BD8152FVM View Datasheet(PDF) - ROHM Semiconductor

Part Name

Description

MFG CO.

BD8152FVM

High-efficiency Step-up Switching Regulators with Built-in Power MOSFET

ROHM Semiconductor 

BD8152FVM, BD8158FVM

Technical Note

(4) Selecting the output rectification diode

Schottky barrier diode is recommended as the rectification diode to use at the DC/DC converter output stage. Select the

diode paying attention to the max. inductor current and max. output voltage.

Max. Inductor current

IINMAX + ¨IL

<

Rating current of diode

Max. output voltage VOMAX

<

Rating voltage of diode

Since each parameter has 30% to 40% of dispersion, be sure to design providing sufficient margins.

(5) Design of the feedback resistor constant

Refer to the following equation to set the feedback resistor. As the setting range, 10 kǡ to 330 kǡ is recommended. If

the resistor is set to 10 kǡ or lower, it causes the reduction of power efficiency. If it is set to 330 kǡ or larger, the offset

voltage becomes larger by the input bias current 0.4 µA (Typ.) in the internal error amp.

Vo = R8 + R9 u 1.245

[V]

R9

Step-up

Vo

R8

R9

Reference voltage 1.245 V

2

FB

㧗ERR

㧙㩷

Fig. 25 Feedback Resistor Setting

(6) Setting the soft start time

Soft start is required to prevent the coil current at the time of startup from

10

increasing and the overshoot of the output voltage at the starting time. Fig.26

shows the relation between the capacitance and soft start time. Please refer

to it to set the capacitance.

1

As the capacitance, 0.001 µF to 0.1 µF is recommended. If the capacitance is

set to 0.001 µF or lower, the overshooting may occur on the output voltage. If

the capacitance is set to 0.1 µF or larger, the excessive back current flow may

occur in the internal parasitic elements when the power is turned off and it

may damage IC. When the capacitor to 0.1 µF or larger is used, be sure to

insert a diode to VCC in series, or a bypass diode between the SS pin and

VCC.

Bypass diode

Back current prevention diode

0.1

0.01

0.001

0.01

0.1

SS CAPACITANCE[uF]

Fig. 26 SS Pin Capacitance vs

Delay Time

VCC

Output pin

Fig. 27 Bypass Diode Example

When there is the startup relation (sequences) with other power supplies, be sure to use the high accuracy product (such as X5R).

Soft start time may vary according to the input voltage, output voltage, loads, coils and output capacity. Be sure to verify

the operation using the actual product.

(7) Setting the ENB pin

When the ENB pin is set to Hi, the internal circuit becomes active and the DC/DC converter starts operating. When it is set

to Low, the shut down is activated and all circuits will be turned off.

(8) Setting the frequency by FCLK

It is possible to change the switching frequency by setting the FCLK pin to Hi or Low. When it is set to Low, the product

operates at 600 kHz (Typ.). When it is set to Hi, the product operates at 1,200 kHz (Typ.).

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© 2011 ROHM Co., Ltd. All rights reserved.

9/17

2011.08 - Rev.C

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