LT3755 datasheet(16/32 Pages) AD

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LT8393

Rev. A

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OPERATION

the cycle. Switches C and D will alternate, behaving like a

typical synchronous boost regulator.

Figure 7. Peak-Boost in Boost Region (VIN << VOUT)

100% ON

100% OFF

A+D

A+C

A+D

A+C

8393 F07

Main Control Loop

The LT8393 is a fixed frequency current mode control-

ler. The inductor current is sensed through the inductor

sense resistor between the LSP and LSN pins. The current

sense voltage is gained up by amplifier A1 and added to

a slope compensation ramp signal from the internal os-

cillator. The summing signal is then fed into the positive

terminals of the buck current comparator A3 and boost

current comparator A4. The negative terminals of A3 and

A4 are controlled by the voltage on the VC pin, which is

the diode-OR of error amplifiers EA1 and EA2.

Depending on the state of the peak-buck peak-boost cur-

rent mode control, either the buck logic or the boost logic

is controlling the four power switches so that either the

FB voltage is regulated to 1V or the current sense voltage

between the ISP and ISN pins is regulated by the CTRL

pin during normal operation. The gains of EA1 and EA2

have been balanced to ensure smooth transition between

constant-voltage and constant-current operation with the

same compensation network.

Light Load Current Operation

At light load, the LT8393 typically still runs at its full

switching frequency in either continuous conduction

modeordiscontinuousconductionmodebecauseboththe

buck and boost reverse current sense thresholds are set

to –4mV. The negative reverse current sense thresholds

allow a small amount of energy flowing from the output

to the input on every cycle, thereby preventing the pulse-

skip frequency from going below 100Hz, which causes

the LED string to flicker.

In the buck region, switch B is turned off whenever the

buck reverse current threshold is triggered during the

“B+D” phase. In the boost region, switch D is turned off

whenever the boost reverse current threshold is triggered

duringthe“A+D”phase.Inthebuck-boostregion,switchD

is turned off whenever the boost reverse current threshold

is triggered during the “A+D” phase, and both switches B

and D are turned off whenever the buck reverse current

threshold is triggered during the “B+D” phase.

However, when a smaller value inductor is used and the

inductor current ripple is bigger, the LT8393 may run in

pulse-skip mode, where the switches are held off for mul-

tiple cycles (i.e., skipping pulses) to maintain regulation.

Internal Charge Path

Each of the two top MOSFET drivers is biased from its

floating bootstrap capacitor, which is normally recharged

byINTVCCthroughboththeexternalandinternalbootstrap

diodes when the top MOSFET is turned off. When the

LT8393 operates exclusively in the buck or boost regions,

oneofthetopMOSFETsisconstantlyon.Aninternalcharge

path, from VOUT and BST2 to BST1 or from VIN and BST1

to BST2, charges the bootstrap capacitor to 4.6V so that

the top MOSFET can be kept on.

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