NCV898031

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8

Internal Soft-Start

To insure moderate inrush current and reduce output

overshoot, the NCV898031 features a soft start which

charges a capacitor with a fixed current to ramp up the

reference voltage.

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.

Oscillator

Slope

Compensation

+

Q

S

R

NCV898031

Voltage Error

VEA

CSA

PWM Comparator

Gate

Drive

Compensation

GDRV

L2

L1

Figure 10. SEPIC Current Mode Schematic

SEPIC Design Methodology

This section details an overview of the component

selection process for the NCV898031 in continuous

conduction mode SEPIC. 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 SEPIC Inductors

4. Select Coupling Capacitor

5. Select Output Capacitors

6. Select Input Capacitors

7. Select Feedback Resistors

8. Select Compensator Components

9. Select MOSFET(s)

10. Select Diode

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.

limit of the NCV898031, the conversion will not be

possible. It is important for a SEPIC converter to have a

a SEPIC 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