ML4880CS datasheet(9/15 Pages) MICRO-LINEAR

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ML4880

BUCK REGULATORS — INPUT CAPACITOR SELECTION

The choice of the input capacitor is based on it’s ripple

current and voltage ratings rather than its capacitance

value. The input capacitor should be a low ESR type and

located as close to the source of the P-MOS switch as

possible. The input capacitor’s ripple current is

determined by the load current and the input voltage, with

the worst case condition occurring at VIN = 2 × VOUT:

RMS C

SENSE MAX

L MAX

OUT

(

)

()

≈+









−













∆

(13)

The capacitor’s voltage rating is based on the maximum

input voltage, VIN(MAX). Capacitor manufacturers typically

recommend derating the capacitor voltage rating by 20%

to 50% for aluminum electrolytic types and 50% to 70%

for tantalum types.

In high current applications it may necessary to add a

µF bulk capacitor and a small 0.1µF ceramic capacitor

to bypass VIN (pin 16) right at the ML4880.

BUCK REGULATORS — OUTPUT CAPACITOR

SELECTION

The output capacitors determine the loop stability and the

output ripple voltage. Use only low ESR capacitors

intended for switching power supply applications, such as

AVX TPS, Sprague 593D Sanyo OS-CON, or Nichicon PL

series. To ensure stability, the minimum capacitance value

is given by:

OUT

ON MAX

SENSE

≥

















()

(14)

The maximum ESR value can be estimated using:

ESR

OUT

L MAX

≤

∆

()

(15)

The selected capacitor must meet both the capacitance

and ESR requirements. As a final check, make sure the

output capacitor can handle the ripple current,

IRMS(COUT):

RMS C

L MAX

OUT

()

≈

∆

(16)

BUCK REGULATORS - OUTPUT VOLTAGE

The output of buck regulator A is adjustable and can be

set to any voltage between 2.5V and 3.5V by connecting a

resistor divider to the feedback pin as shown in Figure 1.

Similarly, the output voltage of buck regulator B is

adjustable and has a usable range of 4.5V to 5.0V. The

resistor values R1 and R2 can be calculated using the

following equation:

OUT =×









125

(17)

The value of R2 should be 125k

Ω or less to minimize bias

current errors.

It is important to note that the accuracy of these resistors

directly affects the accuracy of the output. Use precision

resistors and set the nominal voltage approximately 1% to

2% high with no load in order to make up for the drops

that normally occur when supplying heavy loads. This

offset results in the best overall output accuracy over line

and load.

FLYBACK REGULATOR — INDUCTOR AND SENSE

RESISTOR SELECTION

Figure 7 shows the inductor current of the flyback

regulator. The inductor current is made up of two

components: the DC current level set by the

transconductance amplifier, ISENSE, and the inductor ripple

current,

∆IL. Therefore, the selection of the inductance

value determines how much of the output current is made

up of the inductor ripple current. Higher inductor ripple

current allows smaller inductor values, but results in

higher peak currents, lower efficiency, and higher output

voltage ripple.

∆IL(PEAK)

ISENSE =

VSENSE

RSENSE

IOUT = (ISENSE + 1/2 ∆IL)

TON

VIN

TOFF

VIN

VIN + VOUT

VIN TON

VOUT

∆IL =

Figure 7. Flyback Regulator Inductor Current

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