ld7522 Leadtrend Technology, ld7522 Datasheet - Page 10

• Manufacturer Part Number: ld7522
• Description: Smart Green-mode Pwm Controller With Multiple Protections
• 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
the switching power supplies and switching adapters, the
traditional PWM controllers are not able to support such new
requirements. Furthermore, the cost and size limitation force
the PWM controllers need to be powerful to integrate more
functions to reduce the external part counts. The LD7522
is targeted on such application to provide an easy and cost
effective solution; its detail features are described as below:
Under Voltage Lockout (UVLO)
An UVLO comparator is implemented in it to detect the
voltage on the VCC pin. It would assure the supply voltage
enough to turn on the LD7522 PWM controller and further to
drive the power MOSFET.
hysteresis is built in to prevent the shutdown from the
voltage dip during startup.
threshold level are set at 16V and 10.0V, respectively.
Startup Current and Startup Circuit
The typical startup circuit to generate the LD7522 is shown
in Fig. 20. During the startup transient, the Vcc is lower
than the UVLO threshold thus there is no gate pulse
produced
Therefore, the current through R1 will provide the startup
current and to charge the capacitor C1. Whenever the Vcc
voltage is high enough to turn on the LD7522 and further to
Leadtrend Technology Corporation
LD7522-DS-03 January 2008
UVLO(on)
UVLO(off)
from
Vcc
I(Vcc)
startup current
(~uA)
LD7522
Fig. 19
to
As shown in Fig. 19, a
The turn-on and turn-off
operating current
(~ mA)
drive
www.leadtrend.com.tw
power
t
t
MOSFET.
10
deliver the gate drive signal, the supply current is provided
from the auxiliary winding of the transformer.
startup current requirement on the PWM controller will help
to increase the value of R1 and then reduce the power
consumption on R1. By using CMOS process and the
special circuit design, the maximum startup current of
LD7522 is only 35μA.
If a higher resistance value of the R1 is chosen, it usually
takes more time to start up. To carefully select the value of
R1 and C1 will optimize the power consumption and startup
time.
Output Stage and Maximum Duty-Cycle
An output stage of a CMOS buffer, with typical 500mA
driving capability, is incorporated to drive a power MOSFET
directly. And the maximum duty-cycle of LD7522 is limited
to 75% to avoid the transformer saturation.
Oscillator and Switching Frequency
The switching frequency of LD7522 is fixed as 65KHz
internally to provide the optimized operations by considering
the EMI performance, thermal treatment, component sizes
and transformer design.
Voltage Feedback Loop
The voltage feedback signal is provided from the TL431 in
the secondary side through the photo-coupler to the COMP
Fig. 20
LD7522
Lower