bd9845fv ROHM Co. Ltd., bd9845fv Datasheet - Page 7

• Manufacturer Part Number: bd9845fv
• Description: Single-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 (error amplifier)
3) OSC (triangular wave oscillating unit)
Oscillation frequency is determined by externally mounted C
ICT : CT sink/source current 200 uA 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 is supplied as an
operation voltage of IC internal circuit, as well as output outside through VREF terminal. Insert a capacitor of 1uF 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, as
well as output outside through C5V terminal. Insert a capacitor of 1uF to VCC terminal of C5V terminal.
In step-down application, inverting input INV 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, which is output from error amplifier,
and INV are for feedback of error amplifier, and allows setting of loop
gain.
FB is connected to PWM Comp and supplied as non-inverting input.
Setting of output voltage (Vo) is as follows:
Generates triangular wave for inputting to PWM Comp.
First, timing capacitor C
inside IC. When CT voltage reaches 2.0 V typ, the comparator is switched, and then C
current (200 uA). Then, when CT voltage reaches 1.5V, the comparator is switched again, and C
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 and GND is charged by constant current (200 uA) generated
× 1.0V
7/15
CT
through theoretical formula below:
10000
1000
R 1
R 2
100
10
Vo
10
CT timing capacity (pF)
Rf
Cf
100
12
11
CT
Fig.30
Ta=25℃
is discharged by constant
INV
FB
1V
CT
Fig.31
is charged again. This
1000
ErrAmp
10000