Data Sheet

LT8350S

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Rev. A

22 of 36

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

When VIN is much lower than VOUT, the LT8350S uses peak-boost current mode control in the boost region (See Figure

34). 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

voltage at boost current comparator A2 during the (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.

Figure 34.Peak-Boost in Boost Region (VIN<<VOUT)

Main Control Loop

The LT8350S 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 current 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 a smooth transition between constant-voltage and constant-current operation with the

same compensation network.

Light Load Current Operation

At light load, the LT8350S can be configured to operate in forced continuous conduction mode or discontinuous

conduction mode. The LT8350S runs at their full switching frequency in force continuous conduction mode.

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 output to the input. 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 during the (A + D) phase. In the buck-boost

region, switch D 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. As the load becomes lower and lower, or when a smaller value inductor is used and the inductor current ripple

is bigger, the LT8350S may run in pulse-skipping mode, where the switches are held off for multiple cycles (i.e.,

skipping pulses) to maintain the regulation.

A + C

A + D

A + C

A + D

100% ON

100% OFF

A

B

C

D

IL