ld7577gs Leadtrend Technology, ld7577gs Datasheet - Page 9

• Manufacturer Part Number: ld7577gs
• Description: High Voltage Green-mode Pwm Controller With Brown-out Protection
• Manufacturer: Leadtrend Technology

Application Information
Operation Overview
As long as the green power requirement becomes a trend
and the power saving is becoming 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
LD7577 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’ power on the PWM controller through a
startup resistor to constantly provide current from a rectified
voltage to the capacitor connected to Vcc pin. Nevertheless,
this startup resistor was usually of larger resistance, and it
therefore required more power and longer time to start up.
To achieve the optimized topology, as shown in figure 13,
the LD7577 is built in with high voltage startup circuit to
optimize the power saving. During the startup sequence, a
high-voltage current source sinks current from C
Leadtrend Technology Corporation
LD7577-DS-01 January 2009
Fig. 13
www.leadtrend.com.tw
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capacitor to provide the startup current as well as to charge
the Vcc capacitor C1. During the initialization of the startup,
Vcc voltage is lower than the UVLO(off) threshold thus the
current source is on to supply a current of 1mA.
Meanwhile, the Vcc current consumed by the LD7577 is as
low as 320μA thus most of the HV current is utilized to
charge the Vcc capacitor. By using such configuration, the
turn-on delay time will be almost the same no matter
whether operation condition is under low-line or high-line.
When Vcc voltage reaches UVLO(on) threshold, the
LD7577 is powered on to start issuing the gate drive signal,
the high-voltage current source is then disabled, and the
Vcc supply current will be only provided from the auxiliary
winding of the transformer. Therefore, the power losses on
the startup circuit beyond the startup period can be
eliminated and the power saving can be easily achieved. In
general application, a 39KΩ resistor is still recommended to
be placed in high voltage path to limit the current if there is a
negative voltage applying in any case.
An UVLO comparator is included to detect the voltage on
the V
power on the LD7577 PWM controller and in addition to
drive the power MOSFET.
Hysteresis is provided to prevent the shutdown caused by
the voltage dip during startup.
threshold levels are set at 16V and 10.0V, respectively.
CC
pin to ensure the supply voltage is high enough to
As shown in Fig. 14, a
The turn-on and turn-off
LD7577