BD9845FV

Technical Note

8/15

www.rohm.com

2011.08 - Rev.D

© 2011 ROHM Co., Ltd. All rights reserved.

4) Soft start (soft start function)

It is possible to provide SS terminal (13pin) with soft start function by

Soft start time TSS is shown by the formula below:

Tss = Css・

(Ex) When Css = 0.01 uF

Tss =

=

5 [msec]

In order to function soft start, time must be set longer enough than

start time of power supply and STB.

It is also possible to provide function of soft start by connecting the

on the right.

5) PWM Comp - DEAD TIME (Pause period adjusting circuit - dead time)

Dead time can be set by applying voltage dividing resistance between VREF and GND to DT terminal.

PWM Comp compares the input dead time voltage (DT terminal voltage) and error voltage from Err Amp (FB terminal

voltage) with triangular wave, and turns off and on the output. When dead time voltage < error voltage, duty of output is

determined by dead time voltage. (When dead time setting is not used, pull up DT terminal to VREF terminal with resistor

approx 10 k ohms.)

Dead time voltage VDT in Fig 32 is shown by the formula below:

VDT ＝ VREF・

Relation between VDT and Duty [See the graph on the right.]

Duty 100%

Duty 0%

min

typ

max

min

typ

max

When f = 100kHz

1.9

2.0

2.1

1.4

1.5

1.6

When f = 1.5MHz

1.95

2.1

2.25

1.35

1.5

1.65

Be careful when oscillation frequency is high, upper/lower limit of triangular

wave (Vt100/Vt0) is shifted by delay time of comparator to directions

expanding amplitude.

6) OCP Comp (overcurrent detection circuit)

This function provides protection by forcibly turning off the output when

abnormal overcurrent flows due to shorting of output, etc. When voltage

between terminal OCP+(8pin)/OCP-(9pin) monitoring the current with

sense resistor exceeds overcurrent detection voltage (50 mV typ), it is

determined as overcurrent condition, and switching operation is stopped

immediately by setting OUT to "H" and DT,SS (and FB) to "L".

It is automatically recovered when voltage between terminal OCP+/OCP-

is below overcurrent detection voltage.

In addition, although hysteresis, etc. are not set here, minimum detection

retention time (1.6ms typ) is set for suppressing the heating of FET, etc.

(See the timing chart.)

When the overcurrent detection circuit is not used, short-circuit both

terminal OCP+/OCP- to VCC pin.

R1＋R2

R2

Issso

Vinv

Css : SS terminal connection capacity

Vinv : Error amplifier reference voltage

(1V typ)

Issso : SS source current (2uA typ)

2×10

-6

0.01×10

Fig.32

Fig.33

[Unit : V]

1

1.2

1.4

1.6

1.8

2

2.2

2.4

100

1000

10000

fosc[KHz]

Vt100

Vt0

Fig.34

Fig.35

Direction

of current

Sense

resistor

VREF

DT

R1

R2

1

14

VREF

INV

SS

ErrAmp

2uA

Css

1Vtyp.

13

12

VIN

OCP+

OCP-

OCP Comp

50mVtyp.

8

9