RT9921PQV 查看數據表（PDF） - Richtek Technology

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RT9921PQV

Integrated Multi-Channel DC-DC Converter for TFT LCD Panels

Richtek Technology 

RT9921

Preliminary

Inductor Selection

The minimum inductance value, peak current rating and

series resistance are factors to be considered in the

inductor selecting process. These factors will affect the

converter efficiency, maximum output load capability,

transient response time and output voltage ripple. Physical

size and cost are also important factors to be considered.

The maximum output current, input voltage, output voltage

and switching frequency will determine the inductor value.

Very high inductance values minimize the current ripple

and therefore reduce the peak current, which decreases

core losses in the inductor and I2R losses in the entire

power path. However, large inductor values also require

more energy storage and more turns of wire, which increase

physical size and I2R losses in the inductor. Low

inductance values decrease the physical size but increase

the current ripple and the peak current. Finding the best

inductor involves choosing the best compromise between

circuit efficiency, inductor size and cost.

Choosing an available inductor value from an appropriate

inductor family. Calculating the maximum DC input current

at the minimum input voltage VIN(MIN) using the following

equation.

IIN(DC, MAX)

=

IAVDD(MAX) × VAVDD

VIN(MIN) ×η(MIN)

The expected efficiency at that operating point (ηMIN)

can be taken from an appropriate curve in the Typical

Operating Characteristics. Calculating the ripple current

at that operating point and the peak current required for

the inductor :

IRIPPLE

=

VIN(MIN) × (VAVDD − VIN(MIN) )

L ×V AVDD×fOSC

IPEAK

= IIN(DC,MAX)

+

IRIPPLE

2

The inductor's saturation current rating and the LX over

current protection (IOCP) should exceed IPEAK and the

inductor DC current rating should exceed IIN(DC,MAX). For

good efficiency, choosing an inductor with less than 0.1Ω

series resistance is suggested.

Diode Selection

To achieve high efficiency, Schottky diode is the

recommended diode for lower forward drop voltage and

faster switching time. The output diode rating should be

large enough for maximum output voltage, average power

dissipation and the pulsating diode peak current.

Output Capacitor Selection

For lower output voltage ripple, the low-ESR ceramic

capacitor is recommended. The output voltage ripple

consists of two components: one is the pulsating output

ripple current flowing through the ESR, and the other is

the capacitive ripple caused by charging and discharging.

VRIPPLE = VRIPPLE_ESR + VRIPPLE_C

≅

IPEAK

× RESR

+

IPEAK

COUT

⎜⎛

⎝

VAVDD − VIN

VAVDD × f

⎟⎞

⎠

Input Capacitor Selection

For better input bypassing, low-ESR ceramic capacitor is

recommended for better performance. A 10μF input

capacitor is sufficient and it is flexible to reduce the value

for a lower output power requirement.

http://www.DataSheet4U.net/

Gate-High Regulator

The gate-high regulator is to provide the TFT-LCD gate on

voltage. The charge pump can provide a programmable

output voltage. To regulate the output voltage must set

the resistive voltage-divider sensing at FBP pin. The error

amplifier varies the difference voltage by sensing FBP pin

to regulate the output voltage as the following equation :

VGH = VFBP (1+R9/R10), where VFBP = 1.24V (typ.)

Besides, the Schottky diodes with a current rating should

equal to or greater than two times of the average charge-

pump input current. Note that the voltage difference

between VGH (VGHM) and AVDD should not exceed 18V.

Zener Diode for the Negative Regulator

Bypassing a zener diode(ZD1) after the charge-pump satge

can also stablize the negative voltage.

VGL = −VZ

VD : Zener diode voltage

www.richtek.com

12

DS9921-00 November 2007

datasheet pdf - http://www.DataSheet4U.net/

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