bd9202efs ROHM Co. Ltd., bd9202efs Datasheet - Page 13

• Manufacturer Part Number: bd9202efs
• Description: White Backlight Led Drivers For Medium To Large Lcd Panels Swreg Type
• Manufacturer: ROHM Co. Ltd.

● Phase Compensation Configuration Method
・Stability of Applications
© 2009 ROHM Co., Ltd. All rights reserved.
BD9202EFS
www.rohm.com
Feedback stability conditions for reverse feedback are as follows:
Furthermore, DC/DC converter applications have been sampled by the switching frequency, so the GBW of the entire line is
configured at less than 1/10 of the switching frequency. To sum up, the characteristics aimed for by applications are as
outlined below:
Therefore, to improve the response with GBW limitations, it is necessary to make the switching frequency higher.
The trick to secure stability by phase compensation is to cancel out the second phase lag (-180°) generated by LC
resonance with two phase leads (insert two phase leads).
Phase leads are by the output condenser’s ESR component or the error amp output Comp terminal’s CR.
With DC/DC converter applications, there is always a LC resonance circuit at the output, so the phase lag at that section is
180°. If the output condenser has large ESR (several Ω), such as an aluminum electrolysis condenser, a phase lead of
+90°is generated, and the phase lag is -90°.
condenser, insert R for the ESR component.
With the changes in phase characteristics by caused by ESR, the number of phase leads to be inserted is one.
The frequency configuration to insert phase lead should ideally be configured close to the LC resonance frequency in order to
cancel LC resonance.
This configuration is for simplicity, and no detailed calculations have been carried out, so there are times when adjustments
on the actual product are necessary. These characteristics change depending on substrate layout and load conditions, so
ample confirmation is necessary during design for mass production.
4.
5.
6.
・ Phase-lag of less than 150°(phase margin of over 30°) when gain is 1 (0dB)
・ Phase-lag less than 150°(phase margin over 30°) when gain is 1 (0dB)
・ The GBW (frequency of gain 0dB) at that time is less than 1/10 of switching frequency
VCC
Because there are no switches on the route between the VCC and the VO with regular boost applications, in case of
To initiate soft start of the load switch, insert capacity between the gate and source.
There are no problems as long as the absolute maximum rating of the current rating is L and the pressure threshold
and rectification diode of C are at least VF, but in order to actualize high-speed switching, one with small gate
capacity (injected charge amount) should be selected.
※ Excess of over current protection configuration recommended
※ Higher efficiency can be gained if one with smaller ON resistance is selected.
Select a Schottky barrier diode with higher current ability than the current rating of L and higher reverse pressure
threshold than the threshold of C, particularly with low forward voltage VF.
Selection of MOSFET for Load Switch, and its Soft Start
Selection of Switching MOSFET
Selection of Rectification Diode
3.
an output short circuit the coil or rectification diode may be damaged. To prevent this from happening, a PMOSFET
load switch should inserted between the VCC and the coil. PMOSFET with better ability to withstand pressure
between gate-source and drain-source than VCC should be selected.
VCC
Selection of Input Condenser (Cin)
CO
L
VOUT
I
L
fr
Resonance
phase lag -180°

2
Cin
CO

VOUT
1
LCO
]
[
Hz
point
The input condenser selected should have low ESR with enough capacity to be
The ripple current IRMS can be derived from formula (5).
Furthermore, because it relies heavily on the characteristics of the power
used for input, the wiring pattern on the substrate and MOSFET gate drain
capacity, the usage temperature, load range and MOSFET conditions must
be adequately confirmed.
IRMS
compatible large ripple current, in order to prevent large transient voltage.
When using an output condenser with low ESR such as a ceramic

13/19
IOUT
VCC

(
VOUT
R
CO
ESR
VOUT

VOUT
VCC
fr
fESR
)


VOUT
2


2
1

LCO
RESRCo
]
[
A
1
・・・・・
]
[
Hz
]
[
Hz
Technical Note
2009.04 - Rev.A
Resonance
-180°
Phase lead
Phase lag -90°
(5)
point