ML4890 查看數據表（PDF） - Micro Linear Corporation

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ML4890

High Efficiency, Low Ripple Boost Regulator

Micro Linear Corporation 

ML4890

The DC resistance of the inductor should be kept to a

minimum to reduce losses. A good rule of thumb is to

allow 5 to 10mΩ of resistance for each µH of inductance.

Also, be aware that the DC resistance of an inductor

usually isn‘t specified tightly, so an inductor with a

maximum DC resistance spec of 150mΩ may actually

have 100mΩ of resistance.

Suitable inductors can be purchased from the following

suppliers:

Coilcraft

(708) 639-6400

Coiltronics

(407) 241-7876

Dale

(605) 665-9301

Sumida

(708) 956-0666

BOOST CAPACITOR

The boost capacitor (C2) supplies current to the load

during the ON-time of Q1 and will limit the ripple the

LDO stage has to contend with. The ripple on C2 is

influenced by three capacitor parameters: capacitance,

ESL, and ESR. The contribution due to capacitance can be

determined by looking at the change in the capacitor

voltage required to store the energy delivered by the

inductor in a single charge-discharge cycle, as given by

the following formula:

C2 ≥

TON2 × VIN2

(in Farads)

2 × L × ∆VBOOST × (VOUT – VIN)

(3)

For example, a 2.4V input, a 5V output, a 22µH inductor,

and an allowance of 100mV of ripple on the boost

capacitor results in a minimum C2 value of 15µF.

The boost capacitor‘s Equivalent Series Resistance (ESR)

and Equivalent Series Inductance (ESL), also contribute to

the 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 boost capacitor‘s ESL

causes a high frequency (5ns) spike that can be over 1V in

magnitude. After the ESL spike settles, the boost voltage

still has a ripple component equal to the inductor

discharge current times the ESR. This component will have

a sawtooth waveshape and can be calculated using the

following formula:

ESR ≤ ∆VBOOST (in Ω)

IL(PEAK )

(4)

For example, a 2.4V input, a 22µH inductor, and an

allowance of 100mV of ripple on the boost capacitor

results in a maximum ESR of 200mΩ. Therefore, a boost

capacitor with a capacitance of 22µF or 33µF, an ESR of

less than 200mΩ, and an ESL of less than 5nH is a good

choice. Tantalum capacitors which meet these

requirements can be obtained from the following

suppliers:

AVX

(207) 282-5111

Sprague

(207) 324-4140

OUTPUT CAPACITOR

The LDO stage output capacitor (C1) is required for

stability and to provide a high frequency filter. An output

capacitor with a capacitance of 100µF, an ESR of less than

100mΩ, and an ESL of less than 5nH is a good general

purpose choice.

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

ML4890 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.

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.

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