BD8381EFV-M 查看數據表（PDF） - ROHM Semiconductor

零件编号

产品描述 (功能)

生产厂家

BD8381EFV-M

LED Drivers for Automotive Light

ROHM Semiconductor 

BD8381EFV-M

Technical Note

5. Selection of the output capacitor

Select the output capacitor Cout based on the requirement of the ripple voltage Vpp.

Vpp =

Iout

Cout

×

Vout

Vout+VIN

×

1

Fosc

+ IL_MIN × RESR

Choose Cout that allows the Vpp to settle within the requirement. Allow some margin also, such as the tolerance of the

external components.

6.Selection of the input capacitor

A capacitor at the input is also required as the peak current flows between the input and the output in DC/DC conversion.

We recommend an input capacitor greater than 10µF with the ESR smaller than 100m. The input capacitor outside of

our recommendation may cause large ripple voltage at the input and hence lead to malfunction.

7. Phase Compensation Guidelines

In general, the negative feedback loop is stable when the following condition is met:

 Overall gain of 1 (0dB) with a phase lag of less than 150º (i.e., a phase margin of 30º or more)

However, as the DC/DC converter constantly samples the switching frequency, the gain-bandwidth (GBW) product of the

entire series should be set to 1/10 the switching frequency of the system. Therefore, the overall stability characteristics

of the application are as follows:

 Overall gain of 1 (0dB) with a phase lag of less than 150º (i.e., a phase margin of 30º or more)

 GBW (frequency at gain 0dB) of 1/10 the switching frequency

Thus, to improve response within the GBW product limits, the switching frequency must be increased.

The key for achieving stability is to place fz near to the GBW. GBW is decided by phase delay fp1 by COUT and output

impedance RL. Of each becomes like the next expression.

Phase-lead fz =

1

[Hz]

2πCpcRpc

Phase-lag

fp1 =

1

2πRLCout

[Hz]

Vout

LED

Good stability would be obtained when the fz is set between 1kHz～10kHz.

Please substitute the value at the maximum load for RL.

FB

A

COMP

Rpc

Cpc

In buck-boost applications, Right-Hand-Plane (RHP) Zero exists. This Zero has no gain but a pole characteristic in terms

of phase. As this Zero would cause instability when it is in the control loop, so it is necessary to bring this zero before the

GBW.

Vout+VIN/(Vout+VIN)

fRHP=

[Hz]

2πILOADL

ILOAD: MAXIMUM LOAD CURRENT

It is important to keep in mind that these are very loose guidelines, and adjustments may have to be made to ensure

stability in the actual circuitry. It is also important to note that stability characteristics can change greatly depending on

factors such as substrate layout and load conditions. Therefore, when designing for mass-production, stability should be

thoroughly investigated and confirmed in the actual physical design.

www.rohm.com

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

13/20

2011.04 - Rev.A

Share Link: