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LM3485
SNVS178H – JANUARY 2002 – REVISED DECEMBER 2015
LM3485 Hysteretic PFET Buck Controller
1 Features
3 Description
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The LM3485 is a high-efficiency PFET switching
regulator controller that can be used to quickly and
easily develop a small, low-cost, switching buck
regulator for a wide range of applications. The
hysteretic control architecture provides for simple
design without any control-loop stability concerns
using a wide variety of external components. The
PFET architecture also allows for low component
count as well as ultralow dropout, 100% duty cycle
operation. Another benefit is high efficiency operation
at light loads without an increase in output ripple.
1
Easy-to-Use Control Methodology
No Control-Loop Compensation Required
4.5-V to 35-V Wide Input Range
1.242-V to VIN Adjustable Output Range
High Efficiency 93%
±1.3% (±2% Over Temp) Internal Reference
100% Duty Cycle
Maximum Operating Frequency > 1 MHz
Current Limit Protection
2 Applications
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Set-Top Box
DSL or Cable Modem
PC/IA
Auto PC
TFT Monitor
Battery-Powered Portable Applications
Distributed Power Systems
Always On Power
Current limit protection is provided by measuring the
voltage across the RDS(ON) of the PFET, thus
eliminating the need for a sense resistor. The cycleby-cycle current limit can be adjusted with a single
resistor, ensuring safe operation over a range of
output currents.
Device Information(1)
PART NUMBER
LM3485
PACKAGE
VSSOP (8)
BODY SIZE (NOM)
3.0 mm × 3.0 mm
(1) For all available packages, see the orderable addendum at
the end of the data sheet.
Typical Application Circuit
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.
LM3485
SNVS178H – JANUARY 2002 – REVISED DECEMBER 2015
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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 Overview ................................................................. 10
7.2 Functional Block Diagram ....................................... 10
7.3 Feature Description................................................. 10
7.4 Device Functional Modes........................................ 14
8
Application and Implementation ........................ 15
8.1 Application Information............................................ 15
8.2 Typical Application ................................................. 15
9 Power Supply Recommendations...................... 19
10 Layout................................................................... 19
10.1 Layout Guidelines ................................................. 19
10.2 Layout Example .................................................... 19
11 Device and Documentation Support ................. 21
11.1
11.2
11.3
11.4
11.5
Device Support ....................................................
Community Resources..........................................
Trademarks ...........................................................
Electrostatic Discharge Caution ............................
Glossary ................................................................
21
21
21
21
21
12 Mechanical, Packaging, and Orderable
Information ........................................................... 21
4 Revision History
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
Changes from Revision G (February 2013) to Revision H
•
Added ESD Ratings, Thermal Information 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 F (February 2013) to Revision G
•
2
Page
Page
Changed layout of National Data Sheet to TI format ........................................................................................................... 20
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5 Pin Configuration and Functions
DGK Package
8-Pin VSSOP
Top View
Pin Functions
NO.
NAME
I/O
1
ISENSE
I
The current sense input pin. This pin should be connected to Drain node of the external PFET.
DESCRIPTION
2
GND
G
Signal ground
3
NC
—
No connection
4
FB
I
The feedback input. Connect the FB to a resistor voltage divider between the output and GND for an
adjustable output voltage.
5
ADJ
I
Current limit threshold adjustment. It connects to an internal 5.5-µA current source. A resistor is
connected between this pin and the input Power Supply. The voltage across this resistor is compared with
the VDS of the external PFET to determine if an over-current condition has occurred.
6
PWR GND
G
Power ground
7
PGATE
O
Gate Drive output for the external PFET. PGATE swings between VIN and VIN-5 V.
8
VIN
P/I
Power supply input pin
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6 Specifications
6.1 Absolute Maximum Ratings
over operating free-air temperature range (unless otherwise noted) (1)
MIN
PGATE voltage
−0.3
FB voltage
ISENSE voltage
ADJ voltage
TYP
V
−0.3
5
V
−1.0
36
V
−0.3
36
V
150
Power dissipation (at TA = 25°C)
°C
417
mW
Vapor phase (60 sec.)
215
Infrared (15 sec.)
°C
220
°C
−65
Storage temperature, Tstg
(1)
UNIT
36
Maximum junction temperature
Lead temperature
MAX
160
°C
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.
6.2 ESD Ratings
V(ESD)
(1)
Electrostatic discharge
VALUE
UNIT
±2000
V
Human-body model (HBM), per ANSI/ESDA/JEDEC JS-001 (1)
JEDEC document JEP155 states that 500-V HBM allows safe manufacturing with a standard ESD control process.
6.3 Recommended Operating Conditions
MIN
Supply voltage
TJ
Operating junction temperature
NOM
MAX
UNIT
4.5
35
V
−40
125
°C
6.4 Thermal Information
LM3485
THERMAL METRIC (1)
DGK (VSSOP)
UNIT
8 PINS
RθJA
Junction-to-ambient thermal resistance
163.3
°C/W
RθJC(top)
Junction-to-case (top) thermal resistance
56.7
°C/W
RθJB
Junction-to-board thermal resistance
83.2
°C/W
ψJT
Junction-to-top characterization parameter
5.9
°C/W
ψJB
Junction-to-board characterization parameter
81.9
°C/W
RθJC(bot)
Junction-to-case (bottom) thermal resistance
N/A
°C/W
(1)
4
For more information about traditional and new thermal metrics, see the Semiconductor and IC Package Thermal Metrics application
report, SPRA953.
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6.5 Electrical Characteristics
Specifications are for TJ = 25°C. Unless otherwise specified, VIN = 12 V, VISNS = VIN − 1 V, and VADJ = VIN − 1.1 V. Data sheet
minimum and maximum specification limits are specified by design, test, or statistical analysis.
PARAMETER
TEST CONDITIONS
IQ
Quiescent current at ground FB = 1.5 V (Not Switching)
pin
(TJ = −40°C to 125°C)
VFB
Feedback voltage (3)
VHYST
Comparator hysteresis
VCL (4)
Current limit comparator trip RADJ = 20 kΩ
voltage
RADJ = 160 kΩ
VCL_OFFSET
Current limit comparator
offset
VFB = 1.5 V
ICL_ADJ
Current limit ADJ current
source
VFB = 1.5 V
TONMIN_CLCL
Current limit one shot off
time
RPGATE
IPGATE
Driver resistance
Driver output current
MIN (1)
400
1.242
1.217
(TJ = −40°C to 125°C)
1.258
1.267
10
15
14
20
110
0
−20
20
5.5
(TJ = −40°C to 125°C)
3.0
UNIT
µA
V
mV
mV
880
(TJ = −40°C to 125°C)
VADJ = 11.5 V
VISNS = 11.0 V
VFB = 1.0 V
MAX (1)
250
1.226
(TJ = −40°C to 125°C)
TYP (2)
7.0
mV
µA
9
(TJ = −40°C to 125°C)
6
14
Source
ISOURCE = 100 mA
5.5
Sink
ISink = 100 mA
8.5
Source
VIN = 7 V,
PGATE = 3.5 V
0.44
Sink
VIN = 7 V,
PGATE = 3.5 V
0.32
VFB = 1.0 V
300
µs
Ω
A
IFB
FB pin bias current (5)
TONMIN_NOR
Minimum on time in normal
operation
VISNS = VADJ + 0.1 V
Cload on OUT = 1000 pF (6)
100
ns
T
Minimum on time in current
limit
VISNS = VADJ + 0.1 V
VFB = 1.0 V
Cload on OUT = 1000 pF (6)
175
ns
%VFB/ΔVIN
Feedback voltage line
regulation
4.5 ≤ VIN ≤ 35 V
(1)
(2)
(3)
(4)
(5)
(6)
(TJ = −40°C to 125°C)
750
nA
0.010%
All limits are at room temperature unless otherwise specified. All room temperature limits are 100% tested. All limits at temperature
extremes are specified via correlation using standard Statistical Quality Control (SQC) methods. All limits are used to calculate Average
Outgoing Quality Level (AOQL).
Typical numbers are at 25°C and represent the most likely norm.
The VFB is the trip voltage at the FB pin when PGATE switches from high to low.
VCL = ICL_ADJ × RADJ
Bias current flows out from the FB pin.
A 1000-pF capacitor is connected between VIN and PGATE.
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6.6 Typical Characteristics
Unless otherwise specified, TJ = 25°C
1.255
350
1.250
Tj = -40qC
FB VOLTAGE (V)
QUIESCENT CURRENT (PA)
400
300
Tj = 125qC
250
Tj = 25qC
200
VIN=35V
1.245
VIN=12V
1.240
VIN=4.5V
1.235
150
1.230
100
4
20
12
28
1.225
-40 -20 0 20 40 60 80 100 120 140
JUNCTION TEMPERATURE (°C)
36
INPUT VOLTAGE (V)
FB = 1.5 V
Figure 2. Feedback Voltage vs Temperature
Figure 1. Quiescent Current vs Input Voltage
14
HYSTERESIS VOLTAGE (mV)
HYSTERESIS VOLTAGE (%)
110
105
TJ = 25qC
100
95
12
10
90
12
4
20
28
8
6
4
-40 -20
36
INPUT VOLTAGE (V)
40
60
80 100 120 140
Figure 4. Hysteresis Voltage vs Temperature
12
ONE SHOT OFF TIME (Ps)
6.5
ADJ CURRENT (PA)
20
JUNCTION TEMPERATURE (°C)
Figure 3. Hysteresis Voltage vs Input Voltage
6.0
VIN=12V
VIN=35V
5.5
VIN=4.5V
5.0
4.5
-40 -20
0
20
40
11
10
Figure 5. Current Limit ADJ Current vs Temperature
VIN = 4.5V
9
VIN = 12V
8
-40 -20 0 20 40 60 80 100 120 140
JUNCTION TEMPERATURE (°C)
60 80 100 120 140
JUNCTION TEMPERATURE (qC)
6
0
Figure 6. Current Limit One-Shot OFF-Time vs Temperature
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Typical Characteristics (continued)
10
6.0
5.5
8
TJ =125qC
5.0
TJ = 25qC
4.5
TJ = -40qC
CPGATE = 1800 pF
6
VPGATE (V)
INPUT VOLTAGE - PGATE VOLTAGE (V)
Unless otherwise specified, TJ = 25°C
CPGATE = 1020 pF
4
CPGATE = 540 pF
4.0
CPGATE = 110 pF
2
3.5
0
3.0
12
28
20
INPUT VOLTAGE (V)
4
0
36
50
100
150
T (ns)
VIN = 9 V
Figure 7. PGATE Voltage vs Input Voltage
Figure 8. Typical VPGATE vs Time
20
160
MINIMUM ON TIME (ns)
OPERATING ON TIME (Ps)
VIN= 4.5V
140
120
100
VIN= 12V
80
60
VIN= 24V
40
16
3.3VOUT
12
8
4
20
1.242VOUT
0
0
-20 -40
0
20
40 60
0
80 100 120 140
200
400
600
800
1000
OUTPUT LOAD CURRENT (mA)
JUNCTION TEMPERATURE (°C)
VIN = 4.5 V
Figure 9. Minimum ON-Time vs Temperature
Figure 10. Operating ON-Time vs Output Load Current
100
VIN = 4.5V
90
4
EFFICIENCY (%)
OPERATING ON TIME (Ps)
5
3
5.0VOUT
3.3VOUT
2
80
VIN = 12V
70
60
1.242VOUT
1
50
0
0
200
400
600
800
1000
40
10
OUTPUT LOAD CURRENT (mA)
100
1000
10000
LOAD CURRENT (mA)
VIN = 12 V
VOUT = 3.3 V
Figure 11. Operating ON-Time vs Output Load Current
L = 6.8 µH
Figure 12. Efficiency vs Load Current
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Typical Characteristics (continued)
Unless otherwise specified, TJ = 25°C
100
100
VIN = 4.5V
VIN = 12V
90
90
EFFICIENCY (%)
VIN = 12V
VIN = 24V
VIN = 24V
70
80
EFFICIENCY (%)
80
60
70
60
50
50
40
10
VOUT = 3.3 V
100
1000
40
10
10000
100
1000
10000
LOAD CURRENT (mA)
LOAD CURRENT (mA)
L = 22 µH
VOUT = 5.0 V
L = 22 µH
Figure 14. Efficiency vs Load Current
Figure 13. Efficiency vs Load Current
1A
VIN (10V/div)
0.5A
Inductor Current
0A
(1A/div)
10V
lind@CADJ = 10nF
5V
lind@CADJ = 1nF
0V
SW node Voltage
20mV Output Ripple Voltage
VOUT@CADJ = 1nF
(2V/div)
0mV
VOUT@CADJ = 10nF
(2V/div)
-20mV
TIME (2Ps/div)
VIN = 12 V
TIME (100Ps/div)
VOUT = 3.3 V
L = 22 µH
IOUT = 500 mA
Figure 16. Continuous Mode Operation
Figure 15. Start Up
800
OPERATING FREQUENCY (KHz)
Inductor Current
0.5A
0A
SW node
Voltage
10V
5V
0V
20mV
Output Ripple Voltage
0mV
TIME (5Ps/div)
VOUT = 3.3 V
L = 22 µH
400
L=22PH
200
4
12
20
28
36
INPUT VOLTAGE (V)
IOUT = 50 mA
Figure 17. Discontinuous Mode Operation
8
L=10PH
L=15PH
0
-20mV
VIN = 12 V
600
VOUT = 3.3 V
IOUT = 1 A
Cff = 100 pF
COUT(ESR) = 80 mΩ
Figure 18. Operating Frequency vs Input Voltage
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Typical Characteristics (continued)
Unless otherwise specified, TJ = 25°C
400
L=10PH
OPERATING FREQUENCY (kHz)
60
L=15PH
40
L=22PH
20
0
4
12
20
28
12VIN / 5.0VOUT
200
12VIN / 1.242VOUT
4.5VIN / 1.242VOUT
100
36
4.5VIN / 3.3VOUT
0
INPUT VOLTAGE (V)
VOUT = 3.3 V
IOUT = 1 A
Cff = 100 pF
12VIN / 3.3VOUT
300
0
200
COUT(ESR) = 80 mΩ
L = 22 µH
Figure 19. Output Ripple Voltage vs Input Voltage
OPERATING FREQUENCY (kHz)
300
600
800
1000
Figure 20. Operating Frequency vs Output Load Current
300
Operating Frequency
250
250
200
200
@Cff=100p
150
150
@no Cff
100
400
OUTPUT CURRENT LOAD (mA)
COUT(ESR) = 45 mΩ
Cff = 100 pF
100
Ripple Voltage
@no Cff
50
50
OUTPUT RIPPLE VOLTAGE (mV)
OUTPUT RIPPLE VOLTAGE (mV)
80
@Cff=100p
0
4
12
20
28
0
36
INPUT VOLTAGE (V)
VOUT = 3.3 V
L = 22 µH
IOUT = 500 mA
Figure 21. Feed-Forward Capacitor (Cff) Effect
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7 Detailed Description
7.1 Overview
The LM3485 is buck (step-down) DC-DC controller that uses a hysteretic control scheme. The comparator is
designed with approximately 10 mV of hysteresis. In response to the voltage at the FB pin, the gate drive
(PGATE pin) turns the external PFET on or off. When the inductor current is too high, the current limit protection
circuit engages and turns the PFET off for approximately 9 µs.
Hysteretic control does not require an internal oscillator. Switching frequency depends on the external
components and operating conditions. Operating frequency reduces at light loads resulting in excellent efficiency
compared to other architectures.
Two external resistors can easily program the output voltage. The output can be set in a wide range from
1.242-V (typical) to VIN.
7.2 Functional Block Diagram
7.3 Feature Description
7.3.1 Hysteretic Control Circuit
The LM3485 uses a comparator-based voltage control loop. The feedback is compared to a 1.242-V reference,
and a 10-mV hysteresis is designed into the comparator to ensure noise free operation.
When the FB input to the comparator falls below the reference voltage, the output of the comparator moves to a
low state. This results in the driver output, PGATE, pulling the gate of the PFET low and turning on the PFET.
With the PFET on, the input supply charges Cout and supplies current to the load via the series path through the
PFET and the inductor. Current through the Inductor ramps up linearly and the output voltage increases. As the
FB voltage reaches the upper threshold, which is the internal reference voltage plus 10 mV, the output of the
comparator changes from low to high, and the PGATE responds by turning the PFET off. As the PFET turns off,
the inductor voltage reverses, the catch diode turns on, and the current through the inductor ramps down. Then,
as the output voltage reaches the internal reference voltage again, the next cycle starts.
10
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Feature Description (continued)
The LM3485 operates in discontinuous conduction mode at light load current or continuous conduction mode at
heavy load current. In discontinuous conduction mode, current through the inductor starts at zero and ramps up
to the peak, then ramps down to zero. Next cycle starts when the FB voltage reaches the internal voltage. Until
then, the inductor current remains zero. Operating frequency is lower and switching losses reduce. In continuous
conduction mode, current always flows through the inductor and never ramps down to zero.
The output voltage (VOUT) can be programmed by two external resistors. It can be calculated as Equation 1:
VOUT = 1.242 × ( R1 + R2 ) / R2
(1)
Figure 22. Hysteretic Window
The minimum output voltage ripple (VOUT_PP) can be calculated in the same way.
VOUT_PP = VHYST ( R1 + R2 ) / R2
(2)
For example, with VOUT set to 3.3 V, VOUT_PP is 26.6 mV
VOUT_PP = 0.01 × ( 33K + 20K ) / 20K = 0.0266 V
(3)
Operating frequency (F) is determined by knowing the input voltage, output voltage, inductor, VHYST, equivalent
series resistance (ESR) of output capacitor, and the delay. It can be approximately calculated using Equation 4:
VOUT
(VIN - VOUT ) ´ ESR
F=
´
VIN (VHYST ´ a ´ L) + (VIN ´ delay ´ ESR)
where
•
•
( R1 + R2 ) / R2
delay: It includes the LM3485 propagation delay time and the PFET delay time
(4)
The propagation delay is 90-ns typically (see Figure 23).
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Feature Description (continued)
140
L=22PH
PROPOGATION DELAY (ns)
120
L=10PH
100
80
L=4.7PH
60
40
20
0
0
5
10
15
20
25
30
35
INPUT VOLTAGE - OUTPUT VOLTAGE (V)
Figure 23. Propagation Delay
The operating frequency and output ripple voltage can also be significantly influenced by the speed up capacitor
(Cff). Cff is connected in parallel with the high-side feedback resistor, R1. The location of this capacitor is similar
to where a feed-forward capacitor would be located in a PWM control scheme. However, the effect on hysteretic
operation is much different. The output ripple causes a current to be sourced or sunk through this capacitor. This
current is essentially a square wave. Because the input to the feedback pin, FB, is a high impedance node, the
current flows through R2. The end result is a reduction in output ripple and an increase in operating frequency.
When adding Cff, calculate Equation 4 with α = 1. The value of Cff depends on the desired operating frequency
and the value of R2. A good starting point is 470-pF ceramic at 100-kHz decreasing linearly with increased
operating frequency. Also, as the output voltage is programmed below 2.5 V, the effect of Cff will decrease
significantly.
7.3.2 Current Limit Operation
The LM3485 has a cycle-by-cycle current limit. Current limit is sensed across the VDS of the PFET or across an
additional sense resistor. When current limit is activated, the LM3485 turns off the external PFET for a period of
9 µs (typical). The current limit is adjusted by an external resistor, RADJ.
The current limit circuit is composed of the ISENSE comparator and the one-shot pulse generator. The positive
input of the ISENSE comparator is the ADJ pin. An internal 5.5-µA current sink creates a voltage across the
external RADJ resistor. This voltage is compared to the voltage across the PFET or sense resistor. The ADJ
voltage can be calculated with Equation 5.
VADJ = VIN − (RADJ × 3.0 µA)
where
•
3.0 µA is the minimum ICL-ADJ value
(5)
The negative input of the ISENSE comparator is the ISENSE pin that should be connected to the drain of the
external PFET. The inductor current is determined by sensing the VDS. It can be calculated with Equation 6.
VISENSE = VIN − (RDSON × IIND_PEAK) = VIN − VDS
12
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Feature Description (continued)
Figure 24. Current Sensing by VDS
The current limit is activated when the voltage at the ADJ pin exceeds the voltage at the ISENSE pin. The ISENSE
comparator triggers the 9-µs one shot pulse generator forcing the driver to turn the PFET off. The driver turns the
PFET back on after 9 µs. If the current has not reduced below the set threshold, the cycle will repeat
continuously.
A filter capacitor, CADJ, should be placed as shown in Figure 24. CADJ filters unwanted noise so that the ISENSE
comparator will not be accidentally triggered. A value of 100 pF to 1 nF is recommended in most applications.
Higher values can be used to create a soft-start function (see Start Up).
The current limit comparator has approximately 100 ns of blanking time. This ensures that the PFET is fully on
when the current is sensed. However, under extreme conditions such as cold temperature, some PFETs may not
fully turn on within the blanking time. In this case, the current limit threshold must be increased. If the current limit
function is used, the on time must be greater than 100 ns. Under low duty cycle operation, the maximum
operating frequency will be limited by this minimum on time.
During current limit operation, the output voltage will drop significantly as will operating frequency. As the load
current is reduced, the output will return to the programmed voltage. However, there is a current limit foldback
phenomenon inherent in this current limit architecture. See Figure 25.
Figure 25. Current Limit Fold Back Phenomenon
At high input voltages (>28 V) increased undershoot at the switch node can cause an increase in the current limit
threshold. To avoid this problem, a low Vf Schottky catch diode must be used (see Catch Diode Selection (D1)).
Additionally, a resistor can be placed between the ISENSE pin and the switch node. Any value up to
approximately 600 Ω is recommended.
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7.4 Device Functional Modes
7.4.1 Start Up
The current limit circuit is active during start-up. During start-up the PFET will stay on until either the current limit
or the feedback comparator is tripped
If the current limit comparator is tripped first then the fold back characteristic should be taken into account. Startup into full load may require a higher current limit set point or the load must be applied after start-up.
One problem with selecting a higher current limit is inrush current during start-up. Increasing the capacitance
(CADJ) in parallel with RADJ results in soft-start. CADJ and RADJ create an RC time constant forcing current limit to
activate at a lower current. The output voltage will ramp more slowly when using the soft-start functionality. There
are example start-up plots for CADJ equal to 1 nF and 10 nF in Typical Characteristics. Lower values for CADJ will
have little to no effect on soft-start.
7.4.2 External Sense Resistor
The VDS of a PFET will tend to vary significantly over temperature. This will result an equivalent variation in
current limit. To improve current limit accuracy an external sense resistor can be connected from VIN to the
source of the PFET, as shown in Figure 26.
Figure 26. Current Sensing by External Resistor
7.4.3 PGATE
When switching, the PGATE pin swings from VIN (off) to some voltage below VIN (on). How far the PGATE will
swing depends on several factors including the capacitance, on time, and input voltage.
As shown in the Typical Characteristics, PGATE voltage swing will increase with decreasing gate capacitance.
Although PGATE voltage will typically be around VIN-5 V, with every small gate capacitances, this value can
increase to a typical maximum of VIN-8.3 V.
Additionally, PGATE swing voltage will increase as on time increases. During long on times, such as when
operating at 100% duty cycle, the PGATE voltage will eventually fall to its maximum voltage of VIN-8.3 V (typical)
regardless of the PFET gate capacitance.
The PGATE voltage will not fall below 0.4 V (typical). Therefore, when the input voltage falls below approximately
9 V, the PGATE swing voltage range will be reduced. At an input voltage of 7 V, for instance, PGATE will swing
from 7 V to a minimum of 0.4 V.
14
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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
Hysteretic control is a simple control scheme. However the operating frequency and other performance
characteristics highly depend on external conditions and components. If either the inductance, output
capacitance, ESR, VIN, or Cff is changed, there will be a change in the operating frequency and output ripple.
The best approach is to determine what operating frequency is desirable in the application and then begin with
the selection of the inductor and COUT ESR.
8.2 Typical Application
Figure 27. Typical Application Schematic
8.2.1 Design Requirements
For this design example, use the parameters listed in Table 1.
Table 1. Design Parameters
DESIGN PARAMETERS
VALUE
Input Voltage Range
7 V to 28 V
Output Voltage
3.3 V
Output Current Rating
1A
Output Voltage Ripple
26.6 mV
Operating Frequency (VIN 12 V, Load Current 1 A)
210 kHz
8.2.2 Detailed Design Procedure
8.2.2.1 Step by Step Design Procedure
To
•
•
•
•
•
begin the design process, the following items must be considered:
Output current rating
Input voltage range
Output voltage
Input voltage ripple
Output voltage ripple
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8.2.2.2 Inductor Selection (L1)
The important parameters for the inductor are the inductance and the current rating. The LM3485 operates over
a wide frequency range and can use a wide range of inductance values. A good rule of thumb is to use the
equations used for Simple Switcher®. The equation for inductor ripple (Δi) as a function of output current (IOUT)
for Iout < 2.0 Amps is Equation 7:
Δi ≤ Iout × 0.386827 × Iout−0.366726
(7)
For Iout > 2.0 Amps, follow Equation 8:
Δi ≤ Iout × 0.3
(8)
The inductance can be calculated based upon the desired operating frequency using Equation 9 and
Equation 10:
VIN - VDS - VOUT D
L=
x
f
'i
(9)
VOUT + VD
D=
VIN - VDS + VD
where
•
•
•
D is the duty cycle
VD is the diode forward voltage
VDS is the voltage drop across the PFET
(10)
The inductor should be rated using Equation 11 and Equation 12:
Ipk = (Iout + Δi / 2) × 1.1
IRMS =
iout2 +
'i
3
(11)
2
(12)
The inductance value and the resulting ripple is one of the key parameters controlling operating frequency. The
second is the ESR.
8.2.2.3 Output Voltage Set Point
The output voltage (VOUT) can be programmed by two external resistors. It can be calculated using Equation 13.
VOUT = 1.242 × (R1 + R2) / R2
(13)
Refer to Typical Application . A good starting point is to select R2 to be in the range of 10 kΩ to 20 kΩ.
8.2.2.4 Output Capacitor Selection (COUT)
The ESR of the output capacitor times the inductor ripple current is equal to the output ripple of the regulator.
However, the VHYST sets the first order value of this ripple. As ESR is increased with a given inductance, then
operating frequency increases as well. If ESR is reduced then the operating frequency reduces.
The use of ceramic capacitors has become a common practice of many power supply designers. However,
ceramic capacitors have a very low ESR resulting in a 90° phase shift of the output voltage ripple. This results in
low operating frequency and increased output ripple. To fix this problem a low value resistor should be added in
series with the ceramic output capacitor. Although counter intuitive, this combination of a ceramic capacitor and
external series resistance provides highly accurate control over the output voltage ripple. The other types of
capacitors, such as Sanyo POS CAP and OS-CON, Panasonic SP CAP, Nichicon NA series, are also
recommended and may be used without additional series resistance.
For all practical purposes, any type of output capacitor may be used with proper circuit verification.
8.2.2.5 Input Capacitor Selection (CIN)
A bypass capacitor is required between the input source and ground. It must be located near the source pin of
the external PFET. The input capacitor prevents large voltage transients at the input and provides the
instantaneous current when the PFET turns on.
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The important parameters for the input capacitor are the voltage rating and the RMS current rating. Follow the
manufacturer's recommended voltage derating. For high input voltage application, low ESR electrolytic capacitor,
the Nichicon UD series or the Panasonic FK series, is available. The RMS current in the input capacitor can be
calculated using Equation 14.
1/2
(VOUT ´ (VIN - VOUT ))
IRMS _ CIN = IOUT ´
VIN
(14)
The input capacitor power dissipation can be calculated using Equation 15.
PD(CIN) = IRMS_CIN2 × ESRCIN
(15)
The input capacitor must be able to handle the RMS current and the PD. Several input capacitors may be
connected in parallel to handle large RMS currents. In some cases it may be much cheaper to use multiple
electrolytic capacitors than a single low ESR, high performance capacitor such as OS-CON or Tantalum. The
capacitance value should be selected such that the ripple voltage created by the charge and discharge of the
capacitance is less than 10% of the total ripple across the capacitor.
8.2.2.6 Programming the Current Limit (RADJ)
The current limit is determined by connecting a resistor (RADJ) between input voltage and the ADJ pin.
RADJ = IIND_PEAK × RDSON / ICL_ADJ
where
•
•
•
RDSON : Drain-Source ON resistance of the external PFET
ICL_ADJ : 3.0 µA minimum
IIND_PEAK = ILOAD + IRIPPLE / 2
(16)
Using the minimum value for ICL_ADJ (3.0 µA) ensures that the current limit threshold will be set higher than the
peak inductor current.
The RADJ value must be selected to ensure that the voltage at the ADJ pin does not fall below 3.5 V. With this in
mind,
RADJ_MAX = (VIN – 3.5) / 7 µA
(17)
If a larger RADJ value is needed to set the desired current limit, either use a PFET with a lower RDSON, or use a
current sense resistor as shown in Figure 26.
The current limit function can be disabled by connecting the ADJ pin to ground and ISENSE to VIN.
8.2.2.7 Catch Diode Selection (D1)
The important parameters for the catch diode are the peak current, the peak reverse voltage, and the average
power dissipation. The average current through the diode can be calculated using Equation 18.
ID_AVE = IOUT × (1 − D)
(18)
The off state voltage across the catch diode is approximately equal to the input voltage. The peak reverse
voltage rating must be greater than input voltage. In nearly all cases a Schottky diode is recommended. In low
output voltage applications a low forward voltage provides improved efficiency. For high temperature
applications, diode leakage current may become significant and require a higher reverse voltage rating to
achieve acceptable performance.
8.2.2.8 P-Channel MOSFET Selection (Q1)
The important parameters for the PFET are the maximum Drain-Source voltage (VDS), the on resistance (RDSON),
Current rating, and the input capacitance.
The voltage across the PFET when it is turned off is equal to the sum of the input voltage and the diode forward
voltage. The VDS must be selected to provide some margin beyond the input voltage.
PFET drain current, Id, must be rated higher than the peak inductor current, IIND-PEAK.
Depending on operating conditions, the PGATE voltage may fall as low as VIN – 8.3 V. Therefore, a PFET must
be selected with a VGS greater than the maximum PGATE swing voltage.
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As input voltage decreases below 9 V, PGATE swing voltage may also decrease. At 5.0-V input the PGATE will
swing from VIN to VIN – 4.6 V. To ensure that the PFET turns on quickly and completely, a low threshold PFET
should be used when the input voltage is less than 7 V.
However, PFET switching losses will increase as the VGS threshold decreases. Therefore, whenever possible, a
high threshold PFET should be selected. Total power loss in the FET can be approximated using Equation 19:
PDswitch = RDSON × IOUT2 × D + F × IOUT × VIN × (ton + toff) / 2
where
•
•
ton = FET turnon time
toff = FET turnoff time
(19)
A value from 10 ns to 20 ns is typical for ton and toff.
A PFET should be selected with a turn on rise time of less than 100 ns. Slower rise times will degrade efficiency,
can cause false current limiting, and in extreme cases may cause abnormal spiking at the PGATE pin.
The RDSON is used in determining the current limit resistor value, RADJ.
NOTE
The RDSON has a positive temperature coefficient. At 100°C, the RDSON may be as much
as 150% higher than the 25°C value. This increase in RDSON must be considered it when
determining RADJ in wide temperature range applications. If the current limit is set based
upon 25°C ratings, then false current limiting can occur at high temperature.
Keeping the gate capacitance below 2000 pF is recommended to keep switching losses and transition times low.
This will also help keep the PFET drive current low, which will improve efficiency and lower the power dissipation
within the controller.
As gate capacitance increases, operating frequency should be reduced and as gate capacitance decreases
operating frequency can be increased.
8.2.3 Application Curves
100
80
EFFICIENCY (%)
80
OUTPUT RIPPLE VOLTAGE (mV)
VIN = 4.5V
90
VIN = 12V
70
60
50
L=10PH
60
L=15PH
40
L=22PH
20
0
4
12
40
10
100
VOUT = 3.3 V
1000
VOUT = 3.3 V
Figure 28. Efficiency vs Load Current
18
28
36
INPUT VOLTAGE (V)
10000
LOAD CURRENT (mA)
L = 6.8 µH
20
IOUT = 1 A
Cff = 100 pF
COUT(ESR) = 80 mΩ
Figure 29. Output Ripple Voltage vs Input Voltage
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9 Power Supply Recommendations
The devices are designed to generate from an input voltage supply range between 4.5 V and 35 V. The input
should be well regulated. If the input supply is located more than a few inches from the LM3485 EVM, an
additional bulk capacitor may be required. A tantalum capacitor with a valve of 47 µt as a typical choice.
10 Layout
10.1 Layout Guidelines
The PC board layout is very important in all switching regulator designs. Poor layout can cause switching noise
into the feedback signal and general EMI problems. For minimal inductance, the wires indicated by heavy lines
should be as wide and short as possible. Keep the ground pin of the input capacitor as close as possible to the
anode of the diode. This path carries a large AC current. The switching node, the node with the diode cathode,
inductor, and FET drain, should be kept short. This node is one of the main sources for radiated EMI because it
is an AC voltage at the switching frequency. It is always good practice to use a ground plane in the design,
particularly at high currents.
The two ground pins, PWR GND and GND, should be connected by as short a trace as possible; they can be
connected underneath the device. These pins are resistively connected internally by approximately 50 Ω. The
ground pins should be tied to the ground plane, or to a large ground trace in close proximity to both the FB
divider and COUT grounds.
The gate pin of the external PFET should be located close to the PGATE pin. However, if a very small FET is
used, a resistor may be required between PGATE and the gate of the FET to reduce high frequency ringing.
Because this resistor will slow the rise time of the PFET, the current limit blanking time should be taken into
consideration (see Current Limit Operation).
The feedback voltage signal line can be sensitive to noise. Avoid inductive coupling to the inductor or the
switching node, by keeping the FB trace away from these areas.
10.2 Layout Example
Figure 30. Top Layer,
Typical PCB Layout (3.3-V Output)
Figure 31. Bottom Layer,
Typical PCB Layout (3.3-V Output)
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Layout Example (continued)
C1
VOUT
C2
GND
+
+
Q1
V1
VIN
C3
R3
R2
C4
R1
L1
LM3485 Demo Board
Figure 32. Silk Screen,
Typical PCB Layout (3.3-V Output)
Figure 33. Typical PCB Layout Schematic (3.3-V
Output)
Table 2. Typical Application BOM
DESCRIPTION
PART NUMBER
DISTRIBUTOR
C1
DESIGNATOR
COUT
22-µF to 35-V
EEJL1VD226R
Panasonic
C2
CIN
100-µF to 6.3-V
6TPC100M
C3
CADJ
1-nF ceramic chip capacitor
C4
CFF
100-pF ceramic chip capacitor
D1
1 A to 40 V
MBRS140T3
On Semiconductor
L1
22 µH
QH66SN220M01L
Murata
FDC5614P
Fairchild
Q1
R1
33k-Ω chip resistor
R2
20-kΩ chip resistor
R3
20
RADJ
240-kΩ chip resistor
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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.
Simple Switcher is a registered 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.
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PACKAGE OPTION ADDENDUM
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9-Mar-2021
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)
LM3485MM/NOPB
ACTIVE
VSSOP
DGK
8
1000
RoHS & Green
NIPDAUAG | SN
Level-1-260C-UNLIM
-40 to 125
S29B
LM3485MMX/NOPB
ACTIVE
VSSOP
DGK
8
3500
RoHS & Green
NIPDAUAG | SN
Level-1-260C-UNLIM
-40 to 125
S29B
LM3485Q1MM/NOPB
ACTIVE
VSSOP
DGK
8
1000
RoHS & Green
SN
Level-1-260C-UNLIM
-40 to 125
SVJB
(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
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