ML4851IS-3 Просмотр технического описания (PDF) - Micro Linear Corporation

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ML4851IS-3

Low Current, Voltage Boost Regulator

Micro Linear Corporation 

DESIGN CONSIDERATIONS (Continued)

After the appropriate inductor value is chosen, it is

necessary to find the minimum inductor current rating

required. Peak inductor current is determined from the

following formula:

IL(PEAK)

=

t × V ON(MAX)

IN(MAX)

LMIN

(3)

In the two cell application previously described, a

maximum input voltage of 3V would give a peak current

of 1A. When comparing various inductors, it is important

to keep in mind that suppliers use different criteria to

determine their ratings. Many use a conservative current

level, where inductance has dropped to 90% of its normal

level. In any case, it is a good idea to try inductors of

various current ratings with the ML4851 to determine

which inductor is the best choice. Check efficiency and

maximum output current, and if a current probe is

available, look at the inductor current to see if it looks

like the waveform shown in Figure 3. For additional

information, see Applications Note 29, “Choosing an

Inductor for Your ML4861 Application.”

Suitable inductors can be purchased from the following

suppliers:

Coilcraft (847) 639-6400

Coiltronics (561) 241-7876

Dale

(605) 665-9301

Sumida

(847) 956-0666

XFMRS, Inc. (317) 834-1066

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:

1 6 ∆VOUT

=

t

2

ON

2×L × C×

× VIN2

VOUT

−

VIN

(4)

For a 2.4V input, and 5V output, a 18µH inductor, and a

47µF capacitor, the expected output ripple due to

capacitor value is 33mV.

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

DATASHEET

ML4851

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

I2R losses during the discharge cycle.

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

of less than 0.1W, 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

(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 ML4851 control circuitry, and it also

improves efficiency by reducing I2R losses during the

charge and discharge cycles of the inductor. Again, a low

ESR capacitor (such as tantalum) is recommended.

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 (VREF) with a

small 10nF to 100nF ceramic capacitor. This is

particularly true if the ripple voltage at VIN is greater than

100mV.

SETTING THE RESET THRESHOLD

To use the RESET comparator as an input voltage monitor,

as shown in Figure 1, it is necessary to use an external

resistor divider tied to the DETECT pin as shown in the

block diagram. The resistor values RA and RB can be

1 6 calculated using the following equation:

VIN(MIN) = 0.2 ×

RA + RB

RB

(5)

The value of RB should be 100kW or less to minimize bias

current errors. RA is then found by rearranging the

equation:

July 2000

7

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