Datasheet

UPPER
LOWER
INTERNAL
BIAS
GENERATOR
V
CC
UVLO
S
R
Q
+
HV
VIN
V
CC
REGULATOR
VCC
Rstart
C
VIN
V
IN
UVLO
C
VCC
Enable Driver
TRANSFORMER
BIAS
WINDING
LM5021
SNVS359D –MAY 2005–REVISED MARCH 2013 www.ti.com
DETAILED OPERATING DESCRIPTION
START UP CIRCUIT
Referring to Figure 9, the input capacitor C
VIN
is trickle charged through the start-up resistor Rstart, when the
rectified ac input voltage HV is applied. The VIN current consumed 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 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 MΩ) for improved efficiency with reasonable start-up time.
Figure 9. Start-Up Circuit Block Diagram
RELATIONSHIP BETWEEN INPUT CAPACITOR C
IN
& V
CC
CAPACITOR C
VCC
The internal VCC linear regulator is enabled when VIN reaches 20V. The drop in VIN due to charge transfer from
C
VIN
to C
VCC
after the regulator is enabled can be calculated from the following equations where VIN' is the
voltage on C
VIN
immediately after the VCC regulator charges C
VCC
.
ΔVIN x C
VIN
= ΔVCC x C
VCC
(1)
(20V – V
IN'
) C
VIN
= 8.5V C
VCC
(2)
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