bd9845fv ROHM Co. Ltd., bd9845fv Datasheet - Page 8

• Manufacturer Part Number: bd9845fv
• Description: Single-output High Voltage High-frequency Step-down Switching Regulator Controller Type
• Manufacturer: ROHM Co. Ltd.

4) Soft start (soft start function)
Soft start time TSS is shown by the formula below:
(Ex) When Css = 0.01 uF
5) PWM Comp - DEAD TIME (Pause period adjusting circuit - dead time)
6) OCP Comp (overcurrent detection circuit)
Dead time voltage VDT in Fig 32 is shown by the formula below:
Tss = Css・
Relation between VDT and Duty [See the graph on the right.]
It is possible to provide SS terminal (13pin) with soft start
function by connecting C
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
resistor (R1/R2) and capacitor (C
the right.
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.)
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.
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.
When f = 100kHz
When f = 1.5MHz
Tss =
VDT ＝ VREF・
=
5 [msec]
0.01×10
Issso
Vinv
2×10
1.95
min
1.9
R1＋R2
-6
SS
-6
R2
×1
as shown on the right.
Css : SS terminal connection capacity
Vinv : Error amplifier reference voltage
(1V typ)
Issso : SS source current (2uA typ)
Duty 100%
2.0
2.1
typ
DT
) to DT terminal (14pin) as shown on
max
2.25
2.1
1.35
min
1.4
8/15
Duty 0%
1.5
1.5
typ
[Unit : V]
max
1.65
1.6
Css
of current
Direction
resistor
C
INV
Sense
SS
DT
2.4
2.2
1.8
1.6
1.4
1.2
13
12
2
1
100
VIN
Fig.32
1Vtyp.
OCP+
OCP-
Fig.33
VREF
R 1
R 2
8
9
Fig.35
Fig.34
fosc[KHz]
2uA
50mVtyp.
1000
14
1
VREF
DT
Vt100
OCP Comp
Vt0
ErrAmp
10000