LD7516C

08/31/2016

10

Leadtrend Technology Corporation

www.leadtrend.com.tw

LD7516C-DS-01 August 2016

Load Regulation Compensation

LD7516C

is

implemented

with

load

regulation

compensation to compensate the cable voltage drop and

to achieve a better voltage regulation. The offset voltage

across FB is produced by the internal sink current source

during the sampling period. The internal sink current

source

is

proportional

to

the

value

of

compensate the cable loss as shown in Fig. 17. So, the

full-load to no-load.

It is programmable by adjusting the

resistance of the voltage divider to compensate the drop

for cable lines used in various conditions. The equation of

internal sink current is shown as:

A)

(

75

.

9

0.45)

(V

I

COMP

Load_COMP









0

2.5

0.45

20

)

Fig. 17

Quasi-Resonant Mode Detection

The LD7516C employs quasi-resonant (QR) switching

scheme to switch in valley-mode either in CV or CC

operation. This will greatly reduce the switching loss and

the ratio dv/dt in the entire operating range for the power

supply. Fig. 18 shows the typical QR detection block. The

QR detection block will detect auxiliary winding signal to

drive MOS as FB pin voltage drops to 0.15V. The QR

comparator will not activate if FB pin voltage remains

above 0.2V.

FB

PWM

2R

R

R

S

Q

Error

Amplifier

Load

Compensation

Max. Frequency &

Green Mode

CS

1V

Leading

Edge

Blanking

0.15/0.2 V

QRD

GM

1.0V

Sample

and Hold

QRD

Blanking Time

Time-Out 1

Buffer

QRD

Time-Out 2

S/H

C.C.

FB

Turn-on

Turn-off

Ra

Rb

Fig. 18

Multi-Mode Operation

The LD7516C is a QR controller operating in multi-modes.

The controller changes operation modes according to line

Fig. 19), there might be two situations to meet. If the

system AC input is in low line, the LD7516C will turn on in

first valley. If in high line, the switching frequency will

increase till over the limit of 75 kHz and skip the first valley

to turn on in 2

would vary depending on the line voltage and the load

conditions when the system is operated in QR mode.

the frequency clamp is reduced to 25 kHz maximum.

However, the characteristic in valley switching behaves

well without problem in this condition. The LD7516C will

turn on in 4

decreases, the system will automatically skip some

valleys and the switching frequency is therefore reduced.

A smooth frequency fold-back and high power efficiency

are then achieved.

the system operates in minimum frequency for power

saving. The system modulates the frequency according to