7

ML4761

SETTING THE OUTPUT VOLTAGE

The adjustable output can be set to any voltage between

2.5V and 6V by connecting a resistor divider to the

SENSE pin as shown in the block diagram. The resistor

equation:

V

RR

R

+

02

12

2

.

()

(5)

equation:

RR

12

02

1

=×

−









.

(6)

It is important to note that the accuracy of these resistors

directly affects the accuracy of the output voltage. The

SENSE pin threshold variation is ±3%, and the tolerances

variation.

REFERENCE CAPACITOR

Under some circumstances input ripple cannot be

reduced effectively. This occurs primarily in applications

where inductor currents are high, causing excess output

ripple due to “pulse grouping”, where the charge-

discharge pulses are not evenly spaced in time. In such

cases it may be necessary to decouple the reference pin

than 100mV.

In some applications, input noise may cause output ripple

to become excessive due to “pulse grouping”, where the

charge-discharge pulses are not evenly spaced in time. In

such cases it may be necessary to add a small 20pF to

pin to the SENSE pin.

LAYOUT

Good PC board layout practices will ensure the proper

operation of the ML4761. Important layout considerations

include:

• Use adequate ground and power traces or planes

• Keep components as close as possible to the ML4761

• Use a single point ground for the ML4761 ground pins,

and the input and output capacitors

OUTPUT CAPACITOR

The choice of output capacitor is also important, as it

controls the output ripple and optimizes the efficiency of

the circuit. Output ripple is influenced by three capacitor

parameters: capacitance, ESR, and ESL. The contribution

due to capacitance can be determined by looking at the

change in capacitor voltage required to store the energy

delivered by the inductor in a single charge-discharge

cycle, as determined by the following formula:

∆V

TV

LC

V

V

OUT

ON

IN

OUT

IN

=

×

×× ×

−

22

2(

)

(4)

For a 2.4V input, and 5V output, a 27µH inductor, and a

47µF capacitor, the expected output ripple due to

capacitor value is 87mV.

Capacitor Equivalent Series Resistance (ESR) and

Equivalent Series Inductance (ESL), also contribute to the

output ripple due to the inductor discharge current

waveform. Just after the NMOS transistor turns off, the

output current ramps quickly to match the peak inductor

current. This fast change in current through the output

capacitor’s ESL causes a high frequency (5ns) spike that

can be over 1V in magnitude. After the ESL spike settles,

the output voltage still has a ripple component equal to

the inductor discharge current times the ESR. This

component will have a sawtooth shape and a peak value

equal to the peak inductor current times the ESR. ESR also

has a negative effect on efficiency by contributing

I-squared R losses during the discharge cycle.

An output capacitor with a capacitance of 100µF, an ESR

of less than 0.1ý, and an ESL of less than 5nH is a good

general purpose choice. Tantalum capacitors which meet

these requirements can be obtained from the following

suppliers:

AVX

(207) 282-5111

Sprague

(207) 324-4140

If ESL spikes are causing output noise problems, an EMI

filter can be added in series with the output.

INPUT CAPACITOR

Unless the input source is a very low impedance battery, it

will be necessary to decouple the input with a capacitor

with a value of between 47µF and 100µF. This provides

the benefits of preventing input ripple from affecting the

ML4761 control circuitry, and it also improves efficiency

by reducing I-squared R losses during the charge and

discharge cycles of the inductor. Again, a low ESR

capacitor (such as tantalum) is recommended.