ld7576a Leadtrend Technology, ld7576a Datasheet - Page 9

• Manufacturer Part Number: ld7576a
• Description: Green-mode Pwm Controller With High-voltage Start-up Circuit And Adjustable Olp Delay Time
• Manufacturer: Leadtrend Technology

Application Information
Operation Overview
As long as the green power requirement becomes a trend
and the power saving is getting more and more important
for switching power supplies and switching adaptors, the
traditional PWM controllers are not able to support such
new requirements. Furthermore, the cost and size
limitation forces the PWM controllers to powerfully
integrate more functions, thereby reducing the external
part count. The LD7576A is ideal for these applications to
provide an easy and cost effective solution; and its detailed
features are described as below.
Internal High-Voltage Startup Circuit and
Under Voltage Lockout (UVLO)
Traditional circuits provide the startup current through a
startup resistor to power up the PWM controller.
Nevertheless, it consumes too significant power to meet
the current power saving requirement. In most cases,
startup resistors carry large resistance, which causes
longer startup time.
To achieve the optimized topology, as shown in figure 13,
LD7576A is implemented with a high-voltage startup circuit
for such requirement. During startup, a high-voltage
current source sinks current from the bulk capacitor to
Leadtrend Technology Corporation
LD7576A-DS-00b November, 2009
Fig. 13
www.leadtrend.com.tw
9
provide the startup current as well as to charge the Vcc
capacitor C1. During the startup transie
nt when the Vcc is lower than the UVLO threshold, the
high-voltage current source is enabled to supply 1mA
current. Meanwhile, the Vcc supply current is as low as
100μA such that most of the HV current is adopted to
charge the Vcc capacitor. By using such configuration,
the turn-on delay time will be almost the same no matter
under low-line or high-line condition.
As the Vcc voltage rises higher than UVLO(on) to power
on the LD7576A and further to deliver the gate drive signal,
the high-voltage current source is disabled and the supply
current is solely provided from the auxiliary winding of the
transformer. Therefore, it eliminates the power loss on
the startup circuit and performs highly power saving. An
UVLO comparator is embedded to detect the voltage on
the Vcc pin and to ensure the supply voltage high enough
to power on the LD7576A PWM controller and to drive the
power MOSFET. As shown in Fig. 14, a hysteresis is
provided to prevent undesired shutdown from the voltage
dip during startup.
levels are set at 16V and 10.0V, respectively
The turn-on and turn-off threshold
LD7576A