Data Sheet

LT8350S

analog.com

Rev. A

19 of 36

OPERATION

The LT8350S is a current mode DC/DC converter that can regulate output voltage from input voltage above, below,

or equal to the output voltage. Four internal low-resistance N-channel double-diffused Metal-Oxide-Semiconductor

(DMOS) switches minimize the size of the application circuit and reduce power losses to maximize efficiency. Internal

high-side gate drivers further simplify the design process. The ADI proprietary peak current mode control scheme

directly senses the inductor current across the internal power switches. It provides a smooth transition between

buck region, buck-boost region, and boost region. The LT8350S can be configured to operate over a wide range of

switching frequencies, from 200kHz to 2MHz, allowing applications to be optimized for broad area and efficiency.

See the Block Diagram for the best understanding of the operation.

Power Switch Control

Figure 28 shows the topology of the LT8350S power stage, which is comprised of four N-channel DMOS switches

and their associated gate drivers. Figure 29 shows the current mode control as a function of the VIN/VOUT ratio, and

Figure 30 shows the operation region as a function of the VIN/VOUT ratio. The power switches are properly controlled

to transition smoothly between modes and regions. Hysteresis is added to prevent chattering between modes and

regions.

Figure 28. Power Stage Schematic

There are a total of four states:

(1) Peak-Buck current mode control in Buck region.

(2) Peak-Buck current mode control in the Buck-Boost region.

(3) Peak-Boost current mode control in the Buck-Boost region, and

(4) Peak-Boost current mode control in Boost region.

The following sections provide detailed descriptions for each state with waveforms, in which the shoot-through

protection dead time between switches A and B and between switches C and D are ignored for simplification.

L

SW1

SW2

INTVCC

INTVCC

INTVCC

INTVCC

CBST1

CBST2

VIN

VOUT

GND

GND

A

B

D

C