UPPER

LOWER

INTERNAL

BIAS

GENERATOR

UVLO

S

R

Q

+

HV

VIN

REGULATOR

VCC

Rstart

UVLO

Enable Driver

TRANSFORMER

BIAS

WINDING

LM5021

SNVS359D – MAY 2005 – REVISED MARCH 2013

www.ti.com

DETAILED OPERATING DESCRIPTION

START UP CIRCUIT

rectified ac input voltage HV is applied. The VIN current consumed by the LM5021 is only 18 µA (nominal) while

VIN UVLO threshold of 20V, the internal VCC linear regulator is enabled. 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 secondary

transformer winding(s). A bias winding, shown in Figure 9, 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 bias winding should always be connected to the

VIN pin as shown in Figure 9. Do not connect the bias winding to the VCC pin. The start-up sequence 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 LM5021 is designed for ultra-low start-up current into the VIN pin. To accomplish this very low start-up

current, the VCC regulator of the LM5021 is unique as compared to the VCC regulator used in other controllers

of the LM5xxx family. The LM5021 is designed specifically for applications with the bias winding connected to the

VIN pin as shown in Figure 9. It is not recommended that the bias winding be connected to the VCC pin of

the LM5021.

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.

Figure 9. Start-Up Circuit Block Diagram

The internal VCC linear regulator is enabled when VIN reaches 20V. The drop in VIN due to charge transfer from

(1)

(2)

8

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