BD9845FV_11 Datasheet, PDF (7/16 Pages) Rohm – Single/Dual-output High-frequency Step-down Switching Regulator(Controller type)

BD9845FV
Technical Note
●Operation description of each block and function
1) REG (reference voltage unit)
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.
2) ERR Amp (error amplifier)
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:
Vo = ^R1+R2  1.0V
R2
Vo
R1
1V
ErrAmp
12
Rf
INV
R2
Cf
11
FB
Fig.30
3) OSC (triangular wave oscillating unit)
Generates triangular wave for inputting to PWM Comp.
First, timing capacitor C_CT connected between CT terminal and GND is charged by constant current (200 uA) generated
inside IC. When CT voltage reaches 2.0 V typ, the comparator is switched, and then C_CT is discharged by constant current
(200 µA). Then, when CT voltage reaches 1.5V, the comparator is switched again, and C_CT is charged again. This
repetition generates triangular wave.
Oscillation frequency is determined by externally mounted C_CT through theoretical formula below:
Fosc ≒ ICT/(2・CCT･ΔVosc)
ICT : CT sink/source current 200 uA typ
ΔVosc : Triangular wave amplifying voltage＝(Vt0-Vt100)＝0.50 V typ.
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.
10000
1000
External input voltage range
VCT : 1.4 V < VCT < 2.3 V
Standard external C_CT range
CCT : MIN.47 pF – MAX.3000 pF
100
Ta=25℃
10
10
100
1000
10000
CT timing capacity (pF)
Fig.31
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7/15
2011.08 - Rev.D