NCV8871

http://onsemi.com

10

and will shut down when VIN drops below the UVLO

threshold or the part is disabled.

Internal Soft−Start

To insure moderate inrush current and reduce output

overshoot, the NCV8871 features a soft start which charges a

capacitor with a fixed current to ramp up the reference voltage.

This fixed current is based on the switching frequency, so

that if the NCV8871 is synchronized to twice the default

switching frequency the soft start will last half as long.

VDRV

An internal regulator provides the drive voltage for the

gate driver. Bypass with a ceramic capacitor to ground to

ensure fast turn on times. The capacitor should be between

0.1

mF and 1 mF, depending on switching speed and charge

requirements of the external MOSFET.

APPLICATION INFORMATION

Design Methodology

This section details an overview of the component selection

process for the NCV8871 in continuous conduction mode

boost. It is intended to assist with the design process but does

not remove all engineering design work. Many of the

equations make heavy use of the small ripple approximation.

This process entails the following steps:

1. Define Operational Parameters

2. Select Current Sense Resistor

3. Select Output Inductor

4. Select Output Capacitors

5. Select Input Capacitors

6. Select Feedback Resistors

7. Select Compensator Components

8. Select MOSFET(s)

9. Select Diode

1. Define Operational Parameters

Before beginning the design, define the operating

parameters of the application. These include:

From this the ideal minimum and maximum duty cycles

can be calculated as follows:

Both duty cycles will actually be higher due to power loss

in the conversion. The exact duty cycles will depend on

conduction and switching losses. If the maximum input

voltage is higher than the output voltage, the minimum duty

cycle will be negative. This is because a boost converter

cannot have an output lower than the input. In situations

where the input is higher than the output, the output will

follow the input, minus the diode drop of the output diode

and the converter will not attempt to switch.

the conversion will not be possible. It is important for a boost

conversion ration of a boost converter goes up to infinity as

D approaches 1, a real converter’s conversion ratio starts to

decrease as losses overtake the increased power transfer. If

the converter is in this range it will not be able to regulate

properly.

If the following equation is not satisfied, the device will

2. Select Current Sense Resistor

Current sensing for peak current mode control and current

limit relies on the MOSFET current signal, which is

measured with a ground referenced amplifier. The easiest

method of generating this signal is to use a current sense

resistor from the source of the MOSFET to device ground.

The sense resistor should be selected as follows:

3. Select Output Inductor

The output inductor controls the current ripple that occurs

over a switching period. A high current ripple will result in

excessive power loss and ripple current requirements. A low

current ripple will result in a poor control signal and a slow

current slew rate in case of load steps. A good starting point

for peak to peak ripple is around 10% of the inductor current

choosing a peak current ripple value, calculate the inductor

value as follows:

L +