BD9489F Datasheet, PDF (25/39 Pages) Rohm – DCDC converter with current mode

BD9489F
3.5.Timing Chart
3.5.1 PWM Start up 1 (Input PWM Signal After Input STB Signal)
VCC
7.5V
STB
PWM
REG58
SS
GATE
RT
2.6V
3.7V
0.4V
2.0V
Datasheet
0.4V
STATE
OFF
STANDBY
SS
Normal
STANDBY SS
(*1) (*2)
(*3)
(*4)
(*5) (*6)
Figure 36. PWM Start up 1 (Input PWM Signal After Input STB Signal)
(*1)…REG58 starts up when STB is changed from Low to High. In the state where the PWM signal is not inputted, SS terminal
is not charged and DCDC doesn’t start to boost, either.
(*2)…When REG58 is more than 2.6V, the reset signal is released.
(*3)…The charge of the pin SS starts at the positive edge of PWM=L to H, and the soft start starts. And while the SS is less than
0.4V, the pulse does not output. The pin SS continues charging in spite of the assertion of PWM or OVP level. The GATE
pulse is output by the retaining function of the output voltage, till the OVP level is reached to the certain level, even if
PWM=L. Please refer the section “2.8 the Retaining Function of The Output Voltage” in detail.
(*4)…The soft start interval will end if the voltage of the pin SS, Vss reaches 3.7V. By this time, it boosts V_OUT to the voltage
where the set LED current flows. The abnormal detection of FBMAX starts to be monitored.
(*5)…As STB=L, the boost operation is stopped instantaneously. (Discharge operation continues in the state of STB=L and
REGUVLO=L. Please refer to section 3.5.3)
(*6)…In this diagram, before the charge period is completed, STB is changed to High again. As STB=H again, the boost
operation restarts the next PWM=H. It is the same operation as the timing of (*2). (For capacitance setting of SS terminal,
please refer to the section 3.2.1.
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29.Oct 2014 Rev.002