Richtek Rt8509a User manual

RT8509A

DS8509A-00 November 2013 www.richtek.com

4.5A Step-Up DC/DC Converter

General Description

The RT8509A is a high performance switching Boost

converter that provides a regulated supply voltage for active

matrix thin film transistor (TFT) liquid crystal displays

(LCDs).

The RT8509A incorporates current mode, fixed-frequency,

pulse width modulation (PWM) circuitry with a built in

N-MOSFET to achieve high efficiency and fast transient

response.

The RT8509A has a wide input voltage range from 2.8V to

14V. In addition, the output voltage can be adjusted up to

24V via an external resistive voltage divider. The maximum

peak current is limited to 4.5A (min.). Other features

include adjustable soft-start, over-voltage protection, and

over-temperature protection.

The RT8509A is available in the WDFN-12L 5x5 package.

Features



90%Efficiency



Adjustable Output Up to 24V



2.8V to 14V Input Supply Voltage



Input Supply Under-Voltage Lockout



Fixed 1.2MHz Switching Frequency



Adjustable Soft-Start



VOUT Over-Voltage Protection



Over-Temperature Protection



Thin 12-Lead WDFN Package



RoHS Compliant and Halogen Free

Applications

GIP TFT-LCD Panels

Ordering Information

Note :

Richtek products are :

RoHS compliant and compatible with the current require-

ments of IPC/JEDEC J-STD-020.

Suitable for use in SnPb or Pb-free soldering processes.

Package Type

QW : WDFN-12L 5x5 (W-Type)

RT8509A

Lead Plating System

G : Green (Halogen Free and Pb Free)

RT8509A

VIN

VOUT

COMPGND

COUT

VOUT

CSS

CIN

VIN

L1 D1

Enable

Marking Information

RT8509A

GQW

YMDNN

RT8509AGQW : Product Number

YMDNN : Date Code

Simplified Application Circuit

RT8509A

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Functional Pin Description

Pin No. Pin Name Pin Function

1 COMP

Compensation Node for Error Amplifier. Connect a series RC from COMP to

ground.

2 FB

Feedback Voltage Input. The FB regulation voltage is 1.25V nominal. Connect an

external resistive voltage divider between the step-up regulator’s output (VOUT)

and GND, with the center tap connected to FB. Place the divider close to the IC

and minimize the trace area to reduce noise coupling.

3 EN Enable Control Input. Drive EN low to turn off the Boost.

4, 5, 6, 9,

13 (Exposed Pad) GND Ground. The Exposed Pad must be soldered to a large PCB and connected to

GND for maximum power dissipation.

7, 8 LX Switch Node. LX is the Drain of the internal MOSFET. Connect the

inductor/rectifier diode junction to LX and minimize the trace area for lower EMI.

10 VOUT

Over-Voltage Protection Input for Boost Converter. Bypass VOUT with a

minimum 1F ceramic capacitor directly to GND.

11 VIN Supply Voltage Input. Bypass VIN with a minimum 1μF ceramic capacitor directly

to GND.

12 SS

Soft-Start Time Setting. Connect a soft-start capacitor (CSS) to this pin. The

soft-start capacitor is charged with a constant current of 5A. The soft-start

capacitor is discharged to ground when EN is low.

Pin Configurations

WDFN-12L 5x5

(TOP VIEW)

COMP

GND

VIN

VOUT

GND

GND LX

GND

RT8509A

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Function Block Diagram

Operation

The RT8509A is a high-performance step-up DC/DC

converter that provides a regulated and high precision

supply voltage. It incorporates current mode, fixed-

frequency, pulse-width modulation (PWM) circuitry with

a built-in N-Channel power MOSFET to achieve high

efficiency and fast transient response. The device features

an adjustable soft start time using an external soft-start

capacitor to reduce in-rush current.

Soft

Start

Control

and

Driver

Logic

Current

Sense

Protection

Slope

Compensation

Oscillator

OVP

OTP

Error Amplifier

VDD

1.25V

COMP

VIN

VOUT

Summing

Comparator

GND

Clock

RT8509A

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Electrical Characteristics

Recommended Operating Conditions (Note 4)

Ambient Temperature Range -------------------------------------------------------------------------------------------- −40°C to 85°C

Junction Temperature Range -------------------------------------------------------------------------------------------- −40°C to 125°C

Absolute Maximum Ratings (Note 1)

LX toGND ------------------------------------------------------------------------------------------------------------------- −0.3V to 28V

VIN, EN to GND ------------------------------------------------------------------------------------------------------------ −0.3V to 16.5V

Other Pins------------------------------------------------------------------------------------------------------------------- −0.3V to 6.5V

Power Dissipation, PD@ TA= 25°C

WDFN-12L 5x5 ------------------------------------------------------------------------------------------------------------- 3.38W

Package Thermal Resistance (Note 2)

WDFN-12L 5x5, θJA ------------------------------------------------------------------------------------------------------- 29.5°C/W

WDFN-12L 5x5, θJC ------------------------------------------------------------------------------------------------------- 7.5°C/W

Junction Temperature ----------------------------------------------------------------------------------------------------- 150°C

Storage Temperature Range -------------------------------------------------------------------------------------------- −65°C to 150°C

Lead Temperature (Soldering, 10sec.) -------------------------------------------------------------------------------- 260°C

ESD Susceptibility (Note 3)

HBM (Human Body Model) ---------------------------------------------------------------------------------------------- 2kV

MM (Machine Model) ----------------------------------------------------------------------------------------------------- 200V

(VIN = 3.3V, VOUT = 10V, TA=25°C unless otherwise specified)

Parameter Symbol Test Conditions Min Typ Max Unit

Supply Current

Input Voltage Range VIN 2.8 -- 14 V

Output Voltage Range VOUT -- -- 24 V

Under Voltage Lockout

Threshold VUVLO V

IN Rising -- 2.5 3 V

UVLO Hysteresis VUVLO -- 200 -- mV

VFB = 1.3V, LX Not Switching -- 1 --

VIN Quiescent Current IQVFB = 1V, LX Switching -- 5 -- mA

Thermal Shutdown Threshold TSD Temperature Rising -- 155 -- C

Thermal Shutdown

Hysteresis TSD -- 10 --

C

VOUT Over Voltage

Threshold VOUT Rising -- 26 -- V

Oscillator

Oscillator Frequency fOSC 1000 1200 1500 kHz

Maximum Duty Cycle DMAX -- 90 -- %

Error Amplifier

FB Regulation Voltage VREF 1.2312 1.25 1.2688 V

FB Input Bias Current IFB -- -- 100 nA

FB Line Regulation -- 0.05 0.2 %/V

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Parameter Symbol Test Conditions Min Typ Max Unit

Transconductance gm I = ±2.5μA at VCOMP = 1V -- 100 --

A/V

Voltage Gain AVFB to COMP -- 700 -- V/V

N-MOSFET

Current Limit ILIM 4.5 5 -- A

On-Resistance RDS(ON) -- 100 250

m

Leakage Current ILEAK V

LX = 24V -- 30 45

A

Current Sense

Transresistance RCS -- 0.25 -- V/A

Soft-Start

Charge Current -- 5 -- A

Control Inputs

Logic-High VIH 1.5 -- --

EN Input

Voltage Logic-Low VIL -- -- 0.5

Note 1. Stresses beyond those listed “Absolute Maximum Ratings”may cause permanent damage to the device. These are

stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in

the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions may

affect device reliability.

Note 2. θJA is measured at TA= 25°C on a high effective thermal conductivity four-layer test board per JEDEC 51-7. θJC is

measured at the exposed pad of the package.

Note 3. Devices are ESD sensitive. Handling precaution is recommended.

Note 4. The device is not guaranteed to function outside its operating conditions.

RT8509A

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Typical Application Circuit

RT8509A

VIN

VOUT

COMP

GND

COUT

VOUT

18V

CSS

7, 8

CIN

VIN

12V

4, 5, 6, 9,

13 (Exposed Pad)

10µF x 3

1µF

4.7µH

134k

10k

10µF x 4

33nF

56k

1nF

1µF

Enable

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Boost Reference Voltage vs. Input Voltage

1.1

1.2

1.3

1.4

1.5

2 4 6 8 101214

Input Voltage (V)

Boost Reference Voltage (V)

Typical Operating Characteristics

Boost Efficiency vs. Load Current

100

0 0.3 0.6 0.9 1.2 1.5

Load Current (A)

Boost Efficiency (%)

VOUT = 18V, fOSC = 1.2MHz

VIN = 14V

VIN = 12V

VIN = 10V

Boost Efficiency vs. Load Current

100

0 0.1 0.2 0.3 0.4 0.5

Load Current (A)

Boost Efficiency (%)

VOUT = 13.5V, fOSC = 1.2MHz

VIN = 3.3V

VIN = 5V

Boost Reference Voltage vs. Temperature

1.1

1.2

1.3

1.4

1.5

-50 -25 0 25 50 75 100 125

Temperature (°C)

Boost Reference Voltage (V)

VIN = 3.3V

Boost Frequency vs. Temperature

900

1000

1100

1200

1300

1400

-50 -25 0 25 50 75 100 125

Temperature (°C)

Boost Frequency (kHz

)

VIN = 3.3V

Boost Current Limit vs. Input Voltage

2 4 6 8 10 12 14

Input Voltage (V)

Boost Current Limit (A)

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Application Information

The RT8509A is a high performance step-up DC/DC

converter that provides a regulated supply voltage for panel

source driver ICs. The RT8509A incorporates current mode,

fixed frequency, Pulse Width Modulation (PWM) circuitry

with a built-in N-MOSFET to achieve high efficiency and

fast transient response. The internal driver power is

supplied from the VOUT pin and that will increase efficiency

when low input voltage condition. The following content

contains detailed description and information for

component selection.

BoostRegulator

The RT8509A is a current mode Boost converter integrated

with a 24V/5A power switch, covering a wide VIN range

from 2.8V to 14V. It performs fast transient responses to

generate source driver supplies for TFT-LCD display. The

high operation frequency allows the use of smaller

components to minimize the thickness of the LCD panel.

The output voltage can be adjusted by setting the resistive

voltage-divider sensing at the FB pin. The error amplifier

varies the COMP voltage by sensing the FB pin to regulate

the output voltage. For better stability, the slope

compensation signal summed with the current sense

signal will be compared with the COMP voltage to

determine the current trip point and duty cycle. The Boost

minimum gain ratio depends on minimum on-time. It's

suggested that VOUT higher than 1.2 x VIN for better

performance.

Soft-Start

The RT8509A provides soft-start function to minimize the

inrush current. When powered on, an internal constant

current charges an external capacitor. The rising voltage

rate on the COMP pin is limited from VSS = 0V to 1.24V

and the inductor peak current will also be limited at the

same time. When powered off, the external capacitor will

be discharged until the next soft-start time.

The soft-start function is implemented by the external

capacitor with a 5μA constant current charging to the soft-

start capacitor. Therefore, the capacitor should be large

enough for output voltage regulation. A typical value for

soft-start capacitor is 33nF. The available soft-start capacitor

range is from 10nF to 100nF.

OUT REF REF

V = V x 1 , where V = 1.25V (typ.)









The recommended value for R2 should be at least 10kΩ

without some sacrificing. Place the resistive voltage divider

as close as possible to the chip to reduce noise sensitivity.

Loop Compensation

The voltage feedback loop can be compensated with an

external compensation network consisting of R3. Choose

R3 to set high frequency integrator gain for fast transient

response and C1 to set the integrator zero to maintain

loop stability. For typical application, VIN = 5V,

VOUT = 13.6V, COUT = 4.7μF x 3, L1 = 4.7μH, while the

recommended value for compensation is as follows :

R3 = 56kΩ, C1 = 1nF.

Over-Current Protection

The RT8509A Boost converter has over-current protection

to limit the peak inductor current. It prevents the inductor

and diode from damage due to large current. During the

On-time, once the inductor current exceeds the current

limit, the internal LX switch turns off immediately and

shortens the duty cycle. Therefore, the output-voltage

drops if the over current condition occurs. The current

limit is also affected by the input voltage, duty cycle, and

inductor value.

Over-Temperature Protection

The RT8509A Boost converter has thermal protection

function to prevent the chip from overheating. When the

junction temperature exceeds 155°C, the function shuts

down the device. Once the device cools down by

approximately 10°C, it will automatically restart to normal

operation. To guarantee continuous operation, do not

operate over the maximum junction temperature rating of

125°C.

If CSS < 220pF, the internal soft-start function will be turned

on and period time is approximately 1ms.

Output Voltage Setting

The regulated output voltage is shown as the following

equation :

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where ηis the efficiency of the converter, IIN(MAX) is the

maximum input current, and IRIPPLE is the inductor ripple

current. The input peak current can then be obtained by

adding the maximum input current with half of the inductor

ripple current as shown in the following equation :

PEAK IN(MAX)

I1.2xI

Note that the saturated current of the inductor must be

greater than IPEAK. The inductance can eventually be

determined according to the following equation :

IN OUT IN

OUT OUT(MAX) OSC

x (V ) x(V V )

0.4 x (V ) xI x f







where fosc is the switching frequency. For better system

performance, a shielded inductor is preferred to avoid EMI

problems.

Diode Selection

Schottky diodes are chosen for their low forward voltage

drop and fast switching speed. When selecting a Schottky

diode, important parameters such as power dissipation,

reverse voltage rating, and pulsating peak current should

all be taken into consideration. A suitable Schottky diode's

reverse voltage rating must be greater than the maximum

output voltage and its average current rating must exceed

the average output current. Last of all, the chosen diode

should have a sufficiently low leakage current level, since

it will increase with temperature.

Output Capacitor Selection

The output ripple voltage is an important index for

estimating chip performance. This portion consists of two

parts. One is the product of the inductor current with the

ESR of the output capacitor, while the other part is formed

by the charging and discharging process of the output

OUT OUT(MAX)

IN(MAX)

RIPPLE IN(MAX)

V xI

I = x V

I = 0.4 x I



Inductor Selection

The inductance depends on the maximum input current.

As a general rule, the inductor ripple current range is 20%

to 40% of the maximum input current. If 40% is selected

as an example, the inductor ripple current can be

calculated according to the following equations : IN L OUT IN L OUT

IN OUT OUT1

OUT OSC

11 1

Q xI II I II

22 2

x x C x V

















where fOSC is the switching frequency, and ΔILis the

inductor ripple current. Bring COUT to the left side to

estimate the value of ΔVOUT1 according to the following

equation :

OUT

OUT1

OUT OSC

D x I

Vx C x f





where D is the duty cycle and ηis the Boost converter

efficiency. Finally, taking ESR into account, the overall

output ripple voltage can be determined by the following

equation :

OUT

OUT IN

OUT OSC

D x I

V I x ESR x C x f



 

The output capacitor, COUT, should be selected accordingly.

Time

Inductor Current

Output Current

Output Ripple

Voltage (ac)

(1-D)TS

ΔVOUT1

ΔIL

Input Current

Figure 1. The Output Ripple Voltage without the

Contribution of ESR

Input Capacitor Selection

Low ESR ceramic capacitors are recommended for input

capacitor applications. Low ESR will effectively reduce

the input voltage ripple caused by switching operation. A

10μF capacitor is sufficient for most applications.

capacitor. As shown in Figure 1, ΔVOUT1 can be evaluated

based on the ideal energy equalization. According to the

definition of Q, the Q value can be calculated as the

following equation :

RT8509A

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Nevertheless, this value can be decreased for lower output

current requirement. Another consideration is the voltage

rating of the input capacitor which must be greater than

the maximum input voltage.

Thermal Considerations

For continuous operation, do not exceed absolute

maximum junction temperature. The maximum power

dissipation depends on the thermal resistance of the IC

package, PCB layout, rate of surrounding airflow, and

difference between junction and ambient temperature. The

maximum power dissipation can be calculated by the

following formula :

PD(MAX) = (TJ(MAX) −TA) / θJA

where TJ(MAX) is the maximum junction temperature, TAis

the ambient temperature, and θJA is the junction to ambient

thermal resistance.

For recommended operating condition specifications, the

maximum junction temperature is 125°C. The junction to

ambient thermal resistance, θJA, is layout dependent. For

WDFN-12L 5x5 packages, the thermal resistance, θJA, is

29.5°C/W on a standard JEDEC 51-7 four-layer thermal

test board. The maximum power dissipation at TA= 25°C

can be calculated by the following formula :

PD(MAX) = (125°C −25°C) / (29.5°C/W) = 3.38W for

WDFN-12L 5x5 package

The maximum power dissipation depends on the operating

ambient temperature for fixed TJ(MAX) and thermal

resistance, θJA. The derating curve in Figure 2 allows the

designer to see the effect of rising ambient temperature

on the maximum power dissipation.

Figure 2. Derating Curve of Maximum Power Dissipation

LayoutConsiderations

For high frequency switching power supplies, the PCB

layout is important to get good regulation, high efficiency

and stability. The following descriptions are the guidelines

for better PCB layout.

For good regulation, place the power components as

close as possible. The traces should be wide and short

enough especially for the high current output loop.

The feedback voltage divider resistors must be near the

feedback pin. The divider center trace must be shorter

and the trace must be kept away from any switching

nodes.

The compensation circuit should be kept away from the

power loops and be shielded with a ground trace to

prevent any noise coupling.

Minimize the size of the LX node and keep it wide and

shorter. Keep the LX node away from the FB.

The exposed pad of the chip should be connected to a

strong ground plane for maximum thermal consideration.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0 25 50 75 100 125

Ambient Temperature (°C)

Maximum Power Dissipation (W)1

Four-Layer PCB

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Figure 3. PCB Layout Guide

GND

VOUT

Place the power components

as close as possible. The

traces should be wide and

short especially for the high-

current loop.

COUT

Locate the C2 as close to

the VIN pin as possible.

VIN

CIN

VOUT

VIN

GND R4

VIN

GND

The feedback voltage-divider

resistors must near the feedback

pin. The divider center trace

must be shorter and avoid the

trace near any switching nodes.

COMP

VOUT

GND

VIN

GND 6

9GND

More GND via and layout area for

better thermal performance.

The switching trace should be wide and

short especially for the high-current loop.

The compensation circuit should be kept away from the power loops and

should be shielded with a ground trace to prevent any noise coupling.

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Richtek Technology Corporation

14F, No. 8, Tai Yuen 1st Street, Chupei City

Hsinchu, Taiwan, R.O.C.

Tel: (8863)5526789

Richtek products are sold by description only. Richtek reserves the right to change the circuitry and/or specifications without notice at any time. Customers should

obtain the latest relevant information and data sheets before placing orders and should verify that such information is current and complete. Richtek cannot

assume responsibility for use of any circuitry other than circuitry entirely embodied in a Richtek product. Information furnished by Richtek is believed to be

accurate and reliable. However, no responsibility is assumed by Richtek or its subsidiaries for its use; nor for any infringements of patents or other rights of third

parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Richtek or its subsidiaries.

Outline Dimension

W-Type 12L DFN 5x5 Package