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參數(shù)資料
| 型號(hào)： | LM5021 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | AC-DC Current Mode PWM Controller |
| 中文描述： | 交直流電流模式PWM控制器 |
| 文件頁(yè)數(shù)： | 7/15頁(yè) |
| 文件大?。?/td> | 746K |
| 代理商： | LM5021 |
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Typical Performance Characteristics
Unless otherwise specified: T
J
= 25C. (Continued)
Output Switching Frequency vs RT
20144210
Detailed Operating Description
START UP CIRCUIT
Referring to
Figure 2
, the input capacitor C
is trickle
charged through the start-up resistor Rstart, when the recti-
fied ac input voltage HV is applied. The VIN current con-
sumed by the LM5021 is only 18 μA (nominal) while the
capacitor C
VIN
is initially charged to the start-up threshold.
When the input voltage, VIN reaches the upper VIN UVLO
threshold of 20V, the internal VCC linear regulator is en-
abled. The VCC regulator will remain on until VIN falls to the
lower UVLO threshold of 7.25V (12.5V hysteresis). When
the VCC regulator is turned on, the external capacitor at the
VCC pin begins to charge. The PWM controller, soft-start
circuit and gate driver are enabled when the VCC voltage
reaches the VCC UVLO upper threshold of 7V. The VCC
UVLO has 1.2V hysteresis between the upper and lower
thresholds to avoid chattering during transients on the VCC
pin. When the VCC UVLO enables the switching power
supply, energy is transferred from the primary to the second-
ary transformer winding(s). A bias winding, shown in
Figure
2
, delivers power to the VIN pin to sustain the VCC regulator.
The voltage supplied should be from 11V (VCC regulated
voltage maximum plus VCC regulator dropout voltage) to
30V (maximum operating VIN voltage). The start-up se-
quence is completed and normal operation begins when the
voltage from the bias winding is sufficient to maintain VCC
level greater than the VCC UVLO threshold (5.8V typical).
The size of the start-up resistor Rstart not only affects power
supply start-up time, but also power supply efficiency since
the resistor dissipates power in normal operation. The ultra
low start-up current of the LM5021 allows a large value
Rstart resistor (up to 3 M
) for improved efficiency with
reasonable start-up time.
20144211
FIGURE 2. Start-Up Circuit Block Diagram
L
www.national.com
7
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