BD9489F Datasheet, PDF (17/39 Pages) Rohm – DCDC converter with current mode

BD9489F
3.2.2 Shutdown Method and REG58 Capacitance Setting
When this IC shuts down, VOUT discharge function works. Indicated below is the sequence.
Datasheet
Figure 26. The waveform and diagram at shutdown
○Sequence explanation of shutdown
1. When STB=L, GATE and REG58 stop.
2. While STB=L and REG58UVLO=H, DIMOUT asserts the same logic of PWM. And VOUT is discharged until
REG58=5.8V reaches 2.3V by -5μA(typ.).
3. When VOUT is discharged enough by ILED, ILED doesn’t get to flow.
4. When REG58 voltage reaches under 2.3V(typ), whole system is shutdown.
○Setting method of REG58 capacitance
When REG58 terminal capacitance is defined as C_REG , shutdown time T_OFF is decided by the following equation.
T_OFF

C_REG[F]　 　(5.8 
5 [A]
2.3)
[V]
　[sec]
When discharge function is used, PWM signal must be continuously inputted after STB=L.
VOUT discharge time is longest when PWM is set on mininum DUTY.
Please set C_REG capacitance value with margin so that the system is shutdown after VOUT is discharged enough.
3.2.3 VCC Series Resistance Setting
Here are the following effects of inserting series resistor Rvcc into VCC line.
(i) In order to drop the voltage VCC, it is possible to suppress the heat
generation of the IC.
(ii) It can limit the inflow current to VCC line.
However, if resistance RVCC is set bigger, VCC voltage becomes under
minimum operation voltage (VCC<9V). RVCC must be set to an appropriate
series resistance.
IC’s inflow current line I_IN has the following inflow lines.
・IC’s circuit current…ICC
・Current of RREG connected to REG58…IREG
・Current to drive FET’s Gate…I_GATE
These decide the voltage ΔV at RVCC.
VCC terminal voltage at that time can be expressed as follows.
VCC　[V]　 　VIN　[V]  ICC　[A]  IDCDC　[A]  IREG　[A] RVCC　[Ω]　  9　[V]
Here, judgement is the 9V minimum operation voltage.
Please consider a sufficient margin when setting the series resistor of VCC.
Internal
BLOCK
VIN
ICC
+
-
RVCC
ΔV
VCC
I_IN
IREG ^REG58
IDCDC
RREG
DCDC
DRIVER
GATE ^I_GATE
【setting example】
Above equation is translated as follows.
RVCC　[Ω] 
VIN　[V]  9　[V]
ICC　[A]  IDCDC　[A]  IREG　[A]
Figure 27.VCC series resistance
circuit example
When VIN=24V, ICC=2.0mA, RREG=10kΩ and IDCDC=2mA, RVCC’s value is calculated as follows.
RVCC　[Ω]

0.002　[A]

24　[V]  9　[V]
0.002　[A]  5.8　[V]　1　0000　[Ω]

3.26　[kΩ]
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