®
RT7304
Primary-Side-Regulation LED Driver Controller
with Active PFC
General Description
The RT7304 is a constant current LED driver with active
power factor correction. It supports high power factor
across a wide range of line voltages, and it drives the
converter in the Quasi-Resonant (QR) mode to achieve
higher efficiency. By using Primary Side Regulation (PSR),
the RT7304 controls the output current accurately without
a shunt regulator and an opto-coupler at the secondary
side, reducing the external component count, the cost,
and the volume of the driver board.
The RT7304 embeds comprehensive protection functions
for robust designs, including LED open-circuit protection,
LED short-circuit protection, output diode short-circuit
protection, VDD Under-Voltage lockout (UVLO), VDD Over-
Voltage Protection (VDD OVP), Over-Temperature
Protection (OTP), and cycle-by-cycle current limitation.
Features
Tight LED Current Regulation
No Opto-Coupler and TL431 Required
Power Factor Correction (PFC)
Quasi-Resonant
Maximum/Minimum Switching Frequency
Clamping
Maximum/Minimum On-Time Limitation
Wide VDD Voltage Range (up to 25V)
Multiple Protection Features
½
LED Open-Circuit Protection
LED Short-Circuit Protection
½
Output Diode Short-Circuit Protection
½
VDD Under-Voltage Lockout
½
VDD Over-Voltage Protection
½
Over-Temperature Protection
½
Cycle-by-Cycle Current Limit
RoHS Compliant and Halogen Free
½
Marking Information
0H= : Product Code
0H=DNN
DNN : Date Code
Applications
AC/DC LED Lighting driver
Simplified Application Circuit
Flyback Converter
Buck-Boost Converter
Line
BD
TX1
D
OUT
C
OUT
V
OUT+
Line
BD
TX1
C
OUT
V
OUT-
D
OUT
C
SIN
Neutral
R
ST
RT7304
GD
VDD
CS
R
G
R
PC
V
OUT-
Q1
Neutral
C
SIN
R
ST
RT7304
GD
VDD
CS
R
G
R
PC
V
OUT+
Q1
R
CS
C
VDD
C
COMP
COMP
GND
ZCD
R
ZCD2
C
VDD
C
COMP
COMP
GND
ZCD
R
CS
R
ZCD2
D
AUX
R
ZCD1
D
AUX
R
ZCD1
Copyright
©
2015 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS7304-04 February 2015
www.richtek.com
1
RT7304
Ordering Information
RT7304
Package Type
E : SOT-23-6
Lead Plating System
G : Green (Halogen Free and Pb Free)
Note :
Richtek products are :
½
Pin Configurations
(TOP VIEW)
COMP ZCD CS
6
5
2
4
3
GND VDD GD
RoHS compliant and compatible with the current require-
ments of IPC/JEDEC J-STD-020.
Suitable for use in SnPb or Pb-free soldering processes.
SOT-23-6
½
Functional Pin Description
Pin No.
1
2
3
4
5
6
Pin Name
GND
VDD
GD
CS
ZCD
COMP
Ground of the Controller.
Supply Voltage (V
DD
) Input. The controller will be enabled when V
DD
exceeds V
TH_ON
and disabled when V
DD
is lower than V
TH_OFF
.
Gate Driver Output for External Power MOSFET.
Current Sense Input. Connect this pin to the current sense resistor.
Zero Current Detection Input. This pin is used to sense the voltage at auxiliary winding of
the transformer.
Compensation Node. Output of the internal trans-conductance amplifier.
Pin Function
Function Block Diagram
ZCD
Clamping
Circuit
Ramp
Generator
Starter
Circuit
+
Valley
Detector
Under
Voltage
Lockout
(16V/9V)
VDD Over
Voltage
Protection
V
CLAMP
13V
PWM
Gate
Driver
R
GD
GND
GD
VDD
Constant Current Control
I
CS
Output Over
Voltage
Protection
-
Constant On-Time
Comparator
+
V
CS_CL
-
1V
Current Limit
Comparator
PWM
Control
Logic
VDD OVP
CS
Leading
Edge
Blanking
Output Diode
Short Circuit
Protection
Over
Temperature
Protection
OTP
Output OVP
COMP
Copyright
©
2015 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
2
DS7304-04 February 2015
RT7304
Operation
Critical-Conduction Mode (CRM) with Constant
On-Time Control.
Figure 1 shows a typical flyback converter with input
voltage (V
IN
). When main switch Q1 is turned on with a
fixed on-time (t
ON
), the peak current (I
L_PK
) of the magnetic
inductor (L
m
) can be calculated by the following equation :
V
I
L_PK
=
IN
t
ON
L
m
TX1
I
L
V
IN
+
Primary-Side Constant-Current Regulation
The RT7304 needs no shunt regulator and opto-coupler
at the secondary side to achieve the output current
regulation. Figure 3 shows several key waveforms of a
conventional flyback converter in Quasi-Resonant (QR)
mode, in which V
AUX
is the voltage on the auxiliary winding
of the transformer.
V
DS
V
IN
0
GD
(V
GS
)
V
AUX
0
D
OUT
+
C
OUT
V
OUT
-
I
OUT
R
OUT
L
m
Q1
(V
OUT
+ V
f
) x N
A
/ N
S
Figure 1. Typical Flyback Converter
If the input voltage is the output voltage of the full-bridge
rectifier with sinusoidal input voltage (V
IN_PK
x sin(θ)), the
inductor peak current (I
L_PK
) can be expressed as the
following equation :
I
Q1
V
IN
x N
A
/ N
P
Clamped by
controller
I
DOUT
V
IN_PK
sin(θ)
t
ON
L
m
When the converter operates in CRM with constant on-
time control, the envelope of the peak inductor current
will follow the input voltage waveform with in-phase. Thus,
high power factor can be achieved, as shown in Figure 2.
I
L_PK
=
Figure 3. Key Waveforms of a Flyback Converter
Voltage Clamping Circuit
The RT7304 provides a voltage clamping circuit at ZCD
pin since the voltage on the auxiliary winding is negative
when the main switch is turned on. The lowest voltage on
ZCD pin is clamped near zero to prevent the IC from being
damaged by the negative voltage. Meanwhile, the sourcing
ZCD current (I
ZCD_SH
), flowing through the upper resistor
(R
ZCD1
), is sampled and held to be a line-voltage-related
signal for propagation delay compensation. The RT7304
embeds the programmable propagation delay
compensation through CS pin. A sourcing current I
CS
(equal to I
ZCD_SH
x K
PC
) applies a voltage offset (I
CS
x
R
PC
) which is proportional to line voltage on CS to
compensate the propagation delay effect. Thus, the output
current can be equal at high and low line voltage.
V
IN
I
L_PK
Input Voltage
Peak Inductor Current
I
in_avg
I
DOUT
Average Input Current
Output Diode Current
I
Q1_DS
MOSFET Current
V
Q1_GS
MOSFET Gate Voltage
Figure 2. Inductor Current of CRM with Constant
On-Time Control
Copyright
©
2015 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS7304-04 February 2015
www.richtek.com
3
RT7304
Quasi-Resonant Operation
For improving converter's efficiency, the RT7304 detects
valleys of the Drain-to-Source voltage (V
DS
) of main switch
and turns on it near the selected valley. For the valley
detections, a pulse of the
“valley
signal” is generated
after a 500ns(typ.) delay time which starts at which the
voltage (V
ZCD
) on ZCD pin goes down and reaches the
voltage threshold (V
ZCDT
, 0.4V typ.). During the rising of
the V
ZCD
, the V
ZCD
must reach the voltage threshold
(V
ZCDA
, 0.5V typ.). Otherwise, no pulse of the
“valley
signal” is generated. Moreover, if the timing when the
falling V
ZCD
reaches V
ZCDT
is not later than a mask time
(t
MASK
, 2μs typ.) then the valley signal will be masked and
regards as no valley, as shown in Figure 4.
PWM
½
½
PWM
t
START
Valley
Signal
PWM
t
S(MIN)
Valley
Signal
PWM
t
S(MIN)
Valley
Signal
PWM
t
S(MIN)
5µs
……
……
½
½
V
ZCD
V
ZCDA
V
ZCDT
½
½
Valley
Signal
t
MASK
500ns
Figure 4. Valley Signal Generating Method
Figure 5 illustrates how valley signal triggers PWM. If no
valley signal is detected for a long time, the next PWM is
triggered by a starter circuit at the end of the interval (t
START
,
130μs typ.) which starts at the rising edge of the previous
PWM signal. A blanking time (t
S(MIN)
, 8.5μs typ.), which
starts at the rising edge of the previous PWM signal, limits
minimum switching period. When the t
S(MIN)
interval is
on-going, all of valley signals are not allowed to trigger
the next PWM signal. After the end of the t
S(MIN)
interval,
the coming valley will trigger the next PWM signal. If one
or more valley signals are detected during the t
S(MIN)
interval and no valley is detected after the end of the t
S(MIN)
interval, the next PWM signal will be triggered
automatically at the end of the t
S(MIN)
+ 5μs (typ.).
Figure 5. PWM Triggered Method
Protections
LED Open-Circuit Protection
In an event of output open circuit, the converter will be
shut down to prevent being damaged, and it will be auto-
restarted when the output is recovered. Once the LED is
open, the output voltage and V
ZCD
will rise. When the
sample-and-hold ZCD voltage (V
ZCD_SH
) exceeds its OV
threshold (V
ZCD_OVP
, 3.1V typ.), output OVP will be
activated and the PWM output (GD pin) will be forced low
to turn off the main switch. If the output is still open-circuit
when the converter restarts, the converter will be shut
down again.
Copyright
©
2015 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
4
DS7304-04 February 2015
½
½
Valley
Signal
RT7304
LED Short-Circuit Protection
LED short-circuit protection can be achieved by VDD UVLO
and cycle-by-cycle current limitation. Once LED short-
circuit failure occurs, V
DD
drops related to the output
voltage. When the V
DD
is lower than falling UVLO threshold
(V
TH_OFF
, 9V typ.), the converter will be shut down and it
will be auto-restarted when the output is recovered.
Output Diode Short-Circuit Protection
When the output diode is damaged as short-circuit, the
transformer will be led to magnetic saturation and the main
switch will suffer from a high current stress. To avoid the
above situation, an output diode short-circuit protection
is built-in. When CS voltage V
CS
exceeds the threshold
(V
CS_SD
1.5 typ.) of the output diode short-circuit protection,
the RT7304 will shut down the PWM output (GD pin) in
few cycles to prevent the converter from damage. It will
be auto-restarted when the failure condition is recovered.
VDD Under-Voltage Lockout (UVLO) and
Over-Voltage Protection(VDD OVP)
The RT7304 will be enabled when VDD voltage (V
DD
)
exceeds rising UVLO threshold (V
TH_ON
, 16V typ.) and
disabled when V
DD
is lower than falling UVLO threshold
(V
TH_OFF
, 9V typ.).
When V
DD
exceeds its over-voltage threshold (V
OVP
, 27V
typ.), the PWM output of the RT7304 is shut down. It will
be auto-restarted when the V
DD
is recovered to a normal
level.
Over-Temperature Protection (OTP)
The RT7304 provides an internal OTP function to protect
the controller itself from suffering thermal stress and
permanent damage. It's not suggested to use the function
as precise control of over temperature. Once the junction
temperature is higher than the OTP threshold (T
SD
, 150°C
typ.), the controller will shut down until the temperature
cools down by 30°C (typ.). Meanwhile, if V
DD
reaches falling
UVLO threshold voltage (V
TH_OFF
), the controller will
hiccup till the over-temperature condition is removed.
Copyright
©
2015 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS7304-04 February 2015
www.richtek.com
5
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