Technical Note
10/16
BD9862MUV
www.rohm.com
2010.03 - Rev.A
© 2010 ROHM Co., Ltd. All rights reserved.
●Method to select the application parts
1) Setting of output voltage
The output voltage VOUT is set by dividing the resistance of the external resistor.
CH1,CH2:VOUT=1+R2/R1
CH3:VOUT=-((R4/R3)+Voffset)
R1: Feedback resistor (GND side), R2: Feedback resistor (VOUT side), R3: Feedback resistor (VREF side),
R4: Feedback resistor (VOUT side)
2) Setting of the output inductor
The maximum current I
Lpeak
that flows in the inductor is calculated by the sum of the average current I
―
L
and the maximum
value of ripple current ⊿I
L.
I
Lpeak
= I
―
L
+ ⊿I
L.
Generally ⊿I
L.
is set to about 30% of I
―
L
.
The average current I
―
L
and the ripple current ⊿I
L.
are calculated according to the following formulas.
maxout
minin
out
max
L
I
V
V
I
outosc
inout
min
in
L
VLf2
VVV
I
⊿
L: value of inductance f
OSC
: switching frequency V
inmax
: maximum input voltage V
inmin
: minimum input voltage
Vout: set value of output voltage
Please set in such a way thatI
Lpeak
(the rated value of inductor current) is not exceeded. If I
Lpeak
exceeded, then the
efficiency is lowered extremely and damage to the inductor is caused. Please set in such a way that a good margin is left
because the inductance varies in value.
3) Setting of output capacitor
The capacitance & ESR of the output capacitor is influenced a great deal by output voltage ripple. Moreover, PFM mode
intentionally makes the switching intermittent, so the output voltage ripple becomes larger compared with PWM mode.
Please use an appropriate capacitor according to the service condition. In addition, please be sure to connect a ceramic
capacitor of 1µF to REGOUT terminal.
It is assumed that this IC uses a multilayer ceramic capacitor. For small multilayer ceramic capacitors such as Size 1608
etc., its actual capacitance may be lower than its nominal one because of the voltage that is bypassed. Please check to
confirm various characteristics such as DC bypass etc. before use.
4) Setting of flying capacitor
Please set the capacitance of the flying capacitor of the start-up charge pump not to exceed 1/10 of the capacitance of the
CPOUT output capacitor. If it is more than 1/10 of the capacitance, damage may be caused.
5) Setting of the input capacitor
A bypass capacitor for input is necessary to the VBAT terminal. Due to input & output voltage, load and wiring pattern etc.,
the actual capacitance is different from the necessary one, so please carefully check to confirm.
6) Setting of CR for phase compensation
The CR for phase compensation is varied due to the characteristics of the capacitor & inductor,
which are used in the
output part, the input & output voltage and the load current etc. The phase-compensation CR constant in a recommended
circuit diagram is set according to the service conditions, but applications under other conditions than the various
conditions mentioned will cause oscillation instability etc. Please contact our technical service department if any conditions
are changed.
7) Setting of schottky diode in the output part
Please use a schottky diode with an allowable current more than I
Lpeak
for the output part. Furthermore, it is necessary that
the maximum reverse voltage is more than output voltage. Generally speaking, more lower the forward voltage, the higher
the efficiency.
8) Setting of UVLO voltage
The VULO release voltage VUVLO can be set according to the following formula:
VUVLO=1+R2/R1(R1=GND-side resistance R2=VBAT-side resistance)
If you want to make the start-up of the IC to lag behind the rising edge of VBAT, connect a capacitor to the UVLOSET
terminal and set the time constant.
9) Setting of oscillating frequency
Oscillating frequency can be adjusted by a resistor connected to the RT terminal.
The CH1 oscillating frequency fosc1 is determined by the formula shown below:
fosc1=1/(8×10
-12
×R
RT
+4×10
-8
)
The frequency calculated by the formula shown above is a theoretical value, so please refer to the above-mentioned
reference data「RT resistance vs. CH1 switching frequency characteristic」for actual frequency.
