Texas Instruments TPS40001 User Manual

User’s Guide
TPS40001 Based Converter Delivers 10-A Output
User’ s Gu ide
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Mailing Address:
Texas Instruments Post Office Box 655303 Dallas, Texas 75265
Copyright 2003, Texas Instruments Incorporated
2
DYNAMIC WARNINGS AND RESTRICTIONS
It is important to operate this EVM within the input voltage range of 0 V
to 5.5 VDC.
DC
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 Users 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 Users 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
Copyright 2003, Texas Instruments Incorporated
3
SLUU131A – September 2002 – Revised February 2003
TPS40001 Based Converter Delivers 10-A Output
Mark Dennis DC to DC Controller Products
Contents
1 Introduction 4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2 Features 4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3 Schematic 5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3 Design Procedure 6. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4 PowerPAD Packaging 10. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5 Test Results/Performance Data 11. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6 PCB Layout 14. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7 List of Material 15. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1 Introduction
The TPS40000 and the TPS40001 are voltage-mode, synchronous buck PWM controllers that utilize TI’s proprietary Predictive Gate Drive technology to wring maximum efficiency from step-down converters. This controller family provides a bootstrap 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, both conduction and reverse recovery. This design note provides details on a buck converter that converts an input of 3 V to 5 V down to a 2.5-V output level utilizing either the TPS40000 or TPS40001 controller.
A schematic for the board is shown in Figure 1. The list of material is provided in section 7 of this Users Guide.
2 Features
The specification for this board is as follows:
D V D V D I D Efficiency = >95% with V D Output voltage ripple < 2% V D Power semiconductor devices: Each MOSFET is a single SO-8 package
= 3.0 V to 5 V
IN
= 2.5 V +/–3%
OUT
= 0 A to 10 A
OUT
= 3.3 V, load 4 A
IN
OUT
4
TPS40001 Based Converter Delivers 10-A Output
3 Schematic
SLUU131A – September 2002 – Revised February 2003
+
+
+
+
Figure 1. Application Diagram for the TPS40002/3
TPS40001 Based Converter Delivers 10-A Output
5
SLUU131A – September 2002 – Revised February 2003
4 Design Procedure
4.1 TPS4000X Family Device Selection
The TPS4000X family of devices offers four selections to encompass the frequency and continuous/discontinuous inductor current options. The TPS4000/1 are selected for this high current application because the 300-kHz switching frequency enables higher efficiency. The TPS40002/3 are available for applications needing 600-kHz operation. The TPS4000X family also allows the user to select Discontinuous Current Mode (DCM) operation or Continuous Current Mode (CCM) operation at lighter loads. In this reference design the TPS40001 is selected to maintain continuous mode operation down to zero load. If desired, the TPS40000 can be installed to turn the synchronous MOSFET off when the controller senses the inductor current reaching zero, indicating the circuit is entering the DCM of operation.
4.2 Inductance Value
The output inductor value is selected to set the ripple current to a value most suited to overall circuit functionality. An inductor selection that is too small leads to larger ripple current that increases RMS current losses in the inductor and MOSFETs, and also leads to more ripple voltage on the output. The inductor value is calculated by equation (1),
V
L
+
MIN
in which I keep the inductor small to minimize the R controller that maintains continuous inductor current down to no load eliminates concerns arising from crossing the DCM boundary. A standard value of 1 µH with a resistance of 3.5 mΩ is selected. At full load the power loss is only 0.35 W, which is only 1.4% of the 25-W output power.
RIPPLE
OUT
f I
RIPPLE
is chosen to be 40% of I
ǒ
1 *
V
V
IN(max)
OUT
Ǔ
OUT
+
DS(on)
2.5 V
300 kHz 4A
, or 4 A at max VIN. This high value of ripple current is selected to
losses due to the high output current. A synchronous rectifier
ǒ
1 *
2.5 V 5V
Ǔ
+ 1.0 mH
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 standalone supply, onboard capacitance is added to handle input voltage ripple and RMS current considerations. For this power level, input voltage ripple of 150 mV is reasonable, and a conservative minimum value of capacitance is calculated as
I Dt
C +
In addition to this minimum capacitance requirement, the RMS current stresses must be considered. In this converter, the large duty cycle causes the input RMS current to be nearly as large as the output current. This simplified formula calculates the RMS current for a trapazoidal current waveform, shown in equation (3).
I
DV
+ I DǸ+ I
RMS
10 A 2.5 ms
+
0.15 V
Ǹ
+ 167 mF
V
OUT
V
IN(min)
+ 10 A
Ǹ
2.5 V
3.0 V
+ 9.1 A
(1)
(2)
(3)
Additional terms for the ripple component of the current add only a small amount to the total RMS current, and can be neglected. To meet this initial requirement with small size and cost, a combination of capacitors is considered. To carry the high frequency ripple current, three 22-µF, X5R ceramic capacitors are placed close to the power circuitry. Although these capacitors have an extremely small resistance, the datasheet indicates that the part undergoes a 30_C temperature rise with 2 A needed. Two 330-µF POSCAPs with an RMS current capability of 4.4 A each is selected. In typical embedded converters, these POSCAPs is not required if the upstream converter feeding this buck has sufficient current handling capability.
6
TPS40001 Based Converter Delivers 10-A Output
current at 500 kHz, so more current capability is
RMS
SLUU131A – September 2002 – Revised February 2003
4.4 Output Capacitor Selection
Selection of the output capacitor is based on many application variables, including function, cost, size, and availability. First, the minimum allowable output capacitance should be determined by the amount of inductor ripple current and one-half the allowable output ripple, as given in equation (4).
I
C
OUT(min)
This only affects the capacitive component of the ripple voltage. In addition, the voltage component due to the capacitor ESR must be considered, shown in equation (5).
+
RIPPLE
8 f V
RIPPLE
+
8 300 kHz 25 mV
4A
+ 67 mF
(4)
V
ESR
Cout
To minimize capacitor size while maintaining good transient response, two 470-µF POSCAPs (with an ESR of 10 mΩ each) are fitted in paralleled with a 1-µF ceramic capacitor.
v
RIPPLE
I
RIPPLE
+
25 mV
4A
+ 6.25 mW
4.5 MOSFET Selection
One constraint of this design is the use of one SO-8 MOSFET in the upper switch device and one SO-8 in the lower synchronous rectifier location in the buck converter power stage. The upper device loss is usually dominated by switching loss, so a device with lower gate charge and switching times was selected. Since this application has a relatively high output voltage, the upper device runs at a high duty cycle and needs to have a low R selected. The same device is fitted in the bottom switch location to achieve high efficiency.
to keep conduction losses low, and an 8-m device with a maximum gate charge of 30 nC is
DS(on)
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 R 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 much higher than rated load. In this particular case, R
3 I
R
LIM
+ R2 +
OUT
15 mA
is selected as shown in equation (6).
LIM
R
DS(on)
due to an internal current source of 15 µA inside pin 1. Due to
LIM
3 10 A 0.008 W
+
15 mA
+ 15 kW
(5)
(6)
For this design, a standard value of 16.2 k is selected for R2. The factor of 3 in the equation accounts for the variations in component tolerances (both initial and over temperature) and output current ripple. The component tolerances include MOSFET R 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- resistor has been placed in series with this pin to limit its excursion to safe levels.
DS(on)
, I
sink current, and the VOS offset voltage of SW vs I
LIM
TPS40001 Based Converter Delivers 10-A Output
. The high
LIM
7
SLUU131A – September 2002 – Revised February 2003
4.7 Compensation Design
The TPS40000 uses voltage mode control in conjunction with a high frequency error amplifier. The power circuit L-C double pole corner frequency f
Freq
The output capacitor ESR zero is calculated by equation (8),
LC
+
2 p L
1
Ǹ
OUT
is located in equation (7).
C
+ 5.1 kHz
C
OUT
(7)
ǒ
R6) R
1
4
1
7
1
4
1
ESR
1
C
C
OUT
15
C
11
Ǔ
C
11
+ 33.8 kHz
C
Ǔ
C
7
7
15
C
7
ǒ
)
C
7
F
where the two POSCAPs ESR of 5 m is used in the calculation because the 1 µF is e ffectively out of the picture at these relatively low frequencies.
The feedback compensation network is implemented to provide two zeroes and three poles. The first pole is placed near the origin to improve dc regulation.
The first zero is placed below fC at 2.2 kHz in equation (9).
f
The second zero is placed at 18 kHz shown in equation (10).
f
The first pole is placed near the ESR zero frequency in equation (11),
f
and the second pole is placed at one-half the switching frequency at 150 kHz to allow a high-speed transient response, shown in equation (12).
f
Z(esr)
z1
z2
p1
p2
+
+
+
+
+
2 p R
2 p
2 p R
2 p R
2 p R
(8)
(9)
(10)
(11)
(12)
8
TPS40001 Based Converter Delivers 10-A Output
SLUU131A – September 2002 – Revised February 2003
Figure 2 is presents the measured loop gain and phase characteristics. At the loop crossover frequency of 20 kHz the phase margin is approximately 50 degrees.
GAIN AND PHASE MARGIN
vs
30 50 40
30 20
10
Gain – db
0
–10 –20
–30 –40
100 1000 10000 100000 1000000
FREQUENCY
Phase
Gain
Frequency – Hz
Figure 2.
200
150
100
50
0
–50
–100
–150
–200
Phase – degrees
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 Q2. This ringing noise should be minimized to prevent it from confusing the control circuitry which is monitoring this node for current limit, Predictive Gate Drivet, and DCM control functions.
As a starting point, the snubber capacitor, C12, is generally chosen to be 5 to 8 times larger than the parasitic capacitance at t h e node, which is primarily C to be 10 nF. R3 is empirically determined to be 2.2 Ω, which minimizes the ringing and overshoot at the switch node. With the relatively low input voltage of 5 V, the power loss, ½ CV
of Q2. Since COS is around 1600 pF for Q2 a t 5 V, C12 is chosen
OS
2
f, is relatively small at 37 mW.
TPS40001 Based Converter Delivers 10-A Output
9
SLUU131A – September 2002 – Revised February 2003
5 PowerPAD Packaging
The TPS4000X family is available in the DGQ version of TIs PowerP ADt thermally enhanced package. In the PowerPADt, 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 PowerPADt may not need to soldered to the PCB.
The thermally conductive epoxy bonding the circuit die to the leadframe die pad causes a high resistance from the leadframe die pad to the device ground pin 5. When the PowerPadt package is soldered to the PCB, the leadframe die pad can be connected to ground (pin 5), but this is not required. The leadframe die pad should not be connected to other potentials in the circuit.
The PowerPADt 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 PowerPADt package, a thermal land should be provided directly underneath the package. 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 to transfer 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 footprint pattern is shown in Figure 2, but does not include the additional copper plane which includes the vias above and below the device.
2.92mm
0.5mm
(0.0197)
0.28mm (0.011)
1.40mm (0.055)
Miminum
PowerPad ”X”
1.3mm
(0.050)
(0.115)
Minimum
PowerPad ”Y”
1.7mm
(0.068)
Via Dia.
0.33mm (0.013)
Figure 3. PowerPADt PCB Layout Guidelines
The Texas Instrument document, PowerPADt Thermally Enhanced Package Application Report (Texas Instrument Literature Number SLMA002) should be consulted for more information on the PowerPADt package. This report offers in-depth information on the package, assembly and rework techniques, and illustrative examples of the thermal performance of the PowerPADt package.
10
TPS40001 Based Converter Delivers 10-A Output
6 Test Results/Performance Data
6.1 Test Setup
dc power supply adjustable from
0 V to 5 V
SLUU131A – September 2002 – Revised February 2003
DVM1
Input wires 16
gauge or larger, as
short as feasible
gauge or larger, as
I
IN
VIN
TP1
Output wires 16
short as feasible
J1
GND
TP2
TP7
(+)
SLUP183A
TP3
J2
LOAD
TP6
VOUT
(+)
GND
(–)
Load adjustable
from 0–10Amps
(–)
I
OUT
TP4
SCOPE
TP5
DVM2
(+)
(–)
Figure 4. Test Setup
TPS40001 Based Converter Delivers 10-A Output
11
SLUU131A – September 2002 – Revised February 2003
Typical efficiency curves are shown in Figure 5 for 3.3-V input. It should be stressed that measuring high efficiencies requires the utmost care in instrumentation. The power losses are so low that small errors can lead to large variations in measured efficiency. The input and output voltages are measured on the PCB as shown in the test diagram to avoid the losses associated with the input and output connectors.
EFFICIENCY
vs
Efficiency – %
OUTPUT CURRENT
100
95
90
85
80
75
70
65
60
55
50
0.00 2.00 6.00 8.00 10.00
4.00
I
– Output Current – A
OUT
Figure 5.
Figure 6 shows the switch node at V
= 5 V and I
IN
=10 A. As the picture indicates, there is almost negligible
OUT
body diode conduction using the Predictive Gate Drivet technique.
TYPICAL SWITCH NODE WAVEFORM
2 V/div
12
TPS40001 Based Converter Delivers 10-A Output
t – Time – 500 ns/div
Figure 6.
SLUU131A – September 2002 – Revised February 2003
Figure 7 shows the output voltage ripple at high VIN and full load, which is the worst case condition for output voltage ripple.
OUTPUT VOLTAGE RIPPLE
10 mV/div
t – Time – 1 µs/div
Figure 7.
Figure 8 shows the transient response with a 50% load step from 2.5 A to 7.5 A.
TRANSIENT RESPONSE
50 mV/div
t – Time – 20 µs/div
Figure 8.
TPS40001 Based Converter Delivers 10-A Output
13
SLUU131A – September 2002 – Revised February 2003
7 PCB Layout
The PCB top assembly and copper layers are shown in Figures 9 through 11.
Figure 9
Figure 10
14
TPS40001 Based Converter Delivers 10-A Output
Figure 11
SLUU131A – September 2002 – Revised February 2003
8 List of Material
Table 1 lists the components used in this design. With minor component tweaks this design could be modified to meet a wide range of applications.
Reference Qty Description Manufacturer Part Number
Capacitor
Terminal Block J1, J2 2 4-pin, 15 A, 5.1 mm, 291 126 OST ED2227
Inductor L1 1 Inductor, SMT, 1 µH, 15 A, 3.5 mΩ, 0.51 x 0.51 Vishay IHLP–5050CE–01
MOSFET Q1, Q2 2 MOSFET, N-channel, 12 V, 17 A, 5.5 mΩ, SO8 Siliconix Si4866DY
Resistor
JACK TP1, TP4, TP5, 3 Red, 1 mm, 0.038, 6400 Farnell 240–345
Adapter TP2, TP6 2 3.5-mm probe clip (or 131–5031–00), 72900 Tektronix 131–4244–00
JACK TP3, TP7 1 Black, 1mm, 0.038,6400 Farnell 240–333
Device U1 1 Low Input Voltage Mode,
C1, C2 2 POSCAP, 330 µF, 6.3 V, 10 milliohm, 20%,
7343 (D) C11 1 Ceramic, 180 pF, 50 V, NPO, 10%, 805 Vishay VJ0805A181KXAAT C12 1 Ceramic, 0.01µF, 50 V, X7R, 10%, 805 Vishay VJ0805Y103KXAAT C13 1 Ceramic, 0.0047 µF, 50 V, X7R, 10%, 805 Vishay VJ0805Y472KXAAT C15 1 Ceramic, 6.8 nF, 50 V, X7R, 10%, 805 Vishay VJ0805Y682KXAAT C3, C4, C5 3 Ceramic, 22 µF, 6.3 V, X5R, 20%, 1210 Panasonic ECJ–4YB0J226M C6, C10, C14 3 Ceramic, 1 µF, 10 V, X5R, 10%, 805 Panasonic ECJ–2YB1A105K C7 1 Ceramic, 1.5 nF, 50 V, X7R, 10%, 805 Vishay VJ0805Y152KXAAT C8, C9 2 POSCAP, 470 µF, 4 V, 10 mΩ, 20%, 7343 (D) Sanyo 4TPD470M
R1, R5 1 Chip, 1.8 , 1/10 W, 5%, 805 Std Std R2 1 Chip, 16.2 k, 1/10 W, 1%, 805 Std Std R3 1 Chip, 2.2 , 1/10 W, 5%, 805 Std Std R4 1 Chip, 5.90 k, 1/10 W, 1%, 805 Std Std R6 1 Chip, 10 k, 1/10 W, 1%, 805 Std Std R7 1 Chip, 698 , 1/10 W, 1%, 805 Std Std R8 1 Chip, 3.92 , 1/10 W, 1%, 805 Std Std R9 1 Chip, 3.3 , 1/10 W, 5%, 805 Std Std
Syncronous Buck Controller DGQ10
Sanyo 6TPD330M
TI TPS40001DGQ
TPS40001 Based Converter Delivers 10-A Output
15
IMPORTANT NOTICE
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TI assumes no liability for applications assistance or customer product design. Customers are responsible for their products and applications using TI components. T o minimize the risks associated with customer products and applications, customers should provide adequate design and operating safeguards.
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Copyright 2003, Texas Instruments Incorporated
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