BD9483F Datasheet, PDF (19/32 Pages) Rohm – White LED Driver for large LCD Panels (DCDC Converter type)

BD9483F,FV
Datasheet
●3.5.4 MOSFET selection
Though there is no problem if the absolute maximum rating is larger than the rated current of the inductor L, or is
larger than the sum of the tolerance voltage of C_OUT and the rectifying diode V_F. The product with small gate
capacitance (injected charge) needs to be selected to achieve high-speed switching.
* One with over current protection setting or higher is recommended.
* The selection of one with small on resistance results in high efficiency.
●3.5.5 Rectifying diode selection
A schottky barrier diode which has current ability higher than the rated current of L, the reverse voltage larger than the
tolerance voltage of C_OUT, and the low forward voltage VF especially needs to be selected.
●3.6 Loop compensation
A current mode DCDC converter has each one pole (phase lag) f_p due to CR filter composed of the output capacitor
and the output resistance (= LED current) and zero (phase lead) f_Z by the output capacitor and the ESR of the
capacitor.
Moreover, a step-up DCDC converter has RHP zero (right-half plane zero point) f_ZRHP which is unique with the boost
converter. This zero may cause the unstable feedback. To avoid this by RHP zero, the loop compensation that the
cross-over frequency f_c set as following, is suggested.
fc = f_ZRHP /5 (f_ZRHP: RHP zero frequency)
Considering the response speed, the below calculated constant is not always optimized completely. It needs to be
adequately verified with an actual device.
VIN
VOUT
L
VOUT
RESR
RCS
COUT
ILED
-
FB
gm
+
RFB1
CFB2
CFB1
Figure 31.
The output voltage block
Figure 32.
The error amp block
i.
Calculate the pole frequency fp and the RHP zero frequency f_ZRHP of DC/DC converter
f
p

I
LED
2π  V  C
　[Hz] 　
OUT
OUT
Where I_LED = the summation of LED current,
f _ZRHP

V_OUT  (1
2π  L 
D  ^{VOUT  VIN}
 D)² _{　[Hz]　}
I _LED
　 ^(Continuous
　
Current
Mode)
V_OUT
ii.
Calculate the phase compensation of the error amp output (f_c = f_ZRHP/5)
R
FB1

5
f _RHZP  R _CS 
f _p  gm  V_OUT
I _LED
 (1
D)
　[Ω]
　
Where gm  4.0 10^4[S ]
C _FB1

1
2π  R _FB1  f _p
　[F]　　
iii.
Calculate zero to compensate ESR (R_ESR) of C_OUT (electrolytic capacitor)
C _FB2

R _ESR  C _OUT
R _FB1
　[F]　
*When a ceramic capacitor (with R_ESR of the order of milliohm) is used to C_OUT, the operation is stabilized by
insertion of C_FB2.
To improve the transient response, R_FB1 need to be increase, C_FB1 need to be decrease. It needs to be adequately verified
with an actual device in consideration of vary from parts to parts since phase margin is decreased.
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28.Nov.2013 Rev.003