TPS40040DRBT [TI]
LOW PIN COUNT, LOW VIN (2.5 V TO 5.5 V) SYNCHRONOUS BUCK DC-TO-DC CONTROLLER WITH ENABLE; 低引脚数，低VIN （ 2.5 V至5.5 V ）同步降压型DC - DC控制器具有使能
| 型号： | TPS40040DRBT |
| 厂家： | 
TEXAS INSTRUMENTS |
| 描述： | LOW PIN COUNT, LOW VIN (2.5 V TO 5.5 V) SYNCHRONOUS BUCK DC-TO-DC CONTROLLER WITH ENABLE |
| 文件： | 总36页 (文件大小：768K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
TPS40040, TPS40041
www.ti.com
SLUS700B–MARCH 2006–REVISED MARCH 2006
LOW PIN COUNT, LOW V (2.5 V TO 5.5 V) SYNCHRONOUS BUCK DC-TO-DC
IN
CONTROLLER WITH ENABLE
FEATURES
CONTENTS
•
•
•
•
•
•
•
2.25-V to 5.5-V Input
Device Ratings
2
3
Output Voltage from 0.6 V to 90% of VIN
High-Side Drive for N-Channel FET
Supports Pre-Biased Outputs
Adaptive Anti-Cross Conduction Gate Drive
1%, 0.6-V Reference
Electrical Characteristics
Device Information
Application Information
Design Examples
8
10
19
32
Two Fixed Switching Frequency Versions,
TPS40040 (300 kHz) and TPS40041 (600 kHz)
Additional References
•
Three Selectable Short Circuit Protection
Levels of 105 mV, 180 mV and 310 mV
DESCRIPTION
The TPS40040 and TPS40041 dc-to-dc controllers
are designed to operate from a 2.25-V to 5.5-V input
source. To reduce the number of external
components, several operating parameters are fixed
internally; namely, frequency, soft start time, and
short circuit protection (SCP) levels. For example, the
operating frequencies of TPS40040/1 are 300
kHz/600 kHz, respectively.
•
•
•
•
•
Hiccup Restart from Faults
Voltage Mode Control
Active Low Enable
Thermal Shutdown Protection at 145°C
8-Pin, 3-mm x 3-mm SON with Ground
Connection to Thermal Pad
APPLICATIONS
•
•
•
•
Point of Load
Telecommunications
DC to DC Modules
Set Top Boxes
SIMPLIFIED APPLICATION DIAGRAM
VIN
TPS40040/1
HDRV
8
7
6
5
1
2
3
4
EN
SW
BOOT
LDRV
FB
VOUT
COMP
GND
VDD
Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of Texas
Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.
PowerPAD is a trademark of Texas Instruments.
Predictive Gate Drive is a registered trademark of Texas Instruments.
PRODUCTION DATA information is current as of publication date.
Products conform to specifications per the terms of the Texas
Instruments standard warranty. Production processing does not
necessarily include testing of all parameters.
Copyright © 2006, Texas Instruments Incorporated
TPS40040, TPS40041
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SLUS700B–MARCH 2006–REVISED MARCH 2006
DESCRIPTION (CONT.)
One of three short circuit threshold levels may be selected by the addition of an external resistor from the COMP
pin to circuit ground. During power on, and before the internal soft start commands the output voltage to rise, the
TPS40040/1 enters a calibration cycle, measures the current out of the COMP pin, and selects an internal SCP
threshold voltage. At the end of the 1.6-ms calibration time, the output voltage is allowed to rise for a 4-ms soft
start. During operation, the selected SCP threshold voltage is compared to the upper MOSFET’s voltage drop
during its ON time to determine whether there is an overload condition.
The packaging of the TPS40040/1 is unique in that the PowerPAD™ is used as an electrical ground connection
as well as a thermal connection.
ORDERING INFORMATION
OPERATING FREQUENCY
300 kHz
PACKAGE
TAPE AND REEL QTY.
PART NUMBER
TPS40040DRBT
TPS40040DRBR
TPS40041DRBT
TPS40041DRBR
Plastic 8-pin SON (DRB)
Plastic 8-pin SON (DRB)
Plastic 8-pin SON (DRB)
Plastic 8-pin SON (DRB)
250
2500
250
300 kHz
600 kHz
600 kHz
2500
DEVICE RATINGS
ABSOLUTE MAXIMUM RATINGS
over operating free-air temperature range (unless otherwise noted, all voltages are with respect to GND.)
PARAMETER
VALUE
6.5
UNIT
VDD
SW
-3 to 10.5
-5
SW
transient (< 50 ns)
BOOT
HDRV
EN, FB, LDRV
COMP
SW+6.5
SW to SW+6.5
-0.3 to 6.5
-0.3 to 3
-40 to 150
-55 to 150
V
Operating junction temperature
Storage junction temperature
°C
RECOMMENDED OPERATING CONDITIONS
over operating free-air temperature range (unless otherwise noted)
PARAMETER
MIN
MIN
TYP
MAX
MAX
UNIT
VIN
TJ
Input voltage
2.25
-40
5.5
V
Junction temperature
125
°C
ELECTROSTATIC DISCHARGE (ESD) PROTECTION
PARAMETER
TYP
2500
1500
UNIT
V
Human body model
CDM
V
PACKAGE DISSIPATION RATINGS(1)
THERMAL IMPEDANCE
TA = 25°C POWER RATING
JUNCTION-TO-AMBIENT
TA = 85°C POWER RATING
48°C/W
2W
0.8W
(1) For more information on the DRB package and the test method, refer to TI technical brief, literature number SZZA017.
2
TPS40040, TPS40041
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SLUS700B–MARCH 2006–REVISED MARCH 2006
ELECTRICAL CHARACTERISTICS
TJ = -40 °C to 85°C VDD = 5 V, (unless otherwise noted)
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNIT
Input Supply
VDD
Input voltage range
Shutdown
2.25
5.5
V
IDDsd
IDDq
EN = VDD
100
180
2.0
µA
Quiescent
FB = 0.8 V
1.0
2.0
mA
IDDs
Switching current
Minimum turn-on voltage
No load at HDRV/LDRV
UVLOON
1.95
80
2.05
130
2.15
200
V
UVLOHYS Hysteresis
mV
Oscillator/ Ramp Generator
fPWM
TPS40040 PWM frequency
2.25 V < VDD < 5.5 V
VDD = 5.0 V, 0°C < TJ < 70°C
2.25 V < VDD < 5.5 V
VDD = 5.0 V, 0°C < TJ < 70°C
VPEAK– VVALLEY
250
270
500
540
0.75
300
300
600
600
0.87
0.37
350
330
700
660
1.0
kHz
kHz
kHz
kHz
V
fPWM
TPS40040 PWM frequency
TPS40041 PWM frequency
TPS40041 PWM frequency
Ramp amplitude PP
fPWM
fPWM
VRAMP
VVALLEY
PWM
Ramp valley voltage
V
MAXDUTY Maximum duty cycle, TPS40040
Maximum duty cycle, TPS40041
MINDUTY Minimum duty cycle
MIN pulse
VFB = 0 V, 2.25 V < VDD < 5.5 V
VFB = 0 V, 2.25 V < VDD < 5.5 V
90
88
95
95
%
0
Minimum width control range before
jumping to zero.
Minimum controllable pulse width
90
150
ns
(1)
width
Error Amplifier
VDD = 5.0 V, 0°C < TJ < 70°C
593.5
590
600.0
606.5
610
VFB
FB input voltage
mV
2.25 V < VDD < 5.5 V, -40°C < TJ <
125°C
IFB
FB input bias current
50
150
nA
V
IOH = 0.5 mA, VFB = 0 V, VDD = 5.5
V
VOH
High level output voltage
2.0
2.5
VOL
IOH
Low level output voltage
Output source current
Output sink current
Gain bandwidth
IOL = 0.5 mA, VFB = VDD
VCOMP = 0.7 V, VFB = GND
VCOMP = 0.7 V, VFB = VDD
80
6
150
mV
1
2
mA
IOL
8
(1)
GBW
AOL
5
10
85
MHz
dB
Open loop gain
55
Short Circuit Protection
Resistor COMP to GND = 2.4 kΩ, TJ
= 25°C
Low short circuit threshold voltage
80
145
250
105
180
130
215
370
TH1
Medium short circuit threshold
voltage
Default: No resistor COMP to GND,
TJ = 25°C
VTH2
mV
Resistor COMP to GND = 12 kΩ, TJ
= 25°C
VTH3
High short circuit threshold voltage
Threshold temperature coefficient
310
3100
200
(1)
(1)
VTH(tc)
tON(oc)
ppm
ns
Minimum HDRV pulse time in over
current
tSWOCblank SW leading edge blanking pulse in
100
40
(1)
over current detection
tHICCUP
Hiccup time between restarts
ms
(1) Ensured by design. Not production tested.
3
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SLUS700B–MARCH 2006–REVISED MARCH 2006
ELECTRICAL CHARACTERISTICS (continued)
TJ = -40 °C to 85°C VDD = 5 V, (unless otherwise noted)
PARAMETER
Soft Start/Enable
TEST CONDITIONS
MIN
TYP
MAX
UNIT
Calibration time before softstart
begins
(2)
tCAL
1.0
1.6
2.5
ms
(2)
tSS
Soft start time
FB rise time from 0 V to 600 mV
Sum of tCAL plus tSS
3.0
4.0
4.0
5.6
6.0
8.5
tREG
Time to voltage regulation
Enable threshold
VEN
EN voltage w.r.t. VDD
-0.8
-1.2
50
-1.6
V
VENHYS
Bootstrap
RBOOT3V3
RBOOT5V
Enable hysteresis
mV
VBOOT to VDD, VDD = 3.3 V
VBOOT to VDD, VDD = 5 V
50
30
Bootstrap switch resistances
Ω
Ω
Output Driver
VBOOT - VSW = 3.3 V, ISRCE = 100
mA
RHDHI3V3
HDRV pull-up resistance
3.0
5.5
RHDLO3V3
RLDHI3V3
RLDLO3V3
HDRV pull-down resistance
LDRV pull-up resistance
LDRV pull-down resistance
LDRV, HDRV rise time
LDRV, HDRV fall time
VBOOT - VSW = 3.3 V, ISINK = 100 mA
VDD = 3.3 V, ISOURCE = 100 mA
VDD = 3.3 V, ISINK = 100 mA
CLOAD = 1 nF
1.5
3.0
1.0
15
3
5.5
2.0
35
(3)
tRISE
(3)
tFALL
CLOAD = 1 nF
10
25
ns
TDEAD HL Adaptive timing HDRV to LDRV
TDEAD LH Adaptive timing LDRV to HDRV
SW Node
No load
15
5
30
No load
15
ILEAK
Leakage current
EN = VDD
-2
µA
°C
Thermal Shutdown
(3)
tSD
Shutdown temperature
Hysteresis
145
15
(2) tCAL and tSS track with temperature and input voltage
(3) Ensured by design. Not production tested.
4
TPS40040, TPS40041
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SLUS700B–MARCH 2006–REVISED MARCH 2006
TYPICAL CHARACTERISTICS
Quiescent Current (Non-Switching)
Shutdown Current
1.100
110
105
100
VDD = 2.25 V
VDD = 5.5 V
1.000
0.900
95
0.800
0.700
0.600
0.500
0.400
90
85
80
VDD = 2.25 V
VDD = 5.5 V
75
70
−40 −20
0
20
40 60
80 100 120
−20
0
20
40 60
80 100 120
−40
Temperature − C
Temperature − C
Figure 1.
Figure 2.
UVLO Threshold
EN Threshold
2.200
2.150
2.100
2.050
−0.8
−0.9
−1.0
−1.1
−1.2
−1.3
Turn ON
Turn OFF
VDD = 5 V
2.000
1.950
1.900
1.850
1.800
−1.4
−1.5
−1.6
−40 −20
0
20
40 60
80 100 120
−40 −20
0
20
40 60
80 100 120
Temperature − C
Temperature − C
Figure 3.
Figure 4.
Oscillator Frequency (TPS40040)
Oscillator Frequency (TPS40041)
350
325
300
700
650
VDD = 2.25V
VDD = 3.9V
VDD = 5.5V
VDD =2.25 V
VDD =3.9 V
VDD = 5 V
600
550
500
275
250
−40 −20
0
20
40 60
80 100 120
−40 −20
0
20
40 60
80 100 120
Temperature − C
Temperature − C
Figure 5.
Figure 6.
5
TPS40040, TPS40041
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SLUS700B–MARCH 2006–REVISED MARCH 2006
TYPICAL CHARACTERISTICS (continued)
Soft Start Time
FB Voltage
4.50
4.45
4.40
610
608
VDD = 2.25 V
VDD = 3.9 V
VDD = 5.5 V
VDD = 5 V
606
604
602
600
4.35
598
596
4.30
4.25
594
592
590
4.20
−40 −20
0
20
40 60
80 100 120
−40 −20
0
20
40 60
80
100 120
Temperature − C
Temperature − C
Figure 7.
Figure 8.
PWM Gain (TPS40040)
PWM Gain (TPS40041)
6.1
6.0
6.0
5.9
VDD = 5 V
VDD = 5 V
5.9
5.8
5.8
5.7
5.6
5.5
5.7
5.6
5.5
−40 −20
0
20
40 60
80
100 120
−40 −20
0
20
40 60
80
100 120
Temperature − C
Temperature − C
Figure 9.
Figure 10.
ILIM Threshold
Bootstrap Switch Resistance
450
400
80
VDD = 3.3 V
VDD = 5 V
R
= 2.5 kΩ
R
= 12.5 kΩ
R
=nil
C
C
C
70
60
350
300
250
200
150
100
50
50
40
30
20
−40 −20
0
20
40 60
80
100 120
−40 −20
0
20
40 60
80
100 120
Temperature − C
Temperature − C
Figure 11.
Figure 12.
6
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SLUS700B–MARCH 2006–REVISED MARCH 2006
TYPICAL CHARACTERISTICS (continued)
Minimum Controllable Pulse Width (TPS40040)
Minimum Controllable Pulse Width (TPS40041)
100
130
VDD = 2.25 V
VDD = 5.5 V
95
90
85
80
75
70
65
125
120
115
110
105
100
VDD = 2.25 V
VDD = 5.5 V
60
95
−40 −20
0
20
40 60
80
100 120
−40 −20
0
20
40 60 80 100 120
Temperature − C
Temperature − C
Figure 13.
Figure 14.
Maximum Duty Cycle
SW Node Leakage Current
100
0.00
−0.50
−0.10
VDD = 5.5 V
VDD = 2.25 V
VDD = 5.5 V
98
96
−0.15
−0.20
−0.25
−0.30
94
−0.35
−0.40
92
90
−0.45
−0.50
−40 −20
0
20
40 60
80 100 120
−40 −20
0
20
40 60
80
100 120
Temperature − C
Temperature − C
Figure 15.
Figure 16.
7
TPS40040, TPS40041
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SLUS700B–MARCH 2006–REVISED MARCH 2006
DEVICE INFORMATION
TERMINAL CONFIGURATION
The package is an 8-pin SON (DRB) package. Note: The thermal pad is an electrical ground connection.
TPS40040/1
8
HDRV
SW
1
2
3
EN
FB
7
6
5
BOOT
LDRV
COMP
VDD
GND
4
Figure 17. DRB Package Terminal Configuration (Top View)
Table 1. TERMINAL FUNCTIONS
TERMINAL
I/O
DESCRIPTION
NAME
NO.
Input (bootstrapped) supply to the high-side gate driver for PWM enabling the gate of the
high side FET to be driven above the input supply rail. Connect a ceramic capacitor from this
pin to SW. This capacitor is charged from the VDD pin voltage through an internal switch.
The switch is turned ON during the off time of the converter. To slow down the turn on of the
external MOSFET, a small resistor (1 Ω to 3 Ω) may be placed in series with the bootstrap
capacitor. See Applications Section to calculate the appropriate value.
BOOT
6
I
Output of the error amplifier and connection node for loop feedback components. The
voltage at this pin determines the duty cycle for the PWM. Optionally, a resistor from this pin
to ground is used to determine the voltage threshold used for short circuit protection. (See
Application Section)
COMP
3
O
•
•
•
Low threshold R = 2.4 kΩ, +/-10%
Mid threshold R = not installed
High threshold R = 12 kΩ, +/-10%
Active low enable input allows ON/OFF operation of the controller. If power is applied to the
TPS40040/1 while the EN pin is allowed to float high, the TPS40040/1 remains disabled
(both external switches are held OFF). Only when the EN pin is pulled to 1.2 V below VDD is
the TPS40040/1 allowed to start. An internal 100-kΩ resistor is connected between VDD and
EN to provide pull up. Connect this pin to GND to bypass the enable function.
EN
FB
1
2
I
I
Inverting input of the error amplifier. In closed loop operation, the voltage at this pin is at the
internal reference level of 600 mV. A series resistor divider from the converter output to
ground, with the center connection tied to this pin, determines the value of the regulated
output voltage. This pin is also a connection node for loop feedback components.
This is the gate drive output for the high side N-channel MOSFET switch for PWM. It is
referenced to SW and is bootstrapped for enhancement of the high-side switch.
HDRV
LDRV
VDD
8
5
4
O
O
I
Gate drive output for the low-side synchronous rectifier (SR) N-channel MOSFET.
Power input to the device. This pin should be locally bypassed to GND with a low ESR
ceramic capacitor of 1 µF or greater.
Connection to the switched node of the converter and the power return for the upper gate
driver. There should be a high current return path from the source of the upper MOSFET to
this pin. It is also used by the adaptive gate drive circuits to minimize the dead time between
upper and lower MOSFET conduction.
SW
7
O
Ground connection to the device. This is also the thermal pad used to conduct heat from the
device. This connection serves a twofold purpose. The first is to provide an electrical ground
connection for the device. The second is to provide a low thermal impedance path from the
device die to the PCB. This pad should be tied externally to a ground plane. See Application
Section for PC board layout information.
GND
Thermal Pad
8
TPS40040, TPS40041
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SLUS700B–MARCH 2006–REVISED MARCH 2006
Block Diagram
VDD
4
VDD/2
SW
VDD
UVLO
SDN
100K
2 V
100ns
DELAY
EN
1
FAULT
LOGIC
EN
ILIM SET
CURRENT LIMIT
Vdd−1.2v
COMP
VDD
SDN CLOCK
0.6 V
LDRV
PWM COMP
VREF
Soft Start
+
+
PWM
6
BOOT
−
2
3
FB
PWM
LOGIC
HI
RAMP
COMP
HDRV
SW
8
7
CLOCK
OSCILLATOR
0.6V
ADAPTIVE
GATE
VREF
VDD
DRIVE
VDD
ILIM SET
LO
Reference
Calibration
Circuit
5
LDRV
ILIM voltages
105 mV
Pre−bias
Thermal
Shutdown
180 mV
310 mV
PAD
GND
Figure 18. Functional Block Diagram
9
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SLUS700B–MARCH 2006–REVISED MARCH 2006
APPLICATION INFORMATION
Functional Description
The TPS40040 (300 kHz) and TPS40041 (600 kHz) are fixed-frequency voltage-mode synchronous buck
controllers. In operation, the synchronous rectifier (SR) is allowed to conduct current in both directions, allowing a
converter to operate in continuous mode, even under no load conditions, simplifying feedback loop compensation
requirements. During startup, internal circuitry modulates the switching of the synchronous rectifier to prevent
discharging of the output if a pre-biased condition exists.
Voltage Reference
The 600-mV bandgap reference voltage cell is internally connected to the non-inverting input of the error
amplifier. The voltage reference is trimmed with the error amplifier in a unity gain configuration to remove
amplifier offset from the final regulation voltage.
Voltage Error Amplifier
The error amplifier has a bandwidth of greater than 5 MHz, and open loop gain of at least 55 dB. The output
voltage swing is limited to just above and below the oscillator ramp levels to improve transient response.
Loop Compensation
Voltage mode buck type converters are typically compensated using Type III networks. Please refer to the
Design Example for detailed methodology in designing feedback loops for voltage mode converters.
Oscillator
The oscillator frequency is internally fixed. The TPS40040/1 operating frequencies are 300 kHz/600 kHz,
respectively.
UVLO
When the input voltage is below the UVLO threshold, the TPS40040/1 turns off the internal oscillator and holds
all gate drive outputs in the low (OFF) state. When the input rises above the UVLO threshold, and the EN pin is
below the turn ON threshold, the start-up sequence is allowed to begin.
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SLUS700B–MARCH 2006–REVISED MARCH 2006
APPLICATION INFORMATION (continued)
Enable and Start-Up Sequence
The EN pin of the TPS40040/1s internally pulled to VDD. When power is applied to VDD, the EN pin is allowed
to float high, and the TPS40040/1 remains OFF. Only when the EN pin is externally pulled below the threshold
voltage of VDD - 1.2 V is the TPS40040/1 allowed to start. When enabled, the TPS40040/1 enters a calibration
cycle where the short circuit current threshold is determined. The TPS40040/1 monitors the current out of the
COMP pin and selects a threshold based on the sensed value of the current. See Selecting the Short Circuit
Current Limit Threshold section for for details. When this calibration time is completed, the soft-start cycle is
allowed to begin. See Figure 19 below.
ENB
COMP
V
OUT
4 ms
1.5 ms
Configure ILIM Threshold
Soft Start
Figure 19. Startup
DESIGN
HINT:
If the enable function is not used, the EN pin should be connected to ground
(GND).
DESIGN
HINT:
When designing the feedback loop compensation, ensure the capacitors used
are not so large that they distort the COMP pin calibration waveform.
Soft Start
At the end of a calibration cycle, the TPS40040/1 slowly increases the voltage to the non-inverting input of the
error amplifier. In this way, the output voltage slowly ramps up until the voltage on the non-inverting input to the
error amplifier reaches the internal reference voltage. At that time, the voltage at the non-inverting input to the
error amplifier remains at the reference voltage.
During the soft-start interval, pulse-by-pulse current limiting is active. If seven consecutive current limit pulses are
detected, overcurrent is declared and a timeout period equivalent to seven calibration/soft-start cycles goes into
effect. See Output Short Circuit Protection section for details.
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SLUS700B–MARCH 2006–REVISED MARCH 2006
APPLICATION INFORMATION (continued)
Pre-Bias Startup
The TPS40040/1 supports pre-biased output voltage applications. In cases where the output voltage is held up
by external means while the TPS40040/1 is off, full synchronous rectification is disabled during the initial phase
of soft starting the output voltage. When the first PWM pulses are detected during soft start, the controller slowly
initiates synshronous rectification by starting the synchronous rectifier with a narrow on time. It then increments
that on time on a cycle-by-cycle basis until it coincides with the time dictated by (1-D), where D is the duty cycle
of the converter. This approach prevents the sinking of current from a pre-biased output, and ensures the output
voltage startup and ramp to regulation is smooth and controlled.
NOTE:
If the output is pre-biased, PWM pulses start when the internal soft-start voltage rises
above the error amplifier input (FB pin).
Figure 20 below depicts the waveform of the HDRV and LDRV output signals at the beginning PWM pulses.
When HDRV turns off, diode rectification is enabled. Before the next PWM cycle starts, LDRV is turned on for a
short pulse. With every clock cycle, the leading edge of LDRV is modulated, increasing the on time of the
synchronous rectifier. Eventually, the leading edge of LDRV coincides with the falling edge of HDRV to achieve
full synchronous rectification. During normal operation of the converter, the TPS40040/1 operates in full two
quadrant source/sink mode.
Figure 21 shows the startup waveform of a 1.2-V output converter under three different pre-biased output
conditions. The lowest trace is when there is no pre-bias on the output. The center and top most traces indicate
converter startup with 0.5-V and 1.0-V pre-bias conditions.
V
V
= 5 V
IN
= 1.2 V
OUT
(200 mV/div)
PREBIAS = 1 V
V
HDRV
PREBIAS = 0.5 V
PREBIAS = 0 V
V
LDRV
t − Time − 2 µs/div
Figure 20. MOSFET Drivers at Beginning of Soft Start
t − Time − 500 µs/div
Figure 21. Startup Waveforms
The recommended output voltage pre-bias range is less than or equal to 90% of the final regulation voltage. A
pre-biased output voltage of 90% to 100% of final regulation could lead to the sinking of current from the pre-bias
source. If the pre-biased voltage is greater than the designed converter output regulation voltage, then upon the
completion of the soft-start interval, the TPS40040/1 draws current from the output to bring the output voltage
into regulation.
12
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APPLICATION INFORMATION (continued)
Output Short Circuit Protection
To minimize circuit losses, the TPS40040/1 uses the RDS(on) of the upper MOSFET switch as the current sensing
element. The current limit comparator, initially blanked during the first portion of each switching cycle, senses the
voltage across the high-side MOSFET when it is fully ON. This voltage is compared to an internally selected
short circuit current (SCC) limit threshold voltage. If the comparator senses a voltage drop across the high-side
MOSFET greater than the SCC limit threshold, it outputs an OC pulse. This terminates the current PWM pulse
preventing further current ramp-up, and sets the fault counter to count up one count on the next clock cycle.
Similarly, if no OC pulse is detected, the fault counter decrements by one count. If seven OC pulses are
summed, a fault condition is declared and the upper switch of the PWM output of the chip is immediately
disabled (turned OFF) and remains that way until the fault time-out period has elapsed. Both HDRV and LDRV
drivers are kept OFF during the fault time-out.
The fault time-out period is determined by cycling through seven internal soft-start time periods. At the end of the
fault time-out period, startup is attempted again.
The main purpose is for hard fault protection of the power switches. The internal SCC voltage has a positive
temperature coefficient designed to improve the short circuit threshold tolerance variation with temperature.
However, given the tolerance of the voltage thresholds and the RDS(on) range for a MOSFET, it is possible to
apply a load that thermally damages the external MOSFETs.
Selecting the Short Circuit Current Limit Threshold
The TPS40040/1 uses one of three user selectable voltage thresholds. During the calibration interval at power on
or enable (Figure 19), the TPS40040/1 monitors the current out of the COMP pin and selects a threshold based
on the sensed value. If the current is zero; that is, no resistor is connected between COMP and GND, then the
threshold voltage level is 180 mV. If a 2.4-kΩ resistor is connected between COMP and GND, then the threshold
voltage level is 105 mV. If a 12-kΩ resistor is connected between COMP and GND, then the threshold voltage is
310 mV.
Once calibration is complete, the selected SCP threshold level is latched into place and remains constant. In
addition, the sensing circuits on COMP pin during calibration are disconnected from the COMP pin, and soft start
is allowed to begin.
Synchronous Rectification and Gate Drive
In a buck converter, when the upper switch MOSFET turns off, current is flowing in the inductor to the load. This
current cannot be stopped immediately without using infinite voltage. To give this current a path to flow and
maintain voltage levels at a safe level, a rectifier or catch device is used. This device can be either a diode, or it
can be a controlled active device. The TPS40040/1 provides a signal to drive an N-channel MOSFET as a
synchronous rectifier (SR). This control signal is carefully coordinated with the drive signal for the main switch so
that there is minimum dead time from the time that the SR turns OFF and the upper switch MOSFET turns ON,
and minimum delay from when the upper switch MOSFET turns OFF and the SR turns ON.
NOTE:
The longer the time spent in diode conduction during the rectifier conduction period,
the lower the converter efficiency.
The drivers for the external HDRV and LDRV MOSFETs are capable of driving a gate to source voltage of
approximately 5 V. At VDD = 5 V, the drivers are capable of driving MOSFETs appropriate for a 15-A converter.
The LDRV driver switches between VDD and ground, while HDRV driver is referenced to SW and switches
between BOOT and SW. The drivers have non-overlapping timing that is governed by an adaptive delay circuit
that minimizes body diode conduction in the synchronous rectifier.
13
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APPLICATION INFORMATION (continued)
Gate Drive Resistors
The TPS40040/41’s adaptive gate delay circuitry monitors the HDRV-to-SW and LDRV-to-GND voltages to
determine the state of the external MOSFET switches. Any voltage drop across an external series gate drive
resistor is sensed as reduced gate voltage during turn-off and may interfere with the MOSFET timing.
DESIGN
HINT:
A resistor should never be placed in series with the synchronous rectifiers gate
and the gate trace should be kept as short as practical in the layout.
Total Gate Charge
The internal voltage sensing of the external MOSFET gate voltages used by the TPS40040/1 to control the
dead-times between turn-off and turn-on can be sensitive to large MOSFET gate charges, especially when
different gate charges are used for the high-side and low-side MOSFETs. Increased gate charge increases
MOSFET switching times and decreases the dead-time between the MOSFETs switching.
MOSFETs with no more than 40 nC of total gate charge should be selected.
DESIGN
HINT:
The upper switch MOSFET’s gate charge should be no less than 60% of the
synchronous rectifier’s gate charge to minimize the turn-on/turn-off delay
mismatch between the high-side and low-side MOSFET.
Synchronous Rectifier dV/dt Turn-On
As the upper switch MOSFET turns on, the switch node voltage rises from close to ground to VIN in a very short
period of time (typically 10 ns to 30 ns) resulting in very high voltage spikes on the switch node. The construction
of a MOSFET creates parasitic capacitances between its terminals, particularly the gate-to-drain and
gate-to-source, creating a capacitive divider between the drain and source of the MOSFET with the gate at its
mid-point. If the gate-to-drain charge (QGD) is larger than the gate-to-source charge (QGS), the capacitive divider
places proportionally more charge on the gate of the MOSFET as the switch node voltage rises than is shunted
to GND. In extreme cases, this can cause the synchronous rectifier gate voltage to rise above the turn on
threshold voltage of the MOSFET and causes cross-conduction. This is called dV/dt turn-on. It increases power
dissipation in both the high-side and the low-side MOSFET, reducing efficiency.
Select a synchronous rectifier MOSFET with a QGD to QGS ratio of less than
one and provide a wide, low resistance, low inductance loop in the synchronous
rectifier gate drive circuit. (See Layout Consideration)
DESIGN
HINT:
A resistor in series with the boost capacitor slows the turn on of the high-side
MOSFET, and reduces the dV/dt of the switch node. See Boost Capacitor
Series Resistor section.
DESIGN
HINT:
14
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APPLICATION INFORMATION (continued)
Bootstrap for N-Channel MOSFET Drive
The PWM duty cycle is limited to a maximum of 95%, allowing the bootstrap capacitor to charge during every
cycle. During each PWM OFF period, the voltage on VDD charges the bootstrap capacitor. When the PWM
switch is next commanded to turn ON, the voltage used to drive the MOSFET is derived from the voltage on this
capacitor. Since this is a charge transfer circuit, the value of the bootstrap capacitor must be sized such that the
energy stored in the capacitor on a per cycle basis is greater then the gate charge requirement of the MOSFET
being used. See the Design Example section for details.
Bootstrap Capacitor Series Resistor
Since resistors should not be placed in series with the high-side gate, it may be necessary to place a small 1-Ω
to 3-Ω resistor in series with the bootstrap capacitor to control the turn-on of the main switching MOSFET and
reduce the dV/dt rate of rise of the switch node voltage. A resistor placed between the BOOT pin and the
bootstrap capacitor increases the series resistance during the turn-on of the high-side MOSFET, and has no
effect during the high-side MOSFET’s turn-off period. This prevents the TPS40040/1 from sensing the upper
switch MOSFET’s turn-off too early and reducing the upper switch MOSFET turn-off to the SR MOSFET turn-on
delay timing too far.
DESIGN
HINT:
To reduce EMI, place a small 1-Ω to 3-Ω resistor in series with the boost
capacitor to control the turn-on of the main switching FET.
External Schottky Diode for Low Input Voltage
The TPS40040/1 uses an internal P-channel MOSFET switch between VDD and BOOT to charge the bootstrap
capacitor during synchronous rectifier conduction time. At low input voltages, a MOSFET can not be turned on
hard enough to rapidly replenish the charge required to turn on an (high gate charge) external high-side
MOSFET. For this situation, an external Schottky diode between the VDD and BOOT pins may be added. While
the diode carries very small average current (QG x FSW) it may be required to carry several hundred mA of peak
surge current. The diode should be rated for at least 500 mA of surge current. For higher input voltage
applications, if a resistor is used in series with the boost capacitor, connect the diode to the junction of the
resistor and capacitor to remove the added resistance from the capacitor’s charge path.
For low input voltages, and a high gate charge upper switch MOSFET, a small
Schottky diode should be placed from VDD to BOOT. Do not use a resistor in
series with the boost capacitor.
DESIGN
HINT:
VDD Bypass and Filtering
To prevent switching noise from being injected into the TPS40040/1 control circuitry, a ceramic capacitor (1 µF
minimum) must be placed as close to the VDD pin and GND pad as possible.
15
TPS40040, TPS40041
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SLUS700B–MARCH 2006–REVISED MARCH 2006
APPLICATION INFORMATION (continued)
VDD Filter Resistor
To further limit the noise on VDD, a small 1-Ω to 2-Ω resistor may be placed between the input voltage and the
VDD pin to create a small filter to VDD. The resistor should connect near the drain of the upper switch MOSFET
to prevent trace IR drops from increasing the sensed voltage drop. The resistor itself should be placed close to
Pin 4.
The current through the resistor includes the device's no-load switching current of 2 mA plus gate switching
current. The voltage drop induced across this resistor reduces the VDD-to-SW voltage sensed by the over
current protection circuitry within the device. This results with the apparent voltage drop across the upper switch
MOSFET being increased, thereby decreasing the current at which protection will occur. To minimize this effect,
the resistor value should be selected to yield less than a 25-mV drop.
Thermal Shutdown
If the junction temperature of the device reaches the thermal shutdown level, the PWM and the oscillator are
turned off and HDRV and LDRV are driven off. When the junction cools to the required level, the PWM soft starts
as during a normal power-up cycle.
Package Power Dissipation
The power dissipation in a controller is largely dependent on the MOSFET driver currents and the input voltage.
The driver current is proportional to the total gate charge, QG, of the external MOSFETs, and the operating
frequency of the converter. Driver power, neglecting external gate resistance, is calculated from:
P
* Q V
F
Wńdriver
SW
D(driver)
G
DRIVE
(1)
And the total power dissipation, assuming the same MOSFET is selected for both the high side and synchronous
rectifier is:
2 P
D
t Ǔ
P ń
* I
V
W
DD
T
Q
V
DRIVE
(2)
(3)
or
t
Ǔ
P ń 2 G F
* I V
W
DD
T
Q
SW
Q
where IQ is the quiescent operating current (neglecting drivers).
The max power capability of the PowerPad™ package is dependent on the layout as well as air flow. The
thermal impedance from junction-to-air assuming 2-oz copper trace and thermal pad with solder and no air flow
is detailed in Reference [5].
16
TPS40040, TPS40041
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SLUS700B–MARCH 2006–REVISED MARCH 2006
APPLICATION INFORMATION (continued)
PCB Layout Guidelines
A synchronous BUCK power stage has two primary current loops, the input current loop that carries high ac
discontinuous current and an output current loop that carries high dc continuous current. The output current loop
carries low ac inductor ripple current.
VIN
VDD Filter
(Optional)
Main
TPS40041
Gate
Input
Current
Loop
Drive
EN
HDRV
Enable
VOUT
FB
SW
Enable Bypass
(Optional)
COMP
VDD
BOOT
LDRV
BOOST Resistor
(Optional)
Output
Current
Loop
GND
(Power Pad)
Current
Limit Set
Resistor
VDD
Bypass
SR Gate
Drive
Signal Ground
Power Ground
Locate Parts Over Power Ground
Locate Parts Over Signal Ground Island
Figure 22. Synchronous BUCK Power Stage
Power Component Routing
As shown in Figure 22, the input current loop contains the input capacitors, the switching MOSFET, the inductor,
the output capacitors, and the ground path back to the input capacitors. To keep this loop as small as possible, it
is good practice to place some ceramic capacitance directly between the drain of the main switching MOSFET
and the source of the synchronous rectifier (SR) through a power ground plane directly under the MOSFETs.
The output current loop includes the filter inductor, the output capacitors, and the ground return between the
output capacitors and the source of the synchronous rectifier MOSFET. As with the input current loop, the ground
return between the output capacitor ground and the source of the SR source should be routed under the inductor
and MOSFETs to minimize the power loop area.
17
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SLUS700B–MARCH 2006–REVISED MARCH 2006
APPLICATION INFORMATION (continued)
Device to Power Stage Interface
The TPS40040/1 uses a very fast break-before-make anti-cross conduction circuit to minimize power loss.
Adding external impedance in series with the gates of the switching MOSFETs adversely affects the converter’s
operation and must be avoided. The loop impedance (HDRV-to-gate plus source-to-SW and LDRV-to-SR gate
plus SR source-to-GND) should be kept to less than 20 nH to avoid possible cross-conduction. The HDRV and
LDRV connections should widen to 20 mils as soon as possible out from the device pin.
The return for the main switching MOSFET gate drive is the SW pin of the TPS40040/1. The SW pin should be
routed to the source of the main switching FET with at least a 20-mils wide trace as close to the HDRV trace as
possible to minimize loop impedance.
The return for the SR MOSFET gate drive is the TPS40040/1 GND pad. The GND pad should be connected
directly to the source of the SR with at least a 20-mil wide trace directly under the LDRV trace. Use a minimum of
2 parallel vias to connect the GND pad to the source of the SR if multiple layers are used.
A small, less than 3-Ω resistor may be added in series with the BOOT pin to slow the turn-on of the upper switch
MOSFET, thereby reducing the rising edge slew-rate of the switch node. In turn, this reduces EMI, increases
upper MOSFET OFF to SR ON dead time, and minimizes induced dV/dt turn-on of the SR when the upper switch
MOSFET turns on. It is recommended customers make provisions on their boards for this resistor and not use
resistors in series with MOSFET gate leads.
VDD Filtering
A ceramic capacitor, 1 µF minimum, must be placed as close to the VDD pin and GND pad as possible with a
15-mil wide (or greater) trace. If used, a small series connected resistor (1 Ω to 2 Ω) may be placed less than
100 mils from the TPS40040/1 between the supply input voltage and the VDD pin to further reduce switching
noise on the VDD pin.
NOTE:
The voltage drop across this resistor affects the level at which the over-current circuit
operates by filtering the sensed VDD voltage.
Device Connections
If a current limit resistor is used (COMP to GND), it must be placed within 100 mils of the COMP pin to limit noise
injection into the PWM comparator. Compensation components (feedback divider, and associated error amplifier
components) should be placed over a signal ground island connected to the power ground at the GND pad
through a 10-mil wide trace. If multiple layers are used, connect to GND through a single via on an internal layer
opposite the connection to the source of the synchronous rectifier.
PowerPAD™ Layout
The PowerPAD™ package provides low thermal impedance for heat removal from the device. The PowerPAD™
derives its name and low thermal impedance from the large bonding pad on the bottom of the device. The circuit
board must have an area of solder-tinned-copper underneath the package. The dimensions of this area depend
on the size of the PowerPAD™ package. See PCB Layout Guidelines for further information.
Thermal vias connect this area to internal or external copper planes and should have a drill diameter sufficiently
small so that the via hole is effectively plugged when the barrel of the via is plated with copper. This plug is
needed to prevent wicking the solder away from the interface between the package body and the solder-tinned
area under the device during solder reflow. Drill diameters of 0.33 mm (13 mils) works well when 1-oz copper is
plated at the surface of the board while simultaneously plating the barrel of the via. If the thermal vias are not
plugged when the copper plating is performed, then a solder mask material should be used to cap the vias with a
diameter equal to the via diameter plus 0.1 mm minimum. This capping prevents the solder from being wicked
through the thermal vias and potentially creating a solder void under the package. Refer to PowerPAD™
Thermally Enhanced Package[2] for more information on the PowerPAD™ package.
18
TPS40040, TPS40041
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SLUS700B–MARCH 2006–REVISED MARCH 2006
DESIGN EXAMPLES
Example 1. A 5-V to 1.8-V DC-to-DC Converter Using a TPS40041
The following example illustrates the design process and component selection for a 5-V to 1.8-V point-of-load
synchronous buck converter. The design goal parameters are given in the table below. A list of symbol definitions
is found at the end of this section.
Design Goal Parameters
SYMBOL
VIN
PARAMETER
Input voltage
TEST CONDITION
MIN
TYP
MAX
UNIT
V
4.5
5.5
75
VINripple
VOUT
Input ripple
IOUT = 6 A
mV
V
Output voltage
Line regulation
Load regulation
Output ripple
Transient deviation
Output current
Switching frequency
Size
IOUT = 0 A, VIN = 5 V
VIN = 4.5 A to 5.5 V
IOUT = 0 A to 6 A
1.764
1.8
1.836
0.5%
0.5%
VRIPPLE
VTRANS
IOUT
IOUT = 6 A
36
6
mV
IOUT = 1 A to 5 A, IOUT = 5 A to 1 A
VIN = 4.5 V to 5.5 V
50
0
A
FSW
600
kHz
In2
1
For this example, the schematic shown in Figure 23 is used. The TPS40041, with FSW = 600 kHz, is selected to
reduce inductor and capacitor sizes.
EN
EN
Figure 23. TPS40041 Sample Schematic
19
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SLUS700B–MARCH 2006–REVISED MARCH 2006
Inductor Selection
The inductor is typically sized for 30% peak-to-peak ripple current (IRIPPLE) Given this target ripple current, the
required inductor size is calculated by:
V
* V
V
IN(max)
OUT
OUT
1
L ń
0.3 I
V
F
OUT
IN(max)
SW
(4)
Solving with VIN(max) = 5.5 V, an inductor value of 1.12 µH is obtained. A standard value of 1.0 µH is selected,
resulting in 2-A peak-peak ripple. The RMS current through the inductor is approximated by the equation:
1
12
1
12
I
* I
I
* I
I
RIPPLE
L(rms)
L(avg)
RIPPLE
OUT
(5)
Using Equation 5, the maximum RMS current in the inductor is about 6.15 A
Output Capacitor Selection (C8 & C9)
The selection of the output capacitor is typically driven by the output load transient response requirement.
Equation 6 and Equation 7 estimate the output capacitance required for a given output voltage transient
deviation.
2
I
L
V
TRAN(max)
C
ń
when V
t 2 V
IN(min) OUT
OUT(min)
ǓV
* V
IN(min)
OUT
TRAN
(6)
(7)
2
I
L
TRAN(max)
C
when V
ń 2 V
IN(min) OUT
* t Ǔ
OUT(min)
V
V
OUT
TRAN
For this example, Equation 6 is used in calculating the minimum output capacitance.
Based on a 4-A load transient with a maximum 50-mV deviation, a minimum of 178-µF output of capacitance is
required.
The output ripple is divided into two components. The first is the ripple voltage generated by inductor ripple
current flowing through the output capacitor's capacitance, and the second is the voltage generated by the ripple
current flowing in the output capacitor's ESR. The maximum allowable ESR is then determined by the maximum
ripple voltage and is approximated by:
I
RIPPLE
F
SW
* t Ǔ
V
RIPPLE(total)
C
V
* V
RIPPLE(cap)
OUT
RIPPLE(total)
ESR
ń
ń
MAX
I
I
RIPPLE
RIPPLE
(8)
Based on 178 µF of capacitance, 2-A ripple current, 600-kHz switching frequency and a design goal of 36-mV
ripple voltage, we calculate a capacitive ripple component of 18.7 mV and a maximum ESR of 8.6 mΩ. Two
1206, 100-µF, 6.3-V, X5R ceramic capacitors are selected to provide significantly less than 8.6 mΩ of ESR.
20
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Peak Current Rating of Inductor
With output capacitance known, it is now possible to calculate the charging current during start-up and determine
the minimum saturation current rating for the inductor. The start-up charging current is approximated by:
V
C
OUT
OUT
I
*
CHARGE
T
SS
(9)
Using the TPS40041’s fixed 4.5-ms soft-start time, COUT = 200 µF and VOUT = 1.8 V, ICHARGE is found to be 80
mA. The peak current rating of the inductor is now found by:
1 ń
t
I
CHARGE
L
* I
I
L(peak)
OUT(max)
RIPPLE
2
(10)
The inductor requirements are summarized in the table below.
Inductor Requirements
PARAMETER
SYMBOL
L
VALUE
1.0
UNITS
µH
Inductance
RMS current (thermal rating)
Peak current (saturation rating)
IL(rms)
IL(peak)
6.15
A
7.08
A PG0083.102, 1.0 µH is selected for its small size, low DCR and high current handling capability.
Input Capacitor Selection (C1 & C2)
The input voltage ripple is divided between capacitance and ESR. For this design, VRIPPLE(CAP) = 50 mV and
VRIPPLE(ESR) = 25 mV. The minimum capacitance and maximum ESR are estimated by:
I
V
LOAD
OUT
C
*
IN(min)
V
V F
IN SW
RIPPLE(cap)
(11)
(12)
V
RIPPLE(ESR)
ESR
*
MAX
1 ń RIPPLEt
I
I
LOAD
2
For this design, CIN > 120 µF and ESR < 3.5 mΩ. The RMS current in the input capacitors is estimated by:
V
V
I
OUT OUT
OUT
1
12
ǓI
RIPPLE
t ǓI
Ǹ
I
t I
ń I
*
ń
RMS(cin)
IN(rms)
IN(avg)
OUT
V
V
IN
IN
(13)
With VIN = VIN(max), the input capacitors must support a ripple current of 1.56 ARMS. Two 1206, 100-µF, X5R
ceramic capacitors with about 5-mΩ ESR and a 2-A RMS current rating are selected. It is important to check the
dc bias voltage derating curves to ensure the capacitors provide sufficient capacitance at the working voltage.
21
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MOSFET Switch Selection (Q1 & Q2)
The switching losses for the upper switch MOSFET are estimated by:
I
V
LOAD
OUT
C
*
IN(min)
V
V F
IN SW
RIPPLE(cap)
(14)
For this design, switching losses are higher at low input voltage due to the lower gate drive current. Designing for
1 W of total losses in both MOSFETS and 20% of the total MOSFET losses in switching losses, we can estimate
our maximum gate-to-drain charge for the design at:
P
V
ń V
t
G1SW
DD
1
Q
* Q
t
GD_Q1
GS2_Q1
V
I
R
F
IN
OUT
DRIVE
SW
(15)
For a low-gate threshold MOSFET, and the TPS40041’s 5 Ω and 3 Ω drive resistances, we estimate a maximum
QGS2+QGD of 10.8 nC.
The conduction losses in the upper switch MOSFET are estimated by the RMS current through the MOSFET
times its RDS(on)
:
2
V
OUT
1
12
t
Ǔ
t
RIPPLE Ǔ
P
ń D I
*
I
R
ń
I
R
L(rms) DS(on_Q1)
CON_Q1
OUT
DS(on)
V
IN
(16)
Estimating about 30% of total MOSFET losses to be high-side conduction losses, the maximum RDS(on) of the
high-side MOSFET can be estimated by:
P
CON_Q1
R
*
DS(on_Q1)
V
2
OUT
I
L(rms)
V
IN
(17)
For this design, with IL_RMS = 6 ARMS and 4.5 V to 1.8 V, RDS(on_Q1) is < 19.5 mΩ for the upper switch MOSFET.
Estimating 50% of total MOSFET losses are in the SR as conduction losses, repeat equation 14. Then calculate
the maximum RDS(on) of the SR by the equation:
P
CON_Q2
R
ń
DS(on_Q2)
V
2
OUT
t Ǔ
I
1 *
L(rms)
V
IN
(18)
For this design IL_RMS = 6 A at 5.5 V to 1.8 V RDS(on_Q2) < 19.6 mΩ. The table below summarizes the MOSFET
requirements.
MOSFET Requirements
PARAMETER
SYMBOL
RDS(on_Q1)
QGS2_Q1 +QGD_Q1
RDS(on_Q2)
VALUE
19.5
UNITS
mΩ
High-side FET RDS(on)
High-side FET turn-on charge
Low-side FET RDS(on)
10.8
19.6
nC
mΩ
IRF7910 has an RDSON(max) of 15 mΩ at 4.5-V gate drive,QGD of 6.2 nC, and QGS2 of 2 nC.
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Bootstrap Capacitor (C7)
To ensure proper charging of the upper switch MOSFET gate, limit the ripple voltage on the bootstrap capacitor
to < 5% of the minimum gate drive voltage of 3.0 V.
20 Q
GS_Q1
C
*
BOOST
V
IN(min)
(19)
Based on the IRF7910 MOSFET with a maximum total gate charge of 26 nC, calculate a minimum of 116 nF of
capacitance. The next higher standard value of 220 nF is selected.
VDD Bypass Capacitor (C6)
Select a 1.0-µF ceramic bypass capacitor for VDD.
VDD Filter Resistor (R7)
An optional resistor in series with VDD helps filter switching noise from the device. Driving the two IRF7910
MOSFETs, with a typical total QG of 17 nC each, we calculate a maximum IDD current of 22 mA. The result of
equation 19, leads to selecting a 1-Ω resistor, and limits the voltage drop across this resistor to less than 25 mV.
V
RVDD(max)
25 mV
R
ń
*
VDD
I
t
2 mA Q
G_Q2ǓFSW
DD
Q
G_Q1
(20)
Short Circuit Protection (R2)
The TPS40040/1 use the forward drop across the upper switch MOSFET during the ON time to measure the
inductor current. The voltage drop across the high-side MOSFET is given by:
20 Q
GS_Q1
C
*
BOOST
V
IN(min)
(21)
When VIN = 4.5 V to 5.5 V, IL_PEAK = 7.2A. Using the IRF7910 MOSFET, we calculate the peak voltage drop to
be 108 mV. The TPS40041’s internal 3100-ppm temperature coefficient helps compensate for the MOSFET’s
RDS(on) temperature coefficient. For this design, select the short circuit protection voltage threshold of 180 mV by
selecting R2 = OPEN.
23
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SLUS700B–MARCH 2006–REVISED MARCH 2006
Feedback Loop Design
To design feedback circuit, a small signal average modeling technique is employed. Further information on this
technique may be found in the references.
Modeling the Power Stage
The peak-to-peak ramp voltage given in the Electrical Specification table allows the modulator gain to be
calculated as:
V
IN
A
*
MOD
V
RAMP(p p)
(22)
For this design, a modulator gain of 7.3 (17.3 dB) is calculated.
The LC filter applies a double pole at the resonance frequency:
1
F
*
RES
ń
2 p L C
(23)
For this design, the resonance frequency is about 11.3 kHz. Below this frequency, the power stage has the dc
gain of 17.3 dB and above this frequency the power stage gain drops off at -40 dB per decade. The ESR zero is
approximated by:
1
F
*
ESR
2 p C
R
ESR
OUT
(24)
For COUT = 2 x 100 µF and RESR = 2.5 mΩ FESR = 318 kHz. This is greater than 1/5th the switching frequency
and outside the scope of the error amplifier design. The gain of the power stage would change to -20 dB per
decade above FESR. The straight line approximation the power stage gain is approximated in Figure 24.
F
RES
A
−40dB/dec
−20dB/dec
MOD
0dB
F
ESR
Frequency (Log Scale)
Figure 24. Power Stage Frequency Response Straight Line Approximation
Feedback Divider (R4, R5 & R8)
Select R8 be between 10 kΩ and 100 kΩ. For this design, select 20 kΩ. Next, R5 is selected to produce the
desired output voltage when VFB = 0.600 V using the following formula.
V
R8
FB
R5 in paralell with R4 ń
V
* V
OUT
FB
(25)
VFB = 0.600 V and R8 = 20 kΩ for VOUT = 1.8 V, R5 = 10 kΩ. If the calculated value is not a standard resistor,
select a slightly higher resistor value and add R4 in parallel to reduce the parallel combination of R4 and R5 to
produce desired output voltage.
24
TPS40040, TPS40041
www.ti.com
SLUS700B–MARCH 2006–REVISED MARCH 2006
Error Amplifier Pole-Zero Selection
Place two zeros at 80% and 125% of the resonance frequency to keep the actual resonance frequency between
the two zeros over the L and C tolerance. For FRES = 11.3 kHz, FZ1 = 9.0 kHz and FZ2 = 14 kHz. Selecting the
cross-over frequency (FCO) of the control loop between 3 times the LC filter resonance and 1/5th the switching
frequency. For most applications 1/10th the switching frequency provides a good balance between ease of
design and fast transient response.
If FESR < FCO; FP1 = ½ FCO and FP2 = 2x FCO
If FESR > 2x FCO; FP1 = FCO and FP2 = 4x FCO
For this design with FSW = 600 kHz, FRES = 11.3 kHz and FESR = 318 kHz.
FCO = 60 kHz and since FESR > 2x FCO, FP1 = FCO and FP2 = 4x FCO
.
.
.
Since FCO < FESR the power stage gain at the desired cross-over can be approximated by:
F
CO
* 40 LOGt Ǔ
A
ń A
MOD
PS(fcc)
F
RES
(26)
APS(FCO) = -11.7 dB, so the error amplifier gain between the two poles should be 10(11.7/20) = 3.84.
If the error amplifier gain is greater than 0 dB at FSW, the converter can achieve a stable bi-modal operation with
duty cycles alternating between two stable values, and the output regulated with a output ripple component at ½
FSW. To prevent this effect, check FP2 by the equation:
F
SW
F
P2(max)
A
MID(band)
(27)
Since FP2 > FP2(max), it is possible for this control loop to obtain bi-modal operation. To prevent this bi-modal
operation, reduce FCO and re-calculate APC(FCO), FP1, and FP2(max)
.
Now, FCO = 50 kHz, AMID-BAND = 2.67, FP1 = 50 kHz and FP2 = 200 kHz.
The table below summarizes the error amplifier compensation network design criteria.
Error Amplifier Compensation Network
PARAMETER
SYMBOL
VALUE
UNITS
First zero frequency
Second zero frequency
First pole frequency
Second pole frequency
Mid-band gain
FZ1
FZ2
FP1
FP2
9
14
kHz
50
200
2.67
AMID-BAND
V/V
25
TPS40040, TPS40041
www.ti.com
SLUS700B–MARCH 2006–REVISED MARCH 2006
Feedback Components (R3, R6, C3, C4, C5)
Approximate C5 with the formula:
1
C5 *
2 p R8 F
Z2
(28)
(29)
(30)
C5 = 560 pF (closest standard capacitor value to calculated 568 pF) and approximate R6 with the formula:
1
R6 *
2 p C5 F
P1
R6 = 4.75 kΩ (closest standard resistor value to calculated 4.74 kΩ) Calculate R3 by the formulae:
( )
R6 R8
A
MID(band)
R3 ń
R6 * R8
With AMID_BAND = 3.84, R6 = 4.75 kΩ and R8 = 20 kΩ, R3 = 14.7 kΩ (closest standard resistor value to calculated
14.7 kΩ) Calculate C3 and C4 by the equations:
1
C4 *
2 p R3 F
Z1
(31)
1
C3 *
2 p R3 F
P2
(32)
For R3 = 14.7 kΩ, C3 = 47 pF (closest standard value to 45 pF) C4 = 1200 pF (closest standard value to 1.2 nF)
Error Amplifier straight line approximation transfer function looks like Figure 25.
F
F
P2
P1
A
mid−Band
0dB
F
F
F
Z2
SW
Z1
Frequency (Log Scale)
Figure 25. Error Amplifier Frequency Response Straight Line Approximation
26
TPS40040, TPS40041
www.ti.com
SLUS700B–MARCH 2006–REVISED MARCH 2006
100%
90%
80%
4.5
5
5.5
70%
60%
50%
0
1
2
3
4
5
6
7
I
− Load Current − A
OUT
Figure 26. Typical Efficency for 5-V to 1.8-V at 6-A Converter Using TPS40041
1.818
1.816
1.814
4.5
5
1.812
1.810
5.5
1.808
1.806
1.804
1.802
1.800
0
1
2
3
4
5
6
7
I
− Load Current − A
OUT
Figure 27. Typical Line/Load Regulation for 5-V to 1.8-V at 6-A Converter Using TPS40041
27
TPS40040, TPS40041
www.ti.com
SLUS700B–MARCH 2006–REVISED MARCH 2006
List of Materials
REF
C1
C2
C3
C4
C5
C6
C7
C8
C9
L1
QTY
1
DESCRIPTION
MFR
TDK
TDK
TDK
TDK
TDK
TDK
TDK
TDK
TDK
Pulse
IR
PART NUMBER
C325X5R0J107M
C3225X5R0J107M
C1005C01H271M
C1005X7R1H152M
C1005X7R1H561M
C1005X7R0J105M
C1005X7R0J224M
C3225X5R0J107M
C3225X5R0J107M
PG0083.102
IRF7311
Capacitor, ceramic, 6.3 V, X5R, 20%, 100 µF, 1210
Capacitor, ceramic, 6.3 V, X5R, 20%, 100 µF, 1210
Capacitor, ceramic, 50 V, X7R, 20%, 270pF, 0402
Capacitor, ceramic, 50 V, X7R, 20%, 1500 pF, 0402
Capacitor, ceramic, 50 V, X7R, 20%, 560 pF, 0402
Capacitor, ceramic, 6.3 V, X5R, 20%, 1.0 µF, 0402
Capacitor, ceramic, 6.3 V, X5R, 20%, 0.22 µF, 0402
Capacitor, ceramic, 6.3 V, X5R, 20%, 100 µF, 1210
Capacitor, ceramic, 6.3 V, X5R, 20%, 100 µF, 1210
Inductor, SMT, 1.0 µH, 12 A, 6.6 mΩ, ED1514, 0.268 x 0.268
MOSFET, dual N-channel, 20 V, 6.6 A, 29 mΩ, 1.0 µH, SO8
Resistor, chip, 1/16 W, %, IRF7910, 0402
1
1
1
1
1
1
1
1
1
Q2
R2
R3
R4
R5
R6
R7
R8
1
1
Std
Std
1
Resistor, chip, 1/16 W, 1%, OPEN, 0402
Std
Std
1
Resistor, chip, 1/16 W, 1%, 11.8 kΩ, 0402
Std
Std
1
Resistor, chip, 1/16 W, 1%, OPEN, 0402
Std
Std
1
Resistor, chip, k 1/1 W, 1%, 10.0 kΩ, 0402
Std
Std
1
Resistor, chip, k, 1/16 W, 1%, 5.62 kΩ, 0402
Resistor, chip, k 1/16 W, 1%, 20 kΩ, 0402
Std
Std
1
Std
Std
Device, Low Voltage DC to DC Synchronous Buck Controller,
TPS40041DRB, SON-8P
U1
1
TPS40041DRB
TI
Active High Enable Circuit
R1
Q1
1
1
Resistor, chip, 100 kΩ, 1/16 W, 1%, 100 kΩ, 0402
Std
Std
Mosfet, N-channel, VDS 60 V, RDS 2 Ω, ID 115 mA,
2N7002W, SOT-323 (SC-70)
2N7002W-7
Diodes Inc
28
TPS40040, TPS40041
www.ti.com
SLUS700B–MARCH 2006–REVISED MARCH 2006
Definition of Symbols
SYMBOL
DESCRIPTION
VIN(max)
Maximum operating input voltage
VIN(min)
VINRIPPLE
VOUT
Minimum operating input voltage
Peak-to-peak ac ripple voltage on VIN
Target output voltage
VOUTRIPPLE
IOUT(max)
IRIPPLE
IL_PEAK
IL_RMS
IRMS_CIN
FSW
Peak-to-peak ac ripple voltage on VOUT
Maximum operating load current
Peak-to-peak ripple current through the output filter inductor
Peak ripple current through the output filter inductor
Root mean squared current through the output filter inductor
Root mean squared current in input capacitor
Switching frequency
FCO
Desired control loop cross-over frequency
AMOD
Low frequency gain of the pulse width modulator
VCONTROL
FRES
PWM control voltage (error amplifier output voltage - VCOMP
L-C filter resonant frequency
)
FESR
Output capacitors’ ESR zero frequency
FP1
First pole frequency in error amplifier compensation
Second pole frequency in error amplifier compensation
First zero frequency in error amplifier compensation
Second pole frequency in error amplifier compensation
Total gate charge of upper switch MOSFET
FP2
FZ1
FZ2
QG1_Q1
QG2_Q2
RDS(on_Q1)
RDS(on_Q2)
PCON_Q1
PSW_Q1
PCON_Q2
QGD_Q1
QGS2_Q1
Total gate charge of synchronous rectifier MOSFET
“ON” drain-to-source resistance of upper switch MOSFET
“ON” drain-to-source resistance of synchronous rectifier MOSEFT
Conduction losses in upper switch MOSFET
Switching losses in upper switch MOSFET
Conduction losses in synchronous rectifier MOSFET
Gate-to-drain charge of upper switch MOSFET
Post threshold gate-to-source charge of the upper switch MOSFET. (Estimate from QG vs. VGS if not provided in
MOSFET data sheet)
VFB
Internal reference voltage as measured on FB pin.
Slope of internal PWM ramp
VRAMP_slope
APS(Fco)
AMID-BAND
VCOMP to VOUT gain at desired loop cross-over frequency. (dB)
VOUT to VCOMP gain at desired loop cross-over frequency (V/V)
29
TPS40040, TPS40041
www.ti.com
SLUS700B–MARCH 2006–REVISED MARCH 2006
Example 2. A 2.5-V to 1.2-V DC-to-DC Converter Using a TPS40041
This example illustrates a 2.5-V to 1.2-V at 3-A synchronous buck application using the TPS40041. A diode has
been added to increase the bootstrap capacitor charging current at low input voltage. The highest current limit
threshold has been selected due to the increased RDS(on) at low input voltages.
Figure 28. Schematic for 2.5-V to 1.2-V at 3-A Converter Using the TPS40041
1.210
100%
90%
2.25
2.5
2.75
1.208
1.206
1.204
80%
70%
2.25
2.5
2.75
1.202
1.200
60%
50%
1.198
0.0
0.5
1.0
I
1.5
2.0
2.5
3.0
3.5
0.0
0.5
1.0
I
1.5
2.0
2.5
3.0
3.5
− Load Current − A
− Load Current − A
OUT
OUT
Figure 29. Typical Efficency for 2.5-V to 1.2-V at 3-A
Converter Using TPS40041
Figure 30. Typical Line/Load Regulation for 2.5-V to 1.2-V
at 3-A Converter Using TPS40041
30
TPS40040, TPS40041
www.ti.com
SLUS700B–MARCH 2006–REVISED MARCH 2006
Example 3. A 3.3-V to 1.2-V DC-to-DC Converter Using a TPS40040
This example illustrates a 3.3-V to 1.2-V at 10-A synchronous BUCK application using the TPS40040 switching
at 300 kHz. Separate SO-8 MOSFETs have been chosen to support the higher currents in this application and a
resistor has been added in series with the BOOT pin to slow the rising edge of the switch node and reduce EMI
on the input of the converter.
Figure 31. Schematic for 3.3-V to 1.2-V at 10-A Converter Using the TPS40040
100
1.217
3.3
3
3.6
1.212
1.207
1.202
90
80
70
60
3
3.3
3.6
1.197
1.192
1.187
1.182
0
2
4
6
8
10
12
50
I
− Load Current − A
0
2
4
6
8
10
12
OUT
I
− Load Current − A
OUT
Figure 32. Typical Efficiency for 3.3-V to 1.2-V at 10-A
Converter Using TPS40040
Figure 33. Typicaly Line and Load Regulation for 3.3-V to
1.2-V at 10-A Converter Using TPS40040
31
TPS40040, TPS40041
www.ti.com
SLUS700B–MARCH 2006–REVISED MARCH 2006
ADDITIONAL REFERENCES
Related Parts
The following parts have characteristics similar to the TPS40040/1 and may be of interest.
Related Parts
DEVICE
TPS40007/9
TPS40021
TPS40190
DESCRIPTION
Low Voltage Synchronous Buck Controller with Predictive Gate Drive®
Full Featured Low Voltage Synchronous Buck Controller with Predictive GateTM Drive
Cost Optimized Mid Voltage Synchronous Buck Controller
References
These references may be found on the web at www.power.ti.com under Technical Documents. Many design
tools and links to additional references, including design software, may also be found at www.power.ti.com
1. Under The Hood Of Low Voltage DC/DC Converters, SEM1500 Topic 5, 2002 Seminar Series
2. Understanding Buck Power Stages in Switchmode Power Supplies, SLVA057, March 1999
3. Design and Application Guide for High Speed MOSFET Gate Drive Circuits, SEM 1400, 2001 Seminar
Series
4. Designing Stable Control Loops, SEM 1400, 2001 Seminar Series
5. Additional PowerPADTM information may be found in Applications Briefs SLMA002 and SLMA004
6. QFN/SON PCB Attachment, Texas Instruments Literature Number SLUA271, June 2002
Package Outline
The page following outlines the mechanical dimensions of the DRB package.
Recommended PCB Footprint
The second page following outlines the recommended PCB layout.
32
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