6

Rev.1.2

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necessary since the input switching current is absorbed by input capacitor (Cin). RMS of input current can be

calculated by following formula:

In worst case, when Vin = 2Vout, Icin = Iload/2. It is necessary to select capacitors which tolerate RMS ripple

current that is half of maximum load current. For input capacitors, ELM recommends using electrolytic, tan-

talum or ceramic ones. When using electrolytic or tantalum capacitors, please connect the 0.1μF one which is

high quality with high frequency to the IC as close as possible. When using ceramic capacitors, it is necessary

to provide sufficient capacity to prevent ripple voltage of input . Input voltage ripple for low ESR capacitors can

be calculated by following formula:

ΔVin = [ Iload/(Cin × fs) ] × (Vout/Vin) × (1 − Vout/Vin)

Cin=input capacitance value.

6. Capacitor

Capacitors are used to ensure output voltage of DC; ELM recommends using ceramic, tantalum, or low ESR

electrolytic ones. To keep output voltage ripple low, low ESR capacitors are preferable. Output voltage ripple

can be calculated by following formula:

ΔVout = [ Vout/(fs × L) ] × (1 − Vout/Vin) × [ Resr + 1 / (8 × fs × Cout) ]

Cout=output capacitance value; Resr=equivalent series resistance (ESR) value of the output capacitor.

When using ceramic capacitors, please select by the high frequency impedance capacitance of switching fre-

quency; output voltage ripple is mainly determined by capacitance. Output voltage ripple can be calculated by

following formula:

When using tantalum or electrolytic capacitors, please select by ESR, which is mainly determined by imped-

ance of switching frequency. Output ripple can be calculated by following formula:

ΔVout = [ Vout/(fs × L) ] × (1 − Vout/Vin) × Resr

Stability of DC/DC converter would be effected by capacitance of output capacitor. ELM611DA is designed to

provide wide range of capacitance and stable operation of ESR.

7. Compensation components

ELM611DA realizes simple compensation and fast transient response by adopting current mode control;

COMP, which is output of internal transconductance error amplifier, controls system stability and transient re-

sponse. A capacitor and a resistor in series connection sets a pole-zero combination for compensation. DC gain

of voltage feedback loop can be calculated by following formula:

Avdc = Rload × Gcs × Aea × Vfb/Vout

Aea=error amplifier voltage gain; Gcs=current sense transconductance; Rload=load resistor value

The control loop has two important poles; one is the product of compensation capacitor (C1) and output resis-

tor of error amplifier, and the other one is the product of output capacitor and load resistor.

These poles are located at:

fp1 = Gea / (2π × C1 × Aea), fp2 = 1 / (2π × Cout × Rload)

Gea=error amplifier transconductance

Control system is produced by compensation capacitor (C1) and compensation resistor (R3), and has one zero.

This zero is located at:

fz1 = 1 / (2π × C1 × R3)

high efficiency synchronous PWM step down DC/DC converter