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NCP1271
(Rev.:2006)

Soft−Skip Mode Standby PWM Controller with Adjustable Skip Level and External Latch

ON Semiconductor 

NCP1271

Skip/ latch

1

I skip

Rskip

10V

4.8 V

8V

−

+

13 us filter

latch−off, reset

S

when Vcc < 4V

RQ

Vskip = Rskip * Iskip or

Vskip = 1.2 V when pin 1 is opened

V skip

skip

V FB +

−

4.1 mA when Vcc > 0.6 V

0.2 mA when Vcc < 0.6 V

turn off

8 HV

FB V FB

2

16.7k

2.85 V

TLD

−

+

disable

soft

skip

soft−skip

75.3k

1/3

10V

VFB / 3

0

CS

V CS

3

180 ns

LEB

Vss

Soft start/ soft−skip

(1V max) management

4 ms/ 300 us

soft

−

start

+

130ms

1

delay

V PWM

PWM

−

&

short

circuit

fault

+

R ramp

10V

100uA

S

QR

double

hiccup

B2

Counter

&

12.6/

5.8 V

−

+

UVLO

9.1 V

−

+

20V

V CC

6

R CS

4

Gnd

10

0

jittered ramp

current source

7.5% Jittering

65, 100 kHz

Oscillator

turn on internal bias

OR

V CC

Drv

RQ

S

Max duty

= 80%

5

driver:

+500 mA

/ −800 mA

Figure 2. Functional Block Diagram

PIN FUNCTION DESCRIPTION

Pin No. Symbol

Function

1

Skip/latch Skip Adjust or

Latchoff

2

FB

Feedback

3

CS

Current Sense

4

Gnd

IC Ground

5

Drv

Driver Output

6

VCC

Supply Voltage

8

HV

High Voltage

Description

A resistor to ground provides the adjustable standby skip level. Additionally, if this pin is

pulled higher than 8.0 V (typical), the controller latches off the drive.

An optocoupler collector pulls this pin low during regulation. If this voltage is less than

the Skip pin voltage, then the driver is pulled low and Soft−Skip mode is activated. If this

pin is open (>3 V) for more than 130 ms, then the controller is placed in a fault mode.

This pin senses the primary current for PWM regulation. The maximum primary current

is limited to 1.0 V / RCS where RCS is the current sense resistor. Additionally, a ramp

resistor Rramp between the current sense node and this pin sets the compensation ramp

for improved stability.

−

The NCP1271’s powerful output is capable of driving the gates of large Qg MOSFETs.

This is the positive supply of the device. The operating range is between 10 V (min) and

20 V (max) with a UVLO start threshold 12.6 V (typ).

This pin provides (1) Lossless startup sequence (2) Double hiccup fault mode (3)

Memory for latch−off shutdown and (4) Device protection if VCC is shorted to GND.

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