LD7516CGL datasheet(11/14 Pages) LEADTREND

11 / 14 page

LD7516C

08/31/2016

Leadtrend Technology Corporation

www.leadtrend.com.tw

LD7516C-DS-01 August 2016

Quasi

Resonant

(First Valley)

Discontinuous with

valley switching

(2nd,3rd ,4th ... Valley)

25kHz

0.7kHz

75kHz

Vcomp

Green

Mode

1.2V

0.7V

0.3V

1.6V

Fig. 19

Current

Sensing

and

Leading-edge

Blanking

The typical current mode of PWM controller feedbacks

both current signal and voltage signal to close the control

loop and achieve regulation. As shown in Fig. 20, the

LD7516C detects the primary MOS current from the CS

pin, which is not only for the peak current mode control

but also for the pulse-by-pulse current limit. The maximum

voltage threshold of the current sensing pin is set at 0.8V.

From above, the MOS peak current can be obtained from

below.

PEAK(MAX)

0.8V



A leading-edge blanking (LEB) time is included in the

input of CS pin to prevent the false-trigger from the

turn-on current spike.

LD7516C delivers more constant current at high input

voltage than at low input voltage. To compensate it, an

offset voltage is added to the RS signal by an internal

current source (ICC) and an external resistor (R1) in series

between the sense resistor (Rs) and the CS pin, By

selecting a proper value of the resistor in series with the

CS pin, the amount of compensation can be adjusted. The

value

ICC

(300 μ A)

depends

the

COMP

voltage(VCOMP>1.2V). The equation of ICC (300 μA) is

decreased as:

OUT

COMP

VIN

LD7516C

LEB time

VCS

Fig. 20

Principle of C.C. Operation

The primary side control scheme is applied to eliminate

secondary feedback circuit or opto-coupler, which will

reduce the system cost. The switching waveforms are

shown in Fig. 21. The output current

“Io” can be

expressed as:

DIS













The

primary

peak

current

(iP,PK),

inductor

current

discharge time (TDIS) and switching period (TS) can be

detected by the IC. The ratio of VCS*TDIS/TS will be

modulated as a constant (VCS*TDIS/TS =1/3). So that IO can

be obtained as

DIS









However this is an approximate equation. The user may

fine-tune it according to the experiment result.

)

300

(









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