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LD5718ACGS Datasheet(PDF) 13 Page - Leadtrend Technology

Part # LD5718ACGS
Description  Primary-Side Quasi-Resonant Controller,Operating in CV/CC Mode
PDF  16 Pages
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Manufacturer  LEADTREND [Leadtrend Technology]
Direct Link  https://www.leadtrend.com.tw/
Logo LEADTREND - Leadtrend Technology

LD5718ACGS Datasheet(HTML) 13 Page - Leadtrend Technology

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LD5718AC
09/22/2016
13
Leadtrend Technology Corporation
www.leadtrend.com.tw
LD5718AC-DS-00 September 2016
Quasi
Resonant
(First Valley)
Discontinuous with
valley switching
(2nd,3rd ,4th ... Valley)
25kHz
0.8kHz
100kHz
V
fs
Green
Mode
1.2
0.7
0.3
1.6
COMP
Fig. 18
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.
19, the LD5718AC detects the primary power 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 1V. From above, the power
MOS peak current can be obtained as below.
S
)
MAX
(
PEAK
R
V
1
I
A leading-edge blanking (LEB) time is included in the
input of CS pin to prevent the false-trigger from the current
spike. LD5718AC can deliver more constant current at
high input voltage than at low input voltage. To
compensate this, 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 of ICC (300μA) depends on the
COMP voltage(VCOMP>0.9V). The equation of ICC (300μA)
is decreased as:
OUT
CS
COMP
FB
Na
Ns
Np
Ra
Rb
VIN
LD5718AC
LEB time
R1
RS
VCS
Fig. 19
Principle of CC 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. 20. The output current Io can
be expressed as:
S
DIS
S
CS
S
P
DIS
PK
,
P
S
P
S
DIS
PK
,
S
T
T
R
V
N
N
2
1
Ts
T
i
N
N
2
1
T
T
i
2
1
Io
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 (EX:VCS*TDIS/TS =1/2). IO can be
induced finally by
2
1
1
2
1
2
1
S
S
P
S
DIS
S
CS
S
P
R
N
N
T
T
R
V
N
N
Io
However this is an approximate equation. The user
may fine-tune it according to the experiment result.
)
300
(
1
R
A
V
V
S
CS



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