NCP3012 View Datasheet(PDF) - ON Semiconductor

Part Name

Description

Manufacturer

NCP3012

Synchronous PWM Controller

ON Semiconductor 

NCP3012

Startup and Shutdown

Once enable is asserted the device begins its startup

process. Closed−loop soft−start begins after a 400 ms delay

wherein the boost capacitor is charged, and the current limit

threshold is set. During the 400 ms delay the OTA output is

set to just below the valley voltage of the internal ramp. This

is done to reduce delays and to ensure a consistent pre

soft−start condition. The device increases the internal

reference from 0 V to 0.8 V in 32 discrete steps while

maintaining closed loop regulation at each step. Some

overshoot may be evident at the start of each step depending

on the voltage loop phase margin and bandwidth. See

Figure 19. The total soft−start time is 14 ms.

The soft−stop process begins once the EN pin voltage

goes below the input low threshold. Soft−stop decreases the

internal reference from 0.8 V − 0 V in 32 steps as with

Soft−Start. Soft−Stop finishes with one “last” high side gate

pulse at half the period of the prior pulse. This helps ensure

positive inductor current following turn off at light loads,

which prevents negative output voltage.

Enable low during Soft−Start will result in Soft−Stop

down counting from that step. Likewise, Enable high during

Soft−Stop will result in Soft−Start up counting from that

step.

0.8 V Output Voltage

25 mV Steps

32 Voltage Steps

Internal Reference Voltage

Internal Ramp

OTA Output

0 .7V

0V

Figure 19. Soft−Start Details

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