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NCP1212P

Current Mode PWM Controller for Both Forward and Flyback Converters

ON Semiconductor 

NCP1212

DETAILED OPERATING DESCRIPTIONS

INTRODUCTION

The NCP1212 implements a standard current mode

architecture where the switch−off time is dictated by the

peak current setpoint. This device represents an ideal

candidate where low external part−count is the key system

requirement. Additionally, the device provides extensive

value−added functions, Soft−Start, Brownout Detect, etc.,

that can be applied to low−cost AC−DC adaptor

applications. The NCP1212 incorporates all the necessary

functions normally needed in UC384X based power supply

systems: Oscillator section, PWM Latch section, Current

Sense section, Brownout Detect protection, Soft−Start and

Maximum Duty Cycle Selection. With all those functions,

this device becomes a good alternative to UC384X that can

help to improve both performance and system cost. Also, the

innovative Maximum Duty Cycle Selection feature allows

the device applied to both forward and fly−back mode

configurations. Detailed functions of individual internal

blocks are described in below and a simplified functional

block diagram is shown in Figure 2.

Oscillator Section

The oscillator frequency is programmed by the capacitor

connected to CT pin. The capacitor is charged by a constant

current source to 3.8 V and 2.5 V for 82% and 48%

maximum Duty Cycle condition respectively. Once the

selected voltage is reached, CT is then discharged by another

constant current source down to 1.0 V and this charging and

discharging action will carry on perpetually. Desirable

switching frequency can be selected by choosing proper

value of timing capacitor, CT. The CT pin waveform is

shown in Figure 32.

82%

48%

48%

3.8 V

2.5 V

1.0 V

Figure 32. CT Pin Waveform for Oscillator

PWM Latch Section

NCP1212 works in current mode. The power switch

current is converted to a positive voltage by inserting a

sensing resistor Rsense between the power switch source and

the ground. The power switch peak current is compared with

the level shifted control input voltage on a cycle−by−cycle

basis. Figure 27 illustrated the internal blocks of the

function. The PWM latch is initialized by the Oscillator set

signal and is terminated by the current sense comparator

when the current exceeds the value dictated by the control

input or current limit level. The current sense Comparator

Latch configuration used ensures that only a single pulse

appears at the output during any given oscillator cycle.

+5 V

9K

1

4

3R

2

FB

R

3

− PWM Control

Output

+

CS

Rsense

300 ns

LEB

Current

− Limit

Output

+

+ 1V

−

Oscillator

Reset

Set

Totem Pole Driver

RQ

F/F

SQ

DRV

Figure 33. PWM Latch Function

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