TI1 LMR24220 Lmr24220 simple switcherâ® 42vin, 2.0a step-down voltage regulator in micro smd Datasheet

LMR24220
LMR24220 SIMPLE SWITCHER® 42Vin, 2.0A Step-Down Voltage Regulator in micro
SMD
Literature Number: SNVS737A
LMR24220
SIMPLE SWITCHER® 42Vin, 2.0A Step-Down Voltage
Regulator in micro SMD
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System Performance
Input voltage range of 4.5V to 42V
Output voltage range of 0.8V to 24V
Output current up to 2.0A
Integrated low RDS(ON) synchronous MOSFETs for high
efficiency
Up to 1 MHz switching frequency
Low shutdown Iq, 25 µA typical
Programmable soft-start
No loop compensation required
COT architecture with ERM
28 bump micro SMD (2.45 x 3.64 x 0.60 mm) packaging
Fully enabled for WEBENCH®™ Power Designer
Efficiency vs Load Current
(VOUT = 3.3V)
100
90
EFFICIENCY (%)
Features
70
60
50
Performance Benefits
40
■ Tiny overall solution reduces system cost
■ Integrated synchronous MOSFETs provides high
0.0
efficiency at low output voltages
■ COT with ERM architecture requires no loop
compensation, reduces component count, and provides
ultra fast transient response
■ Stable with low ESR capacitors
0.4
0.8
1.2
1.6
LOAD CURRENT (A)
2.0
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0.8
0.6
0.4
ΔVOUT (%)
Point-of-Load Conversions from 5V, 12V and 24V Rails
Space Constrained Applications
Industrial Distributed Power Applications
Power Meters
VIN = 4.5V
VIN = 9V
VIN = 12V
VIN = 24V
VIN = 42V
VOUT Regulation vs Load Current
(VOUT = 3.3V)
Applications
■
■
■
■
80
0.2
VIN = 4.5V
VIN = 9V
VIN = 12V
VIN = 24V
VIN = 42V
0.0
-0.2
-0.4
-0.6
-0.8
0.0
0.4
0.8
1.2
1.6
LOAD CURRENT (A)
2.0
30167671
SIMPLE SWITCHER® is a registered trademark of National Semiconductor Corporation
© 2011 National Semiconductor Corporation
301676
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LMR24220 SIMPLE SWITCHER® 42Vin, 2.0A Step-Down Voltage Regulator in micro SMD
October 5, 2011
LMR24220
Typical Application
30167601
Connection Diagram
30167669
28–ball micro SMD — Balls Facing Down
NS Package Number TLC28VFA
Ordering Information
Order Number
Package Type
NSC Package Drawing
LMR24220TL
28–ball micro SMD
TLC28VFA
250 Units on Tape and Reel
LMR24220TLX
28–ball micro SMD
TLC28VFA
1000 Units on Tape and Reel
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2
Supplied As
Ball
Name
Description
Application Information
A2, A3, B2, B3,
C2, C3, D2, D3,
D4
SW
Switching Node
Internally connected to the source of the main
MOSFET and the drain of the Synchronous MOSFET.
Connect to the inductor.
A4, B4
VIN
Input supply voltage
Supply pin to the device. Nominal input range is 4.5V
to 42V.
C4
BST
Connection for bootstrap capacitor
Connect a 33 nF capacitor from the SW pin to this pin.
An internal diode charges the capacitor during the main
MOSFET off-time.
E3, E4, F1, F2,
F3, G3
AGND
Analog Ground
Ground for all internal circuitry other than the PGND
pin.
G2
SS
Soft-start
An 8 µA internal current source charges an external
capacitor to provide the soft- start function.
G1
FB
Feedback
Internally connected to the regulation and over-voltage
comparators. The regulation setting is 0.8V at this pin.
Connect to feedback resistors.
G4
EN
Enable
Connect a voltage higher than 1.26V to enable the
regulator. Leaving this input open circuit will enable the
device at internal UVLO level.
F4
RON
On-time Control
An external resistor from the VIN pin to this pin sets the
main MOSFET on-time.
E1, E2
VCC
Start-up regulator Output
Nominally regulated to 6V. Connect a capacitor of not
less than 680 nF between the VCC and AGND pins for
stable operation.
A1, B1, C1, D1
GND
Power Ground
Synchronous MOSFET source connection. Tie to a
ground plane.
3
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LMR24220
Pin Descriptions
LMR24220
Storage Temperature Range
Junction Temperature (TJ)
Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
VIN, RON to AGND
SW to AGND
SW to AGND (Transient)
VIN to SW
BST to SW
All Other Inputs to AGND
ESD Rating (Note 2)
Human Body Model
Operating Ratings
-65°C to +150°C
150°C
(Note 1)
Supply Voltage Range (VIN)
Junction Temperature Range (TJ)
-0.3V to 43.5V
-0.3V to 43.5V
-2V (< 100ns)
-0.3V to 43.5V
-0.3V to 7V
-0.3V to 7V
4.5V to 42V
−40°C to +125°C
Thermal Resistance (θJA) 28 ball
μSMD(Note 5)
50°C/W
For soldering specifications: see product folder at
www.national.com and www.national.com/ms/MS/
MSSOLDERING. pdf
±2kV
Electrical Characteristics
Specifications with standard type are for TJ = 25°C only; limits in boldface type apply
over the full Operating Junction Temperature (TJ) range. 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. Unless otherwise stated the following conditions apply: VIN = 18V, VOUT = 3.3V.(Note 3)
Symbol
Parameter
Conditions
Min
Typ
Max
Units
5.0
6.0
7.2
V
Start-Up Regulator, VCC
VCC
VCC output voltage
CCC = 680nF, no load
VIN - VCC
VIN - VCC dropout voltage
ICC = 20mA
IVCCL
VCC current limit (Note 4)
VCC = 0V
VCC under-voltage lockout threshold
(UVLO)
VIN increasing
VCC-UVLO-HYS
VCC UVLO hysteresis
VIN decreasing – μSMD
package
150
tVCC-UVLO-D
VCC UVLO filter delay
No switching, VFB = 1V
0.7
1
mA
25
40
µA
VCC-UVLO
IIN
IIN-SD
IIN operating current
350
mV
40
65
mA
3.55
3.75
3.95
mV
3
IIN operating current, Device shutdown VEN = 0V
V
µs
Switching Characteristics
RDS-UP-ON
Main MOSFET RDS(on)
0.18
0.375
Ω
RDS- DN-ON
Syn. MOSFET RDS(on)
0.11
0.225
Ω
4.2
V
VG-UVLO
Gate drive voltage UVLO
VBST - VSW increasing
3.3
SS pin source current
VSS = 0.5V
11
Syn. MOSFET current limit threshold
LMR24220
ON timer pulse width
VIN = 10V, RON = 100 kΩ
1.38
VIN = 30V, RON = 100 kΩ
0.47
Soft-start
ISS
µA
Current Limit
ICL
2.156
2.8
3.4
A
ON/OFF Timer
ton
ton-MIN
toff
µs
ON timer minimum pulse width
150
ns
OFF timer pulse width
260
ns
Enable Input
VEN
VEN-HYS
EN Pin input threshold
VEN rising
Enable threshold hysteresis
VEN falling
1.13
1.18
1.23
90
V
mV
Regulation and Over-Voltage Comparator
VFB
VFB-OV
IFB
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In-regulation feedback voltage
VSS ≥ 0.8V
TJ = −40°C to +125°C
Feedback over-voltage threshold
FB pin current
0.784
0.8
0.816
0.888
0.920
0.945
5
4
V
V
nA
Parameter
Conditions
Min
Typ
Max
Units
Thermal Shutdown
TSD
Thermal shutdown temperature
TJ rising
165
°C
TSD-HYS
Thermal shutdown temperature
hysteresis
TJ falling
20
°C
Note 1: Absolute Maximum Ratings are limits beyond which damage to the device may occur. Operating Ratings are conditions under which operation of the
device is intended to be functional. For guaranteed specifications and test conditions, see the Electrical Characteristics.
Note 2: The human body model is a 100pF capacitor discharged through a 1.5kΩ resistor into each pin.
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: VCC provides self bias for the internal gate drive and control circuits. Device thermal limitations limit external loading.
Note 5: θJA calculations were performed in general accordance with JEDEC standards JESD51–1 to JESD51–11.
5
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LMR24220
Symbol
Unless otherwise speficified all curves are taken at VIN = 18V with the configuration in the typical application circuit for VOUT = 3.3V
(Figure 8) TA = 25°C.
VCC vs ICC
VCC vs VIN
30167604
30167605
ton vs VIN
Switching Frequency, fSW vs VIN, VOUT=0.8V,
700
SWITCHING FREQENCY (kHZ)
LMR24220
Typical Performance Characteristics
Ron = 12.4kΩ; L = 2.2μH, Io = 0.5A
Ron = 12.4kΩ; L = 2.2μH, Io = 2A
Ron = 49.9kΩ; L = 3.3μH, Io = 0.5A
Ron = 49.9kΩ; L = 3.3μH, Io = 2A
600
500
400
300
200
100
0
0
10
20
30
VIN (v)
30167606
50
30167675
VFB vs Temperature
RDS(on) vs Temperature
30167608
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40
30167609
6
VOUT Regulation vs Load Current
(VOUT = 3.3V)
100
0.8
0.6
90
VIN = 4.5V
VIN = 9V
VIN = 12V
VIN = 24V
VIN = 42V
0.4
80
ΔVOUT (%)
EFFICIENCY (%)
LMR24220
Efficiency vs Load Current
(VOUT = 3.3V)
70
60
VIN = 4.5V
VIN = 9V
VIN = 12V
VIN = 24V
VIN = 42V
50
40
0.0
0.2
0.0
-0.2
-0.4
-0.6
-0.8
0.4
0.8
1.2
1.6
LOAD CURRENT (A)
2.0
0.0
0.4
0.8
1.2
1.6
LOAD CURRENT (A)
2.0
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Efficiency vs Load Current
(VOUT = 0.8V)
VOUT Regulation vs Load Current
(VOUT = 0.8V)
100
0.6
90
0.4
80
ΔVOUT (%)
EFFICIENCY (%)
VIN = 4.5V
VIN = 9V
VIN = 12V
VIN = 24V
VIN = 42V
0.5
70
60
VIN = 4.5V
VIN = 9V
VIN = 12v
VIN = 24V
VIN = 42v
50
40
0.0
0.4
0.8
1.2
1.6
LOAD CURRENT (A)
0.3
0.2
0.1
0.0
-0.1
-0.2
-0.3
2.0
0.0
0.4
0.8
1.2
1.6
LOAD CURRENT (A)
30167672
2.0
30167673
Power Up
(VOUT = 3.3V, 2A Loaded)
Startup with Enable
(VOUT = 3.3V, 2A Loaded)
30167614
30167676
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LMR24220
Shutdown Transient
(VOUT = 3.3V, 2A Loaded)
Continuous Mode Operation
(VOUT = 3.3V, 2A Loaded)
30167616
30167615
Discontinuous Mode Operation
(VOUT = 3.3V, 0.050A Loaded)
DCM to CCM Transition
(VOUT = 3.3V, 0.50A - 2A Load)
30167617
30167618
Load Transient
(VOUT = 3.3V, 0.20A - 2A Load,)
30167619
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LMR24220
Simplified Functional Block Diagram
30167620
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LMR24220
Functional Description
VOUT = 0.8V x (RFB1 + RFB2)/RFB2
The LMR24220 Step Down Switching Regulator features all
required functions to implement a cost effective, efficient buck
power converter capable of supplying 2A to a load. It contains
Dual N-Channel main and synchronous MOSFETs. The Constant ON-Time (COT) regulation scheme requires no loop
compensation, results in fast load transient response and
simple circuit implementation. The regulator can function
properly even with an all ceramic output capacitor network,
and does not rely on the output capacitor’s ESR for stability.
The operating frequency remains constant with line variations
due to the inverse relationship between the input voltage and
the on-time. The valley current limit detection circuit, with the
limit set internally at 2.8A, inhibits the main MOSFET until the
inductor current level subsides.
The LMR24220 can be applied in numerous applications and
can operate efficiently for inputs as high as 42V. Protection
features include output over-voltage protection, thermal shutdown, VCC under-voltage lock-out and gate drive under-voltage lock-out. The LMR24220 is available in a small micro
SMD chip scale package.
(3)
Startup Regulator (VCC)
A startup regulator is integrated within the LMR24220. The
input pin VIN can be connected directly to a line voltage up to
42V. The VCC output regulates at 6V, and is current limited to
65 mA. Upon power up, the regulator sources current into an
external capacitor CVCC, which is connected to the VCC pin.
For stability, CVCC must be at least 680 nF. When the voltage
on the VCC pin is higher than the under-voltage lock-out (UVLO) threshold of 3.75V, the main MOSFET is enabled and the
SS pin is released to allow the soft-start capacitor CSS to
charge.
The minimum input voltage is determined by the dropout voltage of the regulator and the VCC UVLO falling threshold
(≊3.7V). If VIN is less than ≊4.0V, the regulator shuts off and
VCC goes to zero.
Regulation Comparator
The feedback voltage at the FB pin is compared to a 0.8V
internal reference. In normal operation (the output voltage is
regulated), an on-time period is initiated when the voltage at
the FB pin falls below 0.8V. The main MOSFET stays on for
the on-time, causing the output voltage and consequently the
voltage of the FB pin to rise above 0.8V. After the on-time
period, the main MOSFET stays off until the voltage of the FB
pin falls below 0.8V again. Bias current at the FB pin is nominally 5 nA.
COT Control Circuit Overview
COT control is based on a comparator and a one-shot ontimer, with the output voltage feedback (feeding to the FB pin)
compared with an internal reference of 0.8V. If the voltage of
the FB pin is below the reference, the main MOSFET is turned
on for a fixed on-time determined by a programming resistor
RON and the input voltage VIN, upon which the on-time varies
inversely. Following the on-time, the main MOSFET remains
off for a minimum of 260 ns. Then, if the voltage of the FB pin
is below the reference, the main MOSFET is turned on again
for another on-time period. The switching will continue to
achieve regulation.
The regulator will operate in the discontinuous conduction
mode (DCM) at a light load, and the continuous conduction
mode (CCM) with a heavy load. In the DCM, the current
through the inductor starts at zero and ramps up to a peak
during the on-time, and then ramps back to zero before the
end of the off-time. It remains zero and the load current is
supplied entirely by the output capacitor. The next on-time
period starts when the voltage at the FB pin falls below the
internal reference. The operating frequency in the DCM is
lower and varies larger with the load current as compared with
the CCM. Conversion efficiency is maintained since conduction loss and switching loss are reduced with the reduction in
the load and the switching frequency respectively. The operating frequency in the DCM can be calculated approximately
as follows:
Zero Coil Current Detect
The current of the synchronous MOSFET is monitored by a
zero coil current detection circuit which inhibits the synchronous MOSFET when its current reaches zero until the
next on-time. This circuit enables the DCM operation, which
improves the efficiency at a light load.
Over-Voltage Comparator
The voltage at the FB pin is compared to a 0.92V internal
reference. If it rises above 0.92V, the on-time is immediately
terminated. This condition is known as over-voltage protection (OVP). It can occur if the input voltage or the output load
changes suddenly. Once the OVP is activated, the main
MOSFET remains off until the voltage at the FB pin falls below
0.92V. The synchronous MOSFET will stay on to discharge
the inductor until the inductor current reduces to zero, and
then switches off.
ON-Time Timer, Shutdown
The on-time of the LMR24220 main MOSFET is determined
by the resistor RON and the input voltage VIN. It is calculated
as follows:
(1)
In the continuous conduction mode (CCM), the current flows
through the inductor in the entire switching cycle, and never
reaches zero during the off-time. The operating frequency remains relatively constant with load and line variations. The
CCM operating frequency can be calculated approximately as
follows:
(4)
The inverse relationship of ton and VIN gives a nearly constant
frequency as VIN is varied. RON should be selected such that
the on-time at maximum VIN is greater than 150 ns. The ontimer has a limiter to ensure a minimum of 150 ns for ton. This
limits the maximum operating frequency, which is governed
by the following equation:
(2)
The output voltage is set by two external resistors RFB1 and
RFB2. The regulated output voltage is
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(5)
The LMR24220 can be remotely shutdown by pulling the voltage of the EN pin below 1V. In this shutdown mode, the SS
pin is internally grounded, the on-timer is disabled, and bias
currents are reduced. Releasing the EN pin allows normal
operation to resume because the EN pin is internally pulled
up.
30167625
FIGURE 1. Shutdown Implementation
(6)
During current limit, the LMR24220 operates in a constant
current mode with an average output current IOUT(CL) equal to
2.8A + ILR / 2.
However, due to thermal limitations, the device may not support load currents greater than 2A for extended periods.
Current Limit
Current limit detection is carried out during the off-time by
monitoring the re-circulating current through the synchronous
MOSFET. Referring to the Functional Block Diagram, when
30167626
FIGURE 2. Inductor Current - Current Limit Operation
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LMR24220
the main MOSFET is turned off, the inductor current flows
through the load, the PGND pin and the internal synchronous
MOSFET. If this current exceeds 2.8A, the current limit comparator toggles, and as a result disabling the start of the next
on-time period. The next switching cycle starts when the recirculating current falls back below 2.8A (and the voltage at
the FB pin is below 0.8V). The inductor current is monitored
during the on-time of the synchronous MOSFET. As long as
the inductor current exceeds 2.8A, the main MOSFET will remain inhibited to achieve current limit. The operating frequency is lower during current limit due to a longer off-time.
Figure 2 illustrates an inductor current waveform. On average, the output current IOUT is the same as the inductor
current IL, which is the average of the rippled inductor current.
In case of current limit (the current limit portion of Figure 2),
the next on-time will not initiate until the current drops below
2.8 (assume the voltage at the FB pin is lower than 0.8V).
During each on-time the current ramps up an amount equal
to:
Thermal Protection
The LMR24220 integrates an N-Channel main MOSFET and
an associated floating high voltage main MOSFET gate driver. The gate drive circuit works in conjunction with an external
bootstrap capacitor CBST and an internal high voltage diode.
CBST connecting between the BST and SW pins powers the
main MOSFET gate driver during the main MOSFET on-time.
During each off-time, the voltage of the SW pin falls to approximately -1V, and CBST charges from VCC through the
internal diode. The minimum off-time of 260 ns provides
enough time for charging CBST in each cycle.
The junction temperature of the LMR24220 should not exceed the maximum limit. Thermal protection is implemented
by an internal Thermal Shutdown circuit, which activates (typically) at 165°C to make the controller enter a low power reset
state by disabling the main MOSFET, disabling the on-timer,
and grounding the SS pin. Thermal protection helps prevent
catastrophic failures from accidental device overheating.
When the junction temperature falls back below 145°C (typical hysteresis = 20°C), the SS pin is released and normal
operation resumes.
Soft-Start
Thermal Derating
The soft-start feature allows the converter to gradually reach
a steady state operating point, thereby reducing startup
stresses and current surges. Upon turn-on, after VCC reaches
the under-voltage threshold, an 8 µA internal current source
charges up an external capacitor CSS connecting to the SS
pin. The ramping voltage at the SS pin (and the non-inverting
input of the regulation comparator as well) ramps up the output voltage VOUT in a controlled manner.
The soft start time duration to reach steady state operation is
given by the formula:
tSS=VREFx CSS / 8µA = 0.8V x CSS / 8µA
This equation can be rearranged as follows:
CSS= tSSx 8µA / 0.8V
Use of a 4.7nF capacitor results in a 0.5ms soft-start duration.
This is a recommended value. Note that high values of CSS
capacitance will cause more output voltage drop when a load
transient goes across the DCM-CCM boundary. If a fast load
transient response is desired for steps between DCM and
CCM mode the softstart capacitor value should be less than
18nF (which corresponds to a soft-start time of 1.8ms).
An internal switch grounds the SS pin if any of the following
three cases happens: (i) VCC is below the under-voltage lockout threshold; (ii) a thermal shutdown occurs; or (iii) the EN
pin is grounded. Alternatively, the output voltage can be shut
off by connecting the SS pin to ground using an external
switch. Releasing the switch allows the SS pin to ramp up and
the output voltage to return to normal. The shutdown configuration is shown in Figure 3.
Temperature rise increases with frequency, load current, input voltage and smaller board dimensions. On a typical board,
the LMR24220 is capable of supplying 2A below an ambient
temperature of 50°C under worst case operation with input
voltage of 42V. Figure 4 shows a thermal derating curve for
the output current without thermal shutdown against ambient
temperature up to 125°C. Obtaining 2A output current is possible at higher temperature by increasing the PCB ground
plane area, adding air flow or reducing the input voltage or
operating frequency
2.4
2.0
MAXIMUM IOUT (A)
LMR24220
N-Channel MOSFET and Driver
1.6
1.2
0.8
0.4
0.0
0
25
50
75
100
AMBIENT TEMPERATURE (°C)
125
30167674
FIGURE 4. Thermal Derating Curve, θJA=40°C/W, Vo =
3.3V, fs = 500kHz (tested on the evaluation board)
30167627
FIGURE 3. Alternate Shutdown Implementation
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EXTERNAL COMPONENTS
The following guidelines can be used to select external components.
RFB1 and RFB2 : These resistors should be chosen from standard values in the range of 1.0 kΩ to 10 kΩ, satisfying the
following ratio:
RFB1/RFB2 = (VOUT/0.8V) - 1
(7)
For VOUT = 0.8V, the FB pin can be connected to the output
directly with a pre-load resistor drawing more than 20 µA. This
is needed because the converter operation needs a minimum
inductor current ripple to maintain good regulation when no
load is connected.
RON: Equation (2) can be used to select RON if a desired operating frequency is selected. But the minimum value of
RON is determined by the minimum on-time. It can be calculated as follows:
30167631
FIGURE 6. Inductor selection for VOUT = 3.3V
(8)
If RON calculated from (2) is smaller than the minimum value
determined in (8), a lower frequency should be selected to recalculate RON by (2). Alternatively, VIN(MAX) can also be limited
in order to keep the frequency unchanged. The relationship
of VIN(MAX) and RON is shown in Figure 5.
On the other hand, the minimum off-time of 260 ns can limit
the maximum duty ratio.
30167632
FIGURE 7. Inductor selection for VOUT = 0.8V
Figure 6 and Figure 7 show curves on inductor selection for
various VOUT and RON. For small RON, according to (8), VIN is
limited. Some curves are therefore limited as shown in the
figures.
CVCC: The capacitor on the VCC output provides not only noise
filtering and stability, but also prevents false triggering of the
VCC UVLO at the main MOSFET on/off transitions. CVCC
should be no smaller than 680 nF for stability, and should be
a good quality, low ESR, ceramic capacitor.
COUT and COUT3: COUT should generally be no smaller than
10 µF. Experimentation is usually necessary to determine the
minimum value for COUT, as the nature of the load may require
a larger value. A load which creates significant transients requires a larger COUT than a fixed load.
COUT3 is a small value ceramic capacitor located close to the
LMR24220 to further suppress high frequency noise at
VOUT. A 100 nF capacitor is recommended.
CIN and CIN3: The function of CIN is to supply most of the main
MOSFET current during the on-time, and limit the voltage rip-
30167629
FIGURE 5. Maximum VIN for selected RON
L: The main parameter affected by the inductor is the amplitude of inductor current ripple (ILR). Once ILR is selected, L can
be determined by:
(9)
where VIN is the maximum input voltage and fSW is determined
from (2).
If the output current IOUT is determined, by assuming that
IOUT = IL, the higher and lower peak of ILR can be determined.
Beware that the higher peak of ILR should not be larger than
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LMR24220
the saturation current of the inductor and current limits of the
main and synchronous MOSFETs. Also, the lower peak of
ILR must be positive if CCM operation is required.
Applications Information
LMR24220
ple at the VIN pin, assuming that the voltage source connecting to the VIN pin has finite output impedance. If the voltage
source’s dynamic impedance is high (effectively a current
source), CIN supplies the average input current, but not the
ripple current.
At the maximum load current, when the main MOSFET turns
on, the current to the VIN pin suddenly increases from zero
to the lower peak of the inductor’s ripple current and ramps
up to the higher peak value. It then drops to zero at turn-off.
The average current during the on-time is the load current.
For a worst case calculation, CIN must be capable of supplying
this average load current during the maximum on-time. CIN is
calculated from:
PC BOARD LAYOUT
The LMR24220 regulation, over-voltage, and current limit
comparators are very fast and may respond to short duration
noise pulses. Layout is therefore critical for optimum performance. It must be as neat and compact as possible, and all
external components must be as close to their associated
pins of the LMR24220 as possible. Refer to the functional
block diagram, the loop formed by CIN, the main and synchronous MOSFET internal to the LMR24220, and the PGND
pin should be as small as possible. The connection from the
PGND pin to CIN should be as short and direct as possible.
Vias should be added to connect the ground of CIN to a ground
plane, located as close to the capacitor as possible. The bootstrap capacitor CBST should be connected as close to the SW
and BST pins as possible, and the connecting traces should
be thick. The feedback resistors and capacitor RFB1, RFB2, and
CFB should be close to the FB pin. A long trace running from
VOUT to RFB1 is generally acceptable since this is a low
impedance node. Ground RFB2 directly to the AGND pin. The
output capacitor COUT should be connected close to the load
and tied directly to the ground plane. The inductor L should
be connected close to the SW pin with as short a trace as
possible to reduce the potential for EMI (electromagnetic interference) generation. If it is expected that the internal dissipation of the LMR24220 will produce excessive junction
temperature during normal operation, making good use of the
PC board’s ground plane can help considerably to dissipate
heat. Additionally the use of thick traces, where possible, can
help conduct heat away from the LMR24220. Judicious positioning of the PC board within the end product, along with the
use of any available air flow (forced or natural convection) can
help reduce the junction temperature.
(10)
where IOUT is the load current, ton is the maximum on-time,
and ΔVIN is the allowable ripple voltage at VIN.
CIN3’s purpose is to help avoid transients and ringing due to
long lead inductance at the VIN pin. A low ESR 0.1 µF ceramic
chip capacitor located close to the LMR24220 is recommended.
CBST: A 33 nF high quality ceramic capacitor with low ESR is
recommended for CBST since it supplies a surge current to
charge the main MOSFET gate driver at turn-on. Low ESR
also helps ensure a complete recharge during each off-time.
CSS: The capacitor at the SS pin determines the soft-start
time, i.e. the time for the reference voltage at the regulation
comparator and the output voltage to reach their final value.
The time is determined from the following equation:
Package Considerations
The die has exposed edges and can be sensitive to ambient
light. For applications with direct high intensitiy ambient red,
infrared, LED or natural light it is recommended to have the
device shielded from the light source to avoid abnormal behavior.
(11)
CFB: If the output voltage is higher than 1.6V, CFB is needed
in the Discontinuous Conduction Mode to reduce the output
ripple. The recommended value for CFB is 10 nF.
30167635
FIGURE 8. Typical Application Schematic for VOUT = 3.3V
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14
LMR24220
30167636
FIGURE 9. Typical Application Schematic for VOUT = 0.8V
15
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LMR24220
Physical Dimensions inches (millimeters) unless otherwise noted
28–Ball μSMD
NS Package Number TLC28VFA
X1 = 2449 +/- 30 µm
X2 = 3643 +/- 30 µm
X3 = 600 +/- 75µm
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16
LMR24220
Notes
17
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LMR24220 SIMPLE SWITCHER® 42Vin, 2.0A Step-Down Voltage Regulator in micro SMD
Notes
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