BD9420F Datasheet, PDF (20/31 Pages) Rohm – White LED Driver for large LCD Panels (DCDC Converter Type)

BD9420F
Datasheet
●Phase Compensation Setting Procedure
DC/DC converter application for current mode control includes one each of pole f_p (phase delay) by CR filer consisting
of output capacitor and output resistor (i.e., LED current) and zero (phase lead) f_Z by the output capacitor and capacitor
ESR.
Furthermore, the step-up DC/DC converter includes RHP zero “f_ZRHP” as the second zero. Since the RHP zero has
phase delay (90) characteristics like the pole, the crossover frequency f_c should be set to not more than RHP zero.
VIN
VOUT
L
VOUT
RESR
RCS
COUT
ILED
-
FB
gm
+
RFB1
CFB2
CFB1
i.
Find Pole f_p and RHP zero f_ZRHP 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]　　
Here, _ILED　＝LED Total current[A], _{D  VOUT  VIN 　}
V_OUT
ii.
Find phase compensation to be inserted to error amplifier.(set f_c is 1/5 to f_ZRHP)
R_FB1

5
f _RHZP  R_CS 
f _p  gm V_OUT
I _LED
 (1 
D)
　[]
　
C _FB1

2
1
 R_FB1 
f_p
　[F ]　　
Here, _{gm  4.0 10}4_{[S ]}
iii.
Find zero used to compensate ESR (R_ESR) of C_OUT (electrolytic capacitor).
C _{FB 2}

R_ESR  C_OUT
R_FB1
　[F ]　　
※Even if a ceramic capacitor (R_ESR of the order of milliohms) for C_OUT, it is recommended to insert C_FB2 for
stable operation.
To improve transient response, it is necessary to increase R_FB1 and reduce C_FB1. However, this improvement reduces
a phase margin. To avoid this problem, conduct thorough verification, including variations in external components, on
practical systems.
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TSZ22111 • 15 • 001
20/28
TSZ02201-0F1F0C100300-1-2
31.Jan.2014 Rev.002