LT8350RVPBF datasheet(13/24 Pages) AD

13 / 24 page

LT8350

Rev. 0

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OPERATION

switches A and C are turned on and the inductor current

ramps up. When the inductor current hits the peak boost

current threshold commanded by VC voltage at boost

current comparator A2 during (A+C) phase, switch C is

turned off and switch D is turned on for the rest of the

cycle. After 80% cycle, switch A is turned off and switch

B is turned on for the rest of the cycle.

(4) Peak-Boost in Boost Region (VIN << VOUT)

When VIN is much lower than VOUT, the LT8350 uses peak-

boost current mode control in boost region (Figure 7).

Switch A is always on and switch B is always off. At the

beginning of every cycle, switch C is turned on and the

inductor current ramps up. When the inductor current hits

the peak boost current threshold commanded by VC volt-

age at boost current comparator A2 during (A+C) phase,

switch C is turned off and switch D is turned on for the

rest of the cycle. Switches C and D will alternate, behaving

like a typical synchronous boost regulator.

100% ON

100% OFF

A + D

A + C

A + D

A + C

8350 F06

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

Main Control Loop

The LT8350 is a fixed frequency current mode DC/DC

converter. The inductor current is directly sensed across

the internal switch A. The current sense voltage is added

to a slope compensation ramp signal from the internal

oscillator. The summing signal is then fed into the positive

terminals of the buck current comparator A1 and boost

current comparator A2. The negative terminals of A1 and

A2 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 LT8350 can be configurated to operate

in forced continuous conduction mode or discontinuous

conduction mode.

In force continuous conduction mode, the LT8350 runs

at their full switching frequency.

In discontinuous conduction mode, both buck and boost

reverse current sense thresholds are set to be positive,

thus preventing any reverse current flowing from the out-

put to the input. In the buck region, switch B is turned

off whenever the buck reverse current threshold is trig-

gered during (B+D) phase. In the boost region, switch D

is turned off whenever the boost reverse current thresh-

old is triggered during (A+D) phase. In the buck-boost

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