BD9285F Datasheet, PDF (25/39 Pages) Rohm – Boost 1channel white LED driver For large LCDs

BD9285F
Datasheet
●3.6. Loop compensation
A current mode DCDC converter has each one pole (phase lag) fp due to CR filter composed of the output capacitor
and the output resistance (= LED current) and zero (phase lead) fZ by the output capacitor and the ESR of the
capacitor.
Moreover, a step-up DCDC converter has RHP zero (right-half plane zero point) fZRHP 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 fc set as following, is suggested.
fc = fZRHP /5 (fZRHP: 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 circuit of output stage and the error amplifier
i.
Calculate the pole frequency fp and the RHP zero frequency fZRHP of DC/DC converter
f_p
=
2π
I _LED
×V_OUT × C_OUT
　[Hz]　　
f _ZRHP
=
V_OUT × (1 − D)²
2π × L × I_LED
　[Hz]　　
Where ILED = the summation of LED current,
D = ^{VOUT − VIN} 　　 ^{(Continuous Current Mode)}
ii.
Calculate the phase compensation of the error amp outpVut_OU(f_Tc = fZRHP/5)
R_FB1
=
5×
f _RHZP × R_CS ×
f _p × gm ×V_OUT
I _LED
× (1 −
D)
　[Ω]　　
C _FB1
=
2π
1
× R_FB1
×
f_p
　[F ]　　
Where _{gm = 4.0 ×10−4[S ]　}
iii.
Calculate zero to compensate ESR (RESR) of COUT (electrolytic capacitor)
C_{FB 2}
=
R_ESR × C_OUT
R_FB1
　[F ]　　
*When a ceramic capacitor (with RESR of the order of milliohm) is used to COUT, the operation is stabilized by
insertion of CFB2.
To improve the transient response, RFB1 need to be increase, CFB1 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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11.Jul.2012 Rev.001