RT7304 datasheet(11/14 Pages) RICHTEK

11 / 14 page

RT7304

DS7304-04 February 2015

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

Application Information

Output Current Setting

Considering the conversion efficiency, the programmed

DC level of the average output current (IOUT (t)) can be

derived as :

OUT_CC

TX1

SCS

SEC_PK

TX1

PRI_PK

CTR





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.

According to the above parameters, current sense resistor

RCS can be determined as the following equation :

TX1

SOUT_CC

CTR





Propagation Delay Compensation Design

The VCS deviation (

ΔVCS) caused by propagation delay

effect can be derived as :





ZCD1

= K





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 :

where NA is the turns number of auxiliary winding.

RPC can be designed by :

ZCD1





Minimum On-Time Setting

The RT7304 limits a minimum on-time (tON(MIN)) for each

switching cycle. The tON(MIN) is a function of the sample-

and-hold ZCD current (IZCD_SH) as following :

ON(MIN)

ZCD_SH

375p sec A (typ.)





ZCD_SH

ZCD1



IZCD_SH can be expressed as :

Thus, RZCD1 can be determined by :

ON(MIN)

ZCD1

(typ.)

375p



AC(MAX)

ZCD1

2.5m



In addition, the current flowing out of ZCD pin must be

lower than 2.5mA (typ.). Thus, the RZCD1 is also

determined by :

where the VAC(MAX) is maximum input AC voltage.

Output Over-Voltage Protection Setting

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 :

ZCD2

OUT

ZCD1

ZCD2

120% = 3.1V (typ.)







Component

Range of Typical Value

CVDD

F to 33F

CCOMP

F to 4.7F

CZCD

10pF to 22pF

CCS

NC to 22pF

RST

0.68M

 to 2M

RGP

10k

 to 22k

 to 47

RAUX

 to 100

Table 2. Suggested Component Values Range

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