ML4865 데이터 시트보기 (PDF) - Micro Linear Corporation

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ML4865

High Voltage High Current Boost Regulator

Micro Linear Corporation 

DESIGN CONSIDERATIONS (Continued)

INDUCTOR SELECTION

The ML4865 is able to operate over a wide range of

inductor values. A value of 22µH or 33µH is a good

choice, but any value between 15µH and 50µH is

acceptable. As the inductor value is changed the control

circuitry will automatically adjust to keep the inductor

current under control. Choosing an inductance value of

less than 15µH will reduce the component’s footprint, but

the efficiency and maximum output current may drop.

It is important to use an inductor that is rated to handle

1.5A peak currents without saturating. Also look for an

inductor with low winding resistance. A good rule of

thumb is to allow 5 to 10mW of resistance for each µH of

inductance.

The final selection of the inductor will be based on trade-

offs between size, cost and efficiency. Inductor tolerance,

core and copper loss will vary with the type of inductor

selected and should be evaluated with a ML4865 under

worst case conditions to determine its suitability.

Several manufacturers supply standard inductance values

in surface mount packages:

Coilcraft (847) 639-6400

Coiltronics (561) 241-7876

Dale

(605) 665-9301

Sumida

(847) 956-0666

OUTPUT CAPACITOR

The output capacitor filters the pulses of current from the

switching regulator. Since the switching frequency will

vary with inductance, the minimum output capacitance

required to reduce the output ripple to an acceptable

level will be a function of the inductor used. Therefore, to

maintain an output voltage with less than 100mV of ripple

(due to capacitance) at full load current, use the

following equation:

ML4865

COUT

=

10 ™ L

VOUT

(F)

(2)

The output capacitor’s Equivalent Series Resistance (ESR)

and Equivalent Series Inductance (ESL), also contribute to

the ripple. Just after the NMOS transistor, Q1, turns off,

the current in the output capacitor ramps quickly to

between 0.5A and 1.5A. This fast change in current

through the capacitor’s ESL causes a high frequency (5ns)

spike to appear on the output. After the ESL spike settles,

the output still has a ripple component equal to the

inductor discharge current times the ESR. To minimize

these effects, choose an output capacitor with less than

10nH of ESL and 200mW of ESR.

Suitable tantalum capacitors can be obtained from the

following vendors:

AVX

TPS Series

(207) 282-5111

Sprague 593D Series (207) 324-4140

Kemet

T495 Series (864) 963-6300

INPUT CAPACITOR

Due to the high input current drawn at startup and

possibly during operation, it is recommended to decouple

the input with a capacitor with a value of 22µF to 68µF.

This filtering prevents the input ripple from affecting the

ML4865 control circuitry, and also improves the

efficiency by reducing the I squared R losses during the

charge cycle of the inductor. Again, a low ESR capacitor

(such as tantalum) is recommended.

It is also recommended that low source impedance

batteries be used. Otherwise, the voltage drop across the

source impedance during high input current situations will

cause the ML4865 to fail to start-up or to operate

unreliably. In general, for two cell applications the source

impedance should be less than 200mW, which means that

small alkaline cells should be avoided.

VIN

C1

47µF

ML4865

SENSE

GND

VOUT

SHDN

VIN

VL2

VL1 PWR GND

R1

1MΩ

22µH

(Sumida CD75)

D1

MBR0520L

VOUT

C2

47µF

Figure 7. Typical Application Circuit.

7

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