Application Note: SY8751
High Efficiency, 5-80V Input, 500kHz White LED Driver
Preliminary Specification
General Description
SY8751 is a high efficiency, 5V-80V wide input voltage
range DC/DC regulator targeting at LED applications.
The device integrates the low R
DS(ON)
MOSFET and
internal compensation.SY8751 supports analog dimming
with PWM signal/Analog signal.
Features
Wide input range: 5-80 V
500kHz switching frequency
Integrated low R
DS(ON)
FET
Analog dimming with PWM signal/Analog signal
Maximum output LED current 1.2A
Compact package: SO8E
Ordering Information
SY8751
□(□□)□
Tempera ture Code
Packa ge Code
Optiona l Spec Code
Package type
SO8E
Note
----
Applications
LED lighting
Ordering Number
SY8751FCC
Typical Applications
Figure 1. Schematic diagram
AN_SY8751 Rev. 0.1
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1
SY8751
Pinout (top view)
SO8E
Top Mark: BJAxyz (device
code: BJA,
x=year code, y=week code, z= lot number code)
Pin Name
SEN
IN
TM
GND
LX
EN
CF
S08E
1
2
3
4
5,6
7
8
Negative Current Sense Pin.
Input pin. Decouple this pin to GND pin with 1uF ceramic cap. Also used as
the positive current sense pin.
Connect to GND
Ground pin
Inductor node.
Enable pin and PWM dimming input pin.
Connect a capacitor from this pin to ground to filter out the AC ripple on
reference voltage. Or add 0~1.1V on this pin directly to realize linear
dimming
.
Pin Description
Block Diagram
IN
SEN
LX
Current Sense
Control
Logic
EN
PWM Dimming 500k
processor
Io
reference
Driver
GND
Thermal
Shutdown
UVLO
CF
AN_SY8751 Rev. 0.1
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2
SY8751
Absolute Maximum Ratings
LX, IN, EN------------------------------------------------------------------------------------------------------------------------
CF -----------------------------------------------------------------------------------------------------------------------------------
90V
V
IN
0.6V
SEN -----------------------------------------------------------------------------------------------------------------------
5V
Power Dissipation, PD @ T
A
= 25°C SO8E, ------------------------------------- -------------------------------------------3.3W
Package Thermal Resistance (Note 2)
θ
JA
-----------------------------------------------------------------------------------------------------------------
θ
JC
-----------------------------------------------------------------------------------------------------------------
Junction Temperature Range --------------------------------------------------------------------------------------
Lead Temperature (Soldering, 10 sec.) ------------------------------------------------------------------------------------
Storage Temperature Range --------------------------------------------------------------------------------------
30°C/W
10°C/W
-40°C to150°C
260°C
-65°C to 150°C
Recommended Operating Conditions
IN, LX, EN ----------------------------------------------------------------------------------------------------------------
SEN -----------------------------------------------------------------------------------------------------------------------
CF ------------------------------------------------------------------------------------------------------------------------------
Junction Temperature Range -------------------------------------------------------------------------------------
5V to 80V
IN
0.4V
0-2.0V
-40°C to 150°C
V
Electrical Characteristics
(V
IN
=80V, V
OUT
=70V, I
OUT
=0.6A, TA = 25°C unless otherwise specified)
Parameter
Symbol
Test Conditions
V
IN
Input Voltage Range
I
SHDN
Shutdown Current
EN=0
F
SW
Switching Frequency
V
Current Sense Limit
IN-SEN
I
Main FET Current Limit
LIM
V
EN Rising Threshold
ENH
V
EN Falling Threshold
ENL
IN UVLO Rising Threshold
UVLO Hysteresis
Dimming section:
Analog dimming range on CF
Thermal Shutdown
Temperature
Thermal Hysteresis
Min
5
196
Typ
10
500
200
1.8
1.25
1.05
4.4
1.25
Max
90
204
V
U
Unit
V
µA
kHz
mV
A
V
V
V
V
mV
V
°C
°C
IN_UVLO
VLO_HYS
V
CF
T
SD
I
LED
is 5.7% of full load
I
LED
is 100% of full load
62.5
1.1
150
15
Hyst
Note 1:
Stresses beyond the “Absolute Maximum Ratings” ma y cause permanent damage to the device. These are
stress ratings only. Functional operation of the device at these or any other conditions beyond those indicated in the
operational sections of the specification is not implied. Exposure to absolute maximum rating conditions for extended
periods may affect device reliability.
Note 2:
JA is measured in the natural convection at T
A
= 25°C on a low effective single layer thermal con ductivity test board of
JEDEC 51-3 thermal measurement standard.
θ
Note 3:
The device is not guaranteed to function outside its operating conditions.
AN_SY8751 Rev. 0.1
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SY8751
Typical Performance Characteristics
Efficiency vs Input Voltage
98.5
98.0
97.5
Ef
fic
ie
nc
y
(
%
)
97.0
96.5
96.0
95.5
95.0
94.5
40
45
50
55
60
65
70
75
80
85
V
O
=70V
V
O
=36V
Input Voltage (V)
PWM_Dimming
800
600
I
O
(
m
A)
(%)
400
/I
I
200
20KH
Z
MAX
O_
O
0
0
20
40
60
80
100
PWM_Duty(%)
AN_SY8751 Rev. 0.1
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4
SY8751
Operation
SY8751 is a grounded-switch buck regulator that
integrates controller, power MOSFET on the same die.
With ultra low R
DS(ON)
power switches and proprietary
PWM control, this regulator can achieve the high
efficiency. SY8751 also supports analog dimming
function with both PWM and analog dimming signals.
)
L
½
V (1 V /V
F
SW
I
OUT, MAX
40%
OUT
OUT
IN,MAX
where Fsw is the switching frequency and I
OUT,MAX
is
the LED current.
SY8751 is quite tolerant of different ripple current
amplitude. Consequently, the final choice of inductance
can be slightly off the calculation value without
significantly impacting the performance.
2)
The saturation current rating of the inductor must be
selected to be greater than the peak inductor current
under full load conditions.
I
SAT
,
MIN
I
OUT
,
MAX
Applications Information
Because of the high integration in the SY8751, the
application circuit based on this regulator is rather
simple. The design of external components’ parameter as
below:
Schottky diode:
Select according to input voltage and output current.
Current sense resistor R
SEN
:
Choose R
SEN
to program the proper output Current:
I
LED
(A)
½
V
OUT
(1-V
OUT
/V
IN
,
MAX
)
2
F
SW
L
0.2(V)
R
SEN
()
Input capacitor C
IN
:
The ripple current through input capacitor is calculated
as:
I
CIN_RMS
½
I
OUT
D(1- D)
A typical X7R or better grade ceramic capacitor with
suitable capacitance should be chosen to handle this
ripple current well. To minimize the potential noise
problem, this ceramic capacitor should be placed really
close to IN and GND. The loop area, formed by C
IN
, and
IN/GND pins, should be minimized to achieve better
EMI performance.
Output capacitor C
OUT
:
The output capacitor is selected to handle the output
current ripple. For the best performance, it is
recommended to use X7R or better grade ceramic
capacitor greater than 1uF capacitance.
Output inductor L:
There are several considerations in choosing this
inductor.
1)
Choose the inductance to provide the desired ripple
current. It is suggested to choose the ripple current
to be about 40% of the maximum output current.
The inductance is calculated as:
Dimming Operation:
It is compatible with two dimming signals: PWM signal
and analog signal.
Applied with PWM signal, the PWM is connected to EN
pin, the dimming frequency is limited to larger than
20kHz. If the dimming frequency is lower than 20kHz,
an external capacitor is needed to bypass CF pin.
Applied with analog dimming, EN pin is pulled up to
high, and the analog signal 0-1.1V is connected to CF
pin.
I
O
/
I
O_
MA
X
(
%
)
Soft Start:
Add a
ceramic capacitor C
F
on CF to achieve soft start, the
soft start time can be adjusted by C
F
.
SCP:
If V
VIN
-V
SEN
>=0.6V, the output is disabled.
EN OFF:
If VEN is lower than VENL, IC will shut down after 11ms.
AN_SY8751 Rev. 0.1
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