MCP19035T-AAAAESLASHMF datasheet(17/44 Pages) MICROCHIP

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 2012-2013 Microchip Technology Inc.

DS22326B-page 17

MCP19035

4.11

Soft Start

To control the output voltage during start-up, the

MCP19035 uses a soft-start circuit that allows the

output voltage of the system to monotonically increase.

The soft start circuitry allows the output voltage to rise

up to the desired regulation limit, typically within 8 ms.

The soft start circuit is enabled each time the

MCP19035 starts. This includes initial start-up, start-up

from toggling the SHDN pin, start-up after thermal

shutdown, or start-up after an overcurrent condition.

FIGURE 4-3:

Soft Start-up Diagram.

4.12

Pre-Bias Load Start-up

A special start-up sequence will prevent any current to

be sourced from the output in case of a pre-biased

load. This is accomplished by monitoring the FB pin

and internal reference voltages. If the positive input to

the Error Amplifier (internal reference voltage) is

greater than the feedback voltage (voltage present at

FB pin), the controller will drive the low-side MOSFET

(synchronous rectifier) with a reduced duty cycle. This

sequence ensures a smooth output voltage transition

without sinking any current from the pre-biased

external load.

Note:

Do not use a low impedance source to

back-drive the output voltage during the

pre-bias state. There is no protection

mechanism for positive current flow in the

synchronous

rectifier

MOSFET.

The

converter can reverse the energy flow

(becoming a Boost converter) if the input

voltage is accidentally disconnected.

SHDN

VOUT

Soft Start Time

(TSS = 8 ms)

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