SD6251D6G-R datasheet(7/9 Pages) SHOUDING

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SD6251

85T

Application Information (Continued)

(1) Programming the Output Voltage

The output voltage of the

can be

adjusted with an external resistor divider.

The

typical value of the voltage on the FB pin is

800mV in fixed frequency operation.

The

maximum allowed value for the output voltage is

5.5V.

The current through the resistive divider

should be about 100 times greater than the

current into the FB pin.

The typical current into

the FB pin is 0.01µA, and the voltage across R2

is typically 800mV. Based on those two values,

the recommended value for R2 is in the range of

00kΩ in order to set the divider current at 1µA.

From that, the value of resistor R1, depending

on the needed output voltage (VO), can be

calculated using Equation 1.

R1 R2

O T

800kΩ

O T

800m

1 …..(1)

(2) Inductor Selection

A boost converter normally requires two main

passive components for storing energy during

the conversion.

A boost inductor is required

and a storage capacitor at the output.

To select

the boost inductor, it is recommended to keep

the possible peak inductor current below the

current limit threshold of the power switch in the

chosen configuration.

The second parameter for choosing the inductor

is the desired current ripple in the inductor.

Normally, it is advisable to work with a ripple of

less than 20% of the average inductor current.

A smaller ripple reduces the magnetic hysteresis

losses in the inductor, as well as output voltage

ripple and EMI.

But in the same way, regulation

time at load changes rises.

In addition, a larger

inductor increases the total system cost.

With

those parameters, it is possible to calculate the

value for the inductor by using Equation 2.

O T- N

O T

…..(2)

Parameter

is the switching requency and Δ

L is

the ripple current in the inductor, i.e, 20% x IL.

With this calculated value and currents, it is

possible to choose a suitable inductor.

Care must

be taken that load transients and losses in the

circuit can lead to higher currents.

Also, the

losses in the inductor caused by magnetic

hysteresis losses and copper losses are a major

parameter for total circuit efficiency.

(3) Capacitor Selection

The major parameter necessary to define the

output capacitor is the maximum allowed output

voltage ripple of the converter.

This ripple is

determined by two parameters of the capacitor,

the capacitance and the ESR.

It is possible to

calculate the minimum capacitance needed for

the defined ripple, supposing that the ESR is zero,

by using Equation 3.

M N

O T

O T- N

O T

…..(3)

Parameter f is the switching frequency and △V is

the maximum allowed ripple.

The total ripple is larger due to the ESR of the

output capacitor.

This additional component of

the ripple can be calculated using Equation 4.

ESR

O T RESR …..(4)

The total ripple is the sum of the ripple caused by

the capacitance and the ripple caused by the ESR

of the capacitor.

It is possible to improve the

design by enlarging the capacitor or using smaller

capacitors in parallel to reduce the ESR or by using

better capacitors with lower ESR, like ceramics.

Tradeoffs must be made between performance and

costs of the converter circuit.

A 10µF input capacitor is recommended to

improve transient behavior of the regulator.

ceramic or tantalum capacitor with a 100nF in

parallel placed close to the IC is recommended.

SD6251

http://www.sdw-tw.com

Ver 1.0 Oct 2012

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