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BD8151EFV-E2

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18
BD8151EFV, BD8157EFV
Technical Note
10/17
www.rohm.com
2009.07 - Rev.B
© 2009 ROHM Co., Ltd. All rights reserved.
(4) Selecting the output rectification diode
Schottky barrier diode is recommended as the rectification diode to use at the DC/DC converter output stage. Select the
diode paying attention to the max. inductor current and max. output voltage.
Max. Inductor current I
INMAX
+ ∆I
L
< Rating current of diode
Max. output voltage V
OMAX
< Rating voltage of diode
Since each parameter has 30% to 40% of dispersion, be sure to design providing sufficient margins.
(5) Design of the feedback resistor constant
Refer to the following equation to set the feedback resistor. As the setting range, 10 k to 330 k is recommended. If the
resistor is set to a 10 k or lower, it causes the reduction of power efficiency. If it is set to 330 k or larger, the offset
voltage becomes larger by the input bias current 0.4 µA (Typ.) in the internal error amplifier.
Step-up
Vo =
R8 + R9
1.245 [V]
R9
(6) Setting the soft start time
Soft start is required to prevent the coil current at the time of startup from increasing
and the overshoot of the output voltage at the starting time. Fig. 32 shows the
relation between the capacitance and soft start time. Please refer to it to set the
capacitance.
As the capacitance, 0.001 µF to 0.1µF is recommended. If the capacitance is set to
0.001 µF, the overshooting may occur on the output voltage. If the capacitance is
set to 0.1 µF or larger, the excessive back current flow may occur in the internal
parasitic elements when the power is turned OFF and it may damage IC. When the
capacitor of 0.1 µF or larger is used, be sure to insert a diode to Vcc in series, or a
bypass diode between the SS pin and VCC.
Fig. 33 Bypass Diode Example
When there is the startup relation (sequences) with other power supplies, be sure to use the high accuracy product (such as X5R).
Soft start time may vary according to the input voltage, output voltage loads, coils and output capacity. Be sure to verify the
operation using the actual product.
(7) Setting the ENB pin
When the ENB pin is set to Hi, the internal circuit becomes active and the DC/DC converter starts operating. When it is set
to Low, the shut down is activated and all circuits will be turned OFF.
(8) Setting the frequency by FCLK
It is possible to change the switching frequency by setting the FCLK pin to Hi or Low. When it is set to Low, the product
operates at 600 kHz (Typ.). When it is set to Hi, the product operates at 1,200 kHz (Typ.).
0.01
0.1
1
10
0.001 0.01 0.1
SS CAPACITANCE[uF]
DELAY TIME[ms]
Fig. 31Feedback Resistance Setting
Fig.32 SS Pin Capacitance vs
Delay Time
VCC
Output pin
Back current prevention diode
Bypass diode
＋
－
Vo
R8
R9
ERR
Reference voltage 1.245 V
FB
2
BD8151EFV, BD8157EFV
Technical Note
11/17
www.rohm.com
2009.07 - Rev.B
© 2009 ROHM Co., Ltd. All rights reserved.
(9) Setting R
C
, C
C
of the phase compensation circuit
In the current mode control, since the coil current is controlled, a pole (phase lag) made by the CR filter composed of the
output capacitor and load resistor will be created in the low frequency range, and a zero (phase lead) by the output
capacitor and ESR of capacitor will be created in the high frequency range. In this case, to cancel the pole of the power
amplifier, it is easy to compensate by adding the zero point with C
C
and R
C
to the output from the error amplifier as shown
in the illustration.
Open loop gain
Pole at the power amplification stage
When the output current reduces, the load resistance
R
O
increases and the pole frequency lowers.
Error amplifier phase compensation
Zero at the power amplification stage
When the output capacitor is set larger, the pole
frequency lowers but the zero frequency will not
change. (This is because the capacitor ESR
becomes 1/2 when the capacitor becomes 2 times.)
It is possible to realize the stable feedback loop by canceling the pole fp (Min.), which is created by the output capacitor
and load resistor, with CR zero compensation of the error amplifier as shown below.
fz (Amp.) = fp (Min.)
1
=
1
2 Rc Cc 2
Romax
Co
As the setting range for the resistor, 1 k to 10 k is recommended. When the resistor is set to 1 k or lower, the effect by
phase compensation becomes low and it may cause the oscillation of output voltage. When it is set to 10 k or larger, the
COMP pin becomes Hi-Z and the switching noise becomes easy to superpose. Therefore the stable switching pulse
cannot be generated and the irregular ripple voltage may be generated on the output voltage.
As the setting range for the capacitance, 3,300 pF to 10,000 pF is recommended. When the capacitance is set to 3,300 pF
or lower, the irregular ripple voltage may be generated on the output voltage due to the effect of switching noise. When it is
set to 10,000 pF or larger, the response becomes worse and the output voltage fluctuation becomes large. Accordingly it
may require the output capacitor which is larger than the necessary value.
1
fp =
2 Ro Co
1
fz (ESR) =
2 E
SR Co
1
fp(Min) =
2 Ro
Max Co
[Hz]
(At light-load)
fz(Max) =
1
[Hz]
(At heavy-load)
fp(Min)
fp(Max)
fz(ESR)
A
0
-90
0
Gain
【dB】
Phase
【deg】
l
OUTMin
l
OUTMax
0
0
A
-90
Gain
【dB】
Phase
【deg】
Fig. 34 Gain vs Phase
Fig. 35 Application Circuit Diagram
[Hz]
[Hz]
[Hz]
V
CC
Rc
Cc
Cin
Vcc,PVcc
GND,PGND
SW
COMP
Co
ESR
Ro
Vo
L
2
RoMin Co
1
fp (Amp.) =
2
Rc Cc
[Hz]
BD8151EFV, BD8157EFV
Technical Note
12/17
www.rohm.com
2009.07 - Rev.B
© 2009 ROHM Co., Ltd. All rights reserved.
(10) Using the buffer amp and VCOM
The 4-channel buffer amp and 1-channel VCOM output are used to generate the gamma compensation voltage that is
input to the source driver. The VS pin serves as the power supply for the buffer amp and VCOM.
Use caution as the gamma correction buffer amp and VCOM have different output current capacities. A range from I/O
power supply to ground potentials can be set for the built-in buffer amplifier. If output voltage noise becomes problematic,
insert a 0.1 µF capacitor in the output circuit. A capacitance value of 0 pF to 1 µF is recommended for this capacitor. Large
capacitance values of 1 µF or larger may cause back current to flow through internal parasitic diodes in the event of a
supply voltage ground fault, causing damage to internal IC elements. For applications where such modes are anticipated,
implement a bypass diode or other preventive measure.
Vs
V1
V2
V3
V4
VS
VCOM voltage output
V1
V2
V3
V4
For gamma correction
Gamma correction voltage output
VCOMIN
V
IN1
VIN2
VIN3
V
IN4
Fig. 36 Example Buffer Amp Circuit
Fig. 37 Gamma Correction Voltage Startup Waveform
Wait for trigge
r
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18

BD8151EFV-E2

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ROHM Semiconductor
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LCD Drivers REGULATOR
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