lb11947 Sanyo Semiconductor Corporation, lb11947 Datasheet - Page 7

• Manufacturer Part Number: lb11947
• Description: Pwm Current Control Type Dc Motor Driver + Switching Regulator
• Manufacturer: Sanyo Semiconductor Corporation

(Switching regulator, soft start function)
When the VCC output is about to rise to the target voltage of 5 V instantaneously at start of switching regulator (at VBB
power ON), the IC output pulse operates with the maximum duty, causing the rush current to flow to the output
transistor. It is therefore necessary to raise the VCC target voltage gradually so that the output does not operate with the
maximum duty.
Connection of a capacitor between the CSS pin (pin 8) and GND allows slow rising of VCC output (soft start) when
VBB power is applied.
The soft start time (tss) is approximately set by the following equation.
(Switching regulator, switching frequency set time)
The switching frequency of switching regulator is varied by changing the capacitor connected between the OSC pin (pin
10) and GND.
The switching frequency (fosc) is set approximately by the following equation. Since this equation is the approximate
expression, check it in the mounted condition when it is to be used at a particularly high frequency.
tss . = . 1.4 C/Iss
tss: Soft start time
Iss: Soft start charge current
C: CSS pin capacitor
fosc . = . {1/C(1/Icosc+1/Idosc)} 10
fosc: Switching frequency
Icosc: OSC pin charge current
Idosc: OSC pin discharge current
C: OSC pin capacitor
VBB
CSS
VCC
–3
[s]
[µA]
[µF]
[kHz]
[µA]
[µA]
[pF]
5V
tss
9V
LB11947
1.4V
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