RT8086BGQUF - Richtek Technology

RT8086B

DS8086B-02 February 2016 www.richtek.com

Applications

Smart Handheld devices

Portable Instruments

Battery-Powered Equipment

Distributed Power Systems

3.5A, 1.2MHz, Synchronous Step-Down Converter

General Description

The RT8086B is a high efficiency , synchronous step-down

DC/DC converter. The available input voltage range is from

2.8V to 5.5V and the regulated output voltage is adjustable

from 0.6V to 3.3V while delivering up to 3.5A of output

current.

The internal synchronous low on-resistance power

switches increase efficiency and eliminate the need for

an external Schottky diode.

The current mode constant on-time operation with internal

compensation allows the transient response to be

optimized over a wide range of loads and output ca pacitors.

TheRT8086B is available in the UQFN-12L 2x2 (FC)

package.

Simplified Application Circuit

Marking Information

Features



High Efficiency Up to 95%



Low RDS(ON) Switches : 50mΩΩ

ΩΩ

Ω/40mΩΩ

ΩΩ



0.6V Reference Allows for Low Output Voltage



Internal Compensation



Input Voltage Range : 2.8V to 5.5V



Adjustable Output Voltage from 0.6V to 3.3V



1.2MHz Switching Frequency



Start-Up into Pre-Biased Load



Built in Soft-Start



Power Good Indication



Cycle-by-Cycle Current Limit



Input Under Voltage Lockout



Output Under Voltage Protection (Hiccup)



Thermal Shutdown Protection



RoHS Compliant and Halogen Free

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

QUF : UQFN-12L 2x2 (FC) (U-Type)

RT8086B

Lead Plating System

G : Green (Halogen Free and Pb Free)

0YW

0Y : Product Code

W : Date Code

PVIN

AGND

VIN

PGND

RT8086B

CIN

PGOOD VOUT

VOUT

LX L

VIN

COUT

PGOOD

Enable

RT8086B

2DS8086B-02 February 2016www.richtek.com

Functional Pin Description

Pin No. Pin Name Pin Func tion

1 PVIN

Power Input. The available input voltage range is from 2.8V to 5.5V. A 10F or

larger input capacitor is needed to reduce voltage spikes at the input.

2, 11 LX Switch Node. Output of the internal high side and low side MOSFETs.

3, 12 PGND Power Ground.

4 AGND Analog Ground.

5 NC No Internal Connection.

6 VOUT Sense Input Pin for Output Voltage.

7 FB Feedback Input. This pin used to set the output voltage of the converter to

regulate to the desired value via an external resistive divider. The feedback

reference voltage is 0.6V typically.

8 EN Enable Control Input. A logic-high (1.2V < EN < 5.5V) enables the converter; a

logic-low forces the IC into s hutdown mode.

9 PGOOD

Power Good Indicator. The output of this pin is an open drain with internal pull-up

resister to VIN. The output of this pin is pulled to high when the FB voltage is

within 10%; otherwise it is Low.

10 VIN Supply Voltage for Internal Control Circuit. It is connected to PVIN in s ide the chip.

Pin Configurations (TOP VIEW)

UQFN-12L 2x2 (FC)

PVIN

AGND

LX PGOOD

VOUT

PGND 8

VIN

PGND

RT8086B

DS8086B-02 February 2016 www.richtek.com

Function Block Diagram

Operation

The RT8086B is a synchronous low voltage step-down

converter that can support the input voltage range from

2.8V to 5.5V and the output current can be up to 3.5A.

The RT8086B uses a constant on-time, current mode

architecture. In normal operation, the high side P-MOSFET

is turned on when the switch controller is set by the

comparator and is turned off when the Ton comparator

resets the switch controller.

Low side MOSFET pea k current is measured by internal

RSENSE. The error amplifier EA adjusts COMP voltage

by comparing the feedback signal (VFB) from the output

voltage with the internal 0.6V reference. When the load

current increa ses, it causes a drop in the feedback voltage

relative to the reference, then the COMP voltage rises to

allow higher inductor current to match the load current.

UV Comparator

If the feedback voltage (VFB) is lower than threshold voltage

0.2V, the UV comparator's output will go high and the

switch controller will turn off the high side MOSFET. The

output under voltage protection is designed to operate in

Hiccup mode.

PGOOD Comparator

When the feedback voltage (VFB) is higher tha n threshold

voltage 0.54V and under 0.66V, the PGOOD open drain

output will be high impeda nce.

Enable Comparator

A logic-high enables the converter; a logic-low forces the

IC into shutdown mode. There is an internal pull down

1MΩ resistor at EN pin.

Soft-Start (SS)

An internal current source charges an internal capacitor

to build the soft-start ramp voltage. The VFB voltage will

track the internal ramp voltage during soft-start interval.

The typical soft-start time is 2ms.

Over Current Protection (OCP)

The RT8086B provides over current protection by detecting

low side MOSFET valley inductor current. If the sensed

valley inductor current is over the current limit threshold

(4.5A min.), the OCP will be triggered. When OCP is

tripped, the RT8086B will keep the over current threshold

level until the over current condition is removed.

Thermal Shutdown (OTP)

The device implements an internal thermal shutdown

function when the junction temperature exceeds 140°C.

The thermal shutdown forces the device to stop switching

when the junction temperature exceeds the thermal

shutdown threshold. Once the die temperature decrea ses

below the hysteresis of 20°C, the device reinstates the

power up sequence.

Ton

PVIN

VIN

AGND

-Driver

PGND

UVLO

Current

Limit

Detector

Current

Sense

Logic

Control

VIN

-RC

VREF

Comparator

PGOOD

OTP

Shut Down

Control

Error

Amplifier

EN VOUT

PGND

RT8086B

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Electrical Characteristics

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

Absolute Maximum Ratings (Note 1)

Supply Input V oltage, PVIN, VIN------------------------------------------------------------------------------ −0.3V to 6.5V

Switch Node Voltage, LX---------------------------------------------------------------------------------------- −0.3V to (PVIN + 0.3V)

Other Pins Voltage ----------------------------------------------------------------------------------------------- −0.3V to 6V

Power Dissipation, PD @ TA = 25°C

UQF N-12L 2x2 (FC)---------------------------------------------------------------------------------------------- 1.25W

Pa ckage Thermal Re sistance (Note 2)

UQF N-12L 2x2 (FC), θJA ---------------------------------------------------------------------------------------- 80°C/W

UQFN-12L 2x2 (FC), θJC ---------------------------------------------------------------------------------------- 7°C/W

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

Junction T emperature-------------------------------------------------------------------------------------------- 150°C

Storage T emperature Range ----------------------------------------------------------------------------------- −65°C to 150°C

ESD Susceptibility (Note 3)

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

Recommended Operating Conditions (Note 4)

Supply Input Voltage--------------------------------------------------------------------------------------------- 2.8V to 5.5V

Junction T emperature Range----------------------------------------------------------------------------------- −40°C to 125°C

Ambient T emperature Range----------------------------------------------------------------------------------- −40°C to 85°C

Parameter Symbol Test Conditions Min Typ Max Unit

Input Voltage PVIN 2.8 -- 5.5 V

Feedback Reference Voltage VREF 0.588 0.6 0.612 V

Feedback Leakage Current IFB V

FB = 3.3 V -- -- 1 A

Quiescent Cu rrent Clo se loop, no load current -- 60 --

Shut down Current Shutdo wn -- -- 1 A

Output Voltage Line Regulation VIN = 2.8V to 5.5V -- 0.1 -- %/V

O ut put V olta ge Loa d R eg ul ation VIN = 5V, VOUT = 3.3V, IOUT = 0A to

3.5A -- 0.4 -- %

Switching Frequency V IN = 5V, VOUT = 1.2V -- 1.2 -- MHz

High-Side RPMOS I

LX = 0. 3A , V IN = 5V -- 50 --

Switch

On-Resistance Low-Side RNMOS ILX = 0.3 A , V IN = 5 V -- 40 -- m

Current Limit ILIM Valley current 4.5 -- -- A

Min. Off-Time tOFF -- 100 -- ns

VIN Rising -- 2.5 --

U nder Voltage Lockout

Threshold VIN Falling -- 2.2 -- V

Thermal Shutdown -- 140 -- °C

Logic-High 1.2 -- 5.5

Enable Voltage Logic-Low -- -- 0.4 V

RT8086B

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Parameter Symbol Test Conditions Min Typ Max Unit

EN Input Current VEN = 2V -- 2 -- A

VEN = 0V -- 0 --

PG OO D Pi n Trig ger Delay -- 90 -- s

PGOOD Pin Thres h old

(Relative to VOUT) FB with respect to the Regulation -- ±10 -- %

PGO OD Open Drain

Impedance RPGOOD PGOOD = PVIN -- 500 -- k

PGOOD On-Resistance

Impedance PGOOD = Low -- -- 100 

Soft- Star t T i me TSS -- 2 -- ms

On-Time TON VIN = 5V, VOUT = 1.2V -- 200 -- ns

VIN = 3.6V, VOUT = 1.2V -- 277 --

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.

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

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

RT8086B

6DS8086B-02 February 2016www.richtek.com

Typical Application Circuit

PVIN

AGND

VIN

PGND

RT8086B

10µF

3, 12 4

CIN

PGOOD

9VOUT 6

VOUT

200k

LX 2, 11 L

1µH

200k

VIN

FB 7

*CFF

PGOOD

1.2V

COUT

20µF

Enable

* : The feedforward capacitor CFF is optional for the optimization of transient response by increasing bandwidth and

acceptable pha se margin.

Where fCO is the unity gain crossover frequency of the control loop without the external feedforward ca pa citor installed.

FF CO

1111

C = 2 f R1 R1 R2











Table 1. Suggested Component Values

VOUT (V) R1 (k) R2 (k) L (H) COUT (F)

1.2V 200 200 1 20

1.8V 200 100 1 20

2.5V 200 63.4 1 20

3.3V 200 44.2 1 20

RT8086B

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Typical Operating Characteristics

Output Voltage vs. Output Current

1.10

1.12

1.14

1.16

1.18

1.20

1.22

1.24

1.26

1.28

1.30

0 0.5 1 1.5 2 2.5 3 3.5

Ou tput Cu rrent (A)

Output Vol tage (V)

VOUT = 1.2V

VIN = 3.3V

VIN = 5V

Switching Frequency v s . Te m pe rature

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

-50-25 0 25 50 75100125

Temperature (°C)

Switchi ng Frequency (M Hz) 1

VOUT = 1.8V, IOUT = 0.6A

VIN = 3.3V

VIN = 5V

Switching Frequency vs . Input Voltage

1.10

1.15

1.20

1.25

1.30

1.35

1.40

1.45

1.50

2.5 3 3.5 4 4.5 5 5.5

In put Voltage (V )

Swit ching Frequency (MHz) 1

VOUT = 1.2V, IOUT = 0.6A

Output Voltage vs. Temperature

1.75

1.76

1.77

1.78

1.79

1.80

1.81

1.82

1.83

1.84

1.85

-50 -25 0 25 50 75 100 125

Temperatur e (°C)

Output Vol tage (V)

VOUT = 1.8V, IOUT = 0.6A

VIN = 3.3V

VIN = 5V

O utput Voltage vs. Input Voltage

1.00

1.05

1.10

1.15

1.20

1.25

1.30

1.35

1.40

2.5 3 3.5 4 4.5 5 5.5

In put Voltage (V)

Output Vol tage (V)

VIN = 2.8V to 5.5V, VOUT = 1.2V

Efficiency vs. Output Current

100

0.001 0.01 0.1 1 10

Output Current (A)

Effici en cy (%)

VOUT = 1.2V

VIN = 3.3V

VIN = 5V

RT8086B

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VIN = 3.3V, VOUT = 1.2V, IOUT = 0A

Time (10ms/Div)

Output Ripple Voltage

VOUT

(20mV/Div)

Time (500ns/Div)

Output Ripple Voltage

VOUT

(5mV/Div)

VLX

(2V/Div)

VIN = 3.3V, VOUT = 1.2V, IOUT = 3.5A

Time (2.5ms/Div)

Power On from VIN

VOUT

(1V/Div)

PGOOD

(5V/Div)

ILX

(2A/Div)

VIN

(2V/Div)

VIN = 5V, VOUT = 1.2V, IOUT = 3.5A

IOUT = 0.2A to 2A, COUT = 20μF

Time (50μs/Div)

Load Transient Response

IOUT

(1A/Div)

VOUT

(50mV/Div)

VIN = 3.3V, VOUT = 1.2V,

IOUT = 1A to 2A, COUT = 20μF

Time (50μs/Div)

Load Transient Response

IOUT

(1A/Div)

VOUT

(50mV/Div)

VIN = 3.3V, VOUT = 1.2V,

Current Lim it vs . Input Voltage

4.0

4.5

5.0

5.5

6.0

6.5

7.0

2.5 3 3.5 4 4.5 5 5.5

In put Voltage (V)

Current Li m it (A)

VOUT = 1.2V

RT8086B

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Time (25ms/Div)

Power Off from EN

VOUT

(2V/Div)

ILX

(2A/Div)

VEN

(2V/Div)

VIN = 5V, VOUT = 1.2V, IOUT = 0A

Time (1ms/Div)

Power On from EN

VOUT

(1V/Div)

PGOOD

(5V/Div)

ILX

(2A/Div)

VEN

(2V/Div)

VIN = 5V, VOUT = 1.2V, IOUT = 3.5A

Time (2.5ms/Div)

Power Off from VIN

VOUT

(1V/Div)

VLX

(5V/Div)

ILX

(5A/Div)

VIN

(5V/Div)

VIN = 5V, VOUT = 1.2V, IOUT = 3.5A

RT8086B

10 DS8086B-02 February 2016www.richtek.com

Application Information

The RT8086B is a single-phase step-down converter. It

provides single feedback loop, current mode control with

fa st transient response. An internal 0.6V reference allows

the output voltage to be precisely regulated for low output

voltage applications. A fixed switching frequency (1.2MHz)

oscillator and internal compensation are integrated to

minimize external component count. Protection features

include over current protection, under voltage protection

and over temperature protection.

Output Voltage Setting

Connect a resistive voltage divider at the FB between VOUT

and GND to adjust the output voltage. The output voltage

is set a ccording to the f ollowing equation :

OUT REF R1

V = V 1









where VREF is the feedback reference voltage 0.6V (typ.).

Figure 1. Setting VOUT with a Voltage Divider

GND

VOUT

Chip Enable and Disable

The EN pin allows for power sequencing between the

controller bias voltage and another voltage rail. The

RT8086B remains in shutdown if the EN pin is lower than

400mV. When the EN pin rises above the VEN trip point,

the RT8086B begins a new initialization and soft-start

cycle.

Internal Soft-Start

The RT8086B provides an internal soft-start function to

prevent large inrush current and output voltage overshoot

when the converter starts up. The soft-start (SS)

automatically begins once the chip is enabled. During soft-

start, the internal soft-start capacitor becomes charged

and generates a linear ramping up voltage across the

capa citor. This voltage clamps the voltage at the FB pin,

causing PWM pulse width to increase slowly a nd in turn

reduce the input surge current. The internal 0.6V reference

ta kes over the loop control once the intern al ramping-up

voltage becomes higher than 0.6V .

UVLO Protection

The RT8086B ha s input Under V oltage Lockout protection

(UVLO). If the input voltage exceeds the UVLO rising

threshold voltage (2.5V typ.), the converter resets and

prepares the PWM for operation. If the input voltage falls

below the UVLO falling threshold voltage during normal

operation, the device will stop switching. The UVLO rising

a nd falling thre shold voltage has a hysteresis to prevent

noise-caused reset.

Inductor Selection

The switching frequency (on-time) a nd operating point (%

ripple or LIR) determine the inductor value a s shown below:





OUT IN OUT

SW LOAD(MAX) IN

VV V

L = f LIR I V



 

where LIR is the ratio of the peak-to-peak ripple current to

the average inductor current.

Find a low loss inductor having the lowest possible DC

resistance that fits in the allotted dimensions. The core

must be large enough not to saturate at the peak inductor

current (IPEAK) :

PEAK LOAD(MAX) LOAD(MAX)

LIR

I = I + I









The calculation above serves as a general reference. To

further improve transient response, the output inductor

can be further reduced. This relation should be considered

along with the selection of the output capa citor .

Input Capacitor Selection

High quality ceramic input decoupling ca p a citor , such as

X5R or X7R, with values greater than 10μF are

recommended for the in put ca pa citor. The X5R a nd X7R

ceramic capacitors are usually selected for power regulator

capacitors because the dielectric material has less

ca pacitance variation and more temperature stability .

RT8086B

DS8086B-02 February 2016 www.richtek.com

Voltage rating and current rating are the key parameters

when selecting an input ca pacitor . Generally , selecting an

input capa citor with voltage rating 1.5 tim es greater tha n

the maximum input voltage is a conservatively safe design.

The input capacitor is used to supply the input RMS

current, which can be a pproximately calculated using the

following equation :

OUT OUT

IN_RMS LOAD IN IN

I = I 1









The next step is selecting a proper capacitor for RMS

current rating. One good design uses more than one

capacitor with low equivalent series resistance (ESR) in

parallel to form a capacitor ba nk.

The input capacitance value determines the input ripple

voltage of the regulator. The input voltage ripple can be

approximately calculated using the f ollowing equation :









OUT(MAX) OUT OUT

IN IN SW IN IN

IVV

V = 1

Cf V V

Output Capacitor Selection

The output capacitor and the inductor form a low pass

filter in the Buck topology. In steady state condition, the

ripple current flowing into/out of the capacitor results in

ripple voltage. The output voltage ripple (VP-P) can be

calculated by the following equation :

P_P LOAD(MAX) OUT SW

V= LIRI ESR + 8C f











When load tra nsient occurs, the output ca pacitor supplies

the load current before the controller can respond.

Therefore, the ESR will dominate the output voltage sag

during load transient. The output voltage undershoot (VSAG)

ca n be calculated by the f ollowing equation :

SAG LOAD

V = I ESR

For a given output voltage sag specification, the ESR value

can be determined.

Another para meter that has influence on the output voltage

sag is the equivalent series inducta nce (ESL). The rapid

change in load current results in di/dt during transient.

Therefore, the ESL contributes to part of the voltage sag.

Using a ca pacitor with low ESL ca n obtain better transient

performance. Generally, using several capacitors

connected in parallel can have better transient performance

tha n using a single capa citor f or the same total ESR.

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, TA is

the a mbient temperature, and θJA is the junction to a mbient

thermal resistance.

For recommended operating condition specifications, the

maximum junction temperature is 125°C. The junction to

a mbient thermal resistance, θJA, is layout dependent. For

UQF N-12L 2x2(FC) package, the thermal resistance, θJA,

is 80°C/W on a sta ndard JEDEC 51-7 four-layer thermal

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

ca n be calculated by the f ollowing formula :

PD(MAX) = (125°C − 25°C) / (80°C/W) = 1.25W for

UQF N-12L 2x2 (FC) 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

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

0 25 50 75 100 125

Ambient Tem pera ture (°C)

Maxi mum Power Di ssipation (W ) 1

Four-Layer PCB

RT8086B

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PVIN

AGND

LX PGOOD

VOUT

PGND 8

VIN

PGND

VOUT

GND

CIN1 CIN2

VIN

R1 R2 AGND

Input capacitor must

be placed as close to

the IC as possible.

LX should be connected to

inductor by wide and short

trace. Keep sensitive

components away from

this trace.

COUT

The output capacitor must

be place near the IC. The voltage divider must

be connected as c l ose to

the device as pos sible.

VOUT

LR3

VIN

Figure 3. PCB Layout Guide

Layout Considerations

Layout is very important in high frequency switching

converter design. The PCB can radiate excessive noise

and contribute to converter instability with improper layout.

Certain points must be considered before starting a layout

using the RT8086B.

Ma ke the tra ces of the main current paths a s short a nd

wide as possible.

Put the input ca pacitor as close a s possible to the device

pins (VIN and GND).

LX node encounters high frequency voltage swings so

it should be kept in a small area. Keep sensitive

components away from the LX node to prevent stray

ca pa citive noise pick-up.

Ensure all feedba ck network connections are short and

direct. Pla ce the feedba ck network a s close to the chip

as possible.

The GND pin and Exposed Pad should be connected to

a strong ground plane for heat sinking and noise

protection.

An example of PCB layout guide is shown in Figure 3

for reference.

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Richtek Technology Corporation

14F, No. 8, Tai Yuen 1 st 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

U-Type 12L QFN 2x2 (FC) Package

Min. Max. Min. Max.

A 0.500 0.600 0.020 0.024

A1 0.000 0.050 0.000 0.002

A3 0.100 0.152 0.004 0.006

b 0.200 0.300 0.008 0.012

D 1.900 2.100 0.075 0.083

E 1.900 2.100 0.075 0.083

L 0.350 0.450 0.014 0.018

L1 0.450 0.550 0.018 0.022

Symbol Dime nsions In Millime ters Dime nsions In Inche s

0.500 0.020