NCP3235 datasheet(13/16 Pages) ONSEMI

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NCP3235

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Programmable Soft−Start

An external capacitor connected from the SS pin to

ground sets up the soft start period, which can limit the

start−up inrush current. The soft start period can be

programmed based on the Equation 1.

SS +

SS @ Vref

(eq. 1)

OCP and TSD (thermal shutdown) are the only

protections that are active during a soft−start.

Adaptive Non−Overlap Gate Driver

In a synchronous buck converter, a certain dead time is

required between the low side drive signal and high side

drive signal to avoid shoot through. During the dead time,

the body diode of the low side FET freewheels the current.

The body diode has much higher voltage drop than that of

the MOSFET, which reduces the efficiency significantly.

The longer the body diode conducts, the lower the

efficiency. NCP3235 implements adaptive dead time

control to minimize the dead time, as well as preventing

shoot through.

Precision Enable (EN)

The ENABLE block allows the output to be toggled on

and off and is a precision analog input.

When the EN voltage exceeds V_EN, the controller will

initiate the soft−start sequence as long as the input voltage

and sub−regulated voltage have exceeded their UVLO

thresholds. V_EN_hyst helps to reject noise and allow the

pin to be resistively coupled to the input voltage or

sequenced with other rails.

If the EN voltage is held below typically 0.8 V, the

NCP3235 enters a deep disable state where the internal bias

circuitry is off. As the voltage at EN continues to rise, the

Enable comparator and reference are active and provide a

more accurate EN threshold. The drivers are held off until

the rising voltage at EN crosses V_EN. An internal 2

pullup automatically enables the device when the EN pin is

left floating.

VDD

2uA

1.2 V

Enable

Logic

INPUT SUPPLY / VCC

Figure 28. Enable Functional Block Diagram

Pre−bias Startup

In some applications the controller will be required to start

switching when it’s output capacitors are charged anywhere

from slightly above 0 V to just below the regulation voltage.

This situation occurs for a number of reasons: the

converter’s output capacitors may have residual charge on

them or the converter’s output may be held up by a low

current standby power supply. NCP3235 supports pre−bias

start up by holding off switching until the feedback voltage

rises above the set regulated voltage. If the pre−bias voltage

is higher than the set regulated voltage, switching does not

occur until the output voltage drops back to the regulation

point.

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