ML4880CS datasheet(11/15 Pages) MICRO-LINEAR

11 / 15 page

ML4880

where,

derating factor

OUT

IN MAX

()

(27)

Again a derating factor of 0.8 is typical for most

applications.

FLYBACK REGULATOR - INPUT CAPACITOR

SELECTION

The input capacitor should be a low ESR type and located

as close to the primary winding of the transformer as

possible. The capacitor’s ability to handle ripple current is

the selection criteria used. The input capacitor’s ripple

current is determined by the load current and the input

voltage, with the worst case condition occurring at

VIN = VOUT.

RMS C

SENSE MAX

L MAX

OUT

(

)

()

≈+





















∆

(28)

The capacitor must also be able to handle the maximum

input voltage. For reliable operation derate the capacitor

voltage rating by 20% to 50% for aluminum electrolytic

types and 50% to 70% for tantalum types.

FLYBACK REGULATOR - OUTPUT CAPACITOR

SELECTION

The output capacitors determine the flyback regulator’s

loop stability and the output ripple voltage. Use only low

ESR capacitors intended for switching power supply

applications. To ensure stability, the minimum capacitance

value is given by:

OUT

ON MAX

SENSE

OUT MAX

L MIN

≥

























()

∆

(29)

The maximum ESR value can be estimated using:

ESR

OUT

L MAX

≤

∆

()

(30)

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

OUT

()

(

)

≈

(31)

FLYBACK REGULATOR - OUTPUT VOLTAGE

The output of the flyback regulator is adjustable and can

be set to any voltage between 6.0V and 15.0V by

connecting a resistor divider to the feedback pin as shown

in Figure 3. The resistor values R1 and R2 can be

calculated using the following equation:

OUT =×









125

(32)

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 in order to make up for the ML4880’s load

regulation. This offset results in the best overall output

accuracy over line and load.

LAYOUT

Proximity of passive devices and adequate power and

ground planes are critical for reliable operation of the

circuit. In general, use the top layer for the high current

connections and the bottom layer for the quiet

connections such as GND, feedback and current sense.

Some more specific guidelines follow.

1. The connection from the current sense resistor to the

ISENSE pin should be made by a separate trace and

located as close to the lead of the resistor as

possible. The trace length from the sense resistor to

the ML4880 should be kept as short as possible and

away from switching components and their traces.

2. The trace lengths from the buck regulator’s input

capacitor to the switching MOSFET, from the

MOSFETs to the inductor, from the synchronous

rectifier MOSFET to the SENSE resistor, and from the

inductor to the output capacitor should all be as

short as possible. The flyback regulator’s trace length

from the input capacitor to the inductor, from the

inductor to the MOSFET, from the sense resistor to

the inductor, and from the diode to the output

capacitor should also be kept short to minimize

noise and ground bounce.

3. The high current ground paths need to be kept

separate from the signal ground paths. The GND

connection should be made at a single-point star

ground. It is very important that the ground for the

ML4880 ground pins (pins 6 and 7) be made using a

separate trace.

4. Note: Concentrating on keeping the current sense

and high current connections short as well as

keeping the switching components and traces away

from the sensitive analog components and traces

during layout will eliminate the majority of problems

created by a poor layout.

5. The VIN and VREG bypass capacitors need to be

located close to the ML4880 for adequate filtering of

the IC’s internal bias voltage.

6. Remote sensing the output for improved load

regulation can be implemented with the ML4880.

Any output can be remote sensed by using the top of

the external resistor divider as the remote sense

point.

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