NCP1595, NCP1595A, NCP1595C

http://onsemi.com

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DETAILED DESCRIPTION

Overview

The NCP1595/A/C is a synchronous PWM controller that

incorporates all the control and protection circuitry

necessary to satisfy a wide range of applications. The

NCP1595/A/C employs current mode control to provide fast

transient response, simple compensation, and excellent

stability. The features of the NCP1595/A/C include a

precision reference, fixed 1 MHz switching frequency, a

transconductance error amplifier, an integrated high−side

P−channel MOSFET and low−side N−Channel MOSFET,

internal soft−start, and very low shutdown current. The

protection features of the NCP1595/A/C include internal

soft−start, pulse−by−pulse current limit, and thermal

shutdown.

Reference Voltage

The NCP1595/A/C incorporates an internal reference that

allows output voltages as low as 0.8 V. The tolerance of the

internal reference is guaranteed over the entire operating

temperature range of the controller. The reference voltage is

trimmed using a test configuration that accounts for error

amplifier offset and bias currents.

Oscillator Frequency

A fixed precision oscillator is provided. The oscillator

frequency range is 1 MHz with

$13% variation.

Transconductance Error Amplifier

The transconductance error amplifier’s primary function

is to regulate the converter’s output voltage using a resistor

divider connected from the converter’s output to the FB pin

of the controller, as shown in the applications Schematic. If

a Fault occurs, the amplifier’s output is immediately pulled

to GND and PWM switching is inhibited.

Internal Soft−Start

To limit the startup 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 1 ms

typically.

Output MOSFETs

P−channel and low−side N−channel MOSFETs capable of

delivering up to 1.5 A of current. When the controller is

disabled or during a Fault condition, the controller’s output

stage is tri−stated by turning OFF both the upper and lower

MOSFETs.

Adaptive Dead Time 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. In NCP1595/A/C, the drivers and MOSFETs are

integrated in a single chip. The parasitic inductance is

minimized. Adaptive dead time control method is used to

prevent the shoot through from happening and minimizing

the diode conduction loss at the same time.

Pulse Width Modulation

A high−speed PWM comparator, capable of pulse widths

as low as 50 ns, is included in the NCP1595/A/C. The

inverting input of the comparator is connected to the output

of the error amplifier. The non−inverting input is connected

to the the current sense signal. At the beginning of each

PWM cycle, the CLK signal sets the PWM flip−flop and the

upper MOSFET is turned ON. When the current sense signal

rises above the error amplifier’s voltage then the comparator

will reset the PWM flip−flop and the upper MOSFET will

be turned OFF.

Current Sense

The NCP1595/A/C monitors the current in the upper

MOSFET. The current signal is required by the PWM

comparator and the pulse−by−pulse current limiter.