EL7583IREZ datasheet(10/16 Pages) RENESAS

10 / 16 page

EL7583

FN7335 Rev 5.00

Page 10 of 16

May 12, 2006

The following table gives typical values:

(Margins are considered 10%, 3%, 20%, 10%, and 15% on

VIN, VO, L, FS, and ILMT, respectively)

Component Considerations

Input Capacitor

It is recommended that CIN is larger than 10µF.

Theoretically, the input capacitor has ripple current of

IL.

Due to high-frequency noise in the circuit, the input current

ripple may exceed the theoretical value. Larger capacitor will

reduce the ripple further.

Boost Inductor

The inductor has peak and average current decided by:

The inductor should be chosen to be able to handle this

current. Furthermore, due to the fixed internal

compensation, it is recommended that maximum inductance

of 10µH and 15µH to be used in the 5V and 12V or higher

output voltage, respectively.

The output diode has average current of IO, and peak current

the same as the inductor's peak current. Schottky diode is

recommended and it should be able to handle those currents.

Feedback Resistor Network

An external resistor divider is required to divide the output

voltage down to the nominal reference voltage. Current

drawn by the resistor network should be limited to maintain

the overall converter efficiency. The maximum value of the

resistor network is limited by the feedback input bias current

and the potential for noise being coupled into the feedback

pin. A resistor network in the order of 200k

 is

recommended. The boost converter output voltage is

determined by the following relationship:

where VFBB is 1.300V.

A 1nF compensation capacitor across the feedback resistor

to ground is recommended to keep the converter in stable

operation at low output current and high frequency

conditions.

Schottky Diode

Speed, forward voltage drop, and reverse current are the

three most critical specifications for selecting the Schottky

diode. The entire output current flows through the diode, so

the diode average current is the same as the average load

current and the peak current is the same as the inductor

peak current. When selecting the diode, one must consider

the forward voltage drop at the peak diode current. On the

Elantec demo board, MBRM120 is selected. Its forward

voltage drop is 450mV at 1A forward current.

Output Capacitor

The EL7583 is specially compensated to be stable with

capacitors which have a worst-case minimum value of 10µF

at the particular VOUT being set. Output ripple voltage

requirements also determine the minimum value and the

type of capacitors. Output ripple voltage consists of two

components - the voltage drop caused by the switching

current though the ESR of the output capacitor and the

charging and discharging of the output capacitor:

For low ESR ceramic capacitors, the output ripple is

dominated by the charging/discharging of the output

capacitor.

In addition to the voltage rating, the output capacitor should

also be able to handle the RMS current is given by:

Positive and Negative Charge Pump (VON and

VOFF)

The EL7583 contains two independent charge pumps (see

charge pump block and connection diagram.) The negative

charge pump inverts the VDDN supply voltage and provides

a regulated negative output voltage. The positive charge

pump doubles the VDDP supply voltage and provides a

regulated positive output voltage. The regulation of both the

negative and positive charge pumps is generated by the

internal comparator that senses the output voltage and

compares it with and internal reference. The switching

frequency of the charge pump is set to ½ the boost converter

switching frequency.

The pumps use pulse width modulation to adjust the pump

period, depending on the load present. The pumps are short-

circuit protected to 180mA at 12V supply and can provide

15mA to 60mA for 6V to 12V supply.

TABLE 1. MAXIMUM CONTINUOUS OUTPUT CURRENT

VIN (V)

VO (V)

L (ΜH)

FS (kHz) IOMAX (mA)

3.3

1000

430

3.3

1000

320

3.3

1000

250

1000

650

1000

470

1000

370

1000

830

ILPK

ILAVG

IL

--------

ILAVG

1 - D

-------------

VBOOST

R1 R2

--------------------- VFBB



VRIPPLE

ILPK ESR

VOUT - VIN

VOUT

--------------------------------



IOUT

COUT FS



------------------------------



ICORMS

 - D 

IL

ILAVG

--------------------









------









ILAVG



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