BD9285F ROHM [Rohm], BD9285F Datasheet - Page 25

• Manufacturer Part Number: BD9285F
• Description: Boost 1channel white LED driver For large LCDs
• Manufacturer: ROHM [Rohm]

●3.6. Loop compensation
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© 2012 ROHM Co., Ltd. All rights reserved.
TSZ22111・15・001
BD9285F
To improve the transient response, R
with an actual device in consideration of vary from parts to parts since phase margin is decreased.
A current mode DCDC converter has each one pole (phase lag) f
and the output resistance (= LED current) and zero (phase lead) f
capacitor.
Moreover, a step-up DCDC converter has RHP zero (right-half plane zero point) f
converter. This zero may cause the unstable feedback. To avoid this by RHP zero, the loop compensation that the
cross-over frequency f
fc = f
Considering the response speed, the below calculated constant is not always optimized completely. It needs to be
adequately verified with an actual device.
i.
ii.
iii.
VIN
ZRHP
R
L
CS
/5 (f
Calculate the pole frequency fp and the RHP zero frequency f
Calculate the phase compensation of the error amp output (f
Calculate zero to compensate ESR (R
*When a ceramic capacitor (with R
insertion of C
Where
Where I
C
R
f
ZRHP
p
FB
FB
1
2
: RHP zero frequency)
LED
2
C
R
ESR
OUT
gm
c
5
R
= the summation of LED current,
set as following, is suggested.
FB2.
ESR
Figure 31. the circuit of output stage and the error amplifier
V
V
f
OUT
R
I
OUT
p
4
f
LED
FB
RHZP
0 .
C
1
gm
OUT
FB1
C
10
OUT
R
need to be increase, C
]
[
V
4
CS
F
[
OUT
]
[
S
Hz
]
V
I
OUT
ESR
LED
I
LED
1 (
of the order of milliohm) is used to C
ESR
25/36
D
) of C
)
]
[
FB1
OUT
D
f
ZRHP
need to be decrease. It needs to be adequately verified
(electrolytic capacitor)
p
Z
due to CR filter composed of the output capacitor
V
by the output capacitor and the ESR of the
OUT
V
C
V
OUT
2
OUT
FB
+
-
1
c
V
gm
ZRHP
IN
= f
L
1 (
2
ZRHP
ZRHP
of DC/DC converter
I
TSZ02201-0F1F0C100030-1-2
LED
D
/5)
OUT
R
which is unique with the boost
)
(Continuous Current Mode)
1
2
FB
FB
R
C
, the operation is stabilized by
]
[
1
FB1
FB1
Hz
f
11.Jul.2012 Rev.001
p
]
[
F
C
Datasheet
FB2