TD6815 [ETC]

1.5MHz 1.5A Synchronous Step-Down Regulator Dropout; 1.5MHz的同步1.5A降压稳压器低压差


型号：	TD6815
厂家：	ETC
描述：	1.5MHz 1.5A Synchronous Step-Down Regulator Dropout 1.5MHz的同步1.5A降压稳压器低压差稳压器
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DATASHEET

T^echcode®

1.5MHz 1.5A Synchronous Step-Down Regulator

Dropout

汪工 TEL:13828719410 QQ:1929794238

General Description Features

TD6815

The TD6815 is a high efficiency monolithic synchronous

buck regulator using a constant frequency, current mode

architecture. The device is available in an adjustable

version and fixed output voltages of 1.5V and 1.8V.

Supply current during operation is only 20mA and drops

to ≤1mA in shutdown. The 2.5V to 5.5V input voltage

range makes the TD6815 ideally suited for single Li-Ion

battery-powered applications. 100% duty cycle provides

low dropout operation, extending battery life in portable

systems.Automatic Burst Mode operation increases

efficiency at light loads, further extending battery life.

Switching frequency is internally set at 1.5MHz, allowing

the use of small surface mount inductors and capacitors.

The internal synchronous switch increases efficiency and

eliminates the need for an external Schottky diode. Low

output voltages are easily supported with the 0.6V

feedback reference voltage. The TD6815 is available in

SOP8 package.

High Efficiency: Up to 96%

High Efficiency at light loads

Very Low Quiescent Current: Only 20uA During

Operation

1.5A Output Current

2.5V to 5.5V Input Voltage Range

1.5MHz Constant Frequency Operation

No Schottky Diode Required

Low Dropout Operation: 100% Duty Cycle

0.6V Reference Allows Low Output Voltages

Shutdown Mode Draws ≤1uA Supply Current

Current Mode Operation for Excellent Line and Load

Transient Response

Overtemperature Protected

SOP‐8 Package is Available

Applications

Cellular Telephones

Personal Information Appliances

Wireless and DSL Modems

Digital Still Cameras

MP3 Players

Portable Instruments

Package Types

SOP8

Figure 1. Package Types of TD6815

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T^echcode®

1.5MHz 1.5A Synchronous Step-Down Regulator

Dropout

TD6815

Pin Name Description

Pin Assignments

Run Control Input. Forcing this pin

above 1.5V enables the part.

Forcing this pin below 0.3V shuts

down the device. In shutdown, all

functions are disabled drawing

<1μA supply current. Do not leave

RUN floating.

RUN

4,8 GND

5,6 SW

Ground Pin.

Switch Node Connection to

Inductor. This pin connects to the

drains of the internal main and

synchronous power MOSFET

switches.

SOP8

Main Supply Pin. Must be closely

decoupled to GND, Pin 2, with a

2.2μF or greater ceramic capacitor.

Feedback Pin. Receives the

feedback voltage from an external

resistive divider across the output.

Output Voltage Feedback Pin. An

internal resistive divider divides the

output voltage down for comparison

to the internal reference voltage.

VIN

VFB

VOUT

Ordering Information

TD6815 □ □

Circuit Type

Output Versions

Blank：Adj

12：1.2V

Package

P:SOP8

15：1.5V

18：1.8V

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1.5MHz 1.5A Synchronous Step-Down Regulator

Dropout

TD6815

Functional Block Diagram

Figure2:Functional Block Diagram of TD6815

Type Application Circuit

Figure 3. Type Application Circuit of TD6815

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T^echcode®

1.5MHz 1.5A Synchronous Step-Down Regulator

Dropout

TD6815

Absolute Maximum Ratings

Note1: Stresses greater than those listed under Maximum Ratings may cause permanent damage to the device. This is a stress rating only and functional operation

of the device at these or any other conditions above those indicated in the operation is not implied. Exposure to absolute maximum rating conditions for extended

periods may affect reliability.

Parameter

Value

Unit

Input Supply Voltage

RUN, VFB Voltages

SW Voltage

-0.3 ~6

-0.3 ~ VIN

-0.3V ~(VIN+0.3)

1800

P-Channel Switch Source Current (DC)

N-Channel Switch Sink Current (DC)

Peak SW Sink and Source Current

Operating Temperature Range

Junction Temperature

1800

2.2

-40~+85

125

ºC

Lead Temperature (Soldering, 10 sec)

Storage Temperature Range

300

-65~150

Electrical Characteristics

Unless otherwise specified, VIN= 3.6V TA=25 ºC.

Symbol

Parameter

Conditions

Min.

Typ.

Max.

Unit

IVFB

Feedback Current

ꢀ30

0.5880

0.5865

0.5850

0.6000

0.6120

0.6135

0.6150

TA = 25°C

Regulated Feedback

Voltage

VFB

0°C TA ≤ 85°C

–40°C ≤ TA ≤ 85°C

Reference Voltage Line

Regulation

ꢀVFB

VOUT

VIN = 2.5V to 5.5V

0.04

0.4

%/ V

TD6815-1.5, IOUT = 100mA 1.455

TD6815-1.8, IOUT = 100mA 1.746

1.500

1.800

1.545

1.854

Regulated Output

Voltage

Output Voltage Line

Regulation

ꢀVOUT

VIN = 2.5V to 5.5V

0.04

0.4

%/ V

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1.5MHz 1.5A Synchronous Step-Down Regulator

Dropout

TD6815

Electrical Characteristics(Cont.)

Unless otherwise specified, VIN= 3.6V TA=25 ºC.

Symbol

Parameter

Conditions

Min.

Typ.

Max.

Unit

VIN = 3V, VFB = 0.5V or

VOUT = 90%, Duty Cycle < 1.75

35%

IPK

Peak Inductor Current

1.8

1.9

Output Voltage Load

Regulation

VLOADREG

VIN

0.5

Input Voltage Range

Input DC Bias Current

2.5

5.5

VFB = 0.5V or VOUT =

90%, ILOAD = 0A

Active Mode

300

400

VFB = 0.62V or VOUT =

103%, ILOAD = 0A

Sleep Mode

Shutdown

VRUN = 0V, VIN = 4.2V

0.1

1.5

400

0.25

VFB = 0.6V or VOUT =

fOSC

MHz

KHz

Ω

100%

Oscillator Frequency

VFB = 0V or VOUT = 0V

RDS(ON) of P-Channel

RPFET

RNFET

ILSW

ISW = 100mA

0.35

FET

RDS(ON) of N-Channel

FET

ISW = -100mA

0.25

0.01

Ω

VRUN = 0V, VSW = 0V or

5V, VIN = 5V

SW Leakage

VRUN

IRUN

RUN Threshold

0.3

1.5

RUN Leakage Current

ꢀ0.01

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1.5MHz 1.5A Synchronous Step-Down Regulator

Dropout

TD6815

Typical Operating Characteristics

Reference Voltage

Oscillator Frequency

Oscillator Frequency vs Supply Voltage

RDS(ON) vs Temperature

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TD6815

Typical Operating Characteristics(Cont.)

RDS(ON) vs Input Voltage

Efficiency vs Output Current

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Dropout

TD6815

Typical Operating Characteristics(Cont.)

Efficiency vs Output Current

Output Voltage vs Output Current

Efficiency vs Input Voltage

Dynamic Supply Current vs Supply Voltage

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1.5MHz 1.5A Synchronous Step-Down Regulator

Dropout

TD6815

Typical Operating Characteristics

P-FET Leakage vs Temperature

N-FET Leakage vs Temperature

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1.5MHz 1.5A Synchronous Step-Down Regulator

Dropout

TD6815

Function Description

the EA amplifier’s output rises above the sleep threshold

signaling the BURST comparator to trip and turn the top

MOSFET on. This process repeats at a rate that is

dependent on the load demand.

Main Control Loop

The TD6815 uses a constant frequency, current mode

step-down architecture. Both the main (P-channel

MOSFET) and synchronous (N-channel MOSFET)

switches are internal. During normal operation, the

ShortCircuit Protection

internal top power MOSFET is turned on each cycle When the output is shorted to ground, the frequency of

when the oscillator sets the RS latch, and turned off the oscillator is reduced to about 400kHz, 1/4 the

when the current comparator, ICOMP, resets the RS nominal frequency. This frequency foldback ensures that

latch. The peak inductor current at which ICOMP resets the inductor current has more time to decay, thereby

the RS latch, is controlled by

preventing runaway. The oscillator’s frequency will

the output of error amplifier EA. When the load current progressively increase to 1.5MHz when VFB or VOUT

increases, it causes a slight decrease in the feedback rises above 0V.

voltage, FB, relative to the 0.6V reference, which in turn,

causes the EA amplifier’s output voltage to increase until

Dropout Operation

the average inductor current matches the new load

current. While the top MOSFET is off, the bottom

As the input supply voltage decreases to a value

MOSFET is turned on until either the inductor current

approaching the output voltage, the duty cycle increases

starts to reverse, as indicated by the current reversal

toward the maximum on-time. Further reduction of the

comparator IRCMP, or the beginning of the next clock

supply voltage forces the main switch to remain on for

cycle.

more than one cycle until it reaches 100% duty cycle.

The output voltage will then be determined by the input

Burst Mode Operation

voltage minus the voltage drop across the P-channel

MOSFET and the inductor.

The TD6815 is capable of Burst Mode operation in which An important detail to remember is that at low input

the internal power MOSFETs operate intermittently

based on load demand.

supply voltages, the RDS(ON) of the P-channel switch

increases (see Typical Performance Characteristics).

In Burst Mode operation, the peak current of the inductor Therefore, the user should calculate the power

is set to approximately 200mA regardless of the output dissipation when the TD6815 is used at 100% duty cycle

load. Each burst event can last from a few cycles at light with low input voltage (See Thermal Considerations in

loads to almost continuously cycling with short sleep

intervals at moderate loads. In between these burst

events, the power MOSFETs and any unneeded circuitry

are turned off, reducing the quiescent current to 20mA. In

this sleep state, the load current is being supplied solely

from the output capacitor. As the output voltage droops,

the Applications Information

section).

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1.5MHz 1.5A Synchronous Step-Down Regulator

Dropout

TD6815

Function Description(Cont.)

The basic TD6815 application circuit is shown in Figure

3. External component selection is driven by the load

requirement and begins with the selection of L followed

by CIN and COUT.

Low Supply Operation

The TD6815 will operate with input supply voltages as

low as 2.5V, but the maximum allowable output current is

reduced at this low voltage. Figure 2 shows the reduction

in the maximum output current as a function of input

voltage for various output voltages.

Inductor Selection

For most applications, the value of the inductor will fall in

the range of 1mH to 4.7mH. Its value is chosen based on

the desired ripple current. Large value inductors lower

ripple current and small value inductors result in higher

ripple currents. Higher VIN or VOUT also increases the

ripple current as shown in equation 1. A reasonable

starting point for setting ripple current is DIL = 600mA

(40% of 1500mA).

Slope Compensation and Inductor Peak

Current

Slope compensation provides stability in constant

frequency architectures by preventing subharmonic

oscillations at high duty cycles. It is accomplished

internally by adding a compensating ramp to the inductor

current signal at duty cycles in excess of 40%. Normally,

this results in a reduction of maximum inductor peak

current for duty cycles >40%. However, the TD6815 uses

a patent-pending scheme that counteracts this

compensating ramp, which allows the maximum inductor

peak current to remain unaffected throughout all duty

cycles.

The DC current rating of the inductor should be at least

equal to the maximum load current plus half the ripple

current to prevent core saturation. Thus, a 1620mA rated

inductor should be enough for most applications

(1500mA + 120mA). For better efficiency, choose a low

DC-resistance

inductor.

The inductor value also has an effect on Burst Mode

operation. The transition to low current operation begins

when the inductor current peaks fall to approximately

200mA. Lower inductor values (higher DIL) will cause

this to occur at lower load currents, which can cause a

dip in efficiency in the upper range of low current

operation. In Burst Mode operation, lower inductance

values will cause the burst frequency to increase.

Maximum Output Current vs Input Voltag

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Function Description(Cont.)

This formula has a maximum at VIN = 2VOUT, where

IRMS = IOUT/2. This simple worst-case condition is

commonly used for design because even significant

deviations do not offer much relief. Note that the

capacitor manufacturer’s ripple current ratings are often

based on 2000 hours of life. This makes it advisable to

further derate the capacitor, or choose a capacitor rated

at a higher temperature than required. Always consult

the manufacturer if there is any question.

Inductor Core Selection

Different core materials and shapes will change the

size/current and price/current relationship of an inductor.

Toroid or shielded pot cores in ferrite or permalloy

materials are small and don’t radiate much energy, but

generally cost more than powdered iron core inductors

with similar electrical characteristics. The choice of which

style inductor to use often depends more on the price vs

size requirements and any radiated field/EMI

The selection of COUT is driven by the required effective

series resistance (ESR). Typically, once the ESR

requirement for COUT has been met, the RMS current

rating generally far exceeds the IRIPPLE(P-P)

requirements than on what the TD6815 requires to

operate. Table 1 shows some typical surface mount

inductors that work well in TD6815 applications.

requirement. The output ripple DVOUT is determined by:

where f = operating frequency, COUT = output

capacitanceand DIL = ripple current in the inductor. For a

fixed output voltage, the output ripple is highest at

maximum input voltage since DIL increases with input

voltage.

Aluminum electrolytic and dry tantalum capacitors are

both available in surface mount configurations. In the

case of tantalum, it is critical that the capacitors are

surge tested for use in switching power supplies. An

excellent choice is the AVX TPS series of surface mount

tantalum. These are specially constructed and tested for

low ESR so they give the lowest ESR for a given volume.

Other capacitor types include Sanyo POSCAP, Kemet

T510 and T495 series, and Sprague 593D and 595D

series. Consult the manufacturer for other specific

recommendations.

Table 1. Representative Surface Mount Inductors

CIN and COUT Selection

In continuous mode, the source current of the top

MOSFET is a square wave of duty cycle VOUT/VIN. To

prevent large voltage transients, a low ESR input

capacitor sized for the maximum RMS current must be

used. The maximum RMS capacitor current is given by:

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Dropout

TD6815

Function Description(Cont.)

Using Ceramic Input and Output

Capacitors

Higher values, lower cost ceramic capacitors are now

becoming available in smaller case sizes. Their high

ripple current, high voltage rating and low ESR make

them ideal for switching regulator applications. Because

the TD6815’s control loop does not depend on the output

capacitor’s ESR for stable operation, ceramic capacitors

can be used freely to achieve very low output ripple and

small circuit size.

Figure 4:Setting the output Voltage

Vout

1.2V

1.5V

1.8V

2.5V

3.3V

150K

160K

150K

240K

300K

470K

680K

However, care must be taken when ceramic capacitors

are used at the input and the output. When a ceramic

capacitor is used at the input and the power is supplied

by a wall adapter through long wires, a load step at the

output can induce ringing at the input, VIN. At best, this

ringing can couple to the output and be mistaken as loop

instability. At worst, a sudden inrush of current through

the long wires can potentially cause a voltage spike at

VIN, large enough to damage the part.

Table 2. Vout VS. R1, R2, Cf Select Table

Efficiency Considerations

The efficiency of a switching regulator is equal to the

output power divided by the input power times 100%. It is

often useful to analyze individual losses to determine

what is limiting the efficiency and which change would

produce the most improvement. Efficiency can be

expressed as:

When choosing the input and output ceramic capacitors,

choose the X5R or X7R dielectric formulations. These

dielectrics have the best temperature and voltage

characteristics of all the ceramics for a given value and

size.

Efficiency = 100% – (L1 + L2 + L3 + ...)

where L1, L2, etc. are the individual losses as a

percentage of input power.

Output Voltage Programming

Although all dissipative elements in the circuit produce

losses, two main sources usually account for most of the

losses in TD6815 circuits: VIN quiescent current and I2R

losses. The VIN quiescent current loss dominates the

efficiency loss at very low load currents whereas the I2R

loss dominates the efficiency loss at medium to high load

currents. In a typical efficiency plot, the efficiency curve

at very low load currents can be misleading since the

actual power lost is of no consequence as illustrated in

Figure 5.

In the adjustable version, the output voltage is set by a

resistive divider according to the following formula:

The external resistive divider is connected to the output,

allowing remote voltage sensing as shown in Figure4.

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Dropout

TD6815

Function Description(Cont.)

RSW = (RDS(ON)TOP)(DC) + (RDS(ON)BOT)(1 – DC)

The RDS(ON) for both the top and bottom MOSFETs

can be obtained from the Typical Performance

Charateristics curves. Thus, to obtain I2R losses, simply

add RSW to RL and multiply the result by the square of

the average output current. Other losses including CIN

and COUT ESR dissipative losses and inductor core

losses generally account for less than 2% total additional

loss.

Thermal Considerations

In most applications the TD6815 does not dissipate

much heat due to its high efficiency. But, in applications

where the TD6815 is running at high ambient

temperature with low supply voltage and high duty

cycles, such as in dropout, the heat dissipated may

exceed the maximum junction temperature of the part. If

the junction temperature reaches approximately 150°C,

both power switches will be turned off and the SW node

will become high impedance.

Figure 4:Power Lost VS Load Current

1. The VIN quiescent current is due to two components:

the DC bias current as given in the electrical

characteristics and the internal main switch and

synchronous switch gate charge currents. The gate

charge current results from switching the gate

capacitance of the internal power MOSFET switches.

Each time the gate is switched from high to low to high

again, a packet of charge, dQ, moves from VIN to

ground. The resulting dQ/dt is the current out of VIN that

is typically larger than

To avoid the TD6815 from exceeding the maximum

junction temperature, the user will need to do some

thermal analysis. The goal of the thermal analysis is to

determine whether the power dissipated exceeds the

maximum junction temperature of the part. The

temperature rise is given by:

the DC bias current. In continuous mode, IGATECHG

=f(QT + QB) where QT and QB are the gate charges of

the internal top and bottom switches. Both the DC bias

and gate charge losses are proportional to VIN and

thustheir effects will be more pronounced at higher

supply voltages.

TR = (PD)(qJA)

where PD is the power dissipated by the regulator and

qJA is the thermal resistance from the junction of the die

to the ambient temperature.

2. I2R losses are calculated from the resistances of the

internal switches, RSW, and external inductor RL. In

continuous mode, the average output current flowing

through inductor L is “chopped” between the main switch

and the synchronous switch. Thus, the series resistance

looking into the SW pin is a function of both top and

bottom MOSFET RDS(ON) and the duty cycle (DC) as

follows:

The junction temperature, TJ, is given by:

TJ = TA + TR

where TA is the ambient temperature.

As an example, consider the TD6815 in dropout at an

input voltage of 2.7V, a load current of 800mA and an

ambient temperature of 70°C. From the typical

performance graph of switch resistance, the RDS(ON) of

the P-channel switch at 70°C is approximately 0.52W.

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Function Description(Cont.)

Therefore,

power dissipated by the part is:

PD = ILOAD 2 • RDS(ON) = 187.2mW

For the SOP8 package, the qJA is 250°C/ W. Thus, the

junction temperature of the regulator is:

TJ = 70°C + (0.1872)(250) = 116.8°C

which is below the maximum junction temperature of

125°C.

Note that at higher supply voltages, the junction

temperature is lower due to reduced switch resistance

(RDS(ON)).

Checking Transient Response

The regulator loop response can be checked by looking

at the load transient response. Switching regulators take

several cycles to respond to a step in load current. When

a load step occurs, VOUT immediately shifts by an

amount equal to (ΔILOAD • ESR), where ESR is the

effective series resistance of COUT. ΔILOAD also begins

to charge or discharge COUT, which generates a

feedback error signal. The regulator loop then acts to

return VOUT to its steadystate value. During this

recovery time VOUT can be monitored for overshoot or

ringing that would indicate a stability problem. For a

detailed explanation of switching control loop theory.

A second, more severe transient is caused by switching

in loads with large (>1μF) supply bypass capacitors. The

discharged bypass capacitors are effectively put in

parallel with COUT, causing a rapid drop in VOUT. No

regulator can deliver enough current to prevent this

problem if the load switch resistance is low and it is

driven quickly. The only solution is to limit the rise time of

the switch drive so that the load rise time is limited to

approximately (25 • CLOAD).Thus, a 10μF capacitor

charging to 3.3V would require a 250μs rise time, limiting

the charging current to about 130mA.

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1.5MHz 1.5A Synchronous Step-Down Regulator

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TD6815

Package Information

SOP8 Package Outline Dimensions

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1.5MHz 1.5A Synchronous Step-Down Regulator

Dropout

TD6815

Design Notes

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TD6815 [ETC]

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