7.4.2 Forced Continuous-Conduction Mode

If the MSEL resistor value used selects FCCM, the controller operates in continuous-conduction mode (CCM)

during light-load conditions. Multifunction Select (MSEL) Pin describes the selection in detail. During FCCM, the

switching frequency is maintained to an almost constant level over the entire load range, which is suitable for

applications requiring tight control of the switching frequency and output ripple at the cost of reduced light-load

efficiency. Use Equation 5 to calculate the typical light-load operation boundary. Below this calculated load

current, the device operates in FCCM.

7.4.3 Powering The Device From a Single Bus

The device works well when powered by a single VIN configuration. In a single VIN configuration, the internal

LDO is typically powered by a 5-V or 12-V Bus and generates a 3.0-V output to bias the internal analog circuitry

and power MOSFET gate drivers. The VIN input range under this configuration is 4 V to 16 V for up to 25-A load

current. Figure 7-1 shows an example for this single VIN configuration.

VIN and EN are the two signals to enable the part. For start-up sequence, any sequence between the VIN and EN

signals can power the device up correctly.

CIN

MSEL

VOUT

VIN: 4 V

16 V

SS

AGND

PG

FB

BOOT

SW

GOSNS

VCC

EN

VIN

ILIM

VOSNS+

VOSNS-

CBOOT

COUT

RFB_T

RFB_B

CFF, Optional

CSS

RMSEL

RILIM

RPG

CVCC

PGOOD

EN

LOUT

PGND

Figure 7-1. Single VIN Configuration for a 12-V Bus

7.4.4 Powering The Device From a Split-rail Configuration

When an external bias, which is at a different level from main VIN bus, is applied onto the VCC pin the device

can be configured to split-rail by using both the main VIN bus and VCC bias. Connecting a valid VCC bias to

VCC pin overrides the internal LDO, thus saves power loss on the internal LDO. This configuration helps to

improve overall system level efficiency but requires a valid VCC bias. A 3.3-V or 5.0-V rail is the common choice

as VCC bias. With a stable VCC bias, the recommended VIN input range under this configuration remains the

same, from 4.0 V to 16 V.

The noise of the external bias affects the internal analog circuitry. To ensure a proper operation, a clean,

low-noise external bias and good local decoupling capacitor from VCC pin to PGND pin are required. Figure 7-2

shows an example for this split rail configuration.

The VCC external bias current during nominal operation varies with the bias voltage level and also the operating

frequency. For example, by setting the device to skip-mode, the VCC pin draws less current from the external

bias when the frequency decreases under a light load condition. The typical VCC external bias current under

FCCM operation is listed in Electrical Characteristics. The external bias must be capable of supplying this

current or the external bias voltage may drop and the internal LDO can no longer be overridden by it.

TPS54KB20

SLVSGP3 – MAY 2023

www.ti.com

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