BD9E151NUX Datasheet, PDF (11/22 Pages) Rohm – 1ch Step-Down Switching Regulator

BD9E151NUX
Datasheet
Application Components Selection Method
(1) Inductors
Something of the shield type that fulfills the current rating (Current value
Ipecac below), with low DCR is recommended. Value of Inductance influences
Inductor Ripple Current and becomes the cause of Output Ripple.
In the same way as the formula below, this Ripple Current can be made small
for as big as the L value of Coil or as high as the Switching Frequency.
⊿IL
Ipeak = IOUT +
・・・ (1)
2
⊿IL =
VIN-VOUT _×
L
VOUT
VIN
×
1
f
・・・ (2)
ΔIL
Figure 24. Inductor Current
(⊿IL: Output Ripple Current, VIN: Input Voltage, VOUT: Output Voltage, f: Switching Frequency)
For design value of Inductor Ripple Current, please carry out design tentatively with about 20%～50% of Maximum Input
Current
(2) Output Capacitor
In order for capacitor to be used in output to reduce output ripple, Low ceramic capacitor of ESR is recommended.
Also, for capacitor rating, on top of putting into consideration DC Bias characteristics, please use something whose
maximum rating has sufficient margin with respect to the Output Voltage. Output ripple voltage is looked for using the
following formula. The actual value of the output capacitor is not critical, but some practical limits do exist. Consider the
relationship between the crossover frequency of the design and LC corner frequency of the output filter. In general, it is
desirable to keep the crossover frequency at less than 1/5 of the switching frequency. With high switching frequencies
such as the 600kHz frequency of this design, internal circuit limitations of the BD9E151NUX limit the practical maximum
crossover frequency to about 30kHz. In general, the crossover frequency should be higher than the corner frequency
determined by the load impedance and the output capacitor. This limits the minimum capacitor value for the output filter
to:
1
COUT_min = _{2π×Rl×fc_max}
・・・ (3)
Where: Rl is the output load resistance and fc_max is the maximum crossover frequency. The output ripple voltage can
be estimated by:
Vpp = ⊿IL ^×
1
2π×f×COUT
+
⊿IL
×
RESR
・・・ (4)
Please design in a way that it is held within Capacity Ripple Voltage.
In the BD9E151NUX, it is recommended a ceramic capacitor more than 10µF.
(3) Output Voltage Setting
ERROR AMP internal Standard Voltage is 1.0V. Output Voltage is determined as seen in (5) formula
VOUT
R1
ERROR AMP
FB
R2
VOUT = ^R1+R2
R2
・・・ (5)
VREF
1.0 V
Figure 25. Output Voltage Setting
(4) Bootstrap Capacitor
Please connect from 0.047µF to 0.47µF (Laminate Ceramic Capacitor) between BST Pin and LX Pin.
www.rohm.com
© 2013 ROHM Co., Ltd. All rights reserved
TSZ22111・15・001
11/19
TSZ02210-0Q3Q0AZ00160-1-2
2013.07.17 Rev.001