BD9757MWV_10 Datasheet, PDF (17/24 Pages) Rohm – Switching Regulator IC

BD9757MWV
Technical Note
●Setting method of IC peripheral components
(1) Design of feedback resistor constant
VOUT1～5
VREF6
VOUT7
VOUT8
R1
ERROR AMP
INV
R1
ERROR AMP
NON
R2
R2
R1
ERROR AMP
INV
R2
R1
ERROR AMP8V
INV8
R2
VREF
0.8V
VOUT6
Reference voltage is
connected to GND inside IC
VREF
1.0V
CH1～5 output voltage
VO  ^{R1 R2}  0.8 [V]・・・(1)
R2
CH7,8 output voltage
VO  ^{R1 R2} 1.0 [V]・・・(3)
R2
CH6 output voltage
VO   ^R2  1.0 [V]・・・(2)
R1
CH8 output voltage
IO  ^INV8I [A] ・・・(4)
R3
VREF
1.0V
Output level
L takes priority
ERROR AMP7I
INV8I
R3
VREF
UPIC8 Setting according to
number of counts
(Refer to Fig.29 of page 15)
Fig.31 Feedback resistor setting method
(a) CH1 ~ 5 setting
The reference voltage of CH1 ~ 5’s ERROR AMP is 0.8V. Please refer to Formula (1) in Fig.31 for determining the
output voltage.
This IC incorporates built-in phase compensation. Please refer to Applied Circuit Diagram for setting the values of R1
& R2 and ensure that the setting values of R1 & R2 are of the order of several hundred kΩ.
(b) CH6 setting
The reference voltage of CH6’s ERROR AMP is connected to GND inside the IC. Therefore, a high-accuracy
regulator can be configured if setting by the feedback resistance between the outputs of VREF and CH5 as shown in
Fig.31. Please refer to Formula (2) in Fig.31 for determining the output voltage. R1 is recommended as more than
20kΩ because the current capacity of VREF is about 100μA.
(c) CH7 setting
The reference voltage of CH7’s ERROR AMP is 1.0V. Please refer to Formula (3) in Fig.31 for determining the output
voltage.
(d) CH8 setting
In the CH8 there are two ERROR AMPs which have different standards, and when used with constant current
feedback applied unilaterally to backlight etc., the over voltage protection can be operated unilaterally.
Over voltage setting and output current setting are respectively shown in Formula (3) and (4) in Fig.31.
The outputs of these two ERROR AMPs are controlled with L being given priority.
Therefore, when used under the control of only either of the two ensure that the INV terminal is used as GND Short.
CH6, CH7, and CH8 are of voltage mode control. Ensure that CH6, 7 and 8 are used by means of discontinuous
inductor current so as to secure the oscillation margin.
(2) Points for attention in terms of PCB layout of base-plate
○For a switching regulator, in principle a large current transiently flows through the route of power supply - coil - output
capacitor. Ensure that the wiring impedance is lowered as much as possible by making the pattern as wide as
possible and the layout as short as possible.
○Interference of power supply noise with feedback terminals (INV1 ~ 8I,NON6) may cause the output voltage to
oscillate. Ensure that the power supply noise’s interference is avoided by making the wiring between feedback
resistor and feedback terminal as short as possible.
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2010.11 - Rev.A