RT7300A
PFC Controller with Critical Conduction Mode
General Description
The RT7300A is an active Power Factor Correction
(PFC) controller with critical conduction mode (CRM)
operation that is designed to meet line current
harmonic regulations for the applications of AC/DC
adapters, electronic ballasts and medium off-line power
converters (<300W). The CRM and Feed-Forward
schemes provide near unity power factor across a wide
range of input voltages and output powers.
The totem-pole gate driver with 600mA sourcing
current and 800mA sinking current provides powerful
driving capability for power MOSFET to improve
conversion efficiency. The RT7300A features an extra
low start-up current (20A) and supports a disable
function to reduce power consumption in standby mode,
which makes it easy to comply with energy saving
regulations such as Blue Angel, Energy Star and
Energy 2000.
This controller integrates comprehensive safety
protection functions for robust designs including input
under voltage lockout, output over voltage protection,
under voltage protection and cycle-by-cycle current
limit.
The RT7300A is a cost-effective solution for PFC
power converter with minimum external components. It
is available in the SOP-8 package.
Features
Critical Conduction Mode (CRM) Operation
Constant On-Time Control (Voltage Mode)
Near Unity Power Factor
Ultra Low Start-up Current (<20A)
Input Voltage Feed-Forward Compensation
Wide Supply Voltage Range from 12V to 25V
Totem Pole Gate Driver with 600mA/800mA
Maximum Frequency Clamping (120kHz)
DCM THD Optimization
Fast Dynamic Response
Light Load Burst Mode Operation
Disable Function
Maximum/Minimum On-Time Limit
Cycle-by-Cycle Current Limit
Output Over Voltage Protection (OVP)
Output Under Voltage Protection (UVP)
Under Voltage Lockout (UVLO)
RoHS Compliant and Halogen Free
Applications
Electrical Lamp Ballast
LED Lighting
AC/DC Adapter/Charger for Desktop PC, NB, TV,
Monitor, Etc.
Entry-Level Server, Web Server
Simplified Application Circuit
BD
Line
R
G
C
SIN
R
CS
Neutral
R
ZCD
ZCD
R
AUX
D
VDD
GD
CS
INV
C
INV
R
OUT2
R
OUT1
V
OUT
-
Q1
C
OUT
L
PFC
D
OUT
V
OUT
+
RT7300A
VDD
C
VDD
R
FF1
R
START
FF
COMP
R
COMP
GND
C
COMP1
C
COMP2
R
FF2
C
FF
Copyright © 2014 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation
DS7300A-00
May 2014
www.richtek.com
1
RT7300A
Ordering Information
RT7300A
Package Type
S : SOP-8
Lead Plating System
G : Green (Halogen Free and Pb Free)
Note :
Richtek products are :
Marking Information
RT7300A
GSYMDNN
RT7300AGS : Product Number
YMDNN : Date Code
Pin Configurations
(TOP VIEW)
INV
COMP
FF
CS
2
3
4
8
7
6
5
RoHS compliant and compatible with the current
requirements of IPC/JEDEC J-STD-020.
Suitable for use in SnPb or Pb-free soldering processes.
VDD
GD
GND
ZCD
SOP-8
Functional Pin Description
Pin No.
1
2
3
4
Pin Name
INV
COMP
FF
CS
Pin Function
Inverting Input of the Internal Error Amplifier. Connect a resistive divider from output
voltage to this pin for voltage feedback. It also used for OVP and UVP detections.
Output of the Internal Error Amplifier. Connect a compensation network between this pin
and GND for dynamic load performance.
Feed-Forward Input for Line Voltage. This pin senses the line input voltage via a
resistive divider. Connect a suitable capacitor to filter out the line voltage ripple & noise.
Current Sense Input. The current sense resistor between this pin and GND is used for
current limit setting.
Zero Current Detection Input. Input from secondary winding of PFC choke for detecting
demagnetization timing of PFC choke. This pin also can be used to enable/disable the
controller.
Ground of the Controller.
Gate Driver Output for External Power MOSFET.
Supply Voltage Input. The controller will be enabled when VDD exceeds V
ON_TH
(12.45V typ.) and disabled when VDD decreases lower than V
OFF_TH
(9V typ.).
5
6
7
8
ZCD
GND
GD
VDD
Copyright © 2014 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation
www.richtek.com
2
DS7300A-00
May 2014
RT7300A
Function Block Diagram
Blank & Maximum
Frequency Clamping
Clamping
Circuit
Zero Current Detect
Disable
Leading Edge Blanking
0.85V
1.5V
FF
Feed- Forward &
THD Optimize
Ramp Generator
+
GM
-
+
-
-
+
-
OVP
+
-
UVP
+
S
Q
Start
R
Soft
Driver
GD
ZCD
CS
COMP
UVLO
VDD
1.65V
GND
0.25V
INV
Operation
Critical Conduction Mode (CRM)
The Critical Conduction Mode is also called Transition
Mode or Boundary Mode. Figure 1 shows the CRM
operating at the boundary between Continuous
Conduction Mode (CCM) and Discontinuous Conduction
Mode (DCM).
In CRM, the power switch turns on immediately when
the inductor current decreases to zero. The CRM is the
preferred control method for medium power (<300W)
applications due to the features of zero current
switching and lower peak current than that in DCM.
Inductor Current
Constant On-Time Voltage Mode Control
Figure 2 shows a typical Boost converter. When the
MOSFET turns on with a fixed on-time (T
ON
), the
inductor current can be calculated by the following
equation (1).
L
PFC
D
OUT
Q1
C
OUT
R
LOAD
V
IN
Figure 2. Typical Boost Converter
I
L_PK
=
V
IN
T
ON
L
PFC
(1)
DCM
CRM
CCM
If the input voltage is a sinusoidal waveform and
rectified by a bridge rectifier, the inductor current can
be expressed with equation (2). When the converter
operates in CRM with constant on-time voltage mode
control, the envelope of inductor peak current will follow
the input voltage waveform with in-phase. The average
inductor current will be half of the peak current shown
as Figure 3. Therefore, the near unity power factor is
easy to be achieved by this control scheme.
I
L_pk
|
sinθ
|
=
VIN_pk
|
sinθ
|
T
ON
L
PFC
(2)
Figure 1. Inductor Current of DCM, CRM and CCM
Copyright © 2014 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation
DS7300A-00
May 2014
www.richtek.com
3
RT7300A
I
L_pk
=
I
in_avg
V
IN
T
ON
L
PFC
= 1
I
PK
2
Feedback Voltage Detection
Figure 5 shows the feedback voltage detection circuit.
The INV pin is the inverting input of the Error Amplifier
with 1.5V reference voltage. Over voltage and under
voltage protections are provided with threshold voltage
1.65V and 0.25V respectively. If the INV voltage is over
1.65V or under 0.25V, the gate driver will be disabled to
prevent output over voltage condition or feedback open
condition. Although the INV is an input pin with high
impedance, it is suggested that the bias current of the
potential divider should be over 30A for noise
immunity.
V
IN
I
L_PK
I
Q1
I
in_avg
I
DOUT
Input
Voltage
Peak Inductor Current
MOSFET Current
Average Input Current
Output Diode Current
COMP
V
OUT
+
R
OUT1
INV
R
OUT2
C
INV
1.5V
+
GM
-
-
OVP
+
-
V
Q1_GATE
MOSFET Gate Voltage
1.65V
Figure 3. Inductor Current of CRM with Constant
On-Time Voltage Mode Control
Under Voltage Lockout
The controller will be enabled when VDD exceeds
V
ON_TH
(12.45V typ.) and disabled when VDD
decreases lower than V
OFF_TH
(9V typ.).
The maximum VDD voltage is set at 27V typically for
over voltage protection shown as Figure 4. An internal
29V zener diode is also used to avoid over voltage
stress for the internal circuits.
When the VDD is available, the precise reference is
generated for internal circuitries such as Error
Amplifier, Current Sense, OVP, UVP. The internal
reference equips with excellent temperature coefficient
performance so that the RT7300A can be operated in
varied environments.
Bias &
Band Gap
UVLO
+
-
UVP
0.25V
+
Figure 5. Feedback Voltage Detection
Transconductance Error Amplifier
The RT7300A implements transconductance error
amplifier with non-linear GM design to regulate the
Boost output voltage and provide fast dynamic
response. The transconductance value is 100A/V in
normal operation. When the INV voltage increases over
1.65V or decreases under 1.35V, the output of error
amplifier will source or sink 1mA maximum current at
COMP pin respectively shown as Figure 6. Thus, the
non-linear GM design can provide fast response for the
dynamic load of PFC converters even though the
bandwidth of control loop is lower than line frequency.
Hys. = 3.45V
+
VDD
29V
12.45V
10µs
De-Bounce
OVP
+
-
+
27V
Figure 4. VDD and UVLO
Copyright © 2014 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation
www.richtek.com
4
DS7300A-00
May 2014
RT7300A
ICOMP_Sourcing Current
Max. Sourcing Current
>1mA
In RT7300A, the Ton is implemented by a constant
current charging a capacitor till V
Comp
threshold
voltage is reached. Therefore, the T
ON
is a function of
V
comp
.
T
ON
=
UV
0.25V
1.35V
OV
1.5V 1.65V
GM
=1
00
µA
/V
INV
C
ramp
V
Comp
I
ramp
(4)
Then, the V
Comp
can be derived from equation (3) and
(4).
C
ramp
V
Comp
4
P
i
L
PFC
=
I
ramp
V
IN_pk 2
V
Comp
=
4
P
i
L
PFC
I
ramp
C
ramp
V
IN_pk 2
(5)
According to equation (5), the V
Comp
is reversely
Figure 6. Non-linear GM
Feed-Forward Compensation
The FF pin is an input pin with high impedance to
detect the line input voltage shown as Figure 7. A
proper voltage divider should be applied to sense the
line voltage after bridge diode rectifier. Since the FF
voltage is proportional to the line input voltage, it
provides a feed-forward signal to compensate the loop
bandwidth for high line and low line input conditions.
V
CSIN
proportional to the squared input voltage so that the
V
Comp
has a large variation for the change of line
voltage between high and low input voltages. This
variation will impact T
ON
, Burst mode entry level and
loop bandwidth.
In order to compensate the variation, the I
ramp
is
designed to be proportional to the squared input
voltage shown as equation (6).
I
ramp
(V
pk
) = k
V
IN_pk 2
gm
ramp
V
Comp
(FF) =
2
P
i
L
PFC
gm
ramp
C
ramp
(6)
(7)
When k = 0.5, the V
Comp
is compensated to be
R
FF1
R
FF2
FF
C
FF
1.5V
+
GM
-
Feed- Forward & THD
Optimize Ramp Generator
+
-
proportional to the power only. So, the Ton will be stable
to support good power factor for high and low line voltage
conditions.
Ramp Generator
PWM OFF
INV
COMP
Figure 7. FF Detection Circuit
The constant on-time, T
ON
, can be derived from the
following equations.
P
i
= 1
V
IN_pk
I
L_pk
4
I
L_pk
=
V
IN_pk
T
ON
L
PFC
2
The RT7300A provides constant on-time voltage mode
control to achieve near unity power factor for the CRM
boost converters. Figure 8 shows the Ramp Generator
with Feed-Forward compensation and THD optimization
circuit for the constant on-time operation.
V
IN_pk
V
IN_pk
P
i
= 1
V
IN_pk
T
ON
= 1
T
ON
4
L
PFC
4 L
PFC
4
P
i
L
PFC
T
ON
=
(3)
V
IN_pk 2
Copyright © 2014 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation
DS7300A-00
May 2014
www.richtek.com
5
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