BD9422EFV_15 Datasheet, PDF (19/30 Pages) Rohm – White LED Driver for large LCD panel

BD9422EFV
Datasheet
Selecting switching MOSFET
Though there is no problem if the absolute maximum rating is the rated current of L or (withstand voltage of C_OUT + rectifying
diode) VF or higher, one with small gate capacitance (injected charge) needs to be selected to achieve high-speed
switching.
* One with over current protection setting or higher is recommended.
* Selection of one with small ON resistance results in high efficiency.
Selecting rectifying diode
A schottky barrier diode which has current ability higher than the rated current of L, reverse voltage larger than withstand
voltage of C_OUT, and low forward voltage VF especially needs to be selected.
Selecting MOSFET for load switch and its soft-start
As a normal step-up DC/DC converter does not have a switch on the path from V_IN to V_OUT, output voltage is generated
even though IC is OFF. To keep output voltage at 0 V until IC works, PMOSFET for load switch needs to be inserted
between V_IN and the inductor. FAIL terminal needs to be used to drive the load switch. PMOSFET for the load switch of
which gate-source withstand voltage and drain-source withstand voltage are both higher than V_IN needs to be selected.
To provide soft-start for the load switch, a capacitor must be inserted among gates and sources.
●3.7 How to set phase compensation
DC/DC converter application controlling current mode has each one pole (phase lag) f_p due to CR filter composed of output
capacitor and output resistance (= LED current) and ZERO (phase lead) f_Z by output capacitor and ESR of the capacitor.
Moreover, step-up DC/DC converter has RHP ZERO f_ZRHP as another ZERO. Since RHP ZERO has a characteristic of
phase lag (-90°) as pole does, cross-over frequency f_c needs to be set at RHP ZERO or lower.
VIN
VOUT
L
ILED
VOUT
RESR
RCS
COUT
-
FB
gm
+
RFB1
CFB2
CFB1
Figure 26. Output part
Figure 27. Error Amplifier
i.
Determine Pole f_p and RHPZERO f_ZRHP of DC/DC converter:
f_p

2
I _LED
V_OUT  C_OUT
　 [Hz]
Here,、 I_LED　＝=sum of LED current,
　
D

V_OUT
f _ZRHP
 V_IN

V_OUT  (1  D)²
2  L  I_LED
　
V_OUT
　[Hz]　　
ii.
Determine Phase compensation to be inserted into error amplifier (with f_c set at 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_p
　[F ]　　
Here,
gm  1.036 10^3[S]　
iii.
Determine ZERO to compensate ESR (R_ESR) of C_OUT (electrolytic capacitor)
C _{FB 2}

R_ESR  C_OUT
R_FB1
　[F ]
　
* When a ceramic capacitor (with R_ESR of the order of millimeters) is used to C_OUT, too, operation is
stabilized by insertion of R_ESR and C_FB2.
Though increase in R_FB1 and decrease in C_FB1 are necessary to improve transient response, it needs to be adequately
verified with an actual device in consideration of variation between external parts since phase margin is decreased.
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TSZ22111・15・001
19/26
TSZ02201-0F1F0C100210-1-2
1.Sep.2015 Rev.006