BD9415FS Datasheet, PDF (20/34 Pages) Rohm – White LED Driver for 4Ch Large LCD Panels (DC/DC converter type)

BD9415FS
3.4 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 follows, is suggested.
fc = fZRHP /5 (fZRHP: RHP zero frequency)
Considering the response speed, the calculated constant below is not always optimized completely. It needs to be
adequately verified with an actual device.
VIN
VOUT
L
RCS
VOUT
RESR
COUT
ILED
-
gm
+
FB
RFB1
CFB1
CFB2
Figure 23. Output stage and error amplifier diagram
i.
Calculate the pole frequency fP and the RHP zero frequency fZRHP of DC/DC converter
f_P

2
I _LED
V_OUT  C_OUT
[Hz]
f _ZRHP

V_OUT  (1  D)²
2  L  I_LED
[Hz]
Where I_LED  the summation of LED current,
D  ^{VOUT  VIN}
V_OUT
(Continuous Current Mode)
ii.
Calculate the phase compensation of the error amp output(fc = fZRHP/5)
R_FB1

5
f_RHZP  R_CS 
f_P  gm V_OUT
I _LED
 (1 
[]
D)
C_FB1 
2
1
 R_FB1 
f_C

2
5
 R_FB! 
[F]
f _ZRHP
gm  4.0 10^4[S]
Above equation is described for lighting LED without the oscillation. The value may cause much error if the quick
response for the abrupt change of dimming signal is required.
To improve the transient response, RFB1 needs to be increased, and CFB1 needs to be decreased. It needs to be
adequately verified with an actual device in consideration of variation from parts to parts since phase margin is
decreased.
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TSZ02201-0F2F0C100140-1-2
23.May.2016 Rev.001