Richtek RT8015AGQW Schematic [ru]

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RT8015A

3A, 2MHz, Synchronous Step-Down Converter

General Description

The RT8015A is a high efficiency synchronous, step-down DC/DC converter. Its input voltage range is from 2.6V to 5.5V and provides an adjustable regulated output voltage from 0.8V to 5V while delivering up to 3A of output current.

The internal synchronous low on-resistance power switches increase efficiency and eliminate the need for an external Schottky diode. The switching frequency is set by an external resistor or can be synchronized to an external clock. The 100% duty cycle provides low dropout operation extending battery life in portable systems. Current mode operation with external compensation allows the transient response to be optimized over a wide range of loads and output capacitors.

The RT8015A is operated in forced continuous PWM Mode which minimizes ripple voltage and reduces the noise and RF interference.

The 100% duty cycle in Low Dropout Operation further maximize battery life.

The RT8015A is available in the WDFN-10L 3x3 package.

Ordering Information

RT8015A

Package Type

QW : WDFN-10L 3x3

Lead Plating System P : Pb Free

G : Green (Halogen Free and Pb Free)

Note :

Richtek products are :

`RoHS compliant and compatible with the current requirements of IPC/JEDEC J-STD-020.

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

Features

zHigh Efficiency : Up to 95%

zLow RDS(ON) Internal Switches : 110mΩ

zProgrammable Frequency : 300kHz to 2MHz

zNo Schottky Diode Required

z0.8V Reference Allows for Low Output Voltage

zForced Continuous Mode Operation

zLow Dropout Operation : 100% Duty Cycle

zRoHS Compliant and 100% Lead (Pb)-Free

Applications

zPortable Instruments

zBattery-Powered Equipment

zNotebook Computers

zDistributed Power Systems

zIP Phones

zDigital Cameras

Pin Configurations

(TOP VIEW)

SHDN/RT 1

 

10

COMP

GND 2

 

9

FB

LX 3

 

8

VDD

LX

4

 

7

PVDD

PGND

5

11

9

PVDD

WDFN-10L 3x3

Marking Information

For marking information, contact our sales representative directly or through a Richtek distributor located in your area.

DS8015A-04 March 2011

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1

RT8015A

Typical Application Circuit

VIN

 

 

 

 

 

 

 

5V

 

 

 

 

 

 

 

ROSC

 

RT8015A

RCOMP

CCOMP

 

 

1000pF

 

332k

1

 

COMP 10

30k

 

SHDN/RT

 

 

 

 

 

 

 

 

2

GND

FB 9

 

R2

240k

 

 

3, 4

 

 

 

 

 

LX

VDD 8

 

 

 

 

 

5

PGND

PVDD 6, 7

C1

R1

 

 

 

 

 

CIN

 

 

 

 

 

22pF

510k

 

 

 

 

22µF

 

 

 

 

 

 

L1

 

 

 

 

 

 

 

2µH

 

 

 

VOUT

 

 

 

 

 

 

 

 

 

 

 

 

COUT

2.5V/3A

 

 

 

 

 

22µFx2

 

Note : Using all Ceramic Capacitors

Functional Pin Description

Pin No.

Pin Name

Pin Function

1

SHDN/RT

Oscillator Resistor Input. Connecting a resistor to ground from this pin sets the

 

 

switching frequency. Forcing this pin to VDD causes the device to be shut down.

2

GND

Signal Ground. All small-signal components and compensation components should

connect to this ground, which in turn connects to PGND at one point.

 

 

3, 4

LX

Internal Power MOSFET Switches Output. Connect this pin to the inductor.

 

 

 

5

PGND

Power Ground. Connect this pin close to the negative terminal of CIN and COUT.

 

 

 

6, 7

PVDD

Power Input Supply. Decouple this pin to PGND with a capacitor.

 

 

 

8

VDD

Signal Input Supply. Decouple this pin to GND with a capacitor. Normally VDD is

equal to PVDD.

 

 

9

FB

Feedback Pin. This pin Receives the feedback voltage from a resistive divider

connected across the output.

 

 

 

 

Error Amplifier Compensation Point. The current comparator threshold increases

10

COMP

with this control voltage. Connect external compensation elements to this pin to

 

 

stabilize the control loop.

11 (Exposed Pad)

NC

No Internal Connection. The exposed pad must be soldered to a large PCB and

connected to GND for maximum power dissipation.

 

 

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DS8015A-04 March 2011

2

Richtek RT8015AGQW Schematic

RT8015A

Function Block Diagram

 

 

SHDN/RT

 

 

 

 

 

 

SD

 

ISEN

 

PVDD

 

 

 

 

 

 

 

 

 

 

 

 

 

OSC

Slope

 

 

 

 

 

Com

 

 

 

 

 

 

 

 

 

COMP

 

 

 

 

 

 

0.8V

EA

Output

 

OC

 

 

FB

Clamp

 

Limit

 

 

 

 

 

 

 

Int-SS

 

 

 

Driver

LX

 

 

0.9V

 

Control

 

 

 

 

 

 

 

 

 

 

Logic

 

 

 

 

0.7V

 

 

 

 

 

 

 

 

 

NISEN

PGND

POR

 

 

 

 

NMOS I Limit

 

0.4V

 

 

 

 

 

 

VREF

OTP

 

GND

VDD

 

 

 

 

 

 

Layout Guide

CIN must be placed between VDD

Output capacitor must be

and GND as closer as possible

near RT8015A

VIN

CF R1

VOUT

GND

 

COUT

CIN

 

RT8015A

 

 

PVDD 6

5

PGND

PVDD 7

4

LX

VDD 8

3

LX

FB 9

2

GND

COMP 10

1

SHDN/RT

R2 RCOMP

 

 

CCOMP

 

 

GND

 

 

VOUT

LX should be

 

 

connected to Inductor

 

 

by wide and short

 

 

trace, keep sensitive

L1

components away

 

 

from this trace

 

 

 

 

ROSC

Connect the FB pin directly to feedback resistors. The resistor divider must be connected between VOUT and GND.

DS8015A-04 March 2011

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3

RT8015A

Operation

Main Control Loop

The RT8015A is a monolithic, constant-frequency, current mode step-down DC/DC converter. During normal operation, the internal top power switch (P-Channel MOSFET) is turned on at the beginning of each clock cycle. Current in the inductor increases until the peak inductor current reach the value defined by the voltage on the COMP pin. The error amplifier adjusts the voltage on the COMP pin by comparing the feedback signal from a resistor divider on the FB pin with an internal 0.8V reference. When the load current increases, it causes a reduction in the feedback voltage relative to the reference. The error amplifier raises the COMP voltage until the average inductor current matches the new load current. When the top power MOSFET shuts off, the synchronous power switch (N-MOSFET) turns on until either the bottom current limit is reached or the beginning of the next clock cycle.

The operating frequency is set by an external resistor connected between the RT pin and ground. The practical switching frequency can range from 300kHz to 2MHz. In an over-voltage condition, the top power MOSFET is turned off and the bottom power MOSFET is switched on until either the over voltage condition clears or the bottom MOSFET's current limit is reached.

Dropout Operation

When the input supply voltage decreases toward the output voltage, the duty cycle increases toward the maximum on-time. Further reduction of the supply voltage forces the main switch to remain on for more than one cycle eventually reaching 100% duty cycle.

The output voltage will then be determined by the input voltage minus the voltage drop across the internal P-Channel MOSFET and the inductor.

Low Supply Operation

The RT8015A is designed to operate down to an input supply voltage of 2.6V. One important consideration at low input supply voltages is that the RDS(ON) of the P-Channel and N-Channel power switches increases. The user should calculate the power dissipation when the

RT8015A is used at 100% duty cycle with low input voltages to ensure that thermal limits are not exceeded.

Slope Compensation and Inductor Peak Current

Slope compensation provides stability in constant frequency architectures by preventing sub-harmonic oscillations at duty cycles greater than 50%. It is accomplished internally by adding a compensating ramp to the inductor current signal. Normally, the maximum inductor peak current is reduced when slope compensation is added. In the RT8015A, however, separated inductor current signals are used to monitor over current condition. This keeps the maximum output current relatively constant regardless of duty cycle.

Short Circuit Protection

When the output is shorted to ground, the inductor current decays very slowly during a single switching cycle. A current runaway detector is used to monitor inductor current.As current increasing beyond the control of current loop, switching cycles will be skipped to prevent current runaway from occurring.

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DS8015A-04 March 2011

4

 

 

 

 

 

RT8015A

Absolute Maximum Ratings

(Note 1)

 

 

 

 

 

 

 

z Supply Input Voltage, VDD, PVDD ----------------------------------------------------------------------------

 

 

 

−0.3V to 6V

 

 

z LX Pin Switch Voltage --------------------------------------------------------------------------------------------

 

 

 

 

−0.3V to (PVDD + 0.3V)

<200ns ---------------------------------------------------------------------------------------------------------------

 

 

 

 

−5V to 7.5V

 

 

z Other I/O Pin Voltages -------------------------------------------------------------------------------------------

 

 

 

 

−0.3V to (VDD + 0.3V)

z LX Pin Switch Current --------------------------------------------------------------------------------------------

 

 

 

 

4A

 

 

 

 

z Power Dissipation, PD @ TA = 25°C

 

 

 

 

 

 

 

 

 

WDFN-10L 3x3 -----------------------------------------------------------------------------------------------------

 

 

 

 

909mW

 

 

z Package Thermal Resistance (Note 2)

 

 

 

 

 

 

 

 

 

WDFN-10L 3x3, θJA -----------------------------------------------------------------------------------------------

 

 

 

 

110°C/W

 

 

z Junction Temperature ---------------------------------------------------------------------------------------------

 

 

 

 

150°C

 

 

 

 

z Lead Temperature (Soldering, 10 sec.) -----------------------------------------------------------------------

 

 

 

260°C

 

 

 

 

z Storage Temperature Range ------------------------------------------------------------------------------------

 

 

 

 

−65°C to 150°C

 

 

z ESD Susceptibility (Note 3)

 

 

 

 

 

 

 

 

 

 

HBM (Human Body Mode) --------------------------------------------------------------------------------------

 

 

 

 

2kV

 

 

 

 

MM (Machine Mode) ----------------------------------------------------------------------------------------------

 

 

 

 

200V

 

 

 

 

Recommended Operating Conditions (Note 4)

 

 

 

 

 

 

 

z Supply Input Voltage ----------------------------------------------------------------------------------------------

 

 

 

 

2.6V to 5.5V

 

 

z Junction Temperature Range ------------------------------------------------------------------------------------

 

 

 

 

−40°C to 125°C

 

 

z Ambient Temperature Range ------------------------------------------------------------------------------------

 

 

 

 

−40°C to 85°C

 

 

Electrical Characteristics

 

 

 

 

 

 

 

 

 

(VDD = 3.3V, TA = 25°C, unless otherwise specified)

 

 

 

 

 

 

 

 

Parameter

Symbol

 

Test Conditions

Min

Typ

 

Max

 

Unit

 

 

 

 

 

 

 

 

 

 

 

 

Input Voltage Range

VDD

 

 

2.6

--

 

5.5

 

V

 

 

 

 

 

 

 

 

 

 

 

 

Feedback Reference Voltage

VREF

 

 

0.784

0.8

 

0.816

 

V

 

Feedback Leakage Current

IFB

 

 

--

0.1

 

0.4

 

μA

 

 

 

 

 

 

 

 

 

 

 

DC Bias Current

 

Active , VFB = 0.78V, Not Switching

--

460

 

--

 

μA

 

 

Shutdown

--

--

 

1

 

μA

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Output Voltage Line Regulation

 

VIN = 2.7V to 5.5V

--

0.04

 

--

 

%/V

 

 

 

 

 

 

 

 

 

 

 

Output Voltage Load Regulation

 

Measured in Servo Loop,

−0.2

±0.02

 

0.2

 

%

 

 

V

= 0.2V to 0.7V (Note 5)

 

 

 

 

 

COMP

 

 

 

 

 

 

 

 

Error Amplifier

gm

 

 

--

800

 

--

 

μs

 

Transconductance

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Current Sense Transresistance

RT

 

 

--

0.4

 

--

 

Ω

 

Switching Leakage Current

 

SHDN/RT = VIN = 5.5V

--

--

 

1

 

μA

 

 

 

 

 

 

 

 

 

 

 

Switching Frequency

 

ROSC = 332k

0.8

1

 

1.2

 

MHz

 

 

Switching Frequency

0.3

--

 

2

 

MHz

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Switch On Resistance, High

RPMOS

ISW = 0.5A

--

110

 

160

 

 

Switch On Resistance, Low

RNMOS

ISW = 0.5A

--

110

 

170

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

To be continued

 

 

 

 

 

 

 

 

 

 

DS8015A-04 March 2011

 

 

 

 

 

 

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