BD9394FP Datasheet, PDF (21/31 Pages) Rohm – White LED Driver for large LCD Panels (DCDC Converter type)

BD9394FP, BD9394EFV
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
ILED
VOUT
RESR
RCS
COUT
-
FB
gm
+
RFB1
CFB2
CFB1
Output Block
Error Amplifier Block
i. Find pole f_p and RHP zero f_ZRHP 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 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)
　[Ω]
　
C _FB1
=
2π
1
× R_FB1 ×
f_p
　[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
21/28
TSZ02201-0F1F0C100180-1-2
09.Nov.2012 Rev.002