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參數(shù)資料
| 型號： | SC4911 |
| 廠商： | Semtech Corporation |
| 英文描述： | Bias/Primary Side PWM Controller with Ultra Low Start-up Current |
| 中文描述： | 偏置/小學(xué)側(cè)PWM控制器與超低啟動電流 |
| 文件頁數(shù)： | 6/14頁 |
| 文件大?。?/td> | 228K |
| 代理商： | SC4911 |
6
2006 Semtech Corp.
www.semtech.com
SC4911
POWER MANAGEMENT
Applications Information
The SC4911 is a voltage mode controller designed mainly
for the isolated or non-isolated flyback converters. As
shown in the Block Diagram, it includes start-up circuit,
bandgap, error amplifier, PWM comparator, logic and
output stage. Line undervoltage has dual functions of
line voltage lockout and shutdown. When LUVLO is pull
down to ground, the SC4911 is disabled and the quies-
cent current is reduced to typical 30μA. When it is con-
nected to a resistor voltage divider between input volt-
age and ground, it monitors the line voltage.
Oscillator
The switching frequency of the SC4911 is set by an ex-
ternal resistor connected between RT and GND. Figure
1
shows the relationship between the resistance of the
external resistor and the switching frequency.
Figure 1. Timing Resistor vs Oscillator Frequency
Timing Resistor vs Switching Frequency
0
50
100
150
200
250
100
200
300
400
500
Frequency ( KHz )
R
Line UVLO and VCC UVLO
The Line UVLO monitors the input line voltage and VCC
UVLO monitors the supply voltage to the SC4911. If ei-
ther input line or VCC is below the lockout threshold, the
output is held low and the supply current to the chip is
typically 30μA.
A resistor divider from the input line to GND determines
the desired undervoltage lockout level. To prevent noise
coupling to affect proper UVLO operation, a small capaci-
tor, one hundred to a few hundred pF, is recommended
to be used from Line UVLO pin to GND.
Start-up
As shown in Typical Applications Circuit on the front page,
when input voltage is applied, C1 is trickle charged
through the start-up resistor R1 until the SC4911 Vcc
reaches the turn-on voltage.
The bias current consumed by the device during this pe-
riod is only typical 30μA. Once Vcc exceeds the turn-on
voltage, OUT starts driving the MOSFET, transferring en-
ergy to the secondary and the bias output. If the bias
output voltage builds to and is above the Vcc turnoff
voltage, the start-up is completed and normal operation
begins.
The size of the start-up resistor not only affects power
supply start-up time, but also power supply efficiency.
The resistor dissipates continuous power in normal op-
eration. Due to ultra low start-up current of the SC4911,
large value resistor (several hundred Kohm to 1Mohm)
could be used, particularly for off-line applications to
improve efficiency considering reasonable start-up time.
Current Limit
The current sense resistor is selected by dividing the cur-
rent sense threshold voltage 0.525V by the primary peak
current at the desired current limit point, typically 120%
of the primary peak current.
PK
I
SENSE
2
V
525
×
.
R
=
Care must be taken to ensure proper selection of sense
resistor and good layout to prevent erratic operation. The
non-inductive resistor must be used as the sense resis-
tor. Parasitic inductance in series with the sense resistor
must be minimized. Additional RC filter may be neces-
sary to eliminate the narrow spike time on the leading
edge of the primary current. The RC time constant can
be chosen to be equal to the time constant of the sense
resistor and the parasitic inductance.
PCB Layout
Long power supply and ground traces should be avoided.
A 0.1
μ
F ceramic capacitor closely placed between Vcc
and GND is recommended. The timing resistor used to
program switching frequency should be located close to
pin RT and pin GND in order to have a stable switching
frequency. MOSFET should be located near the device
and a resistor in the range of several ohms could be
used in series with the gate drive to damp the ringing if
the trace between the drive output and MOSFET is not
short enough. Star ground connection is recommended
to avoid ground loops. The Power path for input filter,
MOSFET and transformer should be separated from
signal path for timing resistor and feedback resistor,
and both should be brought to a single ground point.
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