BD9873CP-V5E2 ROHM [Rohm], BD9873CP-V5E2 Datasheet - Page 7

• Manufacturer Part Number: BD9873CP-V5E2
• Description: Simple Step-down Switching Regulators with Built-in Power MOSFET
• Manufacturer: ROHM [Rohm]

●Application component selection and settings
© 2009 ROHM Co., Ltd. All rights reserved.
BD9873CP-V5,BD9874CP-V5
www.rohm.com
A large inductor series impedance will result in deterioration of efficiency. OCP operation greater than 1.6A may cause
inductor overheating, possibly leading to overload or output short.
Note that the current rating for the coil should be higher than I
Iout（MAX）: maximum load current
If you flow more than maximum current rating, coil will become overload, and cause magnetic saturation, and those account
for efficiency deterioration. Select from enough current rating of coil which doesn’t over peak current.
Output Capacitor C4
The output capacitor is one of the important parts for system stability, and when some capacitor is selected, expected
characteristics cannot be provided, depending on ambient temperature, output voltage setting condition, etc.
Fully confirm ESR, temperature characteristics, DC, and bias characteristics before evaluation.
Inductor L1
Schottky Diode D1
Intput Capacitor C1,C2
L1：inductor value, VCC：maximum input voltage, VOUT：output voltage, ⊿IL：coil ripple current value, fosc：oscillation frequency
Select a Schottky diode having an inter-terminal capacity as small as possible (reverse recovery time as short as possible)
and a forward voltage VF as low as possible. (Noise can be reduced and efficiency improved by reduction of switching
noise and switching loss, as well as reduction of voltage drop loss of forward voltage.)
Diode should be selected on the basis of maximum current rating in forward direction, voltage rating in reverse direction,
and power dissipation of diode.
The power dissipation of the diode is expressed by the following formula:
⊿VL : permissible ripple voltage, ⊿IL : coil ripple current
Another factor that must be considered is the permissible ripple current. Select a capacitor with sufficient margin, governed
by the following formula:
Iout（MAX）: maximum load current, Vf: forward voltage, VOUT: output voltage, VCC: input voltage
IRMS: effective value of ripple current to the output capacitor, ⊿IL : coil ripple current
・The maximum current rating is higher than the combined maximum load current and coil ripple current (⊿I
・The reverse voltage rating is higher than the VIN value.
・Power dissipation for the selected diode must be within the rated level.
A suitable output capacitor should satisfy the following formula for ESR:
The input capacitor is the source of current flow to the coil via the built-in Pch FET when the FET is ON. When selecting
the input capacitor sufficient margin must be provided to accommodate capacitor voltage and 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.
⊿IL
ESR≦⊿VL/⊿IL
Pdi=Iout(MAX)×Vf×(1-VOUT/VCC)
IRMS =⊿IL/2√3
IRMS : effective value of the ripple current to the input capacitor
IOUT : output load current, VOUT: output voltage, VCC: input voltage
IRMS＝IOUT×√ (1－VOUT / VCC)×VOUT / VCC
.
=
(VCC－VOUT)
L1
×
VOUT
VCC
×
fosc
1
7/12
OUT
(MAX)＋⊿IL
.
Technical Note
2009.07 - Rev.A
L
).