LM5145 datasheet(25/55 Pages) TI

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8.3.11 OCP Duty Cycle Limiter

CLK

COMP

300 mV

PWML

Short

Applied

RAMP

CLAMP

ILIM Threshold

ValleyPWM

PWML terminated by

VRAMP > VCOMP

PWML terminated by

VRAMP > VCLAMP

Many

cycles

Inductor Current

Figure 8-11. OCP Duty Cycle Limiting Waveforms

In addition to valley current limiting, the LM5146 uses a proprietary duty-cycle limiter circuit to reduce the PWM

on-time during an overcurrent condition. As shown in Figure 8-10, an auxiliary PWM comparator along with a

modulated CLAMP voltage limits how quickly the on-time increases in response to a large step in the COMP

voltage that typically occurs with a voltage-mode control loop architecture.

As depicted in Figure 8-11, the CLAMP voltage, VCLAMP, is normally regulated above the COMP voltage to

provide adequate headroom during a response to a load-on transient. If the COMP voltage rises quickly during

an overloaded or shorted output condition, the on-time pulse terminates, thereby limiting the on-time and peak

inductor current. Moreover, the CLAMP voltage is reduced if additional valley current limit events occur, further

reducing the average output current. If the overcurrent condition exists for 128 continuous clock cycles, a hiccup

event is triggered and SS is pulled low for 8192 clock cycles before a soft-start sequence is initiated.

8.4 Device Functional Modes

8.4.1 Shutdown Mode

The EN/UVLO pin provides ON / OFF control for the LM5146. When the EN/UVLO voltage is below 0.37 V

(typical), the device is in shutdown mode. Both the internal bias supply LDO and the switching regulator are off.

The quiescent current in shutdown mode drops to 13.5 μA (typical) at VIN = 48 V. The LM5146 also includes

undervoltage protection of the internal bias LDO. If the internal bias supply voltage is below its UVLO threshold

level, the switching regulator remains off.

8.4.2 Standby Mode

The internal bias supply LDO has a lower enable threshold than the switching regulator. When the EN/UVLO

voltage exceeds 0.42 V (typical) and is below the precision enable threshold (1.2 V typically), the internal LDO is

on and regulating. Switching action and output voltage regulation are disabled in standby mode.

8.4.3 Active Mode

The LM5146 is in active mode when the VCC voltage is above its rising UVLO threshold of 5 V and the

EN/UVLO voltage is above the precision EN threshold of 1.2 V. The simplest way to enable the LM5146 is to

tie EN/UVLO to VIN. This allows self start-up of the LM5146 when the input voltage exceeds the VCC threshold

plus the LDO dropout voltage from VIN to VCC.

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LM5146

SNVSBV0A – JUNE 2021 – REVISED JUNE 2021

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