Product
Folder
Sample &
Buy
Support &
Community
Tools &
Software
Technical
Documents
LM26400Y
SNVS457D – FEBRUARY 2007 – REVISED OCTOBER 2015
LM26400Y Dual 2-A, 500-kHz Wide Input Range Buck Regulator
1 Features
3 Description
•
•
•
•
•
•
•
•
•
•
•
•
•
The LM26400Y device is a monolithic, two-output
fixed-frequency PWM step-down DC-DC regulator, in
a 16-pin WSON or thermally-enhanced HTSSOP
package. With a minimum number of external
components and internal loop compensation, the
LM26400Y is easy to use.
1
Input Voltage Range of 3 V to 20 V
Dual 2-A Output
Output Voltage Down to 0.6 V
Internal Compensation
500-kHz PWM Frequency
Separate Enable Pins
Separate Soft-Start Pins
Frequency Foldback Protection
175-mΩ NMOS Switch
Integrated Bootstrap Diodes
Overcurrent Protection
HTSSOP and WSON Packages
Thermal Shutdown
2 Applications
•
•
•
•
•
•
•
DTV-LCD
Set-Top Boxes
XDSL
Automotive
Computing Peripherals
Industrial Controls
Points-of-Load
The ability to drive 2-A loads with an internal 175-mΩ
NMOS switch using state-of-the-art 0.5-µm BiCMOS
technology results in a high-power density design.
The world class control circuitry allows for an ONtime as low as 40 ns, thus supporting high-frequency
conversion over the entire input range of 3 V to 20 V
and down to an output voltage of only 0.6 V. The
LM26400Y utilizes peak current-mode control and
internal compensation to provide high-performance
regulation over a wide range of line and load
conditions. Switching frequency is internally set to
500 kHz, optimal for a broad range of applications in
terms of size versus thermal tradeoffs.
Given a nonsynchronous architecture, efficiencies
above 90% are easy to achieve. External shutdown is
included, enabling separate turnon and turnoff of the
two
channels.
Additional
features
include
programmable soft-start circuitry to reduce inrush
current, pulse-by-pulse current limit and frequency
foldback, integrated bootstrap structure, and thermal
shutdown.
Device Information(1)
PART NUMBER
LM26400Y
PACKAGE
BODY SIZE (NOM)
WSON (16)
5.00 mm × 5.00 mm
HTSSOP (16)
4.40 mm × 5.00 mm
(1) For all available packages, see the orderable addendum at
the end of the data sheet.
Typical Application
1
An IMPORTANT NOTICE at the end of this data sheet addresses availability, warranty, changes, use in safety-critical applications,
intellectual property matters and other important disclaimers. PRODUCTION DATA.
LM26400Y
SNVS457D – FEBRUARY 2007 – REVISED OCTOBER 2015
www.ti.com
Table of Contents
1
2
3
4
5
6
7
Features ..................................................................
Applications ...........................................................
Description .............................................................
Revision History.....................................................
Pin Configuration and Functions .........................
Specifications.........................................................
1
1
1
2
3
4
6.1
6.2
6.3
6.4
6.5
6.6
4
4
4
4
5
6
Absolute Maximum Ratings ......................................
ESD Ratings..............................................................
Recommended Operating Conditions.......................
Thermal Information ..................................................
Electrical Characteristics...........................................
Typical Characteristics ..............................................
Detailed Description ............................................ 10
7.1
7.2
7.3
7.4
Overview .................................................................
Functional Block Diagram .......................................
Feature Description.................................................
Device Functional Modes........................................
10
10
10
13
8
Application and Implementation ........................ 15
8.1 Application Information............................................ 15
8.2 Typical Applications ............................................... 15
9
Power Supply Recommendations...................... 19
9.1 Low Input Voltage Considerations .......................... 19
9.2 Programming Output Voltage ................................. 19
10 Layout................................................................... 20
10.1 Layout Guidelines ................................................. 20
10.2 Layout Example .................................................... 24
10.3 Thermal Considerations ........................................ 24
11 Device and Documentation Support ................. 26
11.1
11.2
11.3
11.4
11.5
Device Support......................................................
Community Resources..........................................
Trademarks ...........................................................
Electrostatic Discharge Caution ............................
Glossary ................................................................
26
26
26
26
26
12 Mechanical, Packaging, and Orderable
Information ........................................................... 26
4 Revision History
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
Changes from Revision C (April 2013) to Revision D
•
Added ESD Ratings table, Feature Description section, Device Functional Modes, Application and Implementation
section, Power Supply Recommendations section, Layout section, Device and Documentation Support section, and
Mechanical, Packaging, and Orderable Information section. ................................................................................................. 1
Changes from Revision B (April 2013) to Revision C
•
2
Page
Page
Changed layout of National Data Sheet to TI format ........................................................................................................... 18
Submit Documentation Feedback
Copyright © 2007–2015, Texas Instruments Incorporated
Product Folder Links: LM26400Y
LM26400Y
www.ti.com
SNVS457D – FEBRUARY 2007 – REVISED OCTOBER 2015
5 Pin Configuration and Functions
PWP Package
16-Pin HTSSOP With PowerPAD IC Package
Top View
NHQ Package
16-Pin WSON
Top View
Pin Functions
PIN
NAME
NO.
TYPE
DESCRIPTION
Input supply for generating the internal bias used by the entire IC and for generating the
internal bootstrap bias. Needs to be locally bypassed.
AVIN
4
PWR
BST1
16
O
Supply rail for the gate drive of Channel 1's NMOS switch. A bootstrap capacitor should be
placed between the BST1 and SW1 pins.
BST2
9
O
Supply rail for the gate drive of Channel 2's NMOS switch. A bootstrap capacitor should be
placed between the BST2 and SW2 pins.
EN1
3
I
Enable control input for Channel 1. Logic high enables operation. Do not allow this pin to
float or be greater than VIN + 0.3 V.
EN2
6
I
Enable control input for Channel 2. Logic high enables operation. Do not allow this pin to
float or be greater than VIN + 0.3 V.
FB1
1
I
Feedback pin of Channel 1. Connect FB1 to an external voltage divider to set the output
voltage of Channel 1.
FB2
8
I
Feedback pin of Channel 2. Connect FB2 to an external voltage divider to set the output
voltage of Channel 2.
GND
5
PWR
Signal and Power ground pin. Kelvin connect the lower resistor of the feedback voltage
divider to this pin for good load regulation.
PVIN
11, 12, 13,14
PWR
Input voltage of the power supply. Directly connected to the drain of the internal NMOS
switch. Tie these pins together and connect to a local bypass capacitor.
SS1
2
I
Soft start pin of Channel 1. Connect a capacitor between this pin and ground to program the
start up speed.
SS2
7
I
Soft start pin of Channel 2. Connect a capacitor between this pin and ground to program the
start up speed.
SW2
10
O
Switch node of Channel 2. Connects to the inductor, catch diode, and bootstrap capacitor.
SW1
15
O
Switch node of Channel 1. Connects to the inductor, catch diode, and bootstrap capacitor.
DAP
—
Must be connected to system ground for low thermal impedance and low grounding
inductance.
Die Attach
Pad
Submit Documentation Feedback
Copyright © 2007–2015, Texas Instruments Incorporated
Product Folder Links: LM26400Y
3
LM26400Y
SNVS457D – FEBRUARY 2007 – REVISED OCTOBER 2015
www.ti.com
6 Specifications
6.1 Absolute Maximum Ratings
(1) (2)
See
MIN
MAX
UNIT
AVIN, PVIN
–0.5
22
V
SWx Voltage
–0.5
22
V
BSTx Voltage
–0.5
26
V
BSTx to SW Voltage
–0.5
6
V
FBx Voltage
–0.5
3
V
ENx Voltage (3)
–0.5
22
V
SSx Voltage
–0.5
3
V
150
°C
150
°C
Junction Temperature
Storage Temperature, Tstg
(1)
(2)
(3)
–65
Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. These are stress ratings
only, which do not imply functional operation of the device at these or any other conditions beyond those indicated under Recommended
Operating Conditions. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability.
If Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/Distributors for availability and
specifications.
EN1 and EN2 pins should never be higher than VIN + 0.3 V.
6.2 ESD Ratings
V(ESD)
(1)
(2)
Electrostatic discharge
Human body model (HBM), per ANSI/ESDA/JEDEC JS-001
(1) (2)
VALUE
UNIT
±2000
V
JEDEC document JEP155 states that 500-V HBM allows safe manufacturing with a standard ESD control process.
The human body model is a 100-pF capacitor discharged through a 1.5-kΩ resistor into each pin. Test method is per JESD-22-A114.
6.3 Recommended Operating Conditions
over operating free-air temperature range (unless otherwise noted)
MIN
VIN
Junction Temperature
NOM
MAX
UNIT
3
20
V
–40
125
°C
6.4 Thermal Information
LM26400Y
(1)
THERMAL METRIC
PWP (HTSSOP)
NHQ (WSON)
16 PINS
16 PINS
UNIT
RθJA
Junction-to-ambient thermal resistance (2) (3)
39.4
27.8
°C/W
RθJC(top)
Junction-to-case (top) thermal resistance
24.5
27.2
°C/W
RθJB
Junction-to-board thermal resistance
18
9.9
°C/W
ψJT
Junction-to-top characterization parameter
0.7
0.3
°C/W
ψJB
Junction-to-board characterization parameter
17.8
10.1
°C/W
RθJC(bot)
Junction-to-case (bottom) thermal resistance
2.1
2.8
°C/W
(1)
(2)
(3)
4
For more information about traditional and new thermal metrics, see the Semiconductor and IC Package Thermal Metrics application
report, SPRA953.
Value is highly board-dependent. For comparison of package thermal performance only. Not recommended for prediction of junction
temperature in real applications. See Thermal Considerations for more information.
A standard board refers to a four-layer PCB with the size 4.5”x3”x0.063”. Top and bottom copper is 2 oz. Internal plane copper is 1 oz.
For details refer to JESD51-7 standard. Mount package on a standard board and test per JESD51-7 standard.
Submit Documentation Feedback
Copyright © 2007–2015, Texas Instruments Incorporated
Product Folder Links: LM26400Y
LM26400Y
www.ti.com
SNVS457D – FEBRUARY 2007 – REVISED OCTOBER 2015
6.5 Electrical Characteristics
Unless otherwise stated, the following conditions apply: AVIN = PVIN = VIN = 5 V. Limits are for TJ = 25°C. Minimum and
maximum limits are ensured through test, design, or statistical correlation. Typical values represent the most likely parametric
norm at TJ = 25°C, and are provided for reference purposes only.
PARAMETER
TEST CONDITIONS
Feedback Loop Closed
VFB
Voltages at FB1 and FB2 Pins
Feedback Loop V Closed
ΔVFB_Line
IFB
Line Regulation of FB1 and FB2 Voltages,
Expressed as PPM Change Per Volt of VIN
Variation
Current in FB1 and FB2 Pins
VFB = 0.6 V
Undervoltage Lockout Threshold
VIN Falling From 3.3 V
VUVLO_HYS
UVLO Hysteresis
Switching Frequency
DMAX
Maximum Duty Cycle
DMIN
Minimum Duty Cycle
TJ = 25°C
TJ = –40°C to 125°C
0.611
0.585
UNIT
TJ = 25°C
ppm/V
0.4
TJ = –40°C to 125°C
250
TJ = 25°C
V
0.617
66
nA
2.7
TJ = –40°C to 125°C
2.9
TJ = 25°C
2.3
TJ = –40°C to 125°C
V
2
0.36
0.2
0.55
0.52
TJ = –40°C to 125°C
0.39
TJ = 25°C
0.65
V
MHz
96%
TJ = –40°C to 125°C
90%
2%
WSON, 2-A Drain Current
TJ = 25°C
175
TJ = –40°C to 125°C
320
TJ = 25°C
194
TJ = –40°C to 125°C
3
TJ = –40°C to 125°C
2.5
Shutdown Current of AVIN Pin
EN1 = EN2 = 0 V
IQ
Quiescent Current of AVIN Pin (both
channels are enabled but not switching)
EN1 = EN2 = 5 V, FB1 = FB2 = 0.7 V,
TJ = –40°C to 125°C
VEN_IH
Input Logic High of EN1 and EN2 Pins
TJ = –40°C to 125°C
VEN_IL
Input Logic Low of EN1 and EN2 Pins
TJ = –40°C to 125°C
IEN
EN1 and EN2 Currents (sink or source)
ISW_LEAK
Switch Leakage Current Measured at SW1
and SW2 Pins
EN1 = EN2 = SWx = 0
ΔΦ
Phase Shift Between SW1 and SW2 Rising
Edges
Feedback Loop Closed. Continuous Conduction Mode.
SSx Pin Current
TJ = 25°C
TJ = –40°C to 125°C
ΔISS
Difference Between SS1 and SS2 Currents
VFB_F
FB1 and FB2 Frequency Foldback
Threshold
TSD
Thermal Shutdown Threshold
Junction temperature rises.
TSD_HYS
Thermal Shutdown Hysteresis
Junction temperature falls from above TSD.
4.5
2
2.5
V
5
nA
1
µA
180
19
16
11
21
3
deg
µA
µA
0.35
V
165
°C
15
°C
Submit Documentation Feedback
Product Folder Links: LM26400Y
mA
V
0.4
170
A
nA
4
TJ = –40°C to 125°C
Copyright © 2007–2015, Texas Instruments Incorporated
mΩ
350
TJ = 25°C
ISD
ISS
MAX
0.6
TJ = 25°C
HTSSOP, 2-A Drain Current
ICL
0.591
TJ = 25°C
ON-Resistance of Internal Power MOSFET
Peak Current Limit of Internal MOSFET
TYP
0.6
TJ = 0°C to 85°C
TJ = –40°C to 125°C
FSW
RDS(ON)
TJ = 25°C
VIN = 3 V to 20 V
VIN Rising From 0 V
VUVLO
MIN
5
LM26400Y
SNVS457D – FEBRUARY 2007 – REVISED OCTOBER 2015
www.ti.com
6.6 Typical Characteristics
Unless otherwise specified or thermal-shutdown related, TA = 25°C for efficiency curves, loop gain plots and waveforms, and
TJ = 25°C for all others.
VOUT = 5 V
VOUT = 3.3 V
Figure 1. Efficiency
Figure 2. Efficiency
VOUT = 2.5 V
6
VOUT = 1.2 V
Figure 3. Efficiency
Figure 4. Efficiency
Figure 5. AVIN Shutdown Current vs Temperature
Figure 6. VIN Shutdown Current vs VIN
Submit Documentation Feedback
Copyright © 2007–2015, Texas Instruments Incorporated
Product Folder Links: LM26400Y
LM26400Y
www.ti.com
SNVS457D – FEBRUARY 2007 – REVISED OCTOBER 2015
Typical Characteristics (continued)
Unless otherwise specified or thermal-shutdown related, TA = 25°C for efficiency curves, loop gain plots and waveforms, and
TJ = 25°C for all others.
Figure 7. Switching Frequency vs Temperature
Figure 8. Feedback Voltage vs Temperature
Figure 9. Feedback Voltage vs VIN
Figure 10. Frequency Foldback
Figure 11. SS-Pin Current vs Temperature
Figure 12. FET RDS_ON vs Temperature
Submit Documentation Feedback
Copyright © 2007–2015, Texas Instruments Incorporated
Product Folder Links: LM26400Y
7
LM26400Y
SNVS457D – FEBRUARY 2007 – REVISED OCTOBER 2015
www.ti.com
Typical Characteristics (continued)
Unless otherwise specified or thermal-shutdown related, TA = 25°C for efficiency curves, loop gain plots and waveforms, and
TJ = 25°C for all others.
8
Figure 13. Switch Current Limit vs Temperature
Figure 14. Loop Gain, CCM
Figure 15. Loop Gain, DCM
Figure 16. Loop Gain, CCM
Figure 17. Loop Gain, DCM
Figure 18. Line Transient Response
Submit Documentation Feedback
Copyright © 2007–2015, Texas Instruments Incorporated
Product Folder Links: LM26400Y
LM26400Y
www.ti.com
SNVS457D – FEBRUARY 2007 – REVISED OCTOBER 2015
Typical Characteristics (continued)
Unless otherwise specified or thermal-shutdown related, TA = 25°C for efficiency curves, loop gain plots and waveforms, and
TJ = 25°C for all others.
Figure 19. Shutdown
Figure 20. Thermal Shutdown
Figure 21. Recovery from Thermal Shutdown
Figure 22. Short-Circuit Triggering
Figure 23. Short-Circuit Release
Submit Documentation Feedback
Copyright © 2007–2015, Texas Instruments Incorporated
Product Folder Links: LM26400Y
9
LM26400Y
SNVS457D – FEBRUARY 2007 – REVISED OCTOBER 2015
www.ti.com
7 Detailed Description
7.1 Overview
The LM26400Y device is a dual PWM peak-current mode buck regulator with two integrated power MOSFET
switches. The part is designed to be easy to use. The two regulators are mostly identical and share the same
input voltage and the same reference voltage (0.6 V). The two PWM clocks are of the same frequency but 180°
out of phase. The two channels can have different soft-start ramp slopes and can be turned on and off
independently.
Loop compensation is built in. The feedback loop design is optimized for ceramic output capacitors.
Since the power switches are built in, the achievable output current level also has to do with thermal environment
of the specific application. The LM26400Y enters thermal shutdown when the junction temperature exceeds
approximately 165°C.
7.2 Functional Block Diagram
SHARED CONTROL
EN1
AVIN
EN1_buf
EN2
UVLO
Comparator
uvlo
Thermal
Shutdown
TSD
Vref_LDO
Reference
Internal
Regulator
Vref
EN2_buf
CLK1
Main
OSC
Internal
Regulator
Vbst
Vddi
CLK2
Vbst
Switching Regulator 1
PVIN
+
I-sense
amp
TSD
Bootstrap
Diode
EN_buf
Iref
BST
Current
Limit
Driver
Logic
SW
CLK
Slave
OSC
0.32V
Freq. Foldback
Comparator
OV
Comparator
Reset
Pulse
Isense
+
PWM
Comparator
110% Vref
Gm
Amplifier
-
+
Internal
Compensation
+
+
Corrective Ramp
Vref
Vddi
FB
SS
GND
7.3 Feature Description
7.3.1 Overcurrent Protection
The instantaneous switch current is limited to a typical of 3 Amperes. Any time the switch current reaches that
value, the switch will be turned off immediately. This will result in a smaller duty cycle than normal, which will
cause the output voltage to dip. The output voltage will continue drooping until the load draws a current that is
equal to the peak-limited inductor current. As the output voltage droops, the FB pin voltage will also droop
proportionally. When the FB voltage dips below 0.35 V or so, the PWM frequency will start to decrease. The
lower the FB voltage the lower the PWM frequency. See Figure 10.
10
Submit Documentation Feedback
Copyright © 2007–2015, Texas Instruments Incorporated
Product Folder Links: LM26400Y
LM26400Y
www.ti.com
SNVS457D – FEBRUARY 2007 – REVISED OCTOBER 2015
Feature Description (continued)
The frequency foldback helps two things. One is to prevent the switch current from running away as a result of
the finite minimum ON-time (40 ns or so for the LM26400Y) and the small duty cycle caused by lowered output
voltage due to the current limit. The other is it also helps reduce thermal stress both in the IC and the external
diode.
The current limit threshold of the LM26400Y remains constant over all duty cycles.
One thing to pay attention to is that recovery from an overcurrent condition does not go through a soft-start
process. This is because the reference voltage at the noninverting input of the error amplifier always sits at 0.6 V
during the overcurrent protection. So if the overcurrent condition is suddenly removed, the regulator will bring the
FB voltage back to 0.6 V as quickly as possible. This may cause an overshoot in the output voltage. Generally,
the larger the inductor or the lower the output capacitance the more the overshoot, and vice versa. If the amount
of such overshoot exceeds the allowed limit for a system, add a CFF capacitor in parallel with the upper feedback
resistor to eliminate the overshoot. See Load Step Response for more details on CFF.
When one channel gets into overcurrent protection mode, the operation of the other channel will not be affected.
7.3.2 Loop Stability
To the first order approximation, the LM26400Y has a VFB-to-Inductor Current transfer admittance (that is, ratio of
inductor current to FB pin voltage, in frequency domain) close to the plot in Figure 24. The transfer admittance
has a DC value of 104 dBS (dBS stands for decibel Siemens. The equivalent of 0 dBS is 1 Siemens.). There is a
pole at 1 Hz and a zero at approximately 8 kHz. The plateau after the 8 kHz zero is about 27 dBS. There are
also high frequency poles that are not shown in the figure. They include a double pole at 1.2 MHz or so, and
another double pole at half the switching frequency. Depending on factors such as inductor ripple size and duty
cycle, the double pole at half the switching frequency may become two separate poles near half the switching
frequency.
Figure 24. VFB-to-Inductor Current Transfer Admittance
An easy strategy to build a stable loop with reasonable phase margin is to try to cross over from 20 kHz to 100
kHz, assuming the output capacitor is ceramic. When using pure ceramic capacitors at the output, simply use the
following equation to find out the crossover frequency.
where
•
•
22S (22 Siemens) is the equivalent of the 27 dBS transfer admittance
r is the ratio of 0.6 V to the output voltage
(1)
Use the same equation to find out the needed output capacitance for a given crossover frequency. Phase margin
is typically between 50° and 60°. The above equation is only good for a crossover from 20 kHz to 100 kHz. A
crossover frequency outside this range may result in lower phase margin and less accurate prediction by the
above equation.
Example: VOUT = 2.5 V, COUT = 36 µF, find out the crossover frequency.
Submit Documentation Feedback
Copyright © 2007–2015, Texas Instruments Incorporated
Product Folder Links: LM26400Y
11
LM26400Y
SNVS457D – FEBRUARY 2007 – REVISED OCTOBER 2015
www.ti.com
Feature Description (continued)
Assume the crossover is from 20 kHz to 100 kHz. Then:
(2)
The above analysis serves as a starting point. It is a good practice to always verify loop gain on bench.
7.3.3 Load Step Response
In general, the excursion in output voltage caused by a load step can be reduced by increasing the output
capacitance. Besides that, increasing the small-signal loop bandwidth also helps. This can be achieved by
adding a 27 nF or so capacitor (CFF) in parallel with the upper feedback resistor (assuming the lower feedback
resistor is 5.9 kΩ). See Figure 25 for an illustration.
Figure 25. Adding a CFF Capacitor
The responses to a load step from 0.2 A to 2 A with and without a CFF are shown in Figure 26. The higher loop
bandwidth as a result of CFF reduces the total output excursion by about 80 mV.
Figure 26. CFF Improves Load Step Response
Use the following equation to calculate the new loop bandwidth:
(3)
Again, the assumption is the crossover is from 20 kHz to 100 kHz.
In an extreme case where the load goes to less than 100 mA during a large load step, output voltage may exhibit
extra undershoot. This usually happens when the load toggles high at the time VOUT just ramps down to its
regulation level from an overshoot. Figure 27 shows such a case where the load toggles between 1.7 A and only
50 mA.
12
Submit Documentation Feedback
Copyright © 2007–2015, Texas Instruments Incorporated
Product Folder Links: LM26400Y
LM26400Y
www.ti.com
SNVS457D – FEBRUARY 2007 – REVISED OCTOBER 2015
Feature Description (continued)
Figure 27. Extreme Load Step
In the example, the load first goes down to 50 mA quickly (0.9 A/µs), causing a 90-µs no-switching period, and
then quickly goes up to 1.7 A when VOUT1 just hits its regulation level (1.2 V), resulting in a large dip of 440 mV in
the output voltage.
If it is known in a system design that the load can go down to less than 100 mA during a load step, and that the
load can toggle high any time after it toggles low, take the following measures to minimize the potential extra
undershoot. First is to add the Cff mentioned above. Second is to increase the output capacitance.
For example, to meet a ±10% VOUT excursion requirement for a 100 mA to 2-A load step, approximately 200 µF
output capacitance is needed for a 1.2-V output, and about 44 µF is needed for a 5-V output.
7.4 Device Functional Modes
7.4.1 Start-Up and Shutdown
During a soft start, the ramp of the output voltage is proportional to the ramp of the SS pin. When the EN pin is
pulled high, an internal 16-µA current source starts to charge the corresponding SS pin. The capacitance
between the SS pin and ground determines how fast the SS voltage ramps up. The noninverting input of the
transconductance error amplifier, that is, the moving reference during soft start, will be the lower of SS voltage
and the 0.6-V reference (VREF). So before SS reaches 0.6V, the reference to the error amplifier will be the SS
voltage. When SS exceeds 0.6 V, the noninverting input of the transconductance amplifier will be a constant 0.6
V and that will be the time soft start ends. The SS voltage will continue to ramp all the way up to the internal 2.7V supply voltage before leveling off.
To calculate the needed SS capacitance for a given soft-start duration, use Equation 4.
where
•
•
•
ISS is SS pin charging current, typically 16 µA
VREF is the internal reference voltage, typically 0.6 V
tSS is the desired soft-start duration
(4)
For example, if 1 ms is the desired soft-start time, then the nominal SS capacitance should be 25 nF. Apply
tolerances if necessary. Use the VFB entry in Electrical Characteristics for the VREF tolerance.
Submit Documentation Feedback
Copyright © 2007–2015, Texas Instruments Incorporated
Product Folder Links: LM26400Y
13
LM26400Y
SNVS457D – FEBRUARY 2007 – REVISED OCTOBER 2015
www.ti.com
Device Functional Modes (continued)
Inductor current during soft start can be calculated by Equation 5.
where
•
•
•
VOUT is the target output voltage
IOUT is the load current during start-up
COUT is the output capacitance
(5)
For example, if the output capacitor is 10 µF, output voltage is 2.5 V, soft-start capacitor is 10 nF and there is no
load, then the average inductor current during soft start will be 62.5 mA.
When EN pin is pulled below 0.4 V or so, the 16-µA current source will stop charging the SS pin. The SS pin will
be discharged through a 330-Ω internal FET to ground. During this time, the internal power switch will remain
turned off while the output is discharged by the load.
If EN is again pulled high before SS and output voltage are completely discharged, soft-start will begin with a
non-zero reference and the level of the soft-start reference will be the lower of SS voltage and 0.6 V.
When the output is prebiased, the LM26400Y can usually start up successfully if there is at least a 2-V difference
between the input voltage and the prebias. An output prebias condition refers to the case when the output is
sitting at a non-zero voltage at the beginning of a start-up. The key to a successful start-up under such a
situation is enough initial voltage across the bootstrap capacitor. When an output prebias condition is anticipated,
the power supply designer should check the start-up behavior under the highest potential prebias.
A prebias condition caused by a glitch in the enable signal after start-up or by an input brownout condition
normally is not an issue because the bootstrap capacitor holds its charge much longer than the output
capacitors.
Due to the frequency foldback mechanism, the switching frequency during start-up will be lower than the normal
value before VFB reaches 0.35 V or so. See Figure 10.
It is generally okay to connect the EN pin to VIN to simplify the system design. However, if the VIN ramp is slow
and the load current is relatively high during soft start, the VOUT ramp may have a notch in it and a slight
overshoot at the end of startup. This is due to the reduced load current handling capability of the LM26400Y for
VIN lower than 5 V. If this kind of behavior is a problem for the system designer, there are two solutions. One is to
control the EN pin with a logic signal and do not pull the EN high until VIN is above 5 V or so. Make sure the logic
signal is never higher than VIN by 0.3 V. The other is to use an external 5-V bootstrap bias if it is ready before VIN
hits 2.7 V or so. See Low Input Voltage Considerations for more information.
14
Submit Documentation Feedback
Copyright © 2007–2015, Texas Instruments Incorporated
Product Folder Links: LM26400Y
LM26400Y
www.ti.com
SNVS457D – FEBRUARY 2007 – REVISED OCTOBER 2015
8 Application and Implementation
NOTE
Information in the following applications sections is not part of the TI component
specification, and TI does not warrant its accuracy or completeness. TI’s customers are
responsible for determining suitability of components for their purposes. Customers should
validate and test their design implementation to confirm system functionality.
8.1 Application Information
The LM26400Y device will operate with input voltage from 3 V to 20 V and provide two regulated output voltages.
The device is optimized for high-efficiency operation with minimum number of external components.
8.2 Typical Applications
8.2.1 LM26400Y Design Example 1
8.2.1.1 Design Requirements
The device must be able to operate at any voltage within the recommended operating range. The load current
must be defined in order to properly size the inductor, input, and output capacitors .The inductor must be able to
handle full expected load current as well as the peak current generated load transients and start-up.
8.2.1.2 Detailed Design Procedure
The best capacitors for use with the LM26400Y 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.
Submit Documentation Feedback
Copyright © 2007–2015, Texas Instruments Incorporated
Product Folder Links: LM26400Y
15
LM26400Y
SNVS457D – FEBRUARY 2007 – REVISED OCTOBER 2015
www.ti.com
Typical Applications (continued)
Table 1. Bill of Materials (Circuit 1, VIN = 12 V±10%, Output1 = 1.2 V/2 A, Output2 = 2.5 V/2 A)
PART
DESCRIPTION
PART VALUES
PHYSICAL SIZE
PART NUMBER
MANUFACTURER
C1
Capacitor, Ceramic
10 µF, 16 V, X5R
1210
GRM32DR61C106KA01
Murata
C2
Capacitor, Ceramic
0.22 µF, 16 V, X5R
0603
EMK107BJ224KA-T
Taiyo Yuden
C3
Capacitor, Ceramic
0.1 µF, 6.3 V, X5R
0402
C1005X5R0J104K
TDK
C4
Capacitor, Ceramic
0.1 µF, 6.3 V, X5R
0402
C1005X5R0J104K
TDK
C5
Capacitor, Ceramic
100 µF, 6.3 V, X5R
1210
GRM32ER60J107ME20L
Murata
C6
Capacitor, Ceramic
47 µF, 6.3 V, X5R
1210
GRM32ER60J476ME20L
Murata
C7
Capacitor, Ceramic
0.012 µF, 6.3 V, X5R
0402
C0402C123K9PACTU
Kemet
C8
Capacitor, Ceramic
0.012 µF, 6.3 V, X5R
0402
C0402C123K9PACTU
Kemet
C9
Capacitor, Ceramic
0.027 µF, 6.3 V, X5R
0402
C0402C273K9PACTU
Kemet
C10
Capacitor, Ceramic
0.027 µF, 6.3 V, X5R
0402
C0402C273K9PACTU
Kemet
D1
Diode, Schottky
2 A, 30 V
SMB
MBRS230LT3G
ON Semiconductor
D2
Diode, Schottky
2 A, 30 V
SMB
MBRS230LT3G
ON Semiconductor
3
L1
Inductor
5 µH, 2.2 A
7 × 7 × 2.8 mm
CDRH6D26NP-5R0NC
Sumida
L2
Inductor
8.7 µH, 2.2 A
7 × 7 × 4 mm3
CDRH6D38NP-8R7NC
Sumida
R1
Resistor
10 Ω, 1%
0402
CRCW040210R0FK
Vishay
R2
Resistor
5.9 kΩ, 1%
0402
CRCW04025K90FK
Vishay
R3
Resistor
5.9 kΩ, 1%
0402
CRCW04025K90FK
Vishay
R4
Resistor
18.7 kΩ, 1%
0402
CRCW040218K7FK
Vishay
R5
Resistor
5.9 kΩ, 1%
0402
CRCW04025K90FK
Vishay
U1
Regulator
Dual 2-A Buck
HTSSOP-16
LM26400YMH
Texas Instruments
Table 2. Bill of Materials (Circuit 1, VIN = 7 V to 20 V, Output1 = 3.3 V/2 A, Output2 = 5 V/2 A)
PART
DESCRIPTION
PART VALUES
PHYSICAL SIZE
PART NUMBER
MANUFACTURER
C1
Capacitor, Ceramic
10 µF, 25 V, X5R
1812
GRM43DR61E106KA12
Murata
C2
Capacitor, Ceramic
0.22 µF, 25 V, X5R
0603
TMK107BJ224KA-T
Taiyo Yuden
C3
Capacitor, Ceramic
0.1 µF, 6.3 V, X5R
0402
C1005X5R0J104K
TDK
C4
Capacitor, Ceramic
0.1 µF, 6.3 V, X5R
0402
C1005X5R0J104K
TDK
C5
Capacitor, Ceramic
47 µF, 6.3 V, X5R
1210
GRM32ER60J476ME20
Murata
C6
Capacitor, Ceramic
33 µF, 6.3 V, X5R
1210
GRM32DR60J336ME19
Murata
C7
Capacitor, Ceramic
0.012 µF, 6.3 V, X5R
0402
C0402C123K9PACTU
Kemet
C8
Capacitor, Ceramic
0.012 µF, 6.3 V, X5R
0402
C0402C123K9PACTU
Kemet
C9
Capacitor, Ceramic
0.027 µF, 6.3 V, X5R
0402
C0402C273K9PACTU
Kemet
C10
Capacitor, Ceramic
0.027 µF, 6.3 V, X5R
0402
C0402C273K9PACTU
Kemet
D1
Diode, Schottky
2 A, 30 V
SMB
MBRS230LT3G
ON Semiconductor
D2
Diode, Schottky
2 A, 30 V
SMB
MBRS230LT3G
ON Semiconductor
L1
Inductor
10 µH, 3 A
8.3 × 8.3 × 4 mm3
CDRH8D38NP-100NC
Sumida
L2
Inductor
15 µH, 3 A
8.3 × 8.3 × 4 mm3
CDRH8D43/HP-150NC
Sumida
R1
Resistor
10 Ω, 1%
0402
CRCW040210R0FK
Vishay
R2
Resistor
26.7 kΩ, 1%
0402
CRCW040226K7FK
Vishay
R3
Resistor
5.9 kΩ, 1%
0402
CRCW04025K90FK
Vishay
R4
Resistor
43.2 kΩ, 1%
0402
CRCW040218K7FK
Vishay
R5
Resistor
5.9 kΩ, 1%
0402
CRCW04025K90FK
Vishay
U1
Regulator
Dual 2-A Buck
HTSSOP-16
LM26400YMH
Texas Instruments
16
Submit Documentation Feedback
Copyright © 2007–2015, Texas Instruments Incorporated
Product Folder Links: LM26400Y
LM26400Y
www.ti.com
SNVS457D – FEBRUARY 2007 – REVISED OCTOBER 2015
8.2.1.3 Application Curves
Figure 28. Load Step Response
Figure 29. Load Step Response
Figure 30. Start-Up (No Load)
Figure 31. Start-Up (No Load)
Submit Documentation Feedback
Copyright © 2007–2015, Texas Instruments Incorporated
Product Folder Links: LM26400Y
17
LM26400Y
SNVS457D – FEBRUARY 2007 – REVISED OCTOBER 2015
www.ti.com
8.2.2 LM26400Y Design Example 2
Table 3. Bill of Materials (Circuit 2, VIN = 3 V to 5 V, Output1 = 1.2 V/2 A, Output2 = 1.8 V/2 A)
PART
DESCRIPTION
PART VALUES
PHYSICAL SIZE
PART NUMBER
MANUFACTURER
C1
Capacitor, Ceramic
10 µF, 6.3 V, X5R
1206
GRM319R60J106KE19
Murata
C2
Capacitor, Ceramic
0.22 µF, 6.3 V, X5R
0402
JMK105BJ224KV-F
Taiyo Yuden
C3
Capacitor, Ceramic
0.1 µF, 6.3 V, X5R
0402
C1005X5R0J104K
TDK
C4
Capacitor, Ceramic
0.1 µF, 6.3 V, X5R
0402
C1005X5R0J104K
TDK
C5
Capacitor, Ceramic
100 µF, 6.3 V, X5R
1210
GRM32ER60J107ME20L
Murata
C6
Capacitor, Ceramic
100 µF, 6.3 V, X5R
1210
GRM32ER60J107ME20L
Murata
C7
Capacitor, Ceramic
0.012 µF, 6.3 V, X5R
0402
C0402C123K9PACTU
Kemet
C8
Capacitor, Ceramic
0.012 µF, 6.3 V, X5R
0402
C0402C123K9PACTU
Kemet
C9
Capacitor, Ceramic
0.027 µF, 6.3 V, X5R
0402
C0402C273K9PACTU
Kemet
C10
Capacitor, Ceramic
0.027 µF, 6.3 V, X5R
0402
C0402C273K9PACTU
Kemet
D1
Diode, Schottky
2 A, 30 V
SMB
MBRS230LT3G
ON Semiconductor
D2
Diode, Schottky
2 A, 30 V
SMB
MBRS230LT3G
ON Semiconductor
L1
Inductor
5 µH, 2.2 A
7 × 7 × 2.8 mm3
CDRH6D26NP-5R0NC
Sumida
L2
Inductor
5 µH, 2.2 A
7 × 7 × 2.8 mm3
CDRH6D26NP-5R0NC
Sumida
R1
Resistor
10 Ω, 1%
0402
CRCW040210R0FK
Vishay
R2
Resistor
5.9 kΩ, 1%
0402
CRCW04025K90FK
Vishay
R3
Resistor
5.9 kΩ, 1%
0402
CRCW04025K90FK
Vishay
R4
Resistor
11.8 kΩ, 1%
0402
CRCW040211K8FK
Vishay
R5
Resistor
5.90 kΩ, 1%
0402
CRCW04025K90FK
Vishay
U1
Regulator
Dual 2-A Buck
HTSSOP-16
LM26400YMH
Texas Instruments
18
Submit Documentation Feedback
Copyright © 2007–2015, Texas Instruments Incorporated
Product Folder Links: LM26400Y
LM26400Y
www.ti.com
SNVS457D – FEBRUARY 2007 – REVISED OCTOBER 2015
9 Power Supply Recommendations
9.1 Low Input Voltage Considerations
When VIN is from 3 V to 5 V, TI recommends that an external bootstrap bias voltage and a Schottky diode be
used to handle load currents up to 2 A. See Figure 32 for an illustration.
Figure 32. External Bootstrap for Low VIN
The recommended voltage for the external bias is 5 V. Due to the absolute maximum rating of VBST - VSW, the
external 5-V bias should not be higher than 6 V.
9.2 Programming Output Voltage
First make sure the required maximum duty cycle in steady state is less than 80% so that the regulator will not
lose regulation. The datasheet lower limit for maximum duty cycle is about 90% over temperature (see Electrical
Characteristics for the accurate value). The maximum duty cycle in steady state happens at low line and full load.
The output voltage is programmed through the feedback resistors R1 and R2, as illustrated in Figure 33.
Figure 33. Programming Output Voltage
TI recommends that the lower feedback resistor R2 always be 5.9 kΩ. This simplifies the selection of the CFF
value (for an explanation of CFF, see Load Step Response). The 5.9 kΩ is also a suitable R2 value in
applications that need to increase the output voltage on the fly by paralleling another resistor with R2. Because
the FB pin is 0.6 V during normal operation, the current through the feedback resistors is normally 0.6 V / 5.9 kΩ
= 0.1 mA and the power dissipation in R2 is 0.6 V × 0.6 V / 5.9 kΩ = 61 µW - low enough for 0402 size or
smaller resistors.
Use Equation 6 to determine the upper feedback resistor R1.
(6)
To determine the maximum allowed resistor tolerance, use Equation 7:
where
•
•
TOL is the set point accuracy of the regulator
Φ is the tolerance of VFB
(7)
Submit Documentation Feedback
Copyright © 2007–2015, Texas Instruments Incorporated
Product Folder Links: LM26400Y
19
LM26400Y
SNVS457D – FEBRUARY 2007 – REVISED OCTOBER 2015
www.ti.com
Programming Output Voltage (continued)
Example:
VOUT = 1.2 V, with a set-point accuracy of ±3.5%.
(8)
Choose 1% resistors. R2 = 5.90 kΩ.
(9)
10 Layout
10.1 Layout Guidelines
There are mainly two considerations for PCB layout - thermal and electrical. For thermal details, see Thermal
Considerations. Electrical wise, follow the rules below as much as possible. In general, the LM26400Y is a quite
robust part in terms of insensitivity to different layout patterns or even abuses.
• Keep the input ceramic capacitor(s) as close to the PVIN pins as possible.
• Use internal ground planes when available.
• The SW pins are high current carrying pins so traces connected to them should be short and fat.
• Keep feedback resistors close to the FB pins.
• Keep the AVIN RC filter close to the AVIN pin.
• Keep the voltage feedback traces away from the switch nodes.
• Use six or more vias next to the ground pad of the catch diode.
• Use at least four vias next to the ground pad of output capacitors.
• Use at least four vias next to each pad of the input capacitors.
For low EMI emission, try not to assign large areas of copper to the noisy switch nodes as a heat sinking
method. Instead, assign a lot of copper to the output nodes.
10.1.1 Thermal Shutdown
Whenever the junction temperature of the LM26400Y exceeds 165°C, the MOSFET switch will be kept off until
the temperature drops below 150°C, at which point the regulator will go through a hard start to quickly raise the
output voltage back to normal. Since it is a hard start, there will be an overshoot at the output. See Figure 20.
10.1.2 Power Loss Estimation
The total power loss in the LM26400Y comprises of three parts: the power FET conduction loss, the power FET
switching loss, and the IC's housekeeping power loss. Use Equation 10 to estimate the conduction loss.
where
•
•
TJ is the junction temperature or the target junction temperature if the former is unknown
RDS is the ON resistance of the internal FET at room temperature
(10)
Use 180 mΩ for RDS if the actual value is unknown.
Use Equation 11 to estimate the switching loss.
(11)
Another loss in the IC is the housekeeping loss. The loss is the power dissipated by circuitry in the IC other than
the power FETs. The equation is:
(12)
The 15 mW is gate drive loss. Do the calculation for both channels and find out the total power loss in the IC.
20
Submit Documentation Feedback
Copyright © 2007–2015, Texas Instruments Incorporated
Product Folder Links: LM26400Y
LM26400Y
www.ti.com
SNVS457D – FEBRUARY 2007 – REVISED OCTOBER 2015
Layout Guidelines (continued)
(13)
The power loss calculation can help estimate the overall power supply efficiency.
Example:
VIN = 12 V, VOUT1 = 1.2 V, IOUT1 = 2 A, VOUT2 = 2.5 V, IOUT2 = 2 A. Target junction temperature is 90°C.
So conduction loss in Channel 1 is:
(14)
Conduction loss in Channel 2 is:
(15)
Switching loss in either channel is:
(16)
Housekeeping loss is:
(17)
Finally the total power loss in the LM26400Y is:
(18)
10.1.3 Inductor Selection
TI recommends an inductance value that gives a peak-to-peak ripple current of 0.4 A to 0.8 A. Too large of a
ripple current can reduce the maximum achievable DC load current because the peak current of the switch is
limited to a typical of 3 A. Too small of a ripple current can cause the regulator to oscillate due to the lack of
inductor current ramp signal, especially under high input voltages. Use Equation 19 to determine inductance:
where
•
VIN_MAX is the maximum input voltage of the application.
(19)
The rated current of the inductor should be higher than the maximum DC load current. Generally speaking, the
lower the DC resistance of the inductor winding, the higher the overall regulator efficiency.
Ferrite core inductors are recommended for less AC loss and less fringing magnetic flux. The drawback of ferrite
core inductors is their quick saturation characteristic. Once the inductor gets saturated, its current can spike up
very quickly if the switch is not turned off immediately. The current limit circuit has a propagation delay and so is
oftentimes not fast enough to stop the saturated inductor from going above the current limit. This has the
potential to damage the internal switch. So to prevent a ferrite core inductor from getting into saturation, the
inductor saturation current rating should be higher than the switch current limit ICL. The LM26400Y is quite robust
in handling short pulses of current that is a few amps above the current limit. When a compromise has to be
made, pick an inductor with a saturation current just above the lower limit of the ICL. Be sure to validate the shortcircuit protection over the intended temperature range.
To prevent the inductor from saturating over the entire –40°C to 125°C range, pick one with a saturation current
higher than the upper limit of ICL in Electrical Characteristics.
Inductor saturation current is usually lower when hot. So consult the inductor vendor if the saturation current
rating is only specified at room temperature.
Submit Documentation Feedback
Copyright © 2007–2015, Texas Instruments Incorporated
Product Folder Links: LM26400Y
21
LM26400Y
SNVS457D – FEBRUARY 2007 – REVISED OCTOBER 2015
www.ti.com
Layout Guidelines (continued)
Soft saturation inductors such as the iron powder types can also be used. Such inductors do not saturate
suddenly and therefore are safer when there is a severe overload or even shorted output. Their physical sizes
are usually smaller than the Ferrite core inductors. The downside is their fringing flux and higher power
dissipation due to relatively high AC loss, especially at high frequencies.
Example:
VOUT = 1.2 V; VIN = 9 V to 14 V; IOUT = 2 A maximum; Peak-to-peak Ripple Current ΔI = 0.6 A.
(20)
Choose a 5-µH or so ferrite core inductor that has a saturation current around 3 A at room temperature. For
example, Sumida's CDRH6D26NP-5R0NC.
If the maximum load current is significantly lower than 2 A, pick an inductor with the same saturation rating as a
2-A design but with a lowered DC current rating. That should result in a smaller inductor. There are not many
choices, though. Another possibility is to use a soft saturation type inductor, whose size will be dominated by the
DC current rating.
10.1.4 Output Capacitor Selection
Output capacitors in a buck regulator handles the AC current from the inductor and so have little ripple RMS
current and their power dissipation is not a concern. The concern usually revolves around loop stability and
capacitance retention.
The LM26400Y's internal loop compensation was designed around ceramic output capacitors. From a stability
point of view, the lower the output voltage, the more capacitance is needed.
Below is a quick summary of temperature characteristics of some commonly used ceramic capacitors. So an
X7R ceramic capacitor means its capacitance can vary ±15% over the temperature range of –55°C to 125°C.
Table 4. Capacitance Variation Over Temperature (Class II Dielectric Ceramic Capacitors)
LOW TEMPERATURE
HIGH TEMPERATURE
CAPACITANCE CHANGE RANGE
X: –55°C
5: +85°C
R: ±15%
Y: –30°C
6: +105°C
S: ±22%
Z: +10°C
7: +125°C
U: +22%, –56%
8: +150°C
V: +22%, –82%
Besides the variation of capacitance over temperature, the actual capacitance of ceramic capacitors also vary,
sometimes significantly, with applied DC voltage. Figure 34 illustrates such a characteristic of several ceramic
capacitors of various physical sizes from Murata. Unless the DC voltage across the capacitor is going to be small
relative to its rated value, going to too small a physical size will have the penalty of losing significant capacitance
during circuit operation.
Figure 34. Capacitance vs Applied DC Voltage
22
Submit Documentation Feedback
Copyright © 2007–2015, Texas Instruments Incorporated
Product Folder Links: LM26400Y
LM26400Y
www.ti.com
SNVS457D – FEBRUARY 2007 – REVISED OCTOBER 2015
The amount of output capacitance directly contributes to the output voltage ripple magnitude. A quick way to
estimate the output voltage ripple is to multiply the inductor peak-to-peak ripple current by the impedance of the
output capacitors. For example, if the inductor ripple current is 0.6 A peak-to-peak, and the output capacitance is
44 µF, then the output voltage ripple should be close to 0.6 A × (6.28 × 500 kHz × 44 µF)-1 = 4.3 mV. Sometimes
when a large ceramic capacitor is used, the switching frequency may be higher than the capacitor's self
resonance frequency. In that case, find out the true impedance at the switching frequency and then multiply that
value by the ripple current to get the ripple voltage.
The amount of output capacitance also impacts the stability of the feedback loop. Refer to Loop Stability for
guidelines.
10.1.5 Input Capacitor Selection
The input capacitors provide the AC current needed by the nearby power switch so that current provided by the
upstream power supply does not carry a lot of AC content, generating less EMI. To the buck regulator in
question, the input capacitor also prevents the drain voltage of the FET switch from dipping when the FET is
turned on, therefore providing a healthy line rail for the LM26400Y to work with. Since typically most of the AC
current is provided by the local input capacitors, the power loss in those capacitors can be a concern. In the case
of the LM26400Y regulator, since the two channels operate 180° out of phase, the AC stress in the input
capacitors is less than if they operated in phase. The measure for the AC stress is called input ripple RMS
current. It is strongly recommended that at least one 4.7-µF ceramic capacitor be placed next to the PVIN pins.
Bulk capacitors such as electrolytic capacitors or OSCON capacitors can be added to help stabilize the local line
voltage, especially during large load transient events. As for the ceramic capacitors, use X7R , X6S, or X5R
types. They maintain most of their capacitance over a wide temperature range. Try to avoid sizes smaller than
0805. Otherwise significant drop in capacitance may be caused by the DC bias voltage. See Output Capacitor
Selection section for more information. The DC voltage rating of the ceramic capacitor should be higher than the
highest input voltage.
Capacitor temperature is a major concern in board designs. While using a 4.7-µF or higher MLCC as the input
capacitor is a good starting point, it is a good idea to check the temperature in the real thermal environment to
make sure the capacitors are not over heated. Capacitor vendors may provide curves of ripple RMS current
versus temperature rise, based on a designated thermal impedance. In reality, the thermal impedance may be
very different. So it is always a good idea to check the capacitor temperature on the board.
Because the duty cycles of the two channels may overlap, calculation of the input ripple RMS current is a little
tedious. Use Equation 21:
where
•
•
•
•
•
•
I1 is Channel 1's maximum output current
I2 is Channel 2's maximum output current
d1 is the non-overlapping portion of Channel 1's duty cycle, D1
d2 is the non-overlapping portion of Channel 2's duty cycle, D2
d3 is the overlapping portion of the two duty cycles
Iav is the average input current, Iav= I1 × D1 + I2 × D2
(21)
To quickly determine the values of d1, d2, and d3, refer to the decision tree in Figure 35. To determine the duty
cycle of each channel, use D = VOUT/VIN for a quick result or use the following equation for a more accurate
result.
where
•
•
RDC is the winding resistance of the inductor
RDS is the ON-resistance of the MOSFET switch.
(22)
Example:
VIN = 5 V, VOUT1 = 3.3 V, IOUT1 = 2 A, VOUT2 = 1.2 V, IOUT2 = 1.5 A, RDS = 170 mΩ, RDC = 30 mΩ. (IOUT1 is the
same as I1 in the input ripple RMS current equation, IOUT2 is the same as I2).
Submit Documentation Feedback
Copyright © 2007–2015, Texas Instruments Incorporated
Product Folder Links: LM26400Y
23
LM26400Y
SNVS457D – FEBRUARY 2007 – REVISED OCTOBER 2015
www.ti.com
First, find out the duty cycles. Plug the numbers into the duty cycle equation and we get D1 = 0.75, and D2 =
0.33. Next, follow the decision tree in Figure 35 to find out the values of d1, d2, and d3. In this case, d1 = 0.5, d2
= D2 + 0.5 – D1 = 0.08, and d3 = D1 – 0.5 = 0.25. Iav = IOUT1 × D1 + IOUT2 × D2 = 1.995 A. Plug all the numbers
into the input ripple RMS current equation and the result is Iirrm = 0.77 A.
Figure 35. Determining d1, d2, and d3
10.1.6 Catch Diode Selection
The catch diode should be at least 2-A rated. The most stressful operation for the diode is usually when the
output is shorted under high line. Always pick a Schottky diode for its lower forward drop and higher efficiency.
The reverse voltage rating of the diode should be at least 25% higher than the highest input voltage. The diode
junction temperature is a main concern here. Always validate the diode's junction temperature in the intended
thermal environment to make sure its thermally derated maximum current is not exceeded. There are a few 2-A,
30-V surface mount Schottky diodes available in the market. Diodes have a negative temperature coefficient, so
do not put two diodes in parallel to achieve a lower temperature rise. Current will be hogged by one of the diodes
instead of shared by the two. Use a larger package for that purpose.
10.2 Layout Example
Figure 36. PCB Layout Example
10.3 Thermal Considerations
Due to the low thermal impedance from junction to the die-attach pad (or DAP, exposed metal at the bottom of
the package), thermal performance heavily depends on PCB copper arrangement. The minimum requirement is
to have a top-layer thermal pad that is exactly the same size as the DAP. There should be at least nine 8-mil
thermal vias in the pad. The thermal vias should be connected to internal ground planes (if available) and to a
ground plane on the bottom layer that is as large as allowed.
24
Submit Documentation Feedback
Copyright © 2007–2015, Texas Instruments Incorporated
Product Folder Links: LM26400Y
LM26400Y
www.ti.com
SNVS457D – FEBRUARY 2007 – REVISED OCTOBER 2015
Thermal Considerations (continued)
In boards that have internal ground planes, extending the top-layer thermal pad outside the body of the package
to form a "dogbone" shape offers little performance improvement. However, for two-layer boards, the dogbone
shape on the top layer will provide significant help.
Predicting on paper with reasonable accuracy the junction temperature of the LM26400Y in a real-world
application is still an art. Major factors that contribute to the junction temperature but not directly associated with
the thermal performance of the LM26400Y itself include air speed, air temperature, nearby heating elements and
arrangement of PCB copper connected to the DAP of the LM26400Y. The RθJA value published in the datasheet
is based on a standard board design in a single heating element mode and measured in a standard environment.
The real application is usually completely different from those conditions. So the actual RθJA will be significantly
different from the datasheet number. The best approach is still to assign as much copper area as allowed to the
DAP and prototype the design.
When prototyping the design, it is necessary to know the junction temperature of the LM26400Y to assess the
thermal margin. The best way to measure the LM26400Y's junction temperature when the board is working in its
usual mode is to measure the package-top temperature using an infrared thermal imaging camera. Look for the
highest temperature reading across the case-top. Add two degrees to the measurement result and the number
should be a pretty good estimate of the junction temperature. Due to the high temperature gradient across the
case-top, the use of a thermal couple is generally not recommended. If a thermal couple has to be used, try to
locate the hottest spot on the case-top first and then secure the thermal couple at exactly the same location. The
thermal couple needs to be a light-gauge type (such as 40-gauge). Apply a small blob of thermal compound to
the contact point and then secure the thermal couple on the case-top using thermally non-conductive glue.
If the maximum allowed junction temperature is exceeded, load current has to be lowered to bring the
temperature back in specification. Or better thermal management such as more air flow needs to be provided.
As a summary, here is a list of important items to consider:
• Use multi-layer PC boards with internal ground planes.
• Use nine or more thermal vias to connect the top-layer thermal pad to internal ground planes and ground
copper on the bottom layer.
• Generate as large a ground plane as allowable on outer layers, especially near the package.
• Use 2-oz. copper whenever possible.
• Try to spread out heat generating components.
• The inductors and diodes are heat generating components and should be connected to power or ground
planes using many vias.
Submit Documentation Feedback
Copyright © 2007–2015, Texas Instruments Incorporated
Product Folder Links: LM26400Y
25
LM26400Y
SNVS457D – FEBRUARY 2007 – REVISED OCTOBER 2015
www.ti.com
11 Device and Documentation Support
11.1 Device Support
11.1.1 Third-Party Products Disclaimer
TI'S PUBLICATION OF INFORMATION REGARDING THIRD-PARTY PRODUCTS OR SERVICES DOES NOT
CONSTITUTE AN ENDORSEMENT REGARDING THE SUITABILITY OF SUCH PRODUCTS OR SERVICES
OR A WARRANTY, REPRESENTATION OR ENDORSEMENT OF SUCH PRODUCTS OR SERVICES, EITHER
ALONE OR IN COMBINATION WITH ANY TI PRODUCT OR SERVICE.
11.2 Community Resources
The following links connect to TI community resources. Linked contents are provided "AS IS" by the respective
contributors. They do not constitute TI specifications and do not necessarily reflect TI's views; see TI's Terms of
Use.
TI E2E™ Online Community TI's Engineer-to-Engineer (E2E) Community. Created to foster collaboration
among engineers. At e2e.ti.com, you can ask questions, share knowledge, explore ideas and help
solve problems with fellow engineers.
Design Support TI's Design Support Quickly find helpful E2E forums along with design support tools and
contact information for technical support.
11.3 Trademarks
E2E is a trademark of Texas Instruments.
All other trademarks are the property of their respective owners.
11.4 Electrostatic Discharge Caution
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.
11.5 Glossary
SLYZ022 — TI Glossary.
This glossary lists and explains terms, acronyms, and definitions.
12 Mechanical, Packaging, and Orderable Information
The following pages include mechanical, packaging, and orderable information. This information is the most
current data available for the designated devices. This data is subject to change without notice and revision of
this document. For browser-based versions of this data sheet, refer to the left-hand navigation.
26
Submit Documentation Feedback
Copyright © 2007–2015, Texas Instruments Incorporated
Product Folder Links: LM26400Y
PACKAGE OPTION ADDENDUM
www.ti.com
10-Dec-2020
PACKAGING INFORMATION
Orderable Device
Status
(1)
Package Type Package Pins Package
Drawing
Qty
Eco Plan
(2)
Lead finish/
Ball material
MSL Peak Temp
Op Temp (°C)
Device Marking
(3)
(4/5)
(6)
LM26400YMH/NOPB
ACTIVE
HTSSOP
PWP
16
92
RoHS & Green
SN
Level-1-260C-UNLIM
-40 to 125
L26400
YMH
LM26400YMHX/NOPB
ACTIVE
HTSSOP
PWP
16
2500
RoHS & Green
SN
Level-1-260C-UNLIM
-40 to 125
L26400
YMH
LM26400YSD/NOPB
ACTIVE
WSON
NHQ
16
1000
RoHS & Green
SN
Level-1-260C-UNLIM
L26400Y
LM26400YSDE/NOPB
ACTIVE
WSON
NHQ
16
250
RoHS & Green
SN
Level-1-260C-UNLIM
L26400Y
(1)
The marketing status values are defined as follows:
ACTIVE: Product device recommended for new designs.
LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect.
NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design.
PREVIEW: Device has been announced but is not in production. Samples may or may not be available.
OBSOLETE: TI has discontinued the production of the device.
(2)
RoHS: TI defines "RoHS" to mean semiconductor products that are compliant with the current EU RoHS requirements for all 10 RoHS substances, including the requirement that RoHS substance
do not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, "RoHS" products are suitable for use in specified lead-free processes. TI may
reference these types of products as "Pb-Free".
RoHS Exempt: TI defines "RoHS Exempt" to mean products that contain lead but are compliant with EU RoHS pursuant to a specific EU RoHS exemption.
Green: TI defines "Green" to mean the content of Chlorine (Cl) and Bromine (Br) based flame retardants meet JS709B low halogen requirements of
工商网监
湘ICP备2023018690号
