7

ML4775

Again, a low ESR capacitor (such as tantalum) is

recommended.

DRIVING THE SHDN INPUT

Unlike other boost regulators which use external

Schottky diodes, the ML4775 has the ability to isolate

the load from the battery input when the SHDN pin is

high. Since there may be no other voltage available

when the regulator is in shutdown, the SHDN input

SHDN can be driven directly from an open collector

is driven from a TTL or CMOS output device, a resistor

divider should be used to prevent the SHDN input high

low level is below the 200mV threshold.

SETTING THE OUTPUT VOLTAGE

The adjustable output can be set to any voltage between

2.5V and 5.5V 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

total output variation.

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 100pF ceramic feedforward capacitor

LAYOUT

Good PC board layout practices will ensure the proper

operation of the ML4775. Important layout

considerations include:

• Use adequate ground and power traces or planes

• Keep components as close as possible to the ML4775

• Use a single point ground for the ML4775 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:

Matsuo

(714) 969-2491

Sprague

(603) 224-1961

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 ML4775 control circuitry, and it also

improves efficiency by reducing I-squared R losses

during the charge and discharge cycles of the inductor.