TI1 LM2672LDX-ADJ Power converter high efficiency 1a step-down voltage regulator with feature Datasheet

TI Confidential - NDA Restrictions
LM2731
www.ti.com
SNVS217F – MAY 2004 – REVISED NOVEMBER 2012
LM2731 0.6/1.6 MHz Boost Converters With 22V Internal FET Switch in SOT-23
Check for Samples: LM2731
FEATURES
DESCRIPTION
•
•
The LM2731 switching regulators are current-mode
boost converters operating at fixed frequencies of 1.6
MHz (“X” option) and 600 kHz (“Y” option).
1
2
•
•
•
•
•
•
•
•
22V DMOS FET Switch
1.6 MHz (“X”), 0.6 MHz (“Y”) Switching
Frequency
Low RDS(ON) DMOS FET
Switch Current up to 1.8A
Wide Input Voltage Range (2.7V–14V)
Low Shutdown Current (<1 µA)
5-Lead SOT-23 Package
Uses Tiny Capacitors and Inductors
Cycle-by-Cycle Current Limiting
Internally Compensated
The use of SOT-23 package, made possible by the
minimal power loss of the internal 1.8A switch, and
use of small inductors and capacitors result in the
industry's highest power density. The 22V internal
switch makes these solutions perfect for boosting to
voltages up to 20V.
These parts have a logic-level shutdown pin that can
be used to reduce quiescent current and extend
battery life.
Protection is provided through cycle-by-cycle current
limiting and thermal shutdown. Internal compensation
simplifies design and reduces component count.
APPLICATIONS
•
•
•
•
•
White LED Current Source
PDA’s and Palm-Top Computers
Digital Cameras
Portable Phones and Games
Local Boost Regulator
Table 1. Switch Frequency
X
Y
1.6 MHz
0.6 MHz
Typical Application Circuit
VIN
SHDN
R3
51K
U1
SW
LM2731 ³;´
GND
R1/117K
FB
SHDN
GND
C1
2.2PF
100
R2
13.3K
CF
220pF
12V
OUT
500mA
(TYP)
EFFICIENCY (%)
5 VIN
Efficiency vs Load Current
D1
MBR0520
L1/10PH
C2
4.7PF
5 - 12V Boost
^y_ s Υ]}v
90
80
70
0 100
200 300 400 500
LOAD CURRENT (mA)
VIN
SHDN
R3
51K
U1
GND
SW
LM2731 ³<´
SHDN
GND
C1
2.2PF
100
R1/40.5K
FB
R2
13.3K
CF
470pF
5V
OUT
700mA
(TYP)
C2
22PF
EFFICIENCY (%)
3.3 VIN
Efficiency vs Load Current
D1
MBR0520
L1/6.8PH
3.3 -5V Boost
^z_ s Υ]}v
90
80
70
0
200
400
600 800
LOAD CURRENT (mA)
1
2
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.
All trademarks are the property of their respective owners.
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 © 2004–2012, Texas Instruments Incorporated
TI Confidential - NDA Restrictions
LM2731
SNVS217F – MAY 2004 – REVISED NOVEMBER 2012
www.ti.com
Efficiency vs Load Current
U1
VIN
R3
51K
SW
R1/117K
LM2731 ³<´
FB
SHDN
GND
GND
EFFICIENCY (%)
3.3 VIN
SHDN
D1
MBR0520
L1/6.8PH
C1
2.2PF
CF
270pF
R2
13.3K
12V
OUT
230mA
(TYP)
C2
10PF
100
90
80
70
60
50
40
30
20
10
0
3.3 -12V Boost
^z_ s Υ]}v
50 100 150 200 250
0
LOAD (mA)
R3
51K
U1
GND
SW
R1/84K
LM2731 ³;´
SHDN
GND
C1
2.2PF
Efficiency vs Load Current
9V OUT
240mA (typ)
FB
CF
330pF
R2
13.3K
C2
4.7PF
D2
D4
D3
D5
R4
EFFICIENCY (%)
VIN
SHDN
D1
MBR0520
L1/10PH
3.3 VIN
R5
100
90
80
70
60
50
40
30
20
10
0
3.3 -9V
^y_ s Υ]}v
0
50 100 150 200 250 300
LOAD (mA)
B1
LI-ION
3.3 - 4.2V
L1 / 1.5 PH
VIN
+
-
R3
51K
FLASH
ENABLE
SW
LM2731"Y"
SHDN
GND
0
D1
MBR0520
C1
4.7PF
FB
WHITE
LED's C2
4.7PF
R2
120
Figure 1. White LED Flash Application
Connection Diagram
Top View
Figure 2. 5-Lead SOT-23 Package
See Package Number DBV0005A
2
Submit Documentation Feedback
Copyright © 2004–2012, Texas Instruments Incorporated
Product Folder Links: LM2731
TI Confidential - NDA Restrictions
LM2731
www.ti.com
SNVS217F – MAY 2004 – REVISED NOVEMBER 2012
PIN DESCRIPTIONS
Pin
Name
1
SW
2
GND
3
FB
4
SHDN
5
VIN
Function
Drain of the internal FET switch.
Analog and power ground.
Feedback point that connects to external resistive divider.
Shutdown control input. Connect to Vin if the feature is not used.
Analog and power input.
These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam
during storage or handling to prevent electrostatic damage to the MOS gates.
Absolute Maximum Ratings (1)
Storage Temperature Range
−65°C to +150°C
Operating Junction Temperature Range
−40°C to +125°C
Lead Temp. (Soldering, 5 sec.)
300°C
Power Dissipation (2)
Internally Limited
FB Pin Voltage
−0.4V to +6V
SW Pin Voltage
−0.4V to +22V
−0.4V to +14.5V
Input Supply Voltage
−0.4V to VIN + 0.3V
SHDN Pin Voltage
θJA (SOT23-5)
ESD Rating (3)
(1)
(2)
(3)
265°C/W
Human Body Model
2 kV
Absolute Maximum Ratings indicate limits beyond which damage to the component may occur. Electrical specifications do not apply
when operating the device outside of the limits set forth under the operating ratings which specify the intended range of operating
conditions.
The maximum power dissipation which can be safely dissipated for any application is a function of the maximum junction temperature,
TJ(MAX) = 125°C, the junction-to-ambient thermal resistance for the SOT-23 package, θJ-A = 265°C/W, and the ambient temperature,
TA. The maximum allowable power dissipation at any ambient temperature for designs using this device can be calculated using the
formula:
. If power dissipation exceeds the maximum specified above, the internal thermal protection
circuitry will protect the device by reducing the output voltage as required to maintain a safe junction temperature.
The human body model is a 100 pF capacitor discharged through a 1.5 kΩ resistor into each pin.
Submit Documentation Feedback
Copyright © 2004–2012, Texas Instruments Incorporated
Product Folder Links: LM2731
3
TI Confidential - NDA Restrictions
LM2731
SNVS217F – MAY 2004 – REVISED NOVEMBER 2012
www.ti.com
Electrical Characteristics
Limits in standard typeface are for TJ = 25°C, and limits in boldface type apply over the full operating temperature range
(−40°C ≤ TJ ≤ +125°C). Unless otherwise specified: VIN = 5V, VSHDN = 5V, IL = 0A.
Parameter
VIN
Test Conditions
Input Voltage
VOUT
(MIN)
Minimum Output Voltage
Under Load
Min (1)
Typ (2)
2.7
RL = 43Ω
X Option (3)
VIN = 2.7V
5.4
7
VIN = 3.3V
8
10
VIN = 2.7V
6
7.5
VIN = 3.3V
8.75
11
VIN = 5V
RL = 43Ω
Y Option (3)
V
V
15
VIN = 2.7V
3.75
5
VIN = 3.3V
5
6.5
VIN = 5V
RL = 15Ω
Y Option (3)
Units
14
16
VIN = 5V
RL = 15Ω
X Option (3)
Max (1)
10
VIN = 2.7V
4
VIN = 3.3V
5.5
VIN = 5V
5
7
10
ISW
Switch Current Limit
See (4)
RDS(ON)
Switch ON Resistance
ISW = 100 mA
Vin = 5V
260
400
500
ISW = 100 mA
Vin = 3.3V
300
450
550
SHDNTH
Shutdown Threshold
1.8
1.4
Device ON
2
1.5
Device OFF
ISHDN
0.50
Shutdown Pin Bias Current VSHDN = 0
0
VSHDN = 5V
VIN = 3V
A
1.230
1.255
V
nA
IFB
Feedback Pin Bias Current VFB = 1.23V
60
500
IQ
Quiescent Current
VSHDN = 5V, Switching "X"
2
3.0
VSHDN = 5V, Switching "Y"
1.0
2
VSHDN = 5V, Not Switching
400
500
VSHDN = 0
0.024
1
0.02
ΔVFB/ΔVIN
FB Voltage Line
Regulation
2.7V ≤ VIN ≤ 14V
FSW
Switching Frequency (5)
“X” Option
1
1.6
1.85
“Y” Option
0.40
0.60
0.8
“X” Option
78
86
“Y” Option
88
93
IL
(1)
(2)
(3)
(4)
(5)
4
Maximum Duty Cycle (5)
Switch Leakage
Not Switching VSW = 5V
µA
2
Feedback Pin Reference
Voltage
DMAX
V
0
VFB
1.205
mΩ
mA
µA
%/V
MHz
%
1
µA
Limits are guaranteed by testing, statistical correlation, or design.
Typical values are derived from the mean value of a large quantity of samples tested during characterization and represent the most
likely expected value of the parameter at room temperature.
L = 10 µH, COUT = 4.7 µF, duty cycle = maximum
Switch current limit is dependent on duty cycle (see Typical Performance Characteristics).
Guaranteed limits are the same for Vin = 3.3V input.
Submit Documentation Feedback
Copyright © 2004–2012, Texas Instruments Incorporated
Product Folder Links: LM2731
TI Confidential - NDA Restrictions
LM2731
www.ti.com
SNVS217F – MAY 2004 – REVISED NOVEMBER 2012
Typical Performance Characteristics
Unless otherwise specified: VIN = 5V, SHDN pin tied to VIN.
Iq Vin (Active)
vs
Temperature - "Y"
2.2
1.25
2.15
1.2
2.1
1.15
IQ VIN ACTIVE (mA)
IQ VIN ACTIVE (mA)
Iq Vin (Active)
vs
Temperature - "X"
2.05
2
1.95
1.1
1.05
1
1.9
0.95
1.85
1.8
-50
-25
0
25
50
75
0.9
-50
100 125 150
-25
50
75 100 125 150
Figure 3.
Figure 4.
Oscillator Frequency
vs
Temperature - "X"
Oscillator Frequency
vs
Temperature - "Y"
0.6
1.58
VIN = 5V
1.56
OSCILLATOR FREQUENCY (MHz)
OSCILLATOR FREQUENCY (MHz)
25
TEMPERATURE (oC)
TEMPERATURE (oC)
1.54
1.52
VIN = 3.3V
1.5
1.48
1.46
1.44
1.42
1.4
-50
VIN = 5V
0.58
0.56
VIN = 3.3V
0.54
0.52
0.5
0.48
-25
0
25
50
75
-50 -25
100 125 150
TEMPERATURE (oC)
0
25
50
75
100 125 150
TEMPERATURE (oC)
Figure 5.
Figure 6.
Max. Duty Cycle
vs
Temperature - "X"
Max. Duty Cycle
vs
Temperature - "Y"
93
96.8
92.9
96.7
92.8
96.6
MAX DUTY CYCLE (%)
MAX DUTY CYCLE (%)
0
92.7
92.6
VIN = 5V
92.5
92.4
VIN = 3.3V
92.3
92.2
96.5
VIN = 3.3V
96.4
96.3
VIN = 5V
96.2
96.1
96
92.1
-50
-25
0
25
50
75 100 125 150
95.9
-50
-25
0
25
50
75 100 125 150
TEMPERATURE (oC)
o
TEMPERATURE ( C)
Figure 7.
Figure 8.
Submit Documentation Feedback
Copyright © 2004–2012, Texas Instruments Incorporated
Product Folder Links: LM2731
5
TI Confidential - NDA Restrictions
LM2731
SNVS217F – MAY 2004 – REVISED NOVEMBER 2012
www.ti.com
Typical Performance Characteristics (continued)
Unless otherwise specified: VIN = 5V, SHDN pin tied to VIN.
Iq Vin (Idle)
vs
Temperature
Feedback Bias Current
vs
Temperature
0.09
375
0.08
FEEDBACK BIAS CURRENT (PA)
380
IQ VIN (IDLE) (PA)
370
365
360
355
350
345
340
-50
0
-25
50
25
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0
-50
75 100 125 150
-25
75 100 125 150
Figure 10.
Feedback Voltage
vs
Temperature
RDS(ON)
vs
Temperature
1.231
0.5
1.23
0.45
0.4
1.229
Vin = 3.3V
0.35
1.228
1.227
1.226
1.225
0.3
Vin = 5V
0.25
0.2
0.15
1.224
0.1
1.223
0.05
0
-40
0
-25
25
50
75 100 125
-40
0
-25
TEMPERATURE (oC)
25
50
75 100 125
TEMPERATURE (oC)
Figure 11.
Figure 12.
Current Limit
vs
Temperature
RDS(ON)
vs
VIN
350
2.6
300
2.5
250
2.4
RDS_ON (m:)
CURRENT LIMIT (A)
50
25
Figure 9.
1.222
2.3
2.2
200
150
100
2.1
50
2
0
-40
-25
0
25
50
75 100 125
2.5
TEMPERATURE (oC)
3.5
4.5
5.5
6.5
7.5
8.5
9.5
VIN (V)
Figure 13.
6
0
TEMPERATURE (oC)
RDS(ON) (:)
FEEDBACK VOLTAGE (V)
TEMPERATURE (oC)
Figure 14.
Submit Documentation Feedback
Copyright © 2004–2012, Texas Instruments Incorporated
Product Folder Links: LM2731
TI Confidential - NDA Restrictions
LM2731
www.ti.com
SNVS217F – MAY 2004 – REVISED NOVEMBER 2012
Typical Performance Characteristics (continued)
Unless otherwise specified: VIN = 5V, SHDN pin tied to VIN.
Efficiency
vs
Load Current - "X"
VIN = 2.7V, VOUT = 5V
100
100
90
80
80
EFFICIENCY (%)
90
EFFICIENCY (%)
70
60
50
40
70
60
50
40
30
30
20
20
10
10
0
0
100
50
200
150
250
Efficiency
vs
Load Current - "X"
VIN = 3.3V, VOUT = 5V
0
300
0
100
200
LOAD (mA)
100
300
400
500 600
70
0
LOAD (mA)
Figure 15.
Figure 16.
Efficiency
vs
Load Current - "X"
VIN = 4.2V, VOUT = 5V
Efficiency
vs
Load Current - "X"
VIN = 2.7V, VOUT = 12V
80
90
70
60
70
EFFICIENCY (%)
EFFICIENCY (%)
80
60
50
40
30
50
40
30
20
20
10
10
0
0
200
400
0
800 1000 1200 1400
600
0
10
20
30
40
50
LOAD (mA)
LOAD (mA)
Figure 17.
Figure 18.
Efficiency
vs
Load Current - "X"
VIN = 3.3V, VOUT = 12V
Efficiency
vs
Load Current - "X"
VIN = 5V, VOUT = 12V
80
100
70
90
80
EFFICIENCY (%)
EFFICIENCY (%)
60
50
40
30
70
60
50
40
30
20
20
10
10
0
0
0
20
40
60
80
100 120 140 160
0
100
200
300
400
500
600
LOAD (mA)
LOAD (mA)
Figure 19.
Figure 20.
Submit Documentation Feedback
Copyright © 2004–2012, Texas Instruments Incorporated
Product Folder Links: LM2731
7
TI Confidential - NDA Restrictions
LM2731
SNVS217F – MAY 2004 – REVISED NOVEMBER 2012
www.ti.com
Typical Performance Characteristics (continued)
Unless otherwise specified: VIN = 5V, SHDN pin tied to VIN.
Efficiency
vs
Load Current - "X"
VIN = 5V, VOUT = 18V
100
90
90
80
80
EFFICIENCY (%)
EFFICIENCY (%)
100
Efficiency
vs
Load Current - "Y"
VIN = 2.7V, VOUT = 5V
70
60
50
40
70
60
50
40
30
30
20
20
10
10
0
0
50
0
100
150
200
250
300
350
0
100 150 200 250 300 350 400
50
LOAD (mA)
LOAD (mA)
Figure 22.
Efficiency
vs
Load Current - "Y"
VIN = 3.3V, VOUT = 5V
Efficiency
vs
Load Current - "Y"
VIN = 4.2V, VOUT = 5V
100
90
90
80
80
EFFICIENCY (%)
EFFICIENCY (%)
100
Figure 21.
70
60
50
40
70
60
50
40
30
30
20
20
10
10
0
0
100 200 300 400 500 600 700 800
0
0
200
400
LOAD (mA)
Figure 24.
Efficiency
vs
Load Current - "Y"
VIN = 2.7V, VOUT = 12V
Efficiency
vs
Load Current - "Y"
VIN = 3.3V, VOUT = 12V
100
90
90
80
80
70
70
60
50
40
30
50
40
30
10
10
0
0
20
40
60
80
0
LOAD (mA)
50
100
150
200
250
LOAD (mA)
Figure 25.
8
60
20
20
0
800 1000 1200 1400
Figure 23.
EFFICIENCY (%)
EFFICIENCY (%)
100
600
LOAD (mA)
Figure 26.
Submit Documentation Feedback
Copyright © 2004–2012, Texas Instruments Incorporated
Product Folder Links: LM2731
TI Confidential - NDA Restrictions
LM2731
www.ti.com
SNVS217F – MAY 2004 – REVISED NOVEMBER 2012
Typical Performance Characteristics (continued)
Unless otherwise specified: VIN = 5V, SHDN pin tied to VIN.
Efficiency
vs
Load Current - "Y"
VIN = 5V, VOUT = 12V
100
90
EFFICIENCY (%)
80
70
60
50
40
30
20
10
0
0
100
200
300
400
500
600
LOAD (mA)
Figure 27.
BLOCK DIAGRAM
Submit Documentation Feedback
Copyright © 2004–2012, Texas Instruments Incorporated
Product Folder Links: LM2731
9
TI Confidential - NDA Restrictions
LM2731
SNVS217F – MAY 2004 – REVISED NOVEMBER 2012
www.ti.com
THEORY OF OPERATION
The LM2731 is a switching converter IC that operates at a fixed frequency (0.6 or 1.6 MHz) for fast transient
response over a wide input voltage range and incorporates pulse-by-pulse current limiting protection. Because
this is current mode control, a 33 mΩ sense resistor in series with the switch FET is used to provide a voltage
(which is proportional to the FET current) to both the input of the pulse width modulation (PWM) comparator and
the current limit amplifier.
At the beginning of each cycle, the S-R latch turns on the FET. As the current through the FET increases, a
voltage (proportional to this current) is summed with the ramp coming from the ramp generator and then fed into
the input of the PWM comparator. When this voltage exceeds the voltage on the other input (coming from the
Gm amplifier), the latch resets and turns the FET off. Since the signal coming from the Gm amplifier is derived
from the feedback (which samples the voltage at the output), the action of the PWM comparator constantly sets
the correct peak current through the FET to keep the output voltage in regulation.
Q1 and Q2 along with R3 - R6 form a bandgap voltage reference used by the IC to hold the output in regulation.
The currents flowing through Q1 and Q2 will be equal, and the feedback loop will adjust the regulated output to
maintain this. Because of this, the regulated output is always maintained at a voltage level equal to the voltage at
the FB node "multiplied up" by the ratio of the output resistive divider.
The current limit comparator feeds directly into the flip-flop that drives the switch FET. If the FET current reaches
the limit threshold, the FET is turned off and the cycle terminated until the next clock pulse. The current limit
input terminates the pulse regardless of the status of the output of the PWM comparator.
Application Hints
SELECTING THE EXTERNAL CAPACITORS
The best capacitors for use with the LM2731 are multi-layer ceramic capacitors. They have the lowest ESR
(equivalent series resistance) and highest resonance frequency which makes them optimum for use with high
frequency switching converters.
When selecting a ceramic capacitor, only X5R and X7R dielectric types should be used. Other types such as
Z5U and Y5F have such severe loss of capacitance due to effects of temperature variation and applied voltage,
they may provide as little as 20% of rated capacitance in many typical applications. Always consult capacitor
manufacturer’s data curves before selecting a capacitor. High-quality ceramic capacitors can be obtained from
Taiyo-Yuden, AVX, and Murata.
SELECTING THE OUTPUT CAPACITOR
A single ceramic capacitor of value 4.7 µF to 10 µF will provide sufficient output capacitance for most
applications. If larger amounts of capacitance are desired for improved line support and transient response,
tantalum capacitors can be used. Aluminum electrolytics with ultra low ESR such as Sanyo Oscon can be used,
but are usually prohibitively expensive. Typical AI electrolytic capacitors are not suitable for switching frequencies
above 500 kHz due to significant ringing and temperature rise due to self-heating from ripple current. An output
capacitor with excessive ESR can also reduce phase margin and cause instability.
In general, if electrolytics are used, it is recommended that they be paralleled with ceramic capacitors to reduce
ringing, switching losses, and output voltage ripple.
SELECTING THE INPUT CAPACITOR
An input capacitor is required to serve as an energy reservoir for the current which must flow into the coil each
time the switch turns ON. This capacitor must have extremely low ESR, so ceramic is the best choice. We
recommend a nominal value of 2.2 µF, but larger values can be used. Since this capacitor reduces the amount of
voltage ripple seen at the input pin, it also reduces the amount of EMI passed back along that line to other
circuitry.
10
Submit Documentation Feedback
Copyright © 2004–2012, Texas Instruments Incorporated
Product Folder Links: LM2731
TI Confidential - NDA Restrictions
LM2731
www.ti.com
SNVS217F – MAY 2004 – REVISED NOVEMBER 2012
FEED-FORWARD COMPENSATION
Although internally compensated, the feed-forward capacitor Cf is required for stability (see Figure 29). Adding
this capacitor puts a zero in the loop response of the converter. The recommended frequency for the zero fz
should be approximately 6 kHz. Cf can be calculated using the formula:
Cf = 1 / (2 X π X R1 X fz)
(1)
SELECTING DIODES
The external diode used in the typical application should be a Schottky diode. A 20V diode such as the
MBR0520 is recommended.
The MBR05XX series of diodes are designed to handle a maximum average current of 0.5A. For applications
exceeding 0.5A average but less than 1A, a Microsemi UPS5817 can be used.
LAYOUT HINTS
High frequency switching regulators require very careful layout of components in order to get stable operation
and low noise. All components must be as close as possible to the LM2731 device. It is recommended that a 4layer PCB be used so that internal ground planes are available.
As an example, a recommended layout of components is shown:
Figure 28. Recommended PCB Component Layout
Some additional guidelines to be observed:
1. Keep the path between L1, D1, and C2 extremely short. Parasitic trace inductance in series with D1 and C2
will increase noise and ringing.
2. The feedback components R1, R2 and CF must be kept close to the FB pin of U1 to prevent noise injection
on the FB pin trace.
3. If internal ground planes are available (recommended) use vias to connect directly to ground at pin 2 of U1,
as well as the negative sides of capacitors C1 and C2.
SETTING THE OUTPUT VOLTAGE
The output voltage is set using the external resistors R1 and R2 (see Figure 29). A minimum value of 13.3 kΩ is
recommended for R2 to establish a divider current of approximately 92 µA. R1 is calculated using the formula:
R1 = R2 X (VOUT/1.23 − 1)
(2)
Submit Documentation Feedback
Copyright © 2004–2012, Texas Instruments Incorporated
Product Folder Links: LM2731
11
TI Confidential - NDA Restrictions
LM2731
SNVS217F – MAY 2004 – REVISED NOVEMBER 2012
www.ti.com
SWITCHING FREQUENCY
The LM2731 is provided with two switching frequencies: the “X” version is typically 1.6 MHz, while the “Y” version
is typically 600 kHz. The best frequency for a specific application must be determined based on the trade-offs
involved:
Higher switching frequency means the inductors and capacitors can be made smaller and cheaper for a given
output voltage and current. The down side is that efficiency is slightly lower because the fixed switching losses
occur more frequently and become a larger percentage of total power loss. EMI is typically worse at higher
switching frequencies because more EMI energy will be seen in the higher frequency spectrum where most
circuits are more sensitive to such interference.
Figure 29. Basic Application Circuit
DUTY CYCLE
The maximum duty cycle of the switching regulator determines the maximum boost ratio of output-to-input
voltage that the converter can attain in continuous mode of operation. The duty cycle for a given boost
application is defined as:
VOUT + VDIODE - VIN
Duty Cycle =
VOUT + VDIODE - VSW
(3)
This applies for continuous mode operation.
INDUCTANCE VALUE
The first question we are usually asked is: “How small can I make the inductor?” (because they are the largest
sized component and usually the most costly). The answer is not simple and involves trade-offs in performance.
Larger inductors mean less inductor ripple current, which typically means less output voltage ripple (for a given
size of output capacitor). Larger inductors also mean more load power can be delivered because the energy
stored during each switching cycle is:
E = L/2 X (lp)2
(4)
Where “lp” is the peak inductor current. An important point to observe is that the LM2731 will limit its switch
current based on peak current. This means that since lp(max) is fixed, increasing L will increase the maximum
amount of power available to the load. Conversely, using too little inductance may limit the amount of load
current which can be drawn from the output.
Best performance is usually obtained when the converter is operated in “continuous” mode at the load current
range of interest, typically giving better load regulation and less output ripple. Continuous operation is defined as
not allowing the inductor current to drop to zero during the cycle. It should be noted that all boost converters shift
over to discontinuous operation as the output load is reduced far enough, but a larger inductor stays “continuous”
over a wider load current range.
To better understand these trade-offs, a typical application circuit (5V to 12V boost with a 10 µH inductor) will be
analyzed. We will assume:
VIN = 5V, VOUT = 12V, VDIODE = 0.5V, VSW = 0.5V
(5)
Since the frequency is 1.6 MHz (nominal), the period is approximately 0.625 µs. The duty cycle will be 62.5%,
which means the ON time of the switch is 0.390 µs. It should be noted that when the switch is ON, the voltage
across the inductor is approximately 4.5V.
12
Submit Documentation Feedback
Copyright © 2004–2012, Texas Instruments Incorporated
Product Folder Links: LM2731
TI Confidential - NDA Restrictions
LM2731
www.ti.com
SNVS217F – MAY 2004 – REVISED NOVEMBER 2012
Using the equation:
V = L (di/dt)
(6)
We can then calculate the di/dt rate of the inductor which is found to be 0.45 A/µs during the ON time. Using
these facts, we can then show what the inductor current will look like during operation:
Figure 30. 10 µH Inductor Current, 5V–12V Boost (LM2731X)
During the 0.390 µs ON time, the inductor current ramps up 0.176A and ramps down an equal amount during the
OFF time. This is defined as the inductor “ripple current”. It can also be seen that if the load current drops to
about 33 mA, the inductor current will begin touching the zero axis which means it will be in discontinuous mode.
A similar analysis can be performed on any boost converter, to make sure the ripple current is reasonable and
continuous operation will be maintained at the typical load current values.
MAXIMUM SWITCH CURRENT
The maximum FET switch current available before the current limiter cuts in is dependent on duty cycle of the
application. This is illustrated in the graphs below which show typical values of switch current for both the "X" and
"Y" versions as a function of effective (actual) duty cycle:
3000
SW CURRENT LIMIT (mA)
2500
VIN = 5V
2000
VIN = 3.3V
1500
1000
VIN = 2.7V
500
VIN = 3V
0
20
30
40
50
60
70
80
90
100
DUTY CYCLE (%) = [1 - EFF*(VIN / VOUT)]
Figure 31. Switch Current Limit vs Duty Cycle - "X"
3000
SW CURRENT LIMIT (mA)
2500
VIN = 5V
2000
VIN = 3.3V
1500
VIN = 3V
1000
VIN = 2.7V
500
0
20
30
40
50
60
70
80
90
100
DUTY CYCLE (%) = [1 - EFF*(VIN / VOUT)]
Figure 32. Switch Current Limit vs Duty Cycle - "Y"
Submit Documentation Feedback
Copyright © 2004–2012, Texas Instruments Incorporated
Product Folder Links: LM2731
13
TI Confidential - NDA Restrictions
LM2731
SNVS217F – MAY 2004 – REVISED NOVEMBER 2012
www.ti.com
CALCULATING LOAD CURRENT
As shown in the figure which depicts inductor current, the load current is related to the average inductor current
by the relation:
ILOAD = IIND(AVG) x (1 - DC)
(7)
Where "DC" is the duty cycle of the application. The switch current can be found by:
ISW = IIND(AVG) + ½ (IRIPPLE)
(8)
Inductor ripple current is dependent on inductance, duty cycle, input voltage and frequency:
IRIPPLE = DC x (VIN-VSW) / (f x L)
(9)
combining all terms, we can develop an expression which allows the maximum available load current to be
calculated:
ILOAD(max) = (1 - DC) x (ISW(max) - DC (VIN - VSW))
2fL
(10)
The equation shown to calculate maximum load current takes into account the losses in the inductor or turn-OFF
switching losses of the FET and diode. For actual load current in typical applications, we took bench data for
various input and output voltages for both the "X" and "Y" versions of the LM2731 and displayed the maximum
load current available for a typical device in graph form:
MAX LOAD CURRENT (mA)
1200
1000
800
600
VOUT = 5V
400
VOUT = 10V
VOUT = 8V
VOUT = 12V
200
VOUT = 18V
0
2
3
4
5
6
7
8
9
10
11
VIN (V)
Figure 33. Max. Load Current (typ) vs VIN - "X"
MAX LOAD CURRENT (mA)
1200
1000
800
VOUT = 5V
600
VOUT = 8V
400
VOUT = 10V
VOUT = 12V
200
0
2
3
4
5
6
7
8
VIN (V)
Figure 34. Max. Load Current (typ) vs VIN - "Y"
14
Submit Documentation Feedback
Copyright © 2004–2012, Texas Instruments Incorporated
Product Folder Links: LM2731
TI Confidential - NDA Restrictions
LM2731
www.ti.com
SNVS217F – MAY 2004 – REVISED NOVEMBER 2012
DESIGN PARAMETERS VSW AND ISW
The value of the FET "ON" voltage (referred to as VSW in the equations) is dependent on load current. A good
approximation can be obtained by multiplying the "ON Resistance" of the FET times the average inductor
current.
FET on resistance increases at VIN values below 5V, since the internal N-FET has less gate voltage in this input
voltage range (see Typical Performance Characteristics curves). Above VIN = 5V, the FET gate voltage is
internally clamped to 5V.
The maximum peak switch current the device can deliver is dependent on duty cycle. For higher duty cycles, see
Typical Performance Characteristics curves.
THERMAL CONSIDERATIONS
At higher duty cycles, the increased ON time of the FET means the maximum output current will be determined
by power dissipation within the LM2731 FET switch. The switch power dissipation from ON-state conduction is
calculated by:
P(SW) = DC x IIND(AVE)2 x RDS(ON)
(11)
There will be some switching losses as well, so some derating needs to be applied when calculating IC power
dissipation.
INDUCTOR SUPPLIERS
Recommended suppliers of inductors for this product include, but are not limited to Sumida, Coilcraft, Panasonic,
TDK and Murata. When selecting an inductor, make certain that the continuous current rating is high enough to
avoid saturation at peak currents. A suitable core type must be used to minimize core (switching) losses, and
wire power losses must be considered when selecting the current rating.
SHUTDOWN PIN OPERATION
The device is turned off by pulling the shutdown pin low. If this function is not going to be used, the pin should be
tied directly to VIN. If the SHDN function will be needed, a pull-up resistor must be used to VIN (approximately
50k-100kΩ recommended). The SHDN pin must not be left unterminated.
Submit Documentation Feedback
Copyright © 2004–2012, Texas Instruments Incorporated
Product Folder Links: LM2731
15
IMPORTANT NOTICE
Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, enhancements, improvements and other
changes to its semiconductor products and services per JESD46, latest issue, and to discontinue any product or service per JESD48, latest
issue. Buyers should obtain the latest relevant information before placing orders and should verify that such information is current and
complete. All semiconductor products (also referred to herein as “components”) are sold subject to TI’s terms and conditions of sale
supplied at the time of order acknowledgment.
TI warrants performance of its components to the specifications applicable at the time of sale, in accordance with the warranty in TI’s terms
and conditions of sale of semiconductor products. Testing and other quality control techniques are used to the extent TI deems necessary
to support this warranty. Except where mandated by applicable law, testing of all parameters of each component is not necessarily
performed.
TI assumes no liability for applications assistance or the design of Buyers’ products. Buyers are responsible for their products and
applications using TI components. To minimize the risks associated with Buyers’ products and applications, Buyers should provide
adequate design and operating safeguards.
TI does not warrant or represent that any license, either express or implied, is granted under any patent right, copyright, mask work right, or
other intellectual property right relating to any combination, machine, or process in which TI components or services are used. Information
published by TI regarding third-party products or services does not constitute a license to use such products or services or a warranty or
endorsement thereof. Use of such information may require a license from a third party under the patents or other intellectual property of the
third party, or a license from TI under the patents or other intellectual property of TI.
Reproduction of significant portions of TI information in TI data books or data sheets is permissible only if reproduction is without alteration
and is accompanied by all associated warranties, conditions, limitations, and notices. TI is not responsible or liable for such altered
documentation. Information of third parties may be subject to additional restrictions.
Resale of TI components or services with statements different from or beyond the parameters stated by TI for that component or service
voids all express and any implied warranties for the associated TI component or service and is an unfair and deceptive business practice.
TI is not responsible or liable for any such statements.
Buyer acknowledges and agrees that it is solely responsible for compliance with all legal, regulatory and safety-related requirements
concerning its products, and any use of TI components in its applications, notwithstanding any applications-related information or support
that may be provided by TI. Buyer represents and agrees that it has all the necessary expertise to create and implement safeguards which
anticipate dangerous consequences of failures, monitor failures and their consequences, lessen the likelihood of failures that might cause
harm and take appropriate remedial actions. Buyer will fully indemnify TI and its representatives against any damages arising out of the use
of any TI components in safety-critical applications.
In some cases, TI components may be promoted specifically to facilitate safety-related applications. With such components, TI’s goal is to
help enable customers to design and create their own end-product solutions that meet applicable functional safety standards and
requirements. Nonetheless, such components are subject to these terms.
No TI components are authorized for use in FDA Class III (or similar life-critical medical equipment) unless authorized officers of the parties
have executed a special agreement specifically governing such use.
Only those TI components which TI has specifically designated as military grade or “enhanced plastic” are designed and intended for use in
military/aerospace applications or environments. Buyer acknowledges and agrees that any military or aerospace use of TI components
which have not been so designated is solely at the Buyer's risk, and that Buyer is solely responsible for compliance with all legal and
regulatory requirements in connection with such use.
TI has specifically designated certain components as meeting ISO/TS16949 requirements, mainly for automotive use. In any case of use of
non-designated products, TI will not be responsible for any failure to meet ISO/TS16949.
Products
Applications
Audio
www.ti.com/audio
Automotive and Transportation
www.ti.com/automotive
Amplifiers
amplifier.ti.com
Communications and Telecom
www.ti.com/communications
Data Converters
dataconverter.ti.com
Computers and Peripherals
www.ti.com/computers
DLP® Products
www.dlp.com
Consumer Electronics
www.ti.com/consumer-apps
DSP
dsp.ti.com
Energy and Lighting
www.ti.com/energy
Clocks and Timers
www.ti.com/clocks
Industrial
www.ti.com/industrial
Interface
interface.ti.com
Medical
www.ti.com/medical
Logic
logic.ti.com
Security
www.ti.com/security
Power Mgmt
power.ti.com
Space, Avionics and Defense
www.ti.com/space-avionics-defense
Microcontrollers
microcontroller.ti.com
Video and Imaging
www.ti.com/video
RFID
www.ti-rfid.com
OMAP Applications Processors
www.ti.com/omap
TI E2E Community
e2e.ti.com
Wireless Connectivity
www.ti.com/wirelessconnectivity
Mailing Address: Texas Instruments, Post Office Box 655303, Dallas, Texas 75265
Copyright © 2013, Texas Instruments Incorporated
Similar pages