH A

CLOCK

SWITCH B

SWITCH C

SWITCH D

HIGH

3780 F05

Figure 5. Boost Mode (VIN < VOUT)

LOW CURRENT OPERATION

The FCB pin is used to select among three modes for both

buck and boost operations by accepting a logic input.

Figure 6 shows the different modes.

FCB PIN

BUCK MODE

BOOST MODE

0V to 0.75V

Force Continuous Mode

0.85V to 5V

Skip-Cycle Mode

Burst Mode Operation

>5.3V

DCM with Constant Freq

Figure 6. Different Operating Modes

When the FCB pin voltage is lower than 0.8V, the controller

behavesasacontinuous,PWMcurrentmodesynchronous

switching regulator. In boost mode, switch A is always on.

switch C and synchronous switch D are alternately turned

on to maintain the output voltage independent of direction

of inductor current. Every ten cycles, switch A is forced off

for about 300ns to allow boost capacitor CA (Figure 13) to

recharge. In buck mode, synchronous switch D is always

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