LM5145 datasheet(32/69 Pages) TI

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9.4 Device Functional Modes

9.4.1 Standby Modes

The LM25143 operates with peak current-mode control such that the compensation voltage is proportional to the

peak inductor current. During no-load or light-load conditions, the output capacitor discharges very slowly. As

a result, the compensation voltage does not demand driver output pulses on a cycle-by-cycle basis. When the

LM25143 controller detects 16 missed switching cycles, it enters standby mode and switches to a low IQ state

to reduce the current drawn from the input. For the LM25143 to go into standby mode, the controller must be

programmed for diode emulation (VDEMB < 0.4 V).

There are two standby modes: ultra-low IQ and normal mode. To enter ultra-low IQ mode, connect MODE to

AGND through a 10-kΩ resistor. In ultra-low IQ mode, the transconductance amplifier gain is reduced from 1200

µS to 60 µS. The typical ultra-low IQ is 15 μA with channel 1 set to 3.3 V and the channel 2 disabled. If ultra-low

IQ is not required, connect MODE to AGND. In normal mode, the IQ is 25 μA with channel 1 set to 3.3 V and the

second channel disabled.

9.4.2 Diode Emulation Mode

A fully synchronous buck regulator implemented with a low-side synchronous MOSFET rather than a diode

has the capability to sink negative current from the output during light-load, overvoltage, and prebias start-up

conditions. The LM25143 provides a diode emulation feature that can be enabled to prevent reverse (drain-

to-source) current flow in the low-side MOSFET. When configured for diode emulation (DEM), the low-side

MOSFET is switched off when reverse current flow is detected by sensing of the applicable SW voltage using

a zero-cross comparator. The benefit of this configuration is lower power loss at light-load conditions; the

disadvantage being slower light-load transient response.

The diode emulation feature is configured with the DEMB pin. To enable diode emulation and thus achieve

discontinuous conduction mode (DCM) operation at light loads, connect DEMB to AGND. If FPWM or continuous

conduction mode (CCM) operation is desired, tie DEMB to VDDA. See Table 9-4. Note that diode emulation is

automatically engaged to prevent reverse current flow during a prebias start-up in FPWM. A gradual change

from DCM to CCM operation provides monotonic start-up performance.

Table 9-4. DEMB Settings

DEMB

FPWM/DEM

VDDA

FPWM

AGND

DEM

External clock

FPWM

9.4.3 Thermal Shutdown

The LM25143 includes an internal junction temperature monitor. If the temperature exceeds 175°C (typical),

thermal shutdown occurs. When entering thermal shutdown, the device:

1. Turns off the high-side and low-side MOSFETs

2. Pulls SS1/2 and PG1/2 low

3. Turns off the VCC regulator

4. Initiates a soft-start sequence when the die temperature decreases by the thermal shutdown hysteresis of

15°C (typical)

This is a non-latching protection, and as such, the device cycles into and out of thermal shutdown if the fault

persists.

LM25143

SNVSC10 – MARCH 2022

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