TI LMR10510YMF Simple switcherâ® 5.5vin, 1a step-down voltage regulator in sot-23 and llp Datasheet

LMR10510
LMR10510 SIMPLE SWITCHER ® 5.5Vin, 1A Step-Down Voltage Regulator in
SOT-23 and LLP
Literature Number: SNVS727A
LMR10510
SIMPLE SWITCHER® 5.5Vin, 1A Step-Down Voltage
Regulator in SOT-23 and LLP
Features
Performance Benefits
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■ Extremely easy to use
■ Tiny overall solution reduces system cost
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Input voltage range of 3V to 5.5V
Output voltage range of 0.6V to 4.5V
Output current up to 1A
1.6MHz (LMR10510X) and 3 MHz (LMR10510Y)
switching frequencies
Low shutdown Iq, 30 nA typical
Internal soft-start
Internally compensated
Current-Mode PWM operation
Thermal shutdown
SOT23-5 (2.92 x 2.84 x 1 mm) and LLP-6 (3 x 3 x 0.8 mm)
packaging
Fully enabled for WEBENCH® Power Designer
Applications
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Point-of-Load Conversions from 3.3V, and 5V Rails
Space Constrained Applications
Battery Powered Equipment
Industrial Distributed Power Applications
Power Meters
Portable Hand-Held Instruments
System Performance
Efficiency vs Load Current - "Y" VIN = 5V
100
100
90
90
EFFICIENCY (%)
EFFICIENCY (%)
Efficiency vs Load Current - "X" VIN = 5V
80
70
60
50
40
80
70
60
50
1.8Vout
3.3Vout
40
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
LOAD CURRENT (A)
1.8Vout
3.3Vout
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
LOAD CURRENT (A)
30165696
30165697
Typical Application
30165664
© 2011 Texas Instruments Incorporated
301656
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LMR10510 SIMPLE SWITCHER® 5.5Vin, 1A Step-Down Voltage Regulator in SOT-23 and LLP
October 31, 2011
LMR10510
Connection Diagrams
30165603
30165601
5-Pin SOT-23
6-Pin LLP
Ordering Information
Order Number
Frequency
Option
NSC Package
Drawing
Package Type
Top Mark
LMR10510XMFE
LMR10510XMF
250 units Tape and Reel
1.6MHz
LMR10510XMFX
SH7B
SOT23-5
LMR10510YMFE
LMR10510YSDE
LMR10510YSD
250 units Tape and Reel
SH9B
1000 units Tape and Reel
3000 units Tape and Reel
3MHz
250 units Tape and Reel
LLP-6
SDE06A
LMR10510YSDX
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1000 units Tape and Reel
3000 units Tape and Reel
MF05A
LMR10510YMF
LMR10510YMFX
Supplied As
L268B
1000 units Tape and Reel
4500 units Tape and Reel
2
LMR10510
Pin Descriptions 5-Pin SOT23
Pin
Name
1
SW
2
GND
Function
Switch node. Connect to the inductor and catch diode.
Signal and power ground pin. Place the bottom resistor of the feedback network as close as
possible to this pin.
3
FB
Feedback pin. Connect to external resistor divider to set output voltage.
4
EN
Enable control input. Logic high enables operation. Do not allow this pin to float or be greater
than VIN + 0.3V.
5
VIN
Input supply voltage.
Pin Descriptions 6-Pin LLP
Pin
Name
1
FB
2
GND
Function
Feedback pin. Connect to external resistor divider to set output voltage.
Signal and power ground pin. Place the bottom resistor of the feedback network as close
as possible to this pin.
3
SW
4
VIND
Switch node. Connect to the inductor and catch diode.
Power Input supply.
5
VINA
Control circuitry supply voltage. Connect VINA to VIND on PC board.
6
EN
Enable control input. Logic high enables operation. Do not allow this pin to float or be greater
than VINA + 0.3V.
DAP
Die Attach Pad
Connect to system ground for low thermal impedance, but it cannot be used as a primary
GND connection.
3
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LMR10510
Junction Temperature (Note 2)
150°C
Storage Temperature
−65°C to +150°C
For soldering specifications:
see product folder at www.national.com and
www.national.com/ms/MS/MS-SOLDERING.pdf
Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
please contact the Texas Instruments Sales Office/
Distributors for availability and specifications.
VIN
FB Voltage
EN Voltage
SW Voltage
ESD Susceptibility
-0.5V to 7V
-0.5V to 3V
-0.5V to 7V
-0.5V to 7V
2kV
Operating Ratings
VIN
Junction Temperature
3V to 5.5V
−40°C to +125°C
Electrical Characteristics (Note 3), (Note 4) VIN = 5V unless otherwise indicated under the Conditions
column. Limits in standard type are for TJ = 25°C only; limits in boldface type apply over the junction temperature (TJ) range of
-40°C to +125°C. Minimum and Maximum limits are guaranteed 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.
Symbol
VFB
ΔVFB/VIN
IB
UVLO
Parameter
Conditions
Feedback Voltage
Feedback Voltage Line Regulation
Min
Typ
Max
Units
0.588
0.600
0.612
V
VIN = 3V to 5V
0.02
Feedback Input Bias Current
Undervoltage Lockout
VIN Rising
Switching Frequency
DMAX
Maximum Duty Cycle
DMIN
Minimum Duty Cycle
RDS(ON)
ICL
VEN_TH
Switch On Resistance
Switch Current Limit
2.73
2.90
V
1.85
LMR10510-X
1.2
2.3
1.6
1.95
LMR10510-Y
2.25
3.0
3.75
LMR10510-X
86
94
LMR10510-Y
82
90
0.43
LMR10510-X
5
LMR10510-Y
7
LLP-6 Package
150
SOT23-5 Package
130
VIN = 3.3V
1.2
Switch Leakage
Enable Pin Current
Quiescent Current (switching)
Quiescent Current (shutdown)
V
MHz
%
%
195
1.75
mΩ
A
0.4
Enable Threshold Voltage
IEN
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nA
Shutdown Threshold Voltage
ISW
IQ
100
VIN Falling
UVLO Hysteresis
FSW
%/V
0.1
1.8
V
100
nA
Sink/Source
100
nA
LMR10510X VFB = 0.55
3.3
5
mA
LMR10510Y VFB = 0.55
4.3
6.5
mA
All Options VEN = 0V
30
4
nA
Parameter
θJA
Junction to Ambient
0 LFPM Air Flow (Note 5)
θJC
Junction to Case
TSD
Thermal Shutdown Temperature
Conditions
Min
Typ
LLP-6 Package
80
SOT23-5 Package
118
LLP-6 Package
18
SOT23-5 Package
80
165
Max
Units
°C/W
°C/W
°C
Note 1: Absolute maximum ratings indicate limits beyond which damage to the device may occur. Operating Range indicates conditions for which the device is
intended to be functional, but does not guarantee specfic performance limits. For guaranteed specifications and test conditions, see the Electrical Characteristics.
Note 2: Thermal shutdown will occur if the junction temperature exceeds the maximum junction temperature of the device.
Note 3: Min and Max limits are 100% production tested at 25°C. Limits over the operating temperature range are guaranteed through correlation using Statistical
Quality Control (SQC) methods. Limits are used to calculate National’s Average Outgoing Quality Level (AOQL).
Note 4: Typical numbers are at 25°C and represent the most likely parametric norm.
Note 5: Applies for packages soldered directly onto a 3” x 3” PC board with 2oz. copper on 4 layers in still air.
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LMR10510
Symbol
Unless stated otherwise, all curves taken at VIN = 5.0V with configuration in typical application circuit shown in Figure 3. TJ = 25°
C, unless otherwise specified.
η vs Load "Y" Vin = 5V, Vo = 3.3V & 1.8V
100
100
90
90
EFFICIENCY (%)
EFFICIENCY (%)
η vs Load "X" Vin = 5V, Vo = 1.8V & 3.3V
80
70
60
50
40
80
70
60
50
1.8Vout
3.3Vout
40
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
LOAD CURRENT (A)
1.8Vout
3.3Vout
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
LOAD CURRENT (A)
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30165697
η vs Load "X and Y" Vin = 3.3V, Vo = 1.8V
Load Regulation
Vin = 3.3V, Vo = 1.8V (All Options)
100
90
EFFICIENCY (%)
LMR10510
Typical Performance Characteristics
80
70
60
50
40
LMR10510Y
LMR10510X
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
LOAD CURRENT (A)
30165698
30165683
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LMR10510
Load Regulation
Vin = 5V, Vo = 1.8V (All Options)
Load Regulation
Vin = 5V, Vo = 3.3V (All Options)
30165684
30165685
Oscillator Frequency vs Temperature - "X"
Oscillator Frequency vs Temperature - "Y"
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30165636
Current Limit vs Temperature
Vin = 3.3V
RDSON vs Temperature (LLP-6 Package)
30165687
30165686
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LMR10510
RDSON vs Temperature (SOT23-5 Package)
LMR10510X IQ (Quiescent Current)
30165688
30165628
LMR10510Y IQ (Quiescent Current)
Line Regulation
Vo = 1.8V, Io = 500mA
30165637
30165653
VFB vs Temperature
30165627
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8
LMR10510
Gain vs Frequency
(Vin = 5V, Vo = 1.2V @ 1A)
Phase Plot vs Frequency
(Vin = 5V, Vo = 1.2V @ 1A)
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30165657
Simplified Block Diagram
30165604
FIGURE 1.
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LMR10510
General Description
The LMR10510 regulator is a monolithic, high frequency,
PWM step-down DC/DC converter in a 5 pin SOT23 and a 6
Pin LLP package. It provides all the active functions to provide
local DC/DC conversion with fast transient response and accurate regulation in the smallest possible PCB area. With a
minimum of external components, the LMR10510 is easy to
use. The ability to drive 1.0A loads with an internal 130 mΩ
PMOS switch results in the best power density available. The
world-class control circuitry allows on-times as low as 30ns,
thus supporting exceptionally high frequency conversion over
the entire 3V to 5.5V input operating range down to the minimum output voltage of 0.6V. The LMR10510 is a constant
frequency PWM buck regulator IC that delivers a 1.0A load
current. The regulator has a preset switching frequency of
1.6MHz or 3.0MHz. This high frequency allows the
LMR10510 to operate with small surface mount capacitors
and inductors, resulting in a DC/DC converter that requires a
minimum amount of board space. The LMR10510 is internally
compensated, so it is simple to use and requires few external
components. Even though the operating frequency is high,
efficiencies up to 93% are easy to achieve. External shutdown
is included, featuring an ultra-low stand-by current of 30 nA.
The LMR10510 utilizes current-mode control and internal
compensation to provide high-performance regulation over a
wide range of operating conditions. Additional features include internal soft-start circuitry to reduce inrush current,
pulse-by-pulse current limit, thermal shutdown, and output
over-voltage protection.
30165666
FIGURE 2. Typical Waveforms
SOFT-START
This function forces VOUT to increase at a controlled rate during start up. During soft-start, the error amplifier’s reference
voltage ramps from 0V to its nominal value of 0.6V in approximately 600 µs. This forces the regulator output to ramp up in
a controlled fashion, which helps reduce inrush current.
OUTPUT OVERVOLTAGE PROTECTION
The over-voltage comparator compares the FB pin voltage to
a voltage that is 15% higher than the internal reference
VREF. Once the FB pin voltage goes 15% above the internal
reference, the internal PMOS control switch is turned off,
which allows the output voltage to decrease toward regulation.
Applications Information
THEORY OF OPERATION
The following operating description of the LMR10510 will refer
to the Simplified Block Diagram (Figure 1) and to the waveforms in Figure 2. The LMR10510 supplies a regulated output
voltage by switching the internal PMOS control switch at constant frequency and variable duty cycle. A switching cycle
begins at the falling edge of the reset pulse generated by the
internal oscillator. When this pulse goes low, the output control logic turns on the internal PMOS control switch. During
this on-time, the SW pin voltage (VSW) swings up to approximately VIN, and the inductor current (IL) increases with a linear
slope. IL is measured by the current sense amplifier, which
generates an output proportional to the switch current. The
sense signal is summed with the regulator’s corrective ramp
and compared to the error amplifier’s output, which is proportional to the difference between the feedback voltage and
VREF. When the PWM comparator output goes high, the output switch turns off until the next switching cycle begins.
During the switch off-time, inductor current discharges
through the Schottky catch diode, which forces the SW pin to
swing below ground by the forward voltage (VD) of the Schottky catch diode. The regulator loop adjusts the duty cycle (D)
to maintain a constant output voltage.
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UNDERVOLTAGE LOCKOUT
Under-voltage lockout (UVLO) prevents the LMR10510 from
operating until the input voltage exceeds 2.73V (typ). The
UVLO threshold has approximately 430 mV of hysteresis, so
the part will operate until VIN drops below 2.3V (typ). Hysteresis prevents the part from turning off during power up if VIN is
non-monotonic.
CURRENT LIMIT
The LMR10510 uses cycle-by-cycle current limiting to protect
the output switch. During each switching cycle, a current limit
comparator detects if the output switch current exceeds 1.75A
(typ), and turns off the switch until the next switching cycle
begins.
THERMAL SHUTDOWN
Thermal shutdown limits total power dissipation by turning off
the output switch when the IC junction temperature exceeds
165°C. After thermal shutdown occurs, the output switch
doesn’t turn on until the junction temperature drops to approximately 150°C.
10
LMR10510
30165695
FIGURE 3. Typical Application Schematic
Design Guide
INDUCTOR SELECTION
The Duty Cycle (D) can be approximated quickly using the
ratio of output voltage (VO) to input voltage (VIN):
In general,
ΔiL = 0.1 x (IOUT) → 0.2 x (IOUT)
If ΔiL = 20% of 1A, the peak current in the inductor will be 1.2A.
The minimum guaranteed current limit over all operating conditions is 1.2A. One can either reduce ΔiL, or make the engineering judgment that zero margin will be safe enough. The
typical current limit is 1.75A.
The LMR10510 operates at frequencies allowing the use of
ceramic output capacitors without compromising transient response. Ceramic capacitors allow higher inductor ripple without significantly increasing output ripple. See the output
capacitor section for more details on calculating output voltage ripple. Now that the ripple current is determined, the
inductance is calculated by:
The catch diode (D1) forward voltage drop and the voltage
drop across the internal PMOS must be included to calculate
a more accurate duty cycle. Calculate D by using the following
formula:
VSW can be approximated by:
VSW = IOUT x RDSON
The diode forward drop (VD) can range from 0.3V to 0.7V depending on the quality of the diode. The lower the VD, the
higher the operating efficiency of the converter. The inductor
value determines the output ripple current. Lower inductor
values decrease the size of the inductor, but increase the
output ripple current. An increase in the inductor value will
decrease the output ripple current.
One must ensure that the minimum current limit (1.2A) is not
exceeded, so the peak current in the inductor must be calculated. The peak current (ILPK) in the inductor is calculated by:
Where
When selecting an inductor, make sure that it is capable of
supporting the peak output current without saturating. Inductor saturation will result in a sudden reduction in inductance
and prevent the regulator from operating correctly. Because
of the speed of the internal current limit, the peak current of
the inductor need only be specified for the required maximum
output current. For example, if the designed maximum output
current is 1.0A and the peak current is 1.25A, then the inductor should be specified with a saturation current limit of >
1.25A. There is no need to specify the saturation or peak current of the inductor at the 1.75A typical switch current limit.
The difference in inductor size is a factor of 5. Because of the
operating frequency of the LMR10510, ferrite based inductors
are preferred to minimize core losses. This presents little restriction since the variety of ferrite-based inductors is huge.
Lastly, inductors with lower series resistance (RDCR) will pro-
ILPK = IOUT + ΔiL
30165605
FIGURE 4. Inductor Current
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LMR10510
tances in the inductor to the output. A ceramic capacitor will
bypass this noise while a tantalum will not. Since the output
capacitor is one of the two external components that control
the stability of the regulator control loop, most applications will
require a minimum of 22 µF of output capacitance. Capacitance often, but not always, can be increased significantly
with little detriment to the regulator stability. Like the input capacitor, recommended multilayer ceramic capacitors are X7R
or X5R types.
vide better operating efficiency. For recommended inductors
see Example Circuits.
INPUT CAPACITOR
An input capacitor is necessary to ensure that VIN does not
drop excessively during switching transients. The primary
specifications of the input capacitor are capacitance, voltage,
RMS current rating, and ESL (Equivalent Series Inductance).
The recommended input capacitance is 22 µF.The input voltage rating is specifically stated by the capacitor manufacturer.
Make sure to check any recommended deratings and also
verify if there is any significant change in capacitance at the
operating input voltage and the operating temperature. The
input capacitor maximum RMS input current rating (IRMS-IN)
must be greater than:
CATCH DIODE
The catch diode (D1) conducts during the switch off-time. A
Schottky diode is recommended for its fast switching times
and low forward voltage drop. The catch diode should be
chosen so that its current rating is greater than:
ID1 = IOUT x (1-D)
The reverse breakdown rating of the diode must be at least
the maximum input voltage plus appropriate margin. To improve efficiency, choose a Schottky diode with a low forward
voltage drop.
Neglecting inductor ripple simplifies the above equation to:
OUTPUT VOLTAGE
The output voltage is set using the following equation where
R2 is connected between the FB pin and GND, and R1 is
connected between VO and the FB pin. A good value for R2
is 10kΩ. When designing a unity gain converter (Vo = 0.6V),
R1 should be between 0Ω and 100Ω, and R2 should be equal
or greater than 10kΩ.
It can be shown from the above equation that maximum RMS
capacitor current occurs when D = 0.5. Always calculate the
RMS at the point where the duty cycle D is closest to 0.5. The
ESL of an input capacitor is usually determined by the effective cross sectional area of the current path. A large leaded
capacitor will have high ESL and a 0805 ceramic chip capacitor will have very low ESL. At the operating frequencies of the
LMR10510, leaded capacitors may have an ESL so large that
the resulting impedance (2πfL) will be higher than that required to provide stable operation. As a result, surface mount
capacitors are strongly recommended.
Sanyo POSCAP, Tantalum or Niobium, Panasonic SP, and
multilayer ceramic capacitors (MLCC) are all good choices for
both input and output capacitors and have very low ESL. For
MLCCs it is recommended to use X7R or X5R type capacitors
due to their tolerance and temperature characteristics. Consult capacitor manufacturer datasheets to see how rated
capacitance varies over operating conditions.
VREF = 0.60V
PCB LAYOUT CONSIDERATIONS
When planning layout there are a few things to consider when
trying to achieve a clean, regulated output. The most important consideration is the close coupling of the GND connections of the input capacitor and the catch diode D1. These
ground ends should be close to one another and be connected to the GND plane with at least two through-holes. Place
these components as close to the IC as possible. Next in importance is the location of the GND connection of the output
capacitor, which should be near the GND connections of CIN
and D1. There should be a continuous ground plane on the
bottom layer of a two-layer board except under the switching
node island. The FB pin is a high impedance node and care
should be taken to make the FB trace short to avoid noise
pickup and inaccurate regulation. The feedback resistors
should be placed as close as possible to the IC, with the GND
of R1 placed as close as possible to the GND of the IC. The
VOUT trace to R2 should be routed away from the inductor and
any other traces that are switching. High AC currents flow
through the VIN, SW and VOUT traces, so they should be as
short and wide as possible. However, making the traces wide
increases radiated noise, so the designer must make this
trade-off. Radiated noise can be decreased by choosing a
shielded inductor. The remaining components should also be
placed as close as possible to the IC. Please see Application
Note AN-1229 for further considerations and the LMR10510
demo board as an example of a good layout.
OUTPUT CAPACITOR
The output capacitor is selected based upon the desired output ripple and transient response. The initial current of a load
transient is provided mainly by the output capacitor. The output ripple of the converter is:
When using MLCCs, the ESR is typically so low that the capacitive ripple may dominate. When this occurs, the output
ripple will be approximately sinusoidal and 90° phase shifted
from the switching action. Given the availability and quality of
MLCCs and the expected output voltage of designs using the
LMR10510, there is really no need to review any other capacitor technologies. Another benefit of ceramic capacitors is
their ability to bypass high frequency noise. A certain amount
of switching edge noise will couple through parasitic capaci-
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12
If the inductor ripple current is fairly small, the conduction
losses can be simplified to:
The complete LMR10510 DC/DC converter efficiency can be
calculated in the following manner.
Switching losses are also associated with the internal PFET.
They occur during the switch on and off transition periods,
where voltages and currents overlap resulting in power loss.
The simplest means to determine this loss is to empirically
measuring the rise and fall times (10% to 90%) of the switch
at the switch node.
Switching Power Loss is calculated as follows:
PCOND = IOUT2 x RDSON x D
Or
PSWR = 1/2(VIN x IOUT x FSW x TRISE)
PSWF = 1/2(VIN x IOUT x FSW x TFALL)
PSW = PSWR + PSWF
Another loss is the power required for operation of the internal
circuitry:
Calculations for determining the most significant power losses are shown below. Other losses totaling less than 2% are
not discussed.
Power loss (PLOSS) is the sum of two basic types of losses in
the converter: switching and conduction. Conduction losses
usually dominate at higher output loads, whereas switching
losses remain relatively fixed and dominate at lower output
loads. The first step in determining the losses is to calculate
the duty cycle (D):
PQ = IQ x VIN
IQ is the quiescent operating current, and is typically around
3.3mA for the 1.6MHz frequency option.
Typical Application power losses are:
Power Loss Tabulation
VSW is the voltage drop across the internal PFET when it is
on, and is equal to:
VSW = IOUT x RDSON
VD is the forward voltage drop across the Schottky catch
diode. It can be obtained from the diode manufactures Electrical Characteristics section. If the voltage drop across the
inductor (VDCR) is accounted for, the equation becomes:
VIN
5.0V
VOUT
3.3V
POUT
3.3W
PDIODE
150mW
IOUT
1.0A
VD
0.45V
FSW
1.6MHz
IQ
3.3mA
PQ
17mW
TRISE
4nS
PSWR
16mW
TFALL
4nS
PSWF
16mW
RDS(ON)
150mΩ
PCOND
100mW
INDDCR
70mΩ
PIND
70mW
D
0.667
PLOSS
369mW
η
88%
PINTERNAL
149mW
ΣPCOND + PSW + PDIODE + PIND + PQ = PLOSS
ΣPCOND + PSWF + PSWR + PQ = PINTERNAL
PINTERNAL = 149mW
Thermal Definitions
The conduction losses in the free-wheeling Schottky diode
are calculated as follows:
TJ = Chip junction temperature
TA = Ambient temperature
RθJC = Thermal resistance from chip junction to device case
RθJA = Thermal resistance from chip junction to ambient air
Heat in the LMR10510 due to internal power dissipation is
removed through conduction and/or convection.
Conduction: Heat transfer occurs through cross sectional areas of material. Depending on the material, the transfer of
heat can be considered to have poor to good thermal conductivity properties (insulator vs. conductor).
Heat Transfer goes as:
Silicon → package → lead frame → PCB
Convection: Heat transfer is by means of airflow. This could
be from a fan or natural convection. Natural convection occurs
when air currents rise from the hot device to cooler air.
Thermal impedance is defined as:
PDIODE = VD x IOUT x (1-D)
Often this is the single most significant power loss in the circuit. Care should be taken to choose a Schottky diode that
has a low forward voltage drop.
Another significant external power loss is the conduction loss
in the output inductor. The equation can be simplified to:
PIND = IOUT2 x RDCR
The LMR10510 conduction loss is mainly associated with the
internal PFET:
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LMR10510
Calculating Efficiency, and Junction
Temperature
LMR10510
Thermal impedance from the silicon junction to the ambient
air is defined as:
Once this is determined, the maximum ambient temperature
allowed for a desired junction temperature can be found.
An example of calculating RθJA for an application using the
LMR10510 is shown below.
A sample PCB is placed in an oven with no forced airflow. The
ambient temperature was raised to 147°C, and at that temperature, the device went into thermal shutdown.
From the previous example:
The PCB size, weight of copper used to route traces and
ground plane, and number of layers within the PCB can greatly effect RθJA. The type and number of thermal vias can also
make a large difference in the thermal impedance. Thermal
vias are necessary in most applications. They conduct heat
from the surface of the PCB to the ground plane. Four to six
thermal vias should be placed under the exposed pad to the
ground plane if the LLP package is used.
Thermal impedance also depends on the thermal properties
of the application operating conditions (Vin, Vo, Io etc), and
the surrounding circuitry.
Silicon Junction Temperature Determination Method 1:
To accurately measure the silicon temperature for a given
application, two methods can be used. The first method requires the user to know the thermal impedance of the silicon
junction to case temperature.
RθJC is approximately 18°C/Watt for the 6-pin LLP package
with the exposed pad. Knowing the internal dissipation from
the efficiency calculation given previously, and the case temperature, which can be empirically measured on the bench
we have:
PINTERNAL = 149 mW
Since the junction temperature must be kept below 125°C,
then the maximum ambient temperature can be calculated as:
Tj - (RθJA x PLOSS) = TA
125°C - (121°C/W x 149 mW) = 107°C
LLP Package
where TC is the temperature of the exposed pad and can be
measured on the bottom side of the PCB.
Therefore:
30165668
FIGURE 5. Internal LLP Connection
Tj = (RθJC x PLOSS) + TC
For certain high power applications, the PCB land may be
modified to a "dog bone" shape (see Figure 6). By increasing
the size of ground plane, and adding thermal vias, the RθJA
for the application can be reduced.
From the previous example:
Tj = (RθJC x PINTERNAL) + TC
Tj = 18°C/W x 0.149W + TC
The second method can give a very accurate silicon junction
temperature.
The first step is to determine RθJA of the application. The
LMR10510 has over-temperature protection circuitry. When
the silicon temperature reaches 165°C, the device stops
switching. The protection circuitry has a hysteresis of about
15°C. Once the silicon temperature has decreased to approximately 150°C, the device will start to switch again. Knowing
this, the RθJA for any application can be characterized during
the early stages of the design one may calculate the RθJA by
placing the PCB circuit into a thermal chamber. Raise the
ambient temperature in the given working application until the
circuit enters thermal shutdown. If the SW-pin is monitored, it
will be obvious when the internal PFET stops switching, indicating a junction temperature of 165°C. Knowing the internal
power dissipation from the above methods, the junction temperature, and the ambient temperature RθJA can be determined.
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30165606
FIGURE 6. 6-Lead LLP PCB Dog Bone Layout
14
LMR10510
LMR10510X Design Example 1
30165607
FIGURE 7. LMR10510X (1.6MHz): Vin = 5V, Vo = 1.2V @ 1.0A
LMR10510X Design Example 2
30165608
FIGURE 8. LMR10510X (1.6MHz): Vin = 5V, Vo = 3.3V @ 1.0A
15
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LMR10510
LMR10510Y Design Example 3
30165660
FIGURE 9. LMR10510Y (3MHz): Vin = 5V, Vo = 3.3V @ 1.0A
LMR10510Y Design Example 4
30165662
FIGURE 10. LMR10510Y (3MHz): Vin = 5V, Vo = 1.2V @ 1.0A
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16
LMR10510
Physical Dimensions inches (millimeters) unless otherwise noted
5-Lead SOT-23 Package
NS Package Number MF05A
6-Lead LLP Package
NS Package Number SDE06A
17
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LMR10510 SIMPLE SWITCHER® 5.5Vin, 1A Step-Down Voltage Regulator in SOT-23 and LLP
Notes
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