BD9E151NUX Datasheet, PDF (12/22 Pages) Rohm – 1ch Step-Down Switching Regulator

BD9E151NUX
Datasheet
(5) Soft Start Function
It is highly recommended to program the soft start time
externally to prevent high inrush current because no soft start
time is implemented internally. A capacitor (Css) connected
between the SS pin and ground implements a soft start time.
The BD9E151NUX has an internal pull-up current source of 2uA
that charges the external soft start capacitor. The equation for
the soft start time (10% to 90 %) is shown in below Equation.
The Iss current is 2uA.
Figure 26. Soft Start Time Setting
(6) Catch Diode
TSS ₌
CSS×0.1
・・・ (6)
ISS
The BD9E151NUX is designed to operate using an external catch diode between LX and GND. The selected diode
must meet the absolute maximum ratings for the application: Reverse voltage must be higher than the maximum voltage
at the LX pin, which is VINMAX + 0.5 V. Peak current must be greater than IOUTMAX+⊿IL plus on half the peak to
peak inductor current. Forward voltage drop should be small for higher efficiencies. It is important to note that the catch
diode conduction time is typically longer than the high-side FET on time, so attention paid to diode parameters can
make a marked improvement in overall efficiency. Additionally, check that the device chosen is capable of dissipating
the power losses.
(7) Input Capacitor
The BD9E151NUX requires an input capacitor and depending on the application. Use low ESR capacitors for the best
performance. Ceramic capacitors are preferred, but low-ESR electrolytic capacitors may also suffice. The typical
recommended value for the decoupling capacitor is 10uF. Please place this capacitor as possible as close to the VIN pin.
When using ceramic capacitors, make sure that they have enough capacitance to provide sufficient charge to prevent
excessive voltage ripple at input. The input voltage ripple caused by capacitance can be estimated by:
∆VCC
=
f
IOUT
× CVCC
×
VOUT
VCC
×
⎢⎣⎡^1-
VOUT
VCC
⎤
⎥⎦
・・・ (7)
Since the input capacitor (CVIN) absorbs the input switching current it requires an adequate ripple current rating. The
RMS current in the input capacitor can be estimated by:
I_CVCC = IOUT ×
VOUT _{× (1−} VOUT ₎
VCC
VCC
・・・ (8)
The worst case condition occurs at VIN= 2VOUT, where
I_{CVCC_max}
=
IOUT
2
・・・ (9)
(8) About Adjustment of DC/DC Comparator Frequency Characteristics
Role of Phase compensation element C1, C2, R3 (See P.8 Example of Reference Application Circuit)
Stability and Responsiveness of Loop are controlled through VC Pin which is the output of Error Amp.
The combination of zero and pole that determines Stability and Responsiveness is adjusted by the combination of
resistor and capacitor that are connected in series to the VC Pin.
DC Gain of Voltage Return Loop can be calculated for using the following formula.
・・・ (10)
Here, VFB is Feedback Voltage (1.0V).AEA is Voltage Gain of Error amplifier (typ : 60 dB),
Gcs is the Trans-conductance of Current Detect (typ : 10A/V), and Rl is the Output Load Resistance value.
www.rohm.com
© 2013 ROHM Co., Ltd. All rights reserved
TSZ22111・15・001
12/19
TSZ02210-0Q3Q0AZ00160-1-2
2013.07.17 Rev.001