Output Switching Frequency vs RT

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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

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

Ω) for improved efficiency with

reasonable start-up time.

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FIGURE 2. Start-Up Circuit Block Diagram

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