Low-Cost, Dual-Output, Step-Down

Controller with Fast Transient Response

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21

The output voltage ripple as a consequence of the

ESR, ESL, and output capacitance is:

These equations are suitable for initial capacitor selec-

tion, but final values should be chosen based on a pro-

totype or evaluation circuit. As a general rule, a smaller

ripple current results in less output-voltage ripple. Since

the inductor ripple current is a factor of the inductor

value and input voltage, the output-voltage ripple

decreases with larger inductance, and increases with

higher input voltages. Ceramic, tantalum, or aluminum

polymer electrolytic capacitors are recommended. The

aluminum electrolytic capacitor is the least expensive;

however, it has higher ESR and ESL. To compensate for

this, use a ceramic capacitor in parallel to reduce the

switching ripple and noise. For reliable and safe opera-

tion, ensure that the capacitor’s voltage and ripple-cur-

rent ratings exceed the calculated values.

The response to a load transient depends on the

selected output capacitors. After a load transient, the

output voltage instantly changes by ESR x

Before the controller can respond, the output voltage

deviates further depending on the inductor and output

capacitor values. After a short period of time (see the

Typical Operating Characteristics), the controller

responds by regulating the output voltage back to its

nominal state. The controller response time depends on

its closed-loop bandwidth. With a higher bandwidth,

the response time is faster, thus preventing the output

voltage from further deviation from its regulating value.

Setting the Output Voltages and Voltage

Positioning

Figure 7 shows the feedback network used on the

MAX8664. With this configuration, a portion of the feed-

back signal is sensed on the switched side of the

inductor (LX), and the output voltage droops slightly as

the load current is increased due to the DC resistance

of the inductor (DCR). This allows the load regulation to

be set to match the voltage droop during a load tran-

sient (voltage positioning), reducing the peak-to-peak

output voltage deviation during a load transient, and

reducing the output capacitance requirements.

To set the magnitude of the voltage positioning, select

a value for R2 in the 8kΩ to 24kΩ range, then calculate

the value of R1 as follows:

output voltage at full load.

RR

I

DCR

V

OUT MAX

OUT MAX

12

1

=×

×

−

⎛

⎝⎜

⎞

⎠⎟

()

()

Δ

I

VV

fL

V

V

PP

IN

OUT

S

OUT

IN

−

×

×

V

I

ESR

V

V

L ESL

ESL

V

I

Cf

RIPPLE ESR

P P

RIPPLE ESL

IN

RIPPLE C

PP

OUT

S

()

()

()

=×

=

+

×

=

××

−

−

8

L

DCR

LX_

FB_

OUT

ESR

R1

R2

R3

Cr

Figure 7. Feedback Network