bd9842fv ROHM Co. Ltd., bd9842fv Datasheet - Page 7

• Manufacturer Part Number: bd9842fv
• Description: Dual-output High Voltage High-frequency Step-down Switching Regulator Controller Type
• Manufacturer: ROHM Co. Ltd.

●Operation description of each block and function
1) REG (reference voltage unit)
2) ERR Amp 1/2 (error amplifier)
3) OSC (triangular wave oscillating unit)
Oscillation frequency is determined by externally mounted C
ICT : CT sink/source current 200 micro A typ
ΔVosc : Triangular wave amplifying voltage＝(Vt0-Vt100)＝0.50 V typ.
External input voltage range
Standard external C
As for REG (2.5V), reference voltage (2.5V) stabilized better than supply voltage input to VCC terminal (pin 12) is
supplied as an operation voltage of IC internal circuit, as well as output outside through VREF terminal (pin 20). Insert a
capacitor of 0.1 micro F to VREF terminal.
As for REG (VCC-5V), voltage of VCC-5V is supplied as power supply (LDO) of driver circuit (DRV) of OUT terminal (pin
10 and 11), as well as output outside through C5V terminal (pin 9). Insert a capacitor of 1 micro F to VCC terminal of C5V
terminal.
In step-down application, inverting input INV (pin 4 and 17) of
error amplifier detects output voltage by sending back feedback
current from final output stage (on load side) of switching
regulator. R1 and R2 connected to this input terminal are
resistor for setting output voltage. Non-inverting input of
amplifier is a reference input of error amplifier itself by adding
reference voltage (1.0V) inside IC.
Rf and Cf connected between FB (pin 5 and 16), which is output
from error amplifier, and INV (pin 4 and 17) are for feedback of
error amplifier, and allows setting of loop gain.
FB is connected to PWM Comp 1/2 and supplied as
non-inverting input.
Setting of output voltage (Vo) is as follows:
Generates triangular wave for inputting to PWM Comp 1/2.
First, timing capacitor C
generated inside IC. When CT voltage reaches 2.0 V typ, the comparator is switched, and then C
constant current (200 micro A). Then, when CT voltage reaches 1.5V, the comparator is switched again, and C
charged again. This repetition generates triangular wave.
Here, error from theoretical formula is caused by delay of
internal circuit at a high frequency. See the graph in Fig 31 for
setting.
This triangular wave can be taken out through CT terminal. It is
also possible to input the oscillator externally by switching to
slave mode described later. Waveform input here in principle
must be triangular wave of Vpeak = (1.5V ⇔ 2.0V) equivalent to
internal oscillation circuit.
Fosc ≒ ICT/(2・C
V
C
CT :
CT :
Vo
1.4 V < V
MIN.47 pF – MAX.3000 pF
CT
=
range
CT <
R1+R2
CT
CT
R2
･ΔVosc)
2.3 V
connected between CT terminal (pin 1) and GND is charged by constant current (200 micro A)
× 1.0V
7/15
CT
through theoretical formula below:
R 1
R 2
10000
1000
Vo
100
10
10
Rf
Cf
CT timing capacity (pF)
5/16
4/17
100
INV
Fig.30
1V
FB
Ta=25℃
Fig.31
CT
1000
ErrAmp1/2
is discharged by
10000
CT
is