NCP3133A

http://onsemi.com

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DETAILED DESCRIPTION

Overview

NCP3133A is a low input voltage 3 A high performance

synchronous

buck

converter

with

two

integrated

N−MOSFETs. NCP3133A’s output voltage range is from

0.6 V to 0.84 x Vin and it has wide input voltage range from

2.9 V to 5.5 V. The features of NCP3133A include

supporting pre−bias start−up to protect sensitive loads,

cycle−by−cycle over−current limiting and short circuit

protection, power good monitor, over voltage and under

voltage protection, built in output discharge and thermal

shutdown.

NCP3133A provides two operation modes to fit various

application requirements. The automatic CCM/DCM mode

operation provides reduced power loss and increases the

efficiency at light load. The adaptive power control

architecture enables smooth transition between light load

and heavy load while maintaining fast response to load

transients

.

Operation Mode

NCP3133A offers two operation modes programmed by

PS pin connections, see table below.

Table 5.

PS pin Connection

Operation Mode

Auto Skip at Light Load

(

≤)174 kW to GND

Automatic CCM/DCM

Yes

Floating or pulled to VDD

FCCM

In forced continuous conduction mode (FCCM), the

high−side FET is ON during the on−time and the low−side

FET is ON during the off−time. The switching is

synchronized to an internal clock thus the switching

frequency is fixed.

In Automatic CCM/DCM mode, the high−side FET is ON

during the on−time and low−side FET is ON during the

off−time until the inductor current reaches zero. An internal

zero−crossing comparator detects the zero crossing of the

inductor current from positive to negative. When the

inductor current reaches zero, the comparator sends a signal

to the logic circuitry and turns off the low−side FET.

When the load is increased, the inductor current is always

positive and the zero−crossing comparator does not send any

zero−crossing signal. The converter enters into continuous

conduction mode (CCM) when no zero−crossing is detected

for two consecutive PWM pulses. In CCM mode, the

switching synchronizes to the internal clock and the

switching frequency is fixed.

Reference Voltage

The NCP3133A incorporates 600 mV reference voltage

with 1.0 % tolerance.

Internal Soft−Start

To limit the start−up inrush current, an internal soft start

circuit is used to ramp up the reference voltage from 0 V to

its final value linearly. The internal soft start time is 0.4 ms

typically.

Soft Stop

Soft−Stop or discharge mode is always on during faults or

disable. In this mode, a fault (UVP, OCP) or disable (EN)

causes the output to be discharged through an internal 2

0 W

transistor inside of SW terminal. The time constant of

soft−stop is a function of output capacitance and the

resistance of the discharge transistor.

Automatic Power Saving Mode

In Automatic CCM/DCM mode when the load current

decreases, the converter will enter power saving mode

operation. During power saving mode, the low−side

MOSFET will turn off when the inductor current reaches

zero. So the converter skips switching and operates with

reduced frequency

, which minimizes the quiescent current

and maintains high efficiency.

Forced Continuous Conduction Mode

When PS pin is floating or pulled high, NCP3133A is

operating in forced continuous conduction mode in both

light load and heavy load conditions. In this mode, the

switching frequency remains constant over the entire load

range, making it suitable for applications that need tight

regulation of switching frequency at a cost of lower

efficiency at light load.