7585AILZ Просмотр технического описания (PDF) - Renesas Electronics

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7585AILZ

TFT-LCD Power Supply

Renesas Electronics 

EL7585A

The following table gives typical values (margins are

considered 10%, 3%, 20%, 10%, and 15% on VIN, VO, L, fS,

and IOMAX:

TABLE 2.

VIN (V)

3.3

VO (V)

9

L (µH)

6.8

fS

(MHz)

1

IOMAX

1.040686

3.3

12

6.8

1

0.719853

3.3

15

6.8

1

0.527353

5

9

6.8

1

1.576797

5

12

6.8

1

1.090686

5

15

6.8

1

0.79902

Input Capacitor

An input capacitor is used to supply the peak charging

current to the converter. It is recommended that CIN be

larger than 10µF. The reflected ripple voltage will be smaller

with larger CIN. The voltage rating of input capacitor should

be larger than maximum input voltage.

Boost Inductor

The boost inductor is a critical part which influences the

output voltage ripple, transient response, and efficiency.

Values of 3.3µH to 10µH are to match the internal slope

compensation. The inductor must be able to handle the

following average and peak current:

ILAVG = 1----I-–-O----D---

ILPK = ILAVG + ---2--I--L-

Rectifier Diode

A high-speed diode is necessary due to the high switching

frequency. Schottky diodes are recommended because of

their fast recovery time and low forward voltage. The rectifier

diode must meet the output current and peak inductor

current requirements.

Output Capacitor

The output capacitor supplies the load directly and reduces

the ripple voltage at the output. Output ripple voltage

consists of two components: the voltage drop due to the

inductor ripple current flowing through the ESR of output

capacitor, and the charging and discharging of the output

capacitor.

VRIPPLE = ILPK  ESR + -V----O---V--–---O--V----I--N--  C-----O-I--O--U----T-  -f-1S--

FN7523 Rev 3.00

March 9, 2006

For low ESR ceramic capacitors, the output ripple is

dominated by the charging and discharging of the output

capacitor. The voltage rating of the output capacitor should

be greater than the maximum output voltage.

NOTE: Capacitors have a voltage coefficient that makes their

effective capacitance drop as the voltage across them increases.

COUT in the equation above assumes the effective value of the

capacitor at a particular voltage and not the manufacturer’s stated

value, measured at zero volts.

Compensation

The EL7585A can operate in either P mode or PI mode.

Connecting the CINT pin directly to VIN will enable P mode;

For better load regulation, use PI mode with a 4.7nF

capacitor in series with a 10K resistor between CINT and

ground. This value may be reduced to improve transient

performance, however, very low values will reduce loop

stability.

Boost feedback resistors

As the boost output voltage, AVDD, is reduced below 12V the

effective voltage feedback in the IC increases the ratio of

voltage to current feedback at the summing comparator

because R2 decreases relative to R1. To maintain stable

operation over the complete current range of the IC, the

voltage feedback to the FBB pin should be reduced

proportionally, as AVDD is reduced, by means of a series

resistor-capacitor network (R7 and C7) in parallel with R1,

with a pole frequency (fp) set to approximately 10kHz for C2

effective = 10µF and 4kHz for C2 (effective) = 30µF.

R7 = ((1/0.1 x R2) - 1/R1)^-1

C7 = 1/(2 x 3.142 x fp x R7)

PI Mode CINT (C23) and RINT (R10)

The IC is designed to operate with a minimum C23 capacitor

of 4.7nF and a minimum C2 (effective) = 10µF.

Note that, for high voltage AVDD, the voltage coefficient of

ceramic capacitors (C2) reduces their effective capacitance

greatly; a 16V 10µF ceramic can drop to around 3µF at 15V.

To improve the transient load response of AVDD in PI mode,

a resistor may be added in series with the C23 capacitor. The

larger the resistor the lower the overshoot but at the expense

of stability of the converter loop - especially at high currents.

With L = 10µH, AVDD = 15V, C23 = 4.7nF, C2 (effective)

should have a capacitance of greater than 10µF. RINT (R7)

can have values up to 5k for C2 (effective) up to 20µF and

up to 10K for C2 (effective) up to 30µF.

Larger values of RINT (R7) may be possible if maximum

AVDD load currents less than the current limit are used. To

ensure AVDD stability, the IC should be operated at the

maximum desired current and then the transient load

response of AVDD should be used to determine the

maximum value of RINT.

Page 10 of 19

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