IR3870MBF datasheet(14/20 Pages) IRF

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IR3870MBF

Output Capacitor Selection

Selection of the output capacitor requires

meeting voltage overshoot requirements during

load removal, and meeting steady state output

ripple voltage requirements. The output

capacitor is the most expensive converter

component and increases the overall system

cost. The output capacitor decoupling in the

converter typically includes the low frequency

capacitor, such as Specialty Polymer Aluminum,

and mid frequency ceramic capacitors.

The first purpose of output capacitors is to

provide current when the load demand exceeds

the inductor current, as shown in Figure 17.

Equation 6 shows the charge requirement for a

certain load. The advantage provided by the

IR3870 at a load step is to reduce the delay

compared to a fixed frequency control method (in

microseconds or (1-D)*Ts). If the load increases

right after the PWM signal goes low, the longest

delay will be equal to the minimum lower gate on

as shown in the Electrical Specification table.

The IR3870 also reduces the inductor current

slew time, the time it takes for the inductor

current to reach equality with the output current,

by increasing the switching frequency up to

2.5MHz. The result reduces the recovery time.

VESR is usually much greater than VESL. The

IR3870 requires a total ESR such that the ripple

voltage at the FB pin is greater than 7mV.

The second purpose of the output capacitor is to

minimize the overshoot of the output voltage

when the load decreases as shown in Figure 18.

By using the law of energy before and after the

load removal, equation 7 shows the output

capacitance requirement for a load step.

Figure 17. Charge Requirement during Load Step

()

(6b)

ΔIstep

(6a)

Istep

0.5

OUT

DROP

OUT

⎥

⎦

⎤

⎢

⎣

⎡

⋅

−

The output voltage drops, VDROP, initially

depending on the characteristic of the output

capacitor. VDROP is the sum of the equivalent

series inductance (ESL) of the output capacitor

times the rate of change of the output current and

the ESR times the change of the output current.

(7)

OUT

STEP

OUT

−

⋅

Figure 18. Typical Output Voltage Response

Waveform.

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