BD9E151NUX Datasheet, PDF (14/22 Pages) Rohm – 1ch Step-Down Switching Regulator

BD9E151NUX
Datasheet
3. Examination whether the second Phase compensation Capacitor C2 is necessary or not is done.
If the ESR zero of Output Capacitor exists in a place that is smaller than half of the Switching Frequency, a second
Phase compensation Capacitor is necessary. In other words, it is the case wherein the formula below happens.
・・・ (18)
In this case, add the second Phase compensation Capacitor C2, and match the frequency of the third pole to the
Frequency fp3 of ESR zero.
C2 is looked for using the following formula.
・・・ (19)
Output Voltage Restriction
BD9E151NUX have a function of BSTUVLO to prevent malfunction at low voltage between BST and LX. Therefore
OUTPUT voltage is restricted by BSTUVLO and Max Duty Cycle (min 85 %).
① Restriction by BST-UVLO
When the voltage between BST and Lx is lower than 2.5V, High-Side FET will be made turned off and the charge will
provide from VIN to BST directly to reset BSTUVLO (path①). The below formula is needed to be satisfied to reset
BSTUVLO.
VIN ≥ VOUT + VF + BSTUVLO reset ・・・ (20)
Here, BSTUVLO reset: BSTUVLO reset voltage, VF: the diode forward bias voltage between VIN and BST
Considering the fluctuation of BSTUVLO reset voltage and VF, maximum voltage is more than 5V.
Therefore maximum output voltage is defined as VIN - 5V.
5.5V
BST
②Restriction by Max Duty Cycle
Maximum output voltage is restricted by Max Duty Cycle (min85%).
In this time it is needed to consider the effect of NchFET Ron
, OUTPUT current and forward voltage of SBD. OUTPUT voltage
can be calculated using the following formula.
BSTUVLO
①
VIN
VOUT_max = (VIN - Ron×IOUT)×0.85 - VF×0.15・・・ (21)
Considering the effect of catch diode type and the loss by inductor,
Vomax = (VIN-Ron×Iomax)×0.85 (casually formula)
Considering the negative voltage in the case of pulling diode current,
maximum voltage is more than VIN×0.7.
Therefore maximum output voltage is defined as VIN×0.7.
LX
Figure 27. BST charge pass
Considering above restriction, adopt the lower output voltage as maximum voltage
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TSZ22111・15・001
14/19
TSZ02210-0Q3Q0AZ00160-1-2
2013.07.17 Rev.001