SC2545TSTRT datasheet(11/24 Pages) SEMTECH

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SC2545

POWER MANAGEMENT

The first term is the DC voltage across C

o at time t=0.

The second term is the voltage variation caused by the

charge balance between the load and the converter

output. The third term is voltage ripple due to ESL and

the fourth term is the voltage ripple due to ESR. The

total output voltage ripple is then a vector sum of the

last three terms.

Since the inductor current is a triangular waveform with

peak-to-peak value

δ *I

o, the ripple-voltage caused by

inductor current ripples is

≈

∆

the ripple-voltage due to ESL is

The voltage rating of aluminum capacitors should be at

least 1.5V

o. The RMS current ripple rating should also

be greater than

Usually it is necessary to have several capacitors of the

same type in parallel to satisfy the ESR requirement.

The voltage ripple caused by the capacitor charge/

discharge should be an order of magnitude smaller than

the voltage ripple caused by the ESR. To guarantee this,

the capacitance should satisfy

esr

In many applications, several low ESR ceramic

capacitors are added in parallel with the aluminum

capacitors in order to further reduce ESR and improve

high frequency decoupling. Because the values of

capacitance and ESR are usually different in ceramic and

aluminum capacitors, the following remarks are made

to clarify some practical issues.

esl

ESL

∆

esr

ESR

∆

and the ESR ripple-voltage is

Aluminum capacitors (e.g. electrolytic) have high

capacitances and low ESLs. The ESR has the dominant

effect on the output ripple voltage. It is therefore very

important to minimize the ESR. Other types to choose

are solid OS-CON, POSCAP, and tantalum.

When determining the ESR value, both the steady state

ripple-voltage and the dynamic load transient need to be

considered. To meet the steady state output ripple-

voltage spec, the ESR should satisfy

∆

〈

To limit the dynamic output voltage overshoot/

undershoot within a (say 3%) of the steady state output

voltage from no load to full load, the ESR value should

satisfy

Then, the required ESR value of the output capacitors

should be

esr = min{Resr1,Resr2 }.

Remark 1: High frequency ceramic capacitors may

not carry most of the ripple current. It also depends on

the capacitor value. Only when the capacitor value is set

properly, the effect of ceramic capacitor low ESR starts

to be significant. For example, if a 10

µ F, 4mΩ ceramic

capacitor is connected in parallel with 2x1500

µ F,

90m Ω electrolytic capacitors, the ripple current in the

ceramic capacitor is only about 42% of the current in the

electrolytic capacitors at the ripple frequency. If a 100

µ F,

2m Ω ceramic capacitor is used, the ripple current in

the ceramic capacitor will be about 4.2 times of that in

the electrolytic capacitors. When two 100

µ F, 2m Ω

ceramic capacitors are used, the current ratio increases

to 8.3. In this case most of the ripple current flows in

the ceramic decoupling capacitor. The ESR of the ceramic

capacitors will then determine the output ripple-voltage.

Remark 2: The total equivalent capacitance of the filter

bank is not simply the sum of all the paralleled capacitors.

The total equivalent ESR is not simply the parallel

combination of all the individual ESR’s either. Instead

they should be calculated using the following formula.

〈

Applications Information (Cont.)

)

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