Datasheet

DOFFPd I Vdc max 0.1uA 325Vdc 32.5 W= · = · = m
DOFFI 0.1 A= m
Vdc max 230Vac 2 325Vdc= · =
VCC 10V=
Vin 230Vac=
LM5023
www.ti.com SNVS961D –APRIL 2013–REVISED JANUARY 2014
FUNCTIONAL DESCRIPTION
The LM5023 is a Quasi-Resonant controller which contains all of the features needed to implement a highly
efficient off-line power supply. The LM5023 uses the transformer auxiliary winding for demagnetization detection
to ensure Quasi-Resonant operation (Valley-Switching) to minimize switching losses. For application that need to
meet the ENERGY STAR® low standby power requirements, the LM5023 features an extremely low lq current
(346 µA) and skip cycle mode which reduces power consumption at light loads. The LM5023 uses a feedback
signal from the output to provide a very accurate output voltage regulation <1%. To reduce overheating and
stress during a sustained overload conditions the LM5023 offers a hiccup mode for over current protection and
provides a current limit restart timer to disable the outputs and forcing a delayed restart (hiccup mode).
For offline start-up, an external Depletion Mode N Channel MOSFET can be used. This method is recommended
for applications where a very low standby power (<50 mW) is required. For application where a low standby
power is not as critical an enhancement mode, N Channel MOSFET can be used. If an OVP is detected on the
auxiliary winding (QR pin), the IC permanently latches off, requiring recycling of power to restart Additional
features include line-current-feed forward, pulse-by-pulse current limit, and a maximum frequency clamp of 130
kHz.
START-UP
Referring to Figure 8, when the AC rectified line voltage is applied to the bulk energy storage capacitor; the N
Channel Depletion Mode MOSFET is turned on and supplies the charging current to the VCC capacitor. When
the voltage on the VCC pin reaches 12.5 V typical, the PWM controller, soft-start circuit and gate driver are
enabled.
When the LM5023 is enabled and the OUT drive signal starts switching the Flyback MOSFET, energy is being
stored and then transferred from the transformer primary to the secondary windings. A bias winding, shown in
Figure 8, delivers energy to the VCC capacitor to sustain the voltage on the VCC pin. The voltage supplied from
the auxiliary winding should be within the range of 10 V to 14 V (where 16 V is the absolute maximum rating).
After reaching the VCC
ON
threshold the LM5023 VSD open Drain output, which is pulled up to VCC during start-
up, goes low. This applies a negative Gate to Source voltage on the Depletion Mode MOSFET turning it off. This
disables the high voltage start-up circuit. The high voltage start-up circuit can be implemented in either of two
ways; the first is shown in Figure 8, which uses an N Channel Depletion Mode FET, the second is shown in
Figure 9, which uses an N Channel Enhancement Mode FET. The circuit using the Depletion Mode FET will
have the lowest standby power. The standby power consumption of the FET is the voltage across the start-up
FET multiplied by the Drain to Source Cutoff current with Gate negatively biased, this is typically 0.1 µA.
Standby Power of the Start-up FET calculation:
•
•
•
• , I
DOFF
is the Depletion MODE FETs leakage current
•
When VCC < VCC(on) the current consumption of the I
C
= I
CC(st)
, nominally 340 µA.
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