Texas Instruments TPS40003 Manuale utente

User’s Guide

TPS40003 Based 5–A Converter in

Less Than One Square Inch

User’s Guide

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Post Office Box 655303

Dallas, Texas 75265

SLUU130A –September 2002 –Revised February 2003

4TPS40003-Based 5-A Converter in Less Than One Square Inch

TPS40003-Based 5-A Converter in Less Than One Square

Inch

Mark Dennis Systems Powe

Contents

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

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

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

4 Design Procedure 6. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5 PowerPADPackaging 10. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

6 Test Results/Performance Data 11. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

7 PCB Layout 15. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

8 List of Material 16. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 Introduction

The TPS40002 and the TPS40003 are voltage-mode, synchronous buck PWM controllers that utilize TI’s

proprietary Predictive Gate Drivetechnology to wring maximum efficiency from step-down converters. These

controllers provide a bootstrap charging circuit to allow the use of an N-channel MOSFET as the topside buck

switch to reduce conduction losses and increase silicon device utilization. Predictive Gate Drivetechnology

controls the delay from main switch turn-off to synchronous rectifier turn-on and also the delay from rectifier

turn-off to main switch turn-on. This allows minimization of the losses in the MOSFET body diodes by reducing

conduction and reverse recovery time. This user’s guide provides details on a 5-A buck converter that converts

3.3 V down to a 1.2-V level utilizing the TPS40003 controller, with less than one square inch board area.

A schematic for the board is shown in Figure 1. A list of material is provided in the final section.

2 Features

The specification for this board is as follows:

DVIN = 3.0 V to 3.6 V

DVOUT = 1.2 V ±3%

D0 A ≤IOUT ≤5 A

DEfficiency > 90% with a load of 2 A

DOutput voltage ripple < 2% VOUT

DPhysical size < 1 square inch circuit area

SLUU130A –September 2002 –Revised February 2003

TPS40003-Based 5-A Converter in Less Than One Square Inch

3 Schematic

Figure 1. Application Diagram for the TPS40002/3

SLUU130A –September 2002 –Revised February 2003

6TPS40003-Based 5-A Converter in Less Than One Square Inch

4 Design Procedure

4.1 TPS4000X Family Device Selection

The TPS4000X family of devices offers four selections to encompass the frequency and output current mode

choices. The TPS40003 is selected for the following reasons. First, the internal oscillator components set a fixed

switching frequency of 600 kHz. This allows minimally sized filter components in this compact design. The other

choice related to the TPS4000X family involves the selection of Discontinuous Current Mode (DCM) operation

or Continuous Current Mode (CCM) operation at lighter loads. In this design the TPS40003 is selected to keep

the current continuous all the way to zero load, to provide robust control characteristics.

4.2 Inductance Value

The output inductor value is selected to set the ripple current to a value most suited to overall circuit functionality.

The inductor value is calculated in equation (1).

L+VOUT

f IRIPPLEȧ

Ȣ1*VOUT

VIN(max)ȧ

Ȥ+1.2 V

600 kHz 1.25 A ǒ1*1.2 V

3.6 VǓ+1.07 mH

where IRIPPLE is chosen to be 25% of IOUT, or 1.25 A. A common value of 1 µH is selected.

4.3 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, onboard capacitance is provided to supply the current required

during the top MOSFET on-time while keeping ripple within acceptable limits. For this power level, input voltage

ripple of 150 mV is reasonable, and a conservative minimum value of capacitance is calculated in equation (2).

C+I Dt

DV+5A 606 ns

0.15 V +20 mF

To meet this requirement with the lowest size and cost, a single 22 µF, X5R ceramic capacitor might be

considered. Although these capacitors have an extremely small resistance a typical datasheet indicates that

the part undergoes a 30°C temperature rise with 2 ARMS current at 500 kHz. With VIN= 3.0 V our circuit requires

nearly 2 ARMS of current, so for a conservative design two capacitors are selected to allow for conservative

current derating. These capacitors function as power bypass components and should be located near the

MOSFET package, to keep the high frequency current flow in a tight loop. The low impedance characteristics

of the dual ceramic capacitors help to reduce noise on the VDD supply of the device. Specifically, the high side

MOSFET current sense is referenced to this point, so noise at the device must be kept to a low level.

(1)

(2)

SLUU130A –September 2002 –Revised February 2003

TPS40003-Based 5-A Converter in Less Than One Square Inch

4.4 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 in equation (3).

COUT(min) +IRIPPLE

8 f VRIPPLE +1.25 A

8 600 kHz 12 mV +22 mF

In this design, COUT(min) is 22 µF with VRIPPLE = 12 mV to allow for some margin. However, this only affects the

capacitive component of the ripple voltage. In addition, the voltage component due to the capacitor ESR must

be considered in equation (4).

ESRCout vVRIPPLE

IRIPPLE +0.012 V

1.25 A +9.6 mW

For compactness while maintaining transient response capability, two 22-µF ceramic capacitors are fitted in

parallel. The total ESR of these capacitors is below 3 mΩ, and contributes only a few mV to the output voltage

ripple.

4.5 MOSFET Selection

The small physical size of this design requires the use of a single SO-8 package which contains dual N-channel

MOSFETs. MOSFETs with an RDS(on) of 18 mΩare selected to keep the conduction losses to a manageable

amount at full load.

4.6 Short Circuit Protection

The TPS40003 implements short circuit protection by comparing the voltage across the topside MOSFET while

it is on to a voltage dropped from VDD by RLIM due to an internal current source of 15 µA inside pin 1. Due to

tolerances in the current source and variations in the power MOSFET on-voltage versus temperature, the short

circuit level can protect against gross overcurrent conditions only, and should be set higher than rated load. In

this particular case, RLIM is selected as:

RLIM +R1 +2.5 IOUT 0.018 W

15 mA+15 kW

For this design, RLIM = 15 kΩ, and the factor of 2.5 in the equation accounts for the variations in component

tolerances over temperature and output current ripple. The high currents that are switched under short circuit

conditions may cause SW pin 8 to be driven below ground several volts, possibly injecting substrate current

which can cause improper operation of the device. A 3.3-Ωresistor has been placed in series with this pin to

limit its excursion to safe levels.

(3)

(4)

(5)

SLUU130A –September 2002 –Revised February 2003

8TPS40003-Based 5-A Converter in Less Than One Square Inch

4.7 Compensation Design

The TPS40003 uses voltage mode control in conjunction with a high frequency error amplifier. For the fastest

transient response, the loop crossover frequency is set at 1/10 fS, or 60 kHz. The power circuit L-C double pole

corner frequency fCis situated at 24 kHz, and the output capacitor ESR zero is far higher at approximately 1MHz.

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 approximately 2/3 fC, 18 kHz,

fz1 +1

2 p R2 C4

The second zero is selected at fC,

fz2 +1

2 p ǒR4)R5Ǔ C12

The two poles are placed at approximately 300 kHz, which is one-half the switching frequency,

fp1 +1

2 p R2 ǒC4 C8

C4)C8Ǔ

and

fp2 +1

2 p R5 C12

(6)

(7)

(8)

(9)

SLUU130A –September 2002 –Revised February 2003

TPS40003-Based 5-A Converter in Less Than One Square Inch

Figure 2 shows the plots for the closed loop gain and phase with VIN = 3.3 V and IOUT = 4.4 A. At the crossover

frequency of 60 kHz the phase margin is approximately 51 degrees.

100

150

100

–50

–100

–10

–20

–30

–40

1000 10000 1000000

Frequency – Hz

100000

Phase

–150

Gain – db

GAIN AND PHASE MARGIN

FREQUENCY

Phase – degrees

ILOAD = 4.4 A

VI= 3.3 V

Gain

Figure 2.

4.8 Snubber Component Selection

The switch node where Q1 and L1 come together is very noisy. An R–C network fitted between this node and

ground can help reduce ringing and voltage overshoot on Q1:B. This ringing noise should be minimized to

prevent it from confusing the control circuitry which is monitoring this node for current limit and Predictive Gate

Drive.

As a starting point, the snubber capacitor, C9, is generally chosen to be 5 to 8 times larger than the parasitic

capacitance at the node, which is primarily COS of Q1:B. Since COS is 440 pF for Q1:B, C9 is chosen to be 3.3 nF.

R3 is empirically determined to be 2.2 Ω, which minimizes the ringing and overshoot at the switch node. With

low input voltages the power loss, 1/2×C×V2×f , is relatively small at 24 mW.

SLUU130A –September 2002 –Revised February 2003

10 TPS40003-Based 5-A Converter in Less Than One Square Inch

5 PowerPADPackaging

The TPS4000X family is available in the DGQ version of TI’s PowerPADthermally enhanced package. In the

PowerPAD, a thermally conductive epoxy is utilized to attach the integrated circuit die to the leadframe die

pad, which is exposed on the bottom of the completed package. The leadframe die pad can be soldered to the

PCB using standard solder flow techniques when maximum heat dissipation is required. However, depending

on power dissipation requirements, the PowerPADmay not need to soldered to the PCB.

The PowerPADpackage helps to 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.

To effectively remove heat from the PowerPADpackage, a thermal land should be provided directly

underneath the package whether the package needs to be soldered or not. This thermal land usually has vias

that help to spread heat to internal copper layers and/or the opposite side of the PCB. The vias should not have

thermal reliefs that are often used on ground planes, because this reduces the copper area which transfers heat.

Additionally, the vias should be small enough so that the holes are effectively plugged when plated. This

prevents the solder from wicking away from the connection between the PCB surface and the bottom of the part.

A typical construction utilizes a few vias of 0.013”diameter plated with 1 ounce copper in the land under the

TPS40003. A typical layout pattern is shown in Figure 2, but does not show the copper land which would

encompass the vias above and below the device.

2.92mm

(0.115”)

0.5mm

(0.0197”)

0.28mm

(0.011”)

1.40mm

(0.055”)Via Dia.

0.33mm

(0.013”)

Miminum

PowerPad ”X”

1.3mm

(0.050”)

Minimum

PowerPad ”Y”

1.7mm

(0.068”)

Figure 3. PowerPAD PCB Layout Guidelines

The Texas Instrument document, PowerPADThermally Enhanced Package Application Report (SLMA002)

should be consulted for more information on the PowerPADpackage. This report offers in-depth information

on the package, assembly and rework techniques, and illustrative examples of the thermal performance of the

PowerPADpackage.

Indice

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