lm5021na-2 National Semiconductor Corporation, lm5021na-2 Datasheet - Page 7

• Manufacturer Part Number: lm5021na-2
• Description: Ac-dc Current Mode Pwm Controller
• Manufacturer: National Semiconductor Corporation

Typical Performance Characteristics
Detailed Operating Description
START UP CIRCUIT
Referring to Figure 2, the input capacitor C
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
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
VIN
Output Switching Frequency vs RT
is initially charged to the start-up threshold.
FIGURE 2. Start-Up Circuit Block Diagram
VIN
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is trickle
Unless otherwise specified: T
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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 MΩ) for improved efficiency with
reasonable start-up time.
J
= 25˚C. (Continued)
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