BD9853AFV Datasheet, PDF (10/17 Pages) Rohm – Single/Dual-output High-frequency Step-down Switching Regulato (Controller type)

BD9853AFV
Technical Note
Selecting the synchronous diode
An extremely low forward voltage Schottky barrier diode should be employed as the synchronous diode.
Selection of the specific diode to be used should be made in conformance with the following relative configurations for
maximum forward current, reverse voltage and diode power dissipation.
・The maximum current rating is higher than the combined maximum load current and coil ripple current (⊿I_L).
・The reverse voltage rating is higher than the VIN value.
・Power dissipation for the selected diode must be within the rated level.
Synchronous diode power dissipation (Pdi) is expressed in the following formula:
Pdi=Iout(MAX)×tdt×fosc×Vf
Iout(MAX) : maximum load current, tdt： dead time 60ns typ,fosc : oscillation frequency, Vf：forward voltage
Selecting the output/input coil
The output coil and the output capacitor together form a second-order smoothing filter for the switch waveform and provide
the DC output voltage.
If a coil’s inductor value is low, its physical size is minimized, but the penalty is higher ripple current, with lowered efficiency
and an increase in output noise. Conversely, a higher inductor value increases the size of the coil, but lowers the ripple
current and, consequently. the output ripple current.
Generally speaking, ripple current should be between 20% and 50% of output load current. The following equation is used
to calculate the inductor value that corresponds to the ripple current value being employed.
(VIN－VOUT)
V_OUT
1
L=
⊿I_L
×
V_IN
× _fOSC
⊿I_L ^{=(0.2 to 0.5)×I}OUT
,
L：inductor value, V_IN：maximum input voltage, V_OUT：output voltage, ⊿I_L：coil ripple current value
fosc：oscillation frequency, I_OUT：output load current
Note that the current rating for the coil should be higher than I_OUT(MAX)＋⊿I_L.
Selecting the input capacitor
The input capacitor is the source of current that flows to the coil via the FET whenever the high side MOSFET is ON. In
selecting an input capacitor, sufficient margin must be provided to accommodate capacitor pressure and the permissible
ripple current.
The expression below defines the effective value of the ripple current to the input capacitor. It should be used in
determining the suitability of the capacitor in providing sufficient margin for the permissible ripple current.
I_RMS＝I_OUT×√ _{(1－VOUT/VIN)×VOUT/VIN}
I_RMS : effective value of the ripple current to the input capacitor
I_OUT : output load current
Selecting the output capacitor
The output capacitor should confine ESR and permissible ripple current within a stable region.
Although incorporating a low-ESR capacitor will limit ripple voltage and load fluctuation, it can also hurt the stability of the
feedback network. Therefore, in order to maintain a stable feedback loop when ceramic or other low-ESR capacitors are
employed, special attention must be paid to providing an appropriate phase compensation scheme.
A suitable output capacitor will satisfy the following formula for ESR.
ESR≦⊿V_L/⊿I_L
⊿V_L : permissible ripple voltage, ⊿I_L : coil ripple current
In addition, use the following formula to determine the effective value of the output capacitance permissible ripple current,
and select a capacitor that allows sufficient margin to accommodate this value.
I_RMS =⊿I_L/2√3
I_RMS : effective value of ripple current to the output condenser, ⊿I_L : coil ripple current
Setting the soft start time
To prevent output voltage startup overshoot on either channel, the capacitors connected to the SCP/SOFT 1, 2 pins – in a
discharged state at power-on – are gradually charged during a delay interval, thus providing a soft start. The soft start
period is the time from when the standby pins go from LOW to HIGH, starting the charge, to the time that the output
voltage reaches the programmed setting. The soft start time is calculated in the following equation:
0.8[V] (typ)×Cscp/soft［F］
tsoft＝
Isosoft (typ: 2.3μA)[A]
tsoft : soft start time, Cscp/soft : SCP/SOFT pin connection capacitance, Isosoft : charge current
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10/16
2009.05 - Rev.A