Texas Instruments HPA070 Manual de usuario

User’s Guide

TPS40055-Based Design Converts

12-V Bus to 1.8 V at 15 A (HPA070)

User’s Guide

EVM IMPORTANT NOTICE

Texas Instruments (TI) provides the enclosed product(s) under the following conditions:

This evaluation kit being sold by TI is intended for use for ENGINEERING DEVELOPMENT OR EVALUATION

PURPOSES ONLY and is not considered by TI to be fit for commercial use. As such, the goods being provided

may not be complete in terms of required design-, marketing-, and/or manufacturing-related protective

considerations, including product safety measures typically found in the end product incorporating the goods.

As a prototype, this product does not fall within the scope of the European Union directive on electromagnetic

compatibility and therefore may not meet the technical requirements of the directive.

Should this evaluation kit not meet the specifications indicated in the EVM User’s Guide, the kit may be returned

within 30 days from the date of delivery for a full refund. THE FOREGOING WARRANTY IS THE EXCLUSIVE

WARRANTY MADE BY SELLER TO BUYER AND IS IN LIEU OF ALL OTHER WARRANTIES, EXPRESSED,

IMPLIED, OR STATUTORY, INCLUDING ANY WARRANTY OF MERCHANTABILITY OR FITNESS FOR ANY

PARTICULAR PURPOSE.

The user assumes all responsibility and liability for proper and safe handling of the goods. Further, the user

indemnifies TI from all claims arising from the handling or use of the goods. Please be aware that the products

received may not be regulatory compliant or agency certified (FCC, UL, CE, etc.). Due to the open construction

of the product, it is the user’s responsibility to take any and all appropriate precautions with regard to electrostatic

discharge.

EXCEPT TO THE EXTENT OF THE INDEMNITY SET FORTH ABOVE, NEITHER PARTY SHALL BE LIABLE

TO THE OTHER FOR ANY INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES.

TI currently deals with a variety of customers for products, and therefore our arrangement with the user is not

exclusive.

TI assumes no liability for applications assistance, customer product design, software performance, or

infringement of patents or services described herein.

Please read the EVM User’s Guide and, specifically, the EVM Warnings and Restrictions notice in the EVM

User’s Guide prior to handling the product. This notice contains important safety information about temperatures

and voltages. For further safety concerns, please contact the TI application engineer.

Persons handling the product must have electronics training and observe good laboratory practice standards.

No license is granted under any patent right or other intellectual property right of TI covering or relating to any

machine, process, or combination in which such TI products or services might be or are used.

Mailing Address:

Texas Instruments

Post Office Box 655303

Dallas, Texas 75265

DYNAMIC WARNINGS AND RESTRICTIONS

It is important to operate this EVM within the input voltage range of 0 VDC to 14 VDC.

Exceeding the specified input range may cause unexpected operation and/or irreversible damage to the EVM.

If there are questions concerning the input range, please contact a TI field representative prior to connecting

the input power.

Applying loads outside of the specified output range may result in unintended operation and/or possible

permanent damage to the EVM. Please consult the EVM User’s Guide prior to connecting any load to the EVM

output. If there is uncertainty as to the load specification, please contact a TI field representative.

During normal operation, some circuit components may have case temperatures greater than 50°C. The EVM

is designed to operate properly with certain components above 50°C as long as the input and output ranges are

maintained. These components include but are not limited to linear regulators, switching transistors, pass

transistors, and current sense resistors. These types of devices can be identified using the EVM schematic

located in the EVM User’s Guide. When placing measurement probes near these devices during operation,

please be aware that these devices may be very warm to the touch.

Mailing Address:

Texas Instruments

Post Office Box 655303

Dallas, Texas 75265

SLUU186 − March 2004

4TPS40055-Based Design Converts 12-V Bus to 1.8 V at 15 A (HPA070)

TPS40055-Based Design Converts 12-V Bus to

1.8 V at 15 A (HPA070)

Mark Dennis System Pow

Contents

1 Introduction 4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2 Features 4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3 Schematic 5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4 Component Selection 6. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5 Test Setup 10. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

6 Test Results and Performance Data 11. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

7 EVM Assembly Drawing and PCB Layout 12. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

8 List of Materials 15. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

9 References 16. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 Introduction

In many modern electronic applications there is a growing demand for circuits to convert a12-V bus to digital

voltages as low as, but not limited to 1.8 V. The current requirements can range from below 1 A to over 15 A.

For high-efficiency and small circuit size the TPS40055 wide-input synchronous buck controller can be used

to provide the necessary control and drive functions to implement these converters. The TPS40055EVM−001

operates at 300 kHz and delivers 1.8 V at 15 A with efficiency over 90% for much of the load range, and a full

load efficiency of 88%.

The TPS40055 synchronous buck controller offers a variety of user programmable functions such as operating

frequency, soft-start time, voltage feed-forward, high-side current limit, and external loop compensation. This

controller also provides a regulated 10-V bias voltage which supplies onboard drivers for the N-channel switch

and synchronous rectifier MOSFETs, utilizing adaptive gate drive logic to prevent cross conduction of the power

MOSFETs.[1]

2 Features

The specification of this design is as follows:

D92% peak efficiency at 6 A

D88% peak efficiency at 15 A

D1.8V output at 15 A

DVIN range from 10 VDC to 14 VDC

DSmall circuit size 1.4” x 2.5” SMT design, components on single side

DLine/load regulation < 0.5%

DHigh-frequency 300-kHz operation

DTransient deviation 60 mV with 10-A load step

SLUU186 − March 2004

TPS40055-Based Design Converts 12-V Bus to 1.8 V at 15 A (HPA070)

3 Schematic

Figure 1. TPS40055EVM−001 (HPA070) Schematic

SLUU186 − March 2004

6TPS40055-Based Design Converts 12-V Bus to 1.8 V at 15 A (HPA070)

4 Component Selection

4.1 TPS40055 Device Selection

The TPS4005x family of parts offers a range of output current configurations including source only (TPS40054),

source/sink (TPS40055), or source/sink with VOUT prebias (TPS40057). In this converter the TPS40055 with

source/sink capability is selected. This serves to maintain continuous inductor ripple current all the way to zero

load to improve the small signal loop response by preventing the inductor current from transitioning to the

discontinuous current mode.

The TPS4005x family is packaged in TI’s PWP PowerPAD thermally enhanced package which should be

soldered to the PCB using standard solder flow techniques. The PowerPADtechnology uses a thermally

conductive epoxy to attach the integrated circuit die to the leadframe die pad, which is exposed on the bottom

of the completed package. The PWP PowerPAD package has a θJC = 2°C/W which helps keep the junction

temperature rise relatively low even with the power dissipation inherent in the onboard MOSFET drivers. This

power loss is proportional to switching frequency, drive voltage, and the gate charge needed to enhance the

N-channel MOSFETs. Effective heat removal allows the use of ultra-small packaging while maintaining high

component reliability.

The technical brief, PowerPAD Thermally Enhanced Package[2] contains more information on the PowerPAD

package.

4.2 Frequency of Operation

The clock oscillator frequency for the TPS40055 is programmed with a single resistor from RT (pin 2) to signal

ground. The following equation (1) from the datasheet allows selection of RT in kΩfor a given switching

frequency in kHz.

RT+R2 +1

fSW 17.82 10*6*23 (kW)

For 300-kHz operation, R2 is selected to be 165 kΩ.

For a particular operating frequency, the PWM ramp time must be programmed via the resistor RKFF connected

to VIN. Also, the selection of RKFF programs the VIN voltage at which the circuit starts operation. This prevents

the circuit from starting at low voltages, which can lead to current flow larger than desired. RKFF is programmed

using equation (2).

RKFF +R6 +ǒVIN(min) *3.5Ǔ ǒ58.14 RT)1340Ǔ(kW)

Where VIN(min) is the minimum startup input voltage, and RTis in kΩ. Note that internal tolerances have been

incorporated into this equation, so the actual VIN(min) of the input voltage should be used. For an oscillator

frequency of 300-kHz, the RKFF value of 71.5 kΩis selected.

(1)

(2)

SLUU186 − March 2004

TPS40055-Based Design Converts 12-V Bus to 1.8 V at 15 A (HPA070)

4.3 UVLO Circuitry

The user programmable UVLO built into the TPS4005x provides hysteresis for transients shorter than a total

count of seven cycles. If the input voltage to the converter can be slowly rising around the minimum VIN range,

external hysteresis can be incorporated to prevent multiple on/off cycles during startup or shutdown. These

on/off cycles are a result of line impedance external to the EVM causing VIN to the module to drop when under

load, which causes the programmable UVLO threshold to be crossed repetitively.

In this converter, C1 and D1 are added to form a peak detector from the lower gate drive which is only active

when the converter is operating. This provides a bias source to deliver hysteresis current from the peak detector

voltage to the lower KFF voltage of 3.5 V, enabling the designer to alter the programmable UVLO shutdown

point. The bias is not present during startup, so the circuit starts as expected from the RKFF calculation.

In this application, R4 is selected to provide a hysteresis current of 20% IKFF. R4 can be calculated from equation

(3).

RHYS +R4 +RKFF ǒVPD *3.5Ǔ

0.2 ǒVIN(min) *3.5Ǔ

where

DVPD is the voltage on the peak detector

DVIN(min) is the desired start voltage used in the determination of RKFF

In a typical case, VPD = 8V, and R4 is found to be 247 kΩ, and a standard value of 243kΩis selected. Testing

shows the startup voltage to be 9.2 V, and the shutdown voltage to be 8.5 V.

4.4 Inductance Value

The output inductor L1 value used in the circuit of Figure 2 was selected from equation (4).

L+VOUT

f IRIPPLE 1*VOUT

VIN(min)

in which IRIPPLE is usually chosen to be in the range between 10% and 40% of IOUT. With IRIPPLE = 20% of

IOUT(max) there is a ripple current of 3 A, and the inductance value is 1.7 µH.

4.5 Input capacitor selection

Bulk input capacitor selection is based on allowable input voltage ripple and required RMS current carrying

capability. In typical buck converter applications, the converter is fed from an upstream power converter with

its own output capacitance. In this converter, ceramic capacitors capable of meeting circuit requirements are

provided onboard. For this power level, input voltage ripple of approximately 250 mV is reasonable, and the

minimum capacitance is calculated in (5).

CIN +I Dt

DV+I VO

DV VIN fS+15 A 1.8 V

0.25 V 10 V 300 kHz +36 mF

Also consider the RMS current rating required for the input capacitors (6).

i^IOUT D

Ǹ+IOUT VOUT

VIN

Ǹ+15 1.8

Ǹ+6.4 A

To meet this requirement with the smallest cost and size two 22 µF, 16 V, X5R ceramic capacitors (C12, C14)

are installed on the board. In the 1812 case, the parts are able to carry approximately 4 ARMS each. These

capacitors function as power bypass components and should be located close to the MOSFET packages to

keep the high-frequency current flow in a small, tight loop.

(3)

(4)

(5)

(6)

SLUU186 − March 2004

8TPS40055-Based Design Converts 12-V Bus to 1.8 V at 15 A (HPA070)

4.6 Output Capacitor Selection

Selection of the output capacitor is based on many application variables, including function, cost, size, and

availability. The minimum allowable output capacitance is determined by the amount of inductor ripple current

and the allowable output ripple, as given in equation (7).

COUT(min) +IRIPPLE

8 f VRIPPLE +3A

8 300 kHz 15 mV +83 mF

In this design, COUT(min) is 83 µF with VRIPPLE=15 mV to allow for some margin. However, this only affects the

capacitive component of the ripple voltage, and the final value of capacitance is generally influenced by ESR

and transient considerations. The voltage component due to the capacitor ESR.

CESR vVRIPPLE

IRIPPLE +15 mA

3A +5mW

An additional consideration in the selection of the output inductor and capacitance value can be derived from

examining the transient voltage overshoot which can be initiated with a load step from full load to no load. By

equating the inductive energy with the capacitive energy the equation (9) can be derived:

COvL I2

V2+

L ǒǒIOHǓ2*ǒIOLǓ2Ǔ

ǒVfǓ2*ǒViǓ2+1.7 mH (15 A)

ǒ(1.9 V)2*(1.8 V)2Ǔ+1034 mF

where

•IOH = full load current

•IOL = no load current

•Vf= allowed transient voltage rise

•Vi= initial voltage

For compactness while maintaining transient response capability, two 470-µF POSCAP capacitors (C16, C17)

are fitted in parallel. The total ESR of these capacitors is approximately 5 mΩ. An additional 47-µF, 6.3-V ceramic

capacitor C15 is placed in parallel with the POSCAPs to help suppress high frequency noise generated by the

fast current transitions as the current switches between the input and output circuits during each switching cycle.

4.7 MOSFET selection

Proper MOSFET selection is essential to optimize circuit efficiency. To operate with high current it is important

to choose a package which allows the generated heat to be removed from the package as easily as possible.

Various MOSFETs with a package similar to the SO−8 footprint are considered for this application, and devices

with reduced junction-case thermal impedance are selected.

For the upper switch Q1, a Hitachi HAT2168H MOSFET with low gate charge (typically 27 nC at 10 V) and with

an RDS(on) of 6 mΩis selected to keep the switching losses to a minimum. The low-side rectifier switch Q2 was

chosen as a Hitachi HAT2167H, which has slightly more gate charge (43 nC at 10 V) but lower RDS(on) = 4.2

mΩto minimize conduction losses. A schottky diode, D2, is placed across Q2 in this high current design to carry

some of the high circulating current during short circuit conditions.

(7)

(8)

(9)

SLUU186 − March 2004

TPS40055-Based Design Converts 12-V Bus to 1.8 V at 15 A (HPA070)

4.8 Short Circuit Protection

The TPS40055 implements short circuit protection by comparing the voltage across the topside MOSFET while

it is ON to a voltage developed across RLIM due to an internal current source of 10 µA inside pin 16. Both of these

voltages are negative with respect to VIN. From the datasheet equation, RLIM is defined as:

RLIM +R9 +IOC RDS(on) (max)

1.12 ISINK +VOS

ISINK +(W)

where

•IOC is the overcurrent set point equal to the DC output current plus one-half the inductor ripple current

•VOS is the overcurrent comparator offset, and Isink is the current into ILIM (pin 16).

Using worst case tolerances the value of RLIM should be maximized to ensure that the converter can deliver full

rated current under all conditions. In a worst case condition, RLIM=R9 and

RLIM +(15 A )1.5 A) (7.9 mW 1.45)

1.12 8.65 mA)*30 mV

8.65 mA+16.0 kW

The standard value of 16.2 kΩwas selected. This ensures that we can deliver a minimum of 15 A before current

limit is activated. There is also a small capacitor, C7, placed in parallel with R9 to filter the signal.

4.9 Snubber Component Selection

Initially, the junction of Q1, Q2, and L1 was ringing at a frequency near 100 MHz with a peak voltage near 30

V. This was due to the extremely fast switching speed of the MOSFETs and the lack of any cross−conduction.

C13 was added to shunt the high-frequency ringing to ground and the peak voltage is now below 25 V.

4.10 Compensation Components

The TPS40055 uses voltage mode control with feed-forward in conjunction with a high-frequency error amplifier

to implement closed loop control. The power circuit L-C double pole corner frequency fCoccurs at 3.8 kHz, and

the output capacitor ESR zero is located at approximately 38 kHz. The feedback compensation network is

implemented to provide two zeroes and three poles. The first pole is placed at the origin to improve DC

regulation.

The first zero is placed at 2.8 kHz, just below the L-C corner frequency.

fZ1 +1

2p R5 C5

The second zero is selected to be coincident with the L-C corner frequency of 3.8 kHz,

fZ2 +1

2p (R7 )R8) C6

The second pole is placed near the ESR zero frequency at 37 kHz.

fP1 +1

2p R5 ǒC4 C5

C4)C5Ǔ

and the third pole is placed at 150 kHz, which is one-half the switching frequency.

fP2 +1

2p R8 C6

(10)

(11)

(12)

(13)

(14)

(15)

SLUU186 − March 2004

10 TPS40055-Based Design Converts 12-V Bus to 1.8 V at 15 A (HPA070)

5 Test Setup

Figure 2 illustrates the basic test setup needed to evaluate the TPS40055EVM−001.

5.1 DC Input Source

The input voltage source should be capable of supplying between 10 VDC and 14 VDC and rated for at least 4

A of current. For best results the input leads should be made with a wire of 18AWG or larger.

5.2 Output Load

The output load can be either an electronic load or a resistive load configured to draw between 0 A and 15 A.

The output leads should be made with a wire of 16AWG or larger diameter wire. Monitor the output voltage on

the PCB by connecting a voltmeter to TP9 and TP10 to prevent voltage drops through PCB traces and the output

terminal block which can lead to substantial measurement errors.

5.3 Oscilloscope Probe Test Jacks

An oscilloscope probe test jack (TP8) has been included to allow monitoring the ourput voltage ripple.

5.4 Fan

There is no cover to prevent the user from probing the internal circuit nodes. There are components that can

get hot to the touch (above 60°C) in normal operation. A small fan delivering more than 15 cfm should be used

when operating at and near full load.

−

+−

Fan

VIN

10 V to 14 V VIN Test Points

TP1 = VIN(+)

TP2 = VIN(−)

VOUT Test Points

TP9 = VOUT(+)

TP10 = VOUT(−)

VLOAD

1.8 V / 15 A

Figure 2. Test Setup

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