參數(shù)資料
型號(hào)： LC5521D
元件分類： 穩(wěn)壓器
英文描述： 2.5 A POWER FACTOR CONTROLLER, 18 kHz SWITCHING FREQ-MAX, DIP7
封裝： DIP-8/7
文件頁(yè)數(shù)： 14/40頁(yè)
文件大?。?/td> 888K
代理商： LC5521D
21
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
28106.02
Operation Modes at Startup
Figure 24 shows the operation modes during the startup phase
of the LC551xD, and figure 25 shows those for the LC552xD
and LC552xF. Note that OCP pin voltage, which determines the
timing of quasi-resonant operation, is shown in reference to the
S/GND pin.
During two periods below at startup, IC operation is set to PWM,
with fOSC = 14 kHz:
While the COMP pin voltage (for LC551xD) and FB pin volt-
age (for LC552xD and LC552xF), in reference to S/GND, are
0 to 0.9 V (the control voltage lower limit for the COMP pin,
VCOMP(MIN), and FB pin, VFB(MIN)): During this period, on-time
is fixed at the Leading Edge Blanking Time, tBW = 500 ns.
Until the quasi-resonant signal (OCP pin voltage) reaches the
Quasi-Resonant Operation Threshold Voltage-1, VBD(TH1)=
0.24 V: During this period, the output voltage is low; therefore,
the auxiliary winding voltage, VD , is low. Thus the quasi-reso-
nant signal is low.
After those startup operations the output voltage starts increasing,
when the OCP pin voltage reaches VBD(TH1) = 0.24 V, the device
is switched to quasi-resonant operation (figure 26).
Soft-Start Function
The soft-start function reduces power stress on the incorporated
MOSFET and secondary diode during the startup phase.
LC551xD series (non-isolated designs)
The soft-start operation
begins when the COMP pin voltage reaches VCOMP(MIN) = 0.9 V
and lasts until the output current becomes constant. During that
period, the output power gradually increases.
During this period, check the below items:
VCC pin voltage does not drop to the Operation Stop Voltage,
VCC(OFF)
Output current reaches the target value before the overload pro-
tection (OLP) is activated by the COMP pin voltage reaching
VCOMP(OLP)2 = 4.5 V
V
CC Pin
Voltage
COMP Pin
Voltage
Output (LED)
Current, I
OUT
Drain
Current, I
D
S/GND
IC turn on
S/GND
GND(I
OUT)
Duration
PWM
(QR)
VCOMP(MIN)= 0.90 V
VCC(BIAS 1
) = 11.0 V
Soft-Start Period
Target
Current
GND(I
D )
Constant current operation
t
ON= tBW(500 ns)
VCC Pin
Voltage
FB Pin
Voltage
Output (LED)
Current, I
OUT
Drain
Current, I
D
S/GND
IC turn on
S/GND
GND(I
OUT)
Duration
PWM
(QR)
VFB (MIN)= 0.90 V
VCC(BIAS 1
) = 11.0 V
Soft-Start Period
Target
Current
GND(I
D )
Constant current operation
t
ON= tBW(500 ns)
Figure 24. Soft-start operation waveforms at startup (LC551xD)
Figure 25. Soft-start operation waveforms at startup (LC552xD/ LC552xF)
GND(ID)
S/GND
VBD(TH1)
PWM operation Quasi-resonant operation (QR)
OCP Pin
Voltage
Drain
Current, ID
Figure 26. OCP Pin Voltage (with time scale expanded)
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