BD9489F Datasheet, PDF (13/39 Pages) Rohm – DCDC converter with current mode

BD9489F
Datasheet
2.8 The Retaining Function of The Output Voltage
In the conventional method, as PWM=L, IC operates no boost pulse (GATE=L), no LED load (DIMOUT=L), FB is the high
impedance. But if PWM=L continues for a long time, Vout and FB is discharged gradually. And at the timing of PWM=H again,
the problem is caused such as the in-rush current to the output capacitor Cout.
To solve this problem, IC built in the retaining function of the output voltage VOUT during PWM=L. As the high level of the
STB pin is set in the following table, IC holds the output voltage (OVP pin voltage), the FB voltage at the negative edge of PWM.
Based on those hold values, IC output the boost pulse and keep the output voltage nevertheless PWM=L.
State
Enable of the keep voltage
Disenable of the keep voltage
STB input voltage
STB=8.5V to 19.0V
STB=2.0V to 6.5V
○The operation of the retaining function of the output voltage
Figure 18. The block diagam of the retaining function
IC holds the OVP terminal voltage VOVP and FB terminal voltage VFB at the timing PWM=High turns to Low. We defines those
as VOVPkeep, VFBkeep. The OVP terminal voltage is smaller than VOVPkeep, the boost pulse is output even if the interval
PWM=Low. The pulse width is decided by the VFBkeep. This function enables IC keep the output voltage VOUT as PWM=0%
and decrease the in-rush current when PWM asserts high again. And that enables the fast startup in low PWM duty.
The resolution of the holding the FB and OVP terminal voltage is 0.1V.
This retaining function of the output voltage is described as below focusing the startup and the low duty PWM.
○The startup operation of the retaining function of the output voltage
Figure 19. the startup operation of the retaining function
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TSZ22111・15・001
13/36
TSZ02201-0F1F0C100250-1-2
29.Oct 2014 Rev.002