AMC2596
APPLICATION INFORMATION
Input Capacitors (CIN)
It is required that VIN must be bypassed with at least a 100µF electrolytic capacitor for stability. Also, it is strongly
recommended the capacitor’s leads must be dept short, and located near the regulator as possible.
For low operating temperature range, for example, below -25°C, the input capacitor value may need to be larger. This
is due to the reason that the capacitance value of electrolytic capacitors decreases and the ESR increases with lower
temperatures and age. Paralleling a ceramic or solid tantalum capacitor will increase the regulator stability at cold
temperatures.
Output Capacitors (COUT
)
An output capacitor is also required to filter the output voltage and is needed for loop stability. The capacitor should
be located near the AMC2596 using short PC board traces. Low ESR types capacitors are recommended for low
output ripple voltage and good stability. Generally, low value or low voltage (less than 12V) electrolytic capacitors
usually have higher ESR numbers. For example, the lower capacitor values (220µF–1000µF) will yield typically 50
mV to 150 mV of output ripple voltage, while larger-value capacitors will reduce the ripple to approximately 20 mV
to 50 mV.
The amount of output ripple voltage is primarily a function of the ESR (Equivalent Series Resistance) of the output
capacitor and the amplitude of the inductor ripple current (∆IIND).
Output Ripple Voltage = (∆IIND) × (ESR of COUT
)
Some capacitors called “high-frequency,” “low-inductance,” or “low-ESR.” are recommended to use to further reduce
the output ripple voltage to 10 mV or 20 mV. However, very low ESR capacitors, such as Tantalum capacitors, should
be carefully evaluated.
Output Voltage Ripple and Transients
The output ripple voltage is due mainly to the inductor sawtooth ripple current multiplied by the ESR of the output
capacitor.
The output voltage of a switching power supply will contain a sawtooth ripple voltage at the switcher frequency,
typically about 1% of the output voltage, and may also contain short voltage spikes at the peaks of the sawtooth
waveform.
Due to the fast switching action, and the parasitic inductance of the output filter capacitor, there is voltage spikes
presenting at the peaks of the sawtooth waveform. Cautions must be taken for stray capacitance, wiring inductance,
and even the scope probes used for transients evaluation. To minimize these voltage spikes, shortening the lead length
and PCB traces is always the first thought. Further more, an additional small LC filter (3µH & 180µF) (as shown in
Figure 3) will possibly provide a 10X reduction in output ripple voltage and transients.
AMC2596-ADJ
4
2
FB
L2
1
7V – 40V
DC INPUT
VIN
GND
3uH
VOUT
ENABLE
5
OUTPUT
R2
50K
L1
C1
3
CIN
470uH
COUT
1000µF
R1
1.21K
68uH
180uF
Figure 3. LC Filter for Low Output Ripple
Copyright © 2006 ADDtek Corp.
7
DD008_A -- JUNE 2006
ADDTEK [ ADDTEK CORP ]
