AN6846 Ver la hoja de datos (PDF) - Fairchild Semiconductor
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AN6846
Fairchild Semiconductor
AN6846 Datasheet PDF : 8 Pages
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AN-6846
APPLICATION NOTE
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
input brownout protection with hysteresis and VDD over-
voltage protection. For over-temperature protection, an
external NTC thermistor can be applied to sense the ambient
temperature. When OCP, VDD OVP, or OTP are activated,
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
(SG6846LX). When the latch is triggered by VDD over-
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.
RI
RI
SG6846
practical. As the voltage of VDD reaches the startup threshold
voltage VTH-ON, the SG6846 activates and drives the entire
power supply. The IC then operates based on the auxiliary
winding energy.
Using one startup resistor (510KΩ) 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
VDD must be large enough to hold VDD above the off
threshold voltage, VTH-OFF, at a step change of load. When
an output short circuit occurs, the duration before VDD drops
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
The maximum power dissipation of RIN is:
PRIN,max
≅
VA2C,max
2 × RIN
(1)
where VAC,max is the maximum rectified input voltage.
Taking a wide-range input (90VAC-264VAC) as an example,
VAC,max =264V:
PRIN,max
=
2642
2 × 510 ×103
≅ 68mW
(2)
In addition to the low startup current, SG6846 consumes less
operating current than a traditional UC384x.
GND
Figure 5. Setting PWM Frequency
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
Oscillator and Green-Mode Operation
Resistor RI programs the frequency of the internal oscillator.
A 26Ω resistor RI determines PWM frequency as 65kHz:
fPWM
(kHz)
=
1690
RI (kΩ)
(3)
A PWM frequency range of between 47kHz ~ 109kHz is
recommended.
© 2008 Fairchild Semiconductor Corporation
Rev. 1.3.2 • 9/26/08
3
www.fairchildsemi.com
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