AN-6846

APPLICATION NOTE

© 2008 Fairchild Semiconductor Corporation

www.fairchildsemi.com

Rev. 1.3.2 • 9/26/08

3

Description

The SG6846 is a highly integrated PWM controller IC that

provides special features satisfying the needs for low

standby power consumption. It also incorporates multiple

protection functions. With low start-up current and low

operating current, high-efficiency power conversion is

achieved. The typical start-up current is only 8µA and the

operating current is around 3.7mA. In nominal loading

conditions, the SG6846 operates at a fixed PWM frequency.

To minimize standby power consumption, a proprietary

green-mode function provides off-time modulation to

continuously decrease the switching frequency under light-

load conditions. Under no-load conditions, the power supply

enters burst-mode. Green mode dramatically cuts the power

loss at no-load and light-load conditions, enabling the power

supply to meet stringent power conservation requirements.

The SG6846 is specially designed for SMPS with surge-

current output, incorporated with a two-level OCP function.

Besides the cycle-by-cycle current limiting; if the switching

current is higher than OCP threshold for a delay time, over-

current protection is activated such that the SG6846 is

totally shutdown. Other protection functions include AC

voltage protection. For over-temperature protection, an

external NTC thermistor can be applied to sense the ambient

an internal latch circuit is used to latch-off the controller. If

the latch circuit is triggered by over-temperature conditions,

it resets when the temperature cools off sufficiently

(SG6846CX) or when the AC supply is disconnected

voltage conditions, the latch resets only when the AC supply

is disconnected.

Other

features

of

this

controller

include

built-in

synchronized slope compensation and proprietary internal

compensation for constant output power limit over universal

AC input range. The gate output is clamped at 18V to

protect the external MOSFET from over-voltage damage.

Startup Circuitry

When the power is turned on, the input voltage charges the

hold-up capacitor C1 via the startup resistors. As shown in

Figure 2, one or two resistors from the AC utility are both

power supply. The IC then operates based on the auxiliary

winding energy.

Using one startup resistor (510K

Ω) with a 10µF/50V hold-

up capacitor C1 can satisfy the three-second maximum

power-on delay requirement in most applications. After

startup, the energy is supplied from C2. The capacitance at

to below the off threshold voltage must be longer than the

OCP delay time or it fails to trigger the internal latch circuit.

A 100µF/50V capacitor is suggested for C2.

Figure 6. Circuit Providing Power

IN

AC

RIN

R

V

P

×

≅

2

2

max

,

max

,

(1)

mW

68

10

510

2

264

3

2

max

,

≅

×

×

=

(2)

In addition to the low startup current, SG6846 consumes less

operating current than a traditional UC384x.

Oscillator and Green-Mode Operation

A 26

Ω resistor R

()

()

Ω

=

k

R

kHz

f

I

PWM

1690

(3)

A PWM frequency range of between 47kHz ~ 109kHz is

recommended.

RI

GND

SG6846

Figure 5. Setting PWM Frequency