NCV8870

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8

Current Limit

The NCV8870 features two current limit protections,

peak current mode and over current latch off. When the

current sense amplifier detects a voltage above the peak

current limit between ISNS and GND after the current limit

leading edge blanking time, the peak current limit causes the

power switch to turn off for the remainder of the cycle. Set

the current limit with a resistor from ISNS to GND, with

If the voltage across the current sense resistor exceeds the

over current threshold voltage the device enters over current

hiccup mode. The device will remain off for the hiccup time

and then go through the soft−start procedure.

Short Circuit Protection

If the short circuit enable bit is set (SCE = Y) the device

will attempt to protect the power MOSFET from damage.

When the output voltage falls below the short circuit trip

voltage, after the initial short circuit blanking time, the

device enters short circuit latch off. The device will remain

off for the hiccup time and then go through the soft-start.

EN/SYNC

The Enable/Synchronization pin has three modes. When

a dc logic high (CMOS/TTL compatible) voltage is applied

to this pin the NCV8870 operates at the programmed

frequency. When a dc logic low voltage is applied to this pin

the NCV8870 enters a low quiescent current sleep mode.

switching frequency is applied to this pin, the switcher

operates at the same frequency as the square wave. If the

signal is slower than this, it will be interpreted as enabling

and disabling the part. The falling edge of the square wave

corresponds to the start of the switching cycle. If device is

disabled, it must be disabled for 7 clock cycles before being

re-enabled.

EN/SYNC pin is at logic−high, the IC may not power up

when VIN returns back above the UVLO. To resume a

normal operating state, the EN/SYNC pin must be cycled

with a single logic−low to logic−high transition.

UVLO

Input Undervoltage Lockout (UVLO) is provided to

ensure that unexpected behavior does not occur when VIN

is too low to support the internal rails and power the

controller. The IC will start up when enabled and VIN

surpasses the UVLO threshold plus the UVLO hysteresis

and will shut down when VIN drops below the UVLO

threshold or the part is disabled.

To avoid any lock state under UVLO conditions, the

EN/SYNC pin should be in logic−low state. For further

details, please refer to EN/SYNC paragraph.

Internal Soft-Start

To insure moderate inrush current and reduce output

overshoot, the NCV8870 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 NCV8870 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.

GDRV

W (typical) GDRV−GND resistor is

strongly recommended.

APPLICATION INFORMATION

Design Methodology

This section details an overview of the component selection

process for the NCV8870 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

10. Determine Feedback Loop Compensation Network

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:

D

V

IN(max)

V

OUT

V

IN(min)

V

OUT