BD93942F Datasheet, PDF (19/30 Pages) Rohm – White LED Driver for large LCD Panels (DCDC Converter type)

BD93942F
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
Figure 21. Output stage and error amplifier diagram
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]　　
Where I_LED　 Total LED current [A],
D  ^{VOUT  VIN} 　
V_OUT
ii. Find phase compensation to be inserted in the error amplifier. (Set f_c to 1/5 of 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_C
　
2
5
 R_FB1 
f _ZRHP
[F]
　
where _{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 ]
　
Note: 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
19/26
TSZ02201-0F5F0C100050-1-2
12.Aug.2015 Rev.002