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參數(shù)資料
| 型號： | SG3842GAD |
| 廠商： | Electronic Theatre Controls, Inc. |
| 英文描述： | BiCMOS Green-Mode PWM Controllers |
| 中文描述： | BiCMOS工藝綠色模式PWM控制器 |
| 文件頁數(shù)： | 13/19頁 |
| 文件大?。?/td> | 394K |
| 代理商： | SG3842GAD |
Product Specification
BiCMOS Green-Mode PWM Controllers
SG3842G/ SG3843G
System General Corp.
Version 1.4 ( IRO33.0007.B3)
- 13 -
www.sg.com.tw
Jun.28,2004
the IC starts-up and begins normal operation, one of the
transformer’s auxiliary windings generates the supply
voltage Vcc, which supplies the operating current of the
SG384xG controller.
Slew Rate Controlled Output Driver
The BiCMOS output stage directly drives the external
power MOSFET up to the full supply voltage. The output
driver, with a low ON-resistance and high current-driving
capability, can easily drive an external capacitive load
larger than 1000pF. If operating under recommended
conditions, the switching frequency can go up to 500kHz.
The output stage is designed to ensure zero cross-
conduction current. This minimizes heat dissipation,
increases efficiency, and enhances reliability. The output
driver is also slew-rate controlled to minimize EMI.
Green-Mode: Linearly Decreasing
Frequency and Burst-Mode
System General’s patented Green-mode function reduces
the switching frequency under light-load and zero-load
conditions. Modulation of the PWM frequency can reduce
power consumption under light-load and zero-load
conditions, because the power loss is directly proportional
to the switching frequency.
Most of the power loss in a power supply occurs due to the
switching loss of the transistor, the core loss of the
transformer and inductors, and the power loss of the
snubber. These sources of power loss all lose power in
proportion to the switching frequency.
The controller uses the output of the error amplifier as a
feedback voltage to calculate load conditions. When the
feedback voltage goes below the Green-mode threshold
voltage, the switching frequency will be reduced. Under
normal-load and high-load conditions, the PWM operates
as usual, and the frequency modulation feature does not
affect its operation.
There are two factors that determine the PWM frequency:
1. The resistor R
T
and the capacitor C
T
determine the
RC charge and discharge times, and therefore, the
circuit frequency. They are both connected to the pin
RT/CT.
2. Internal comparator threshold voltages. Under
normal-load conditions, the internal comparator
threshold voltages are fixed at 1.3V (V
comp,L
) and
2.9V (V
comp,H
). Under light-load conditions, the
V
comp,H
internal threshold voltage gradually increases.
This will increase the RC charging/discharging time,
therefore decreasing the frequency. Under ultra-light
or zero-load conditions, the V
comp,H
voltage is
increased to 4.6V. This will put the circuit into the
lowest frequency it can operate at. Assuming R
T
=
10k and C
T
= 3.3nF, this is about 12kHz. The
frequency vs. COMP voltage (feedback from the
output load) is shown in Fig.1.
Fig.1 Oscillator Frequency vs. COMP Voltage
If 12kHz is not low enough to meet stand-by power
conservation requirements, a shunt resistor R
C
can be
connected in parallel with the capacitor C
T
between
RT/CT and GND. This will allow the SG384xG to enter
into burst-mode.
For example, assuming R
T
= 10k
and R
C
= 47k
, the
peak RT/CT voltage would only be:
(
)
V
V
12
.
47
10
47
+
5
=
×
Oscillator Frequency (Fosc) vs COMP Voltage
0
10
20
30
40
50
60
1
1.5
2
2.5
3
COMP VOLTAGE (V)
F
Temperature=25
℃
VCC=15V
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