Datasheet

Datasheet

14/34

TSZ02201-0F1F0C100240-1-2

© 2013 ROHM Co., Ltd. All rights reserved.

15.Feb.2016 Rev.006

www.rohm.com

TSZ22111・15・001

BD9486F

●3.2 External Components Selection

●3.2.1 Start Up Operation and Soft Start External Capacitance Setting

The below explanation is the start up sequence of this IC

Figure 20. Startup waveform

Figure 21. Circuit behavior at startup

○Explanation of start up sequence

1. Reference voltage REF50 starts by STB=H.

2. SS starts to charge at the time of first PWM=H. At this moment, the SS voltage of slow-start starts to equal FB

voltage,and the circuit becomes FB=SS regardless of PWM logic.

3. When FB=SS reaches the lower point of internal sawtooth waveform, GATE terminal outputs pulse and starts to boost

VOUT.

4. It boosts VOUT and VOUT reaches the voltage to be able to flow LED current.

5. If LED current flows over decided level, FB=SS circuit disconnects and startup behavior completes.

6. Then it works normal operation by feedback of ISENSE terminal. If LED current doesn't flow when SS becomes over

3.7V, SS=FF circuit completes forcibly and FBMAX protection starts.

○Method of setting SS external capacitance

According to the sequence described above, start time Tss that startup completes with FB=SS condition is the time that

FB voltage reaches the feedback point.

The capacitance of SS terminal is defined as Css and the feedback voltage of FB terminal is defined as VFB. The

If Css is set to a very small value, rush current flows into the inductor at startup.

On the contrary, if Css is enlarged too much, LED will light up gradually.

Since Css differs in the constant set up with the characteristic searched for and differs also by factors, such as a voltage

rise ratio, an output capacitance, DCDC frequency, and LED current, please confirm with the system.

【Setting example】

When Css=0.1μF,Iss=3μA,and startup completes at VFB=3.7V, SS setting time is as follows.

[sec]

]

A

μ

[

3

]

V

[

VFB

]

F

μ

[

C

T

ss

ss

　

　





[sec]

123

.

0

]

A

[

10

3

]

V

[

7

.

3

]

F

[

10

1

.

0

T

6

6

ss

　

　

　

　















STB

SS

SLOPE

PWM

GATE

VOUT

ILED

LED_OK

FB

OSC

2

1

3

4

5

6

LED_OK

SS=FB

Circuit

SS

FB

5V

OSC

DRIVER

COMP

GATE

DIMOUT

VOUT

PWM

Css

ISENSE

CS