RT7339P
Functional Block Diagram
Valley Detector
Valley Signal
DMAG
Clamping
Circuit
Input Over / Under
Voltage Protection
Knee Detector
THD
Opimizer
Ramp
Generator
+
CMP
-
Constant-Voltage
Comparator
PWM
Control
Logic
+
CMP
-
Current-Limit
Comparator
VDD OVP
Under-
Voltage
Lockout
VDD
Constant
Voltage Control
Parameter
setting
Voltage
Control
Oscillator
VDD
Over-voltage
Protection
PWM
V
CL
CS
Leading-Edge
Blanking
V
CLAMP
13V
PWM
Gate
Driver
Parameter
setting
OTP
GND
Output OVP
GATE
Output Diode
Short-Circuit
Protection
Over-
Temperature
Protection
Output
Over-Voltage
Protection
COMP
Operation
Critical-Conduction Mode (CRM) with Constant
On-Time Control
Figure 1 shows a typical flyback converter with the
input voltage (V
IN
). When the main switch Q1 is turned
on with a fixed on-time (t
ON
), the peak current (I
L_PK
) of
the magnetizing inductance (L
m
) can be calculated by
the following equation :
V
I
L_PK
�½
IN
t
ON
L
m
TX1
N
P
:N
S
D
OUT
I
L
L
m
V
IN
Q1
V
IN
I
L_PK
I
Q1_DS
Input Voltage
Peak Inductor Current
MOSFET Current
Average Input Current
Output Diode Current
MOSFET Gate Voltage
I
L_PK
�½
V
IN_PK
sin(θ)
t
ON
L
m
As shown in Figure 2, when the converter operates in
CRM with the constant on-time control, the envelope of
the peak inductor current is in phase with the input
voltage. Thus, the high power factor can be achieved.
+
C
OUT
V
OUT
-
I
OUT
R
OUT
Figure 1. Typical Flyback Converter
If the input voltage is the output voltage of the
full-bridge rectifier (V
IN_PK
x |sin|), the inductor peak
current (I
L_PK
) can be expressed as the following
equation :
I
in_avg
I
Do
V
Q1_GS
Figure 2. Inductor Current of CRM with Constant
On-Time Control
Copyright © 2019 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
DS7339P-03
August 2019
www.richtek.com
3
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