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Large Current External FET Controller Type Switching Regulators
Dual-output Step-up, Negative Voltage, Step-down Switching Regulators (Controller type)
Sep. 2008
2/16
3/16
4/16
5/16
6/16
7/16
8/16
9/16
4) SCP Comp ( Timer-latch type short-circuit prevention circuit ) When the output voltage malfunctions because of a short circuit, this function is used to turn OFF the output transistor forcedly, in order to protect the system. If the output voltage drops in realtion to the set voltage level, an error will be amplified through the Err Amp and the voltage at the FB1/FB2 pins will reach the low voltage side. If SCP Comp sets the voltage below the threshold level for either the FB1 or FB2, (i.e., 1.05Vtype for the BA9741F/FS / BA9743AFV or 0.25Vtype for the BA9744FV), the short prevention circuit will be activated to start the charging of the capacitor of SCP pin. When VIT=1.64Vtyp for the BA9741F/FS / BA9743AFV or VIT=1.18Vtyp for the BA9744FV is achieved, the output transistor will turn OF F. The time is set by the capacitor (Cscp) connected to the SCP pin (pin 15) . The time TSCP is obtained by the formula shown below: TSCP = V IT +VSTB . Cscp ISCP
For example, in the case of the BA9743AFV, assuming that C SCP =0.1 F, V IT =1.64 V, V STB =0.05V and ISCP =2.5A.
TSCP =
1.64 - 0.05 2.5
0.1 = 63.6mS
In order to stop the function of the short prevention circuit, short-circuit the SC P pin to GND .
5) PWM Comp 1/2 DEAD TIME (Rest period adjustment circuit / Dead time) This function can be set by dividing the resistance of DT1 and DT2 pins (pins 6 and 11) between V REF and GND. With PWM Comp, comparing the dead time voltage input and the error voltage from the Err Amp with the triangular wave, the output drive transistors are turned ON/OFF. When Dead time voltage>Error voltage, the output duty is determined by the dead time voltage. The dead time voltage VDT is obtained by the expression shown below. VDT = V REF . R2 R1 + R2 (BA9744FV) When VDT=0.49Vtyp: Duty 100% When VDT=1.01Vtyp: Duty 0% (Provided, however, that fosc=10kHz )
(BA9741F/FS/BA9743AFV) When VDT=1.48Vtyp: Duty 100% When VDT=1.97Vtyp: Duty 0% (Provided, however, that fosc=10kHz)
Note: If the oscillation frequency is at a high level, the upper/lower limits (Vt0/Vt100) of the triangular wave will be shifted in the direction in which the amplitude is developed. 6) PWM Comp 1/2 SS (Soft start function) By connecting the resistors R1 and R2, and the capacitor CD T to D T pins (pins 6 and 11), as shown in figure on the right, the soft start function is enabled.
7) Handling of pins of unused channel
Fig.35
Vcc
BA9741F/FS, BA9743AFV
BA9744FV
Fig.36
If channel 1 is only used, the unused channel should be handled as shown above.
Fig.37

Ordering part number
B
A
9
Part No. 9743 9744 9741
7
4
3
F
V
-
E
2
Part No.
Package F: SOP16 (BA9741F) FS: SSOP-A16 (BA9741FS) AFV: SSOP-B16 (BA9743AFV) FV: SSOP-B16 (BA9744FV)
Packaging and forming specification E2: Embossed tape and reel
Catalog No.08T678A '08.9 ROHM (c)
Appendix
Notes
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Appendix-Rev4.0

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