RT7304
Application Information
Output Current Setting
Minimum On-Time Setting
Considering the conversion efficiency, the programmed
DC level of the average output current (IOUT (t)) can be
derived as :
The RT7304 limits a minimum on-time (tON(MIN)) for each
switching cycle. The tON(MIN) is a function of the sample-
and-hold ZCDcurrent (IZCD_SH) as following :
N
K
R
1
P
CC
I
=
CTR
TX1
OUT_CC
tON(MIN) IZCD_SH 375psecA (typ.)
2 N
S
CS
I
I
SEC_PK
N
N
P
S
IZCD_SH can be expressed as :
CTR
=
TX1
PRI_PK
V
N
A
N
IN
I
=
ZCD_SH
in which CTRTX1 is the current transfer ratio of the
transformer TX1, ISEC_PK is the peak current of secondary
side, and IPRI_PK is the peak current of the primary side.
CTRTX1 can be estimated to be 0.9.
R
ZCD1
P
Thus, RZCD1 can be determined by :
V
t
ON(MIN)
IN
N
N
P
A
R
=
(typ.)
ZCD1
375p
According to the above parameters, current sense resistor
RCS can be determined as the following equation :
In addition, the current flowing out of ZCD pin must be
lower than 2.5mA (typ.). Thus, the RZCD1 is also
determined by :
NP
KCC
1
RCS
=
CTRTX1
2 NS IOUT_CC
2 V
AC(MAX)
N
N
P
A
R
>
ZCD1
Propagation Delay Compensation Design
2.5m
The VCS deviation (ΔVCS) caused by propagation delay
where the VAC(MAX) is maximum input AC voltage.
effect can be derived as :
Output Over-Voltage Protection Setting
V
IN
t R
D CS
V
=
CS
Output OVP is achieved by sensing the knee voltage on
the auxiliary winging. It is recommended that output OV
level (VOUT_OVP) is set at 120% of nominal output voltage
(VOUT). Thus, RZCD1 and RZCD2 can be determined by the
equation as :
L
m
in which tD is the delay period which includes the
propagation delay of the RT7304 and the turn-off transition
of the main MOSFET. The sourcing current from CS pin
of the RT7304 (ICS) can be expressed as :
N
N
R
ZCD2
A
V
OUT
120% = 3.1V (typ.)
N
N
P
1
ZCD1
A
R
R
ZCD2
I
= K V
IN
S
ZCD1
CS
PC
R
where NA is the turns number of auxiliary winding.
RPC can be designed by :
Table 2. Suggested Component Values Range
Component
Range of Typical Value
10F to 33F
1F to 4.7F
C
VDD
V
t R R
N
P
N
A
CS
D
CS
ZCD1
R
PC
=
=
I
L
K
C
CS
m
PC
COMP
C
10pF to 22pF
NC to 22pF
ZCD
C
CS
R
0.68M to 2M
10k to 22k
10 to 47
ST
R
GP
R
G
R
AUX
10 to 100
Copyright 2015 Richtek Technology Corporation. All rights reserved.
©
is a registered trademark of Richtek Technology Corporation.
DS7304-04 February 2015
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11
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