Technical Note
13/20
www.rohm.com
2011.08 - Rev.B
© 2011 ROHM Co., Ltd. All rights reserved.
BD8119FM-M
5. Selection of the output capacitor
Select the output capacitor Cout based on the requirement of the ripple voltage Vpp.
Vpp = × × + 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.
Phase-lead fz = [Hz]
Phase-lag fp1 = [Hz]
Good stability would be obtained when the fz is set between 1kHz～10kHz.
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.
fRHP= [Hz] I
LOAD
: 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.
1
Fosc
1
2πCpcRpc
FB
COMP
V
out
Rpc
LED
Cpc
1
2πRLCout
2πI
LOAD
L
Vout+VIN/(Vout+VIN)
Vout
Vout+V
IN
Iout
Cout
