User's Guide
SNVA268A–October 2007–Revised April 2013
AN-1678 LM3103 Demonstration Board Reference Design
1 Introduction
The LM3103 Step Down Switching Regulator features all required functions to implement a cost effective,
efficient buck power converter capable of supplying 0.75A to loads. The Constant On-Time (COT)
regulation scheme requires no loop compensation, results in a fast load transient response and simple
circuit implementation which allows a low component count, and consequently very small overall board
space is required for a typical application. 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. Protection features include output over-voltage protection, thermal shutdown, VCC under-
voltage lock-out, gate drive under-voltage lock-out. The LM3103 is available in the thermally enhanced
HTSSOP-16 package.
2 Demonstration Board Schematic and PCB
Figure 1. LM3103 Demonstration Board Schematic
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Demonstration Board Schematic and PCB
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Figure 2. LM3103 Demonstration Board PCB Top Overlay
Figure 3. LM3103 Demonstration Board PCB Top View
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Demonstration Board Schematic and PCB
Figure 4. LM3103 Demonstration Board PCB Bottom View
Table 1. Demonstration Board Quick Setup Procedures
Step Description Notes
1 Connect a power supply to VIN terminals VIN range: 8V to 42V
2 Connect a load to VOUT terminals IOUT range: 0A to 0.75A
3 SD (JP1) should be left open for normal operation. Short this jumper to
shutdown
4 Set VIN = 18V, with 0A load applied, check VOUT with a voltmeter Nominal 3.3V
5 Apply 0.75A load and check VOUT Nominal 3.3V
6 Short output terminals and check the short circuit current with an ammeter Nominal 1.05A
7 Short SD (JP1) to check the shutdown function
Table 2. Demonstration Board Performance Characteristic
Description Symbol Condition Min Typ Max Unit
Input Voltage VIN 8 18 42 V
Output Voltage VOUT 3.2 3.3 3.4 V
Output Current IOUT 0 - 0.75 A
Output Voltage Ripple VOUT(Ripple) - - 50 mVp-p
Output Voltage Regulation ΔVOUT ALL VIN and IOUT Conditions -2 +2 %
Efficiency VIN = 8V 85 91 %
VIN = 24V 71 84 %
VIN = 42V 59 78 %
(IOUT = 0.1A to 0.75A)
Output Short Current Limit ILIM-SC 1.05 A
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VOUT x (VIN - VOUT)
ILR x fSW x VIN
L1 =
8.3 x 10-11
VIN(MAX) x 100 ns
R1 t
VOUT
8.3 x 10-11 x fSW
R1 =
10 k:= 2.22 k:
- 1
VOUT
0.6
R4 =
=VOUT
0.6
R3
R4 - 1
Design Procedure
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3 Design Procedure
The LM3103 is easy to use compared with other devices available on the market because it integrates all
key components, including both the main and synchronous power MOSFETs, in a single package and
requires no loop compensation owing to the use of the Constant On-Time (COT) hysteretic control
scheme. The design of the demonstration board in this application note is detailed below.
Design Parameters:
VIN = 8V to 42V, typical 18V
VOUT = 3.3V
IOUT = 0.75A
Step 1: Calculate the feedback resistors
The ratio of the feedback resistor can be calculated from the following equation:
(1)
As a general practice, R3 and R4 should be chosen from standard 1% resistor values in the range of 1.0
kΩto 10 kΩsatisfying the above ratio. Now, select R3 = 10 kΩ, with VOUT = 3.3V,
(2)
Step 2: Calculate the on-time setting resistor
The switching frequency fSW of the demonstration board is affected by the on-time ton of the LM3103, which
is determined by R1. If fSW and VOUT are determined, R1 can be calculated as follows:
(3)
For this demonstration board design, VOUT = 3.3V and fSW = 500 kHz are chosen. As a result, R1 = 78.52
kΩ. To ensure that the on-time is larger than the minimum limit, which is 100 ns, the value of R1 must
satisfy the following equation:
(4)
Now the maximum VIN is 42V, the calculated R1 satisfies the above equation.
Step 3: Determine the inductance
The main parameter affected by the inductor is the amplitude of the inductor current ripple ILR. Once ILR is
selected, L1 can be determined by:
(5)
For this demonstration board design, ILR = 0.3A is selected. Now VIN = 18V, VOUT = 3.3V, and fSW = 500
kHz. As a result, L = 17.97 µH.
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C5 x 0.6V
tSS > 180 Ps + 70 PA
IOUT x ton
'VIN
C1 =
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Design Procedure
Figure 5. Inductor Selection for VOUT = 3.3V
Step 4: Determine the value of other components
C1: The function of C1 is to supply most of the main MOSFET current during the on-time, and limit the
voltage ripple 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), C1 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, C1 must be capable
of supplying this average load current during the maximum on-time. C1 is calculated from:
(6)
where IOUT is the load current, ton is the maximum on-time, and ΔVIN is the allowable ripple voltage at VIN.
In this demonstration board, a 10 µF capacitor is used.
C3: C3’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 LM3103 is used in this demonstration board.
C4: A 33 nF high quality ceramic capacitor with low ESR is used for C4 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.
C5: 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 soft-start time is affected by
the output capacitor, which can lengthen the time, and the charging of C5. The minimum soft-start time is
determined by the following equation:
(7)
In this demonstration board, a 33 nF capacitor is used for C5, and the corresponding soft-start time is
about 600 µs.
C8: 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. C8 should be no smaller than 1 µF for
stability, and should be a good quality, low ESR, ceramic capacitor.
C9: If the output voltage is higher than 1.6V, C9 is needed in the Discontinuous Conduction Mode to
reduce the output ripple. In this demonstration board, a 10 nF capacitor is used.
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PC Board Layout
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C10: The output capacitor should generally be no smaller than 10 µF. Experiment is usually necessary to
determine the minimum value for the output capacitor, as the nature of the load may require a larger
value. A load which creates significant transients requires a larger output capacitor than a fixed load. In
this demonstration board, a 47 µF capacitor is used to provide a low output ripple.
C12: C12 is a small value ceramic capacitor located close to the LM3103 to further suppress high
frequency noise at VOUT. A 47 nF capacitor is used in this demonstration board.
4 PC Board Layout
The LM3103 regulation, over-voltage, and current limit comparators are very fast so they will 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
LM3103 as possible. The loop formed by C1, the main and synchronous MOSFET internal to the LM3103,
and the PGND pin should be as small as possible. The connection from the PGND pin to the input
capacitors should be as short and direct as possible. Vias should be added to connect the ground of the
input capacitors to a ground plane, located as close to the capacitor as possible. The bootstrap capacitor
C4 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 R3, R4, and C9 should be close to the FB pin. A long trace
running from VOUT to R3 is generally acceptable since this is a low impedance node. Ground R4 directly to
the AGND pin (pin 7). The output capacitor C10 should be connected close to the load and tied directly to
the ground plane. The inductor L1 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 LM3103 will produce excessive junction temperature during normal operation,
making good use of the PC board’s ground plane can help considerably to dissipate heat. The exposed
pad on the bottom of the LM3103 IC package can be soldered to the ground plane, which should extend
out from beneath the LM3103 to help dissipate heat. The exposed pad is internally connected to the
LM3103 IC substrate. Additionally the use of thick traces, where possible, can help conduct heat away
from the LM3103. Using numerous vias to connect the die attached pad to the ground plane is a good
practice. 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.
5 Bill of Materials
Designation Description Size Manufacturer Part # Vendor
C1 Cap 10µF 50V Y5V 1210 GRM32DF51H106ZA01L Murata
C3 0603/X7R/0.1µF/50V 0603 ECJ1VB1H104K Panasonic
C4, C5 0603/X7R/33000pF/50V 0603 ECJ1VB1H333K Panasonic
C8 0603/X5R/1µF/10V 0603 GRM188R61A105KA61B Murata
C9 0603/X7R/10000pF/50V 0603 ECJ1VB1H103K Panasonic
GRM188R71H103KA01B Murata
C10 1210/X5R/47µF/6.3V 1210 ECJ4YB0J476M Panasonic
GRM32ER60J476ME20B Murata
C12 0603/X7R/47000pF/50V 0603 ECJ1VB1H473K Panasonic
R1 Resistor Chip 78.7kΩF 0603 CRCW06037872F Vishay
R3 Resistor Chip 10kΩF 0603 CRCW06031002F Vishay
R4 Resistor Chip 2.21kΩF 0603 CRCW06032211F Vishay
L1 Power Inductor 18µH 1.45A 6.8×6.8×3 CDR6D28MNNP-180NC Sumida
Power Inductor 18µH 1.7A 7.3×7.3×3.2 7447789118 Wurth
U1 IC LM3103 HTSSOP-16 LM3103 Texas Instruments
PCB LM3103 demo board Texas Instruments
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Typical Performance and Waveforms
6 Typical Performance and Waveforms
All curves and waveforms are taken at VIN = 18V with the demonstration board and TA= 25°C unless
otherwise specified.
Efficiency VOUT Regulation
vs vs
Load Current Load Current
(VOUT = 3.3V) (VOUT = 3.3V)
Continuous Mode Operation Discontinuous Mode Operation
(VOUT = 3.3V, 0.75A Loaded) (VOUT = 3.3V, 0.02A Loaded)
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Typical Performance and Waveforms
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Load Transient
DCM to CCM Transition (VOUT = 3.3V, 0.075A - 0.75A Load,
(VOUT = 3.3V, 0.01A - 0.75A Load) Current slew-rate: 2.5A/µs)
Power Up Enable Transient
(VOUT = 3.3V, 0.75A Loaded) (VOUT = 3.3V, 0.75A Loaded)
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