BD9845FV Datasheet, PDF (8/16 Pages) Rohm – Single/Dual-output High-frequency Step-down Switching Regulator(Controller type)

BD9845FV
Technical Note
4) Soft start (soft start function)
It is possible to provide SS terminal (13pin) with soft start function by
connecting C_SS as shown on the right.
Soft start time TSS is shown by the formula below:
Tss = Css・
Vinv
Issso
Css : SS terminal connection capacity
Vinv : Error amplifier reference voltage
(1V typ)
Css
Issso : SS source current (2uA typ)
(Ex) When Css = 0.01 uF
Tss =
0.01×10^-6×1
2×10^-6
= 5 [msec]
VREF
2uA
SS
13
1Vtyp.
INV
12
ErrAmp
Fig.32
1 _VREF
R1
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_DT) to DT terminal (14pin) as shown
on the right.
C DT
14 _DT
R2
Fig.33
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・
R2
R1＋R2
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
[Unit : V]
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.
2.4
2.2
2
1.8
1.6
1.4
1.2
1
100
Direction
of current
VIN
Vt100
Vt0
1000
fosc[KHz]
Fig.34
10000
Sense
resistor
OCP+
8
OCP-
9
50mVtyp.
OCP Comp
Fig.35
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8/15
2009.05 - Rev.A