BD9489F Datasheet, PDF (24/39 Pages) Rohm – DCDC converter with current mode

BD9489F
Datasheet
3.4.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 follows, is suggested.
fc = f_ZRHP /5 (f_ZRHP: 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.
Figure 35. Output stage and error amplifier diagram
i.
Calculate the pole frequency fp and the RHP zero frequency f_ZRHP of DC/DC converter
f_p

I_LED
2  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} (C_　ontinuous Current Mode)
V_OUT
ii.
Calculate the phase compensation of the error amp output (f_c = f_ZRHP/5)
R_FB1

f_RHZP  R_CS  I_LED
5  f_p  gm  V_OUT  (1  D)
　[]
　
CFB1 
1
2π  RFB1 fc

2π

5
RFB1 
fZRHP
[F]
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, R_FB1 needs to be increased, and C_FB1 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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