BD9845FV Datasheet, PDF (12/16 Pages) Rohm – Single/Dual-output High-frequency Step-down Switching Regulator(Controller type)

BD9845FV
Technical Note
2) Example of overcurrent protection circuit
Insert a sense resistor between the source and VIN of output
Pch-FET for detecting overcurrent as shown in the figure.
Refer to the formula below for determining a sense resistor and
select permissible loss ensuring a margin.
VIN
OCP+
R1
C4
8
Rsense ＝ ^Vocpth
Vocpth=50mV
C1
C3
Iocp
Vocpth : Overcurrent detection voltage (50 mV typ)
OCP-
R2
Sre_セesni_{ンssteスor抵抗}
OCP comp
9
Iocp : Overcurrent detection setting current
Iocp is a peak current Isw (peak) here, and the amperage for
output load is an overcurrent setting amperage minus ripple
current component (Delta I_L/2), etc. (See the formula on P10.)
There is a time delay approx 200ns from detection until stop of
output is made (pulse of approx 100 ns causes delay time but
detection is made), and an error may be caused from the value
above.
In addition, input to overcurrent detection unit is such a
sensitive circuit, and wrong detection by noise may be possible.
When wrong detection occurs, try to eliminate noise by the
resistor R1 and R2 or capacitance C1, C2, C3, and C4 shown
above.
C2
OUT
6
Fig.38
3) Example of output ON/OFF control circuit
When stopping the whole circuit, set STB terminal to
"Low (STB<0.5V) to stop switching and reduce power
consumption of IC to 0 microA (typ).
Also when switching ON and OFF for each channel,
control is fixed to OFF by setting DT terminal of
desired channel to "Low (DT<1.25V)". This control is
independent for each channel, and when DT="L", SS
terminal and FB terminal are also discharged, and soft
start is enabled in restarting.
Each Ch
control signal
VREF
D
T
14
SS
13
To OUT for each Ch
DTcomp
1.25Vtyp.
Digital transistor, etc.
Fig.39
4) Example of master/slave (sync multi-ch output) operation circuit
This IC is set to slave mode by setting the input of STB terminal at 2.5V0.1V, and multi-channel output is enabled with
frequency synchronized. (Fig.40) However, CT terminal has high impedance in slave mode status, and triangular wave is
generated by CT waveform of master mode IC. Therefore the example of master slave circuit below is recommended
when starting and stopping in order to avoid malfunction by start/stop timing of master IC and slave IC. As for output, it is
recommended to control ON/OFF reliably with DT terminal.
Also, oscillation frequency is determined by capacitor (CCT)
connected to CT. When the slave IC is large in number as
well as oscillation frequency is high, parasitic capacity by
board wiring in contact with CT cannot be ignored, and preset
Mas^マte^スr S^タTB
STB信号
CT
Master IC
マスタIC
STB
CT
C_共o_通mmon
S_スlav_レe_ブIC_{IC ×N}
STB
frequency may be drifted. Be careful.
Example of master/slave circuit configuration is shown below.
Inマpuスt タa とsig同na期l oしf たhigh voltage
2_H.5_iV_ghs_電yn_圧ch₂r_.o₅n_Viz_のed_ä¿¡w_号ith_をm_入as_力ter.
If any other configuration is to be applied, inform our
personnel in charge.
Fig.40
Stand-by
ON/OFF
CT
Master IC
SEL VREF
STB
DT
CCT
0.1uF
10kΩ
CT
Slave IC
SEL VREF
STB
DT
0.1uF
10kΩ
Ch
ON/OFF
control
Ch
ON/OFF
control
DTC114Y
Fig 41. Example of master/slave
DTC114Y
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12/15
2009.05 - Rev.A