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SM72485
SNVS697E – JANUARY 2011 – REVISED DECEMBER 2016
SM72485 100-V, 150-mA Constant On-Time Buck Switching Regulator
1 Features
3 Description
•
•
•
•
•
•
•
The SM72485 step-down switching regulator features
all of the functions required to implement a low cost,
efficient, Buck bias regulator. This high voltage
regulator contains an 100-V N-channel buck switch.
The device is easy to implement and is provided in
the VSSOP and the thermally enhanced WSON
package. The regulator is based on a control scheme
using an on-time inversely proportional to VIN. This
feature allows the operating frequency to remain
relatively constant. The control scheme requires no
loop compensation. An intelligent current limit is
implemented with forced off-time, which is inversely
proportional to VOUT. This scheme ensures short
circuit control while providing minimum foldback.
Other features include: Thermal shutdown, VCC
undervoltage lockout, gate drive undervoltage
lockout, maximum duty cycle limiter, and a precharge
switch.
1
•
•
•
•
•
•
•
Operating Input Voltage: 6 V to 95 V
Integrated 100-V, N-Channel Buck Switch
Internal Start-Up Regulator
No Loop Compensation Required
Ultra-Fast Transient Response
On-time Varies Inversely With Input Voltage
Operating Frequency Remains Constant With
Varying Line Voltage and Load Current
Adjustable Output Voltage From 2.5 V
Highly Efficient Operation
Precision Internal Reference
Low Bias Current
Intelligent Current Limit
Thermal Shutdown
Package
– VSSOP (3 mm × 3 mm)
– WSON (4 mm × 4 mm)
Device Information(1)
PART NUMBER
SM72485
2 Applications
•
•
•
•
PV Panel Smart Junction Boxes
Nonisolated Telecommunication Buck Regulator
Secondary High Voltage Post Regulator
42-V Automotive Systems
PACKAGE
BODY SIZE (NOM)
VSSOP (8)
3.00 mm × 3.00 mm
WSON (8)
4.00 mm × 4.00 mm
(1) For all available packages, see the orderable addendum at
the end of the data sheet.
Typical Application, Basic Step-Down Regulator
6V - 95V
Input
VIN
VCC
VIN
C1
C3
SM72485
RT
BST
GND
C4
L1
RT/SD
VOUT
SW
RCL
D1
SHUTDOWN
RFB2
RCL
RTN
R3
C2
FB
GND
RFB1
Copyright © 2016, Texas Instruments Incorporated
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.
SM72485
SNVS697E – JANUARY 2011 – REVISED DECEMBER 2016
www.ti.com
Table of Contents
1
2
3
4
5
6
7
Features ..................................................................
Applications ...........................................................
Description .............................................................
Revision History.....................................................
Pin Configuration and Functions .........................
Specifications.........................................................
1
1
1
2
3
4
6.1
6.2
6.3
6.4
6.5
6.6
4
4
4
4
5
6
Absolute Maximum Ratings ......................................
ESD Ratings..............................................................
Recommended Operating Conditions.......................
Thermal Information ..................................................
Electrical Characteristics...........................................
Typical Characteristics ..............................................
Detailed Description .............................................. 7
7.1 Overview ................................................................... 7
7.2 Functional Block Diagram ......................................... 7
7.3 Feature Description................................................... 7
7.4 Device Functional Modes.......................................... 9
8
Application and Implementation ........................ 12
8.1 Application Information............................................ 12
8.2 Typical Application .................................................. 12
9 Power Supply Recommendations...................... 17
10 Layout................................................................... 18
10.1 Layout Guidelines ................................................. 18
10.2 Layout Example .................................................... 18
11 Device and Documentation Support ................. 19
11.1
11.2
11.3
11.4
11.5
11.6
Documentation Support ........................................
Receiving Notification of Documentation Updates
Community Resources..........................................
Trademarks ...........................................................
Electrostatic Discharge Caution ............................
Glossary ................................................................
19
19
19
19
19
19
12 Mechanical, Packaging, and Orderable
Information ........................................................... 19
4 Revision History
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
Changes from Revision D (April 2013) to Revision E
Page
•
Added Device Information table, Pin Configuration and Functions section, ESD Ratings table, Thermal Information
table, Detailed Description section, Application and Implementation section, Power Supply Recommendations
section, Layout section, Device and Documentation Support section, and Mechanical, Packaging, and Orderable
Information section ................................................................................................................................................................. 1
•
Deleted Renewable Energy Grade from Features ................................................................................................................. 1
•
Deleted Lead temperature (260°C maximum) from Absolute Maximum Ratings table.......................................................... 4
•
Changed Junction to Ambient, RθJA, value in Thermal Information table From: 200°C/W To: 139.6°C/W (VSSOP)
and From: 40°C/W To: 42.3°C/W (WSON) ............................................................................................................................ 4
Changes from Revision C (April 2013) to Revision D
•
2
Page
Changed layout of National Seminconductor Data Sheet to TI format .................................................................................. 1
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SNVS697E – JANUARY 2011 – REVISED DECEMBER 2016
5 Pin Configuration and Functions
DGK Package
8-Pin VSSOP
Top View
NGU Package
8-Pin WSON
Top View
SW
1
8
VIN
BST
2
7
VCC
RCL
3
6
RT/SD
RTN
4
5
FB
SW
1
8
VIN
BST
2
7
VCC
RCL
3
6
RT/SD
RTN
4
5
FB
Exposed Pad on Bottom
Connect to Ground
Pin Functions
PIN
TYPE (1)
DESCRIPTION
2
I
An external capacitor is required between the BST and the SW pins. TI recommends a
0.01-µF ceramic capacitor. An internal diode charges the capacitor from VCC during each
off-time.
5
5
I
This pin is connected to the inverting input of the internal regulation comparator. The
regulation threshold is 2.5 V.
RCL
3
3
I
A resistor between this pin and RTN sets the off-time when current limit is detected. The
off-time is preset to 35 µs if FB = 0 V.
RT/SD
6
6
I
A resistor between this pin and VIN sets the switch on-time as a function of VIN. The
minimum recommended on-time is 400 ns at the maximum input voltage. This pin can be
used for remote shutdown.
RTN
4
4
G
Ground for the entire circuit.
SW
1
1
O
Power switching node. Connect to the output inductor, re-circulating diode, and bootstrap
capacitor.
VCC
7
7
I
This regulated voltage provides gate drive power for the internal Buck switch. An internal
diode is provided between this pin and the BST pin. A local 0.47-µF decoupling capacitor
is required. The series pass regulator is current limited to 9 mA.
VIN
8
8
I
Input operating voltage: 6 V to 95 V.
—
Thermal
Pad
NC
NAME
VSSOP
WSON
BST
2
FB
Exposed
Pad
(1)
The exposed pad has no electrical contact. Connect to system ground plane for reduced
thermal resistance.
G = Ground, I = Input, O = Output, NC = No Contact
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6 Specifications
6.1 Absolute Maximum Ratings
over operating free-air temperature range (unless otherwise noted) (1) (2) (3)
MIN
MAX
UNIT
VIN to GND
–0.3
100
V
BST to GND
–0.3
114
V
SW to GND (steady-state)
–1
V
BST to VCC
100
V
BST to SW
14
V
VCC to GND
14
V
All other inputs to GND
–0.3
7
V
Storage temperature, Tstg
–55
150
°C
(1)
(2)
(3)
Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. These are stress ratings
only, which do not imply functional operation of the device at these or any other conditions beyond those indicated under Recommended
Operating Conditions. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability.
If Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/Distributors for availability and
specifications.
For detailed information on soldering plastic VSSOP and WSON packages, see Absolute Maximum Ratings for Soldering (SNOA549).
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
over operating free-air temperature range (unless otherwise noted)
VIN
Input voltage
TJ
Operating junction temperature
MIN
MAX
6
95
UNIT
V
–40
125
°C
6.4 Thermal Information
SM72485
THERMAL METRIC (1)
DGK (VSSOP)
NGU (WSON)
8 PINS
8 PINS
UNIT
RθJA
Junction-to-ambient thermal resistance
139.6
42.3
°C/W
RθJC(top)
Junction-to-case (top) thermal resistance
41.8
41.4
°C/W
RθJB
Junction-to-board thermal resistance
68.4
20.1
°C/W
ψJT
Junction-to-top characterization parameter
4.2
0.4
°C/W
ψJB
Junction-to-board characterization parameter
67.5
20.3
°C/W
RθJC(bot)
Junction-to-case (bottom) thermal resistance
—
4.1
°C/W
(1)
4
For more information about traditional and new thermal metrics, see the Semiconductor and IC Package Thermal Metrics application
report.
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6.5 Electrical Characteristics
Typical values apply for TA = TJ = 25°C, Minimum and Maximum limits apply for TA = TJ = –40°C to 125°C, and VIN = 48 V
(unless otherwise noted) (1)
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
7
7.4
UNIT
VCC SUPPLY
VCCREG
VCC regulator output
VIN = 48 V
Dropout voltage, VIN – VCC
VIN = 6 V to 8.5 V
VCC bypass threshold
VIN rising
6.6
100
VCC bypass hysteresis
8.5
V
300
mV
VIN = 6 V
100
VIN = 10 V
8.8
VIN = 48 V
0.8
VCC current limit
VIN = 48 V
9.2
VCC UVLO
VCC rising
5.3
V
190
mV
VCC output impedance
VCC UVLO hysteresis
VCC UVLO filter delay
ICC
V
mV
Ω
mA
3
µs
Operating current
VFB = 3 V, VIN = 48 V
550
750
µA
Shutdown current
VRT/SD = 0 V
110
176
µA
2.2
4.6
Ω
3.8
4.8
SWITCH CHARACTERISTICS
Buckswitch RDS(ON)
ITEST = 200 mA
Gate drive UVLO
VBST – VSW rising
2.8
Gate drive UVLO hysteresis
Precharge switch voltage
490
At 1 mA
Precharge switch on-time
V
mV
0.8
V
150
ns
CURRENT LIMIT
Current limit threshold
0.24
Current limit response time
ISW overdrive = 0.1 A, time to switch OFF
tOFF_1
Off-time generator
VFB = 0 V, RCL = 100 kΩ
tOFF_2
Off-time generator
VFB = 2.3 V, RCL = 100 kΩ
0.3
0.36
350
A
ns
35
µs
2.56
ON-TIME GENERATOR
tON_1
On-time generator
VIN = 10 V, RON = 200 kΩ
2.15
2.77
3.5
µs
tON_2
On-time generator
VIN = 95 V, RON = 200 kΩ
200
300
420
ns
Remote shutdown threshold
Rising
0.4
0.7
1.05
Remote shutdown hysteresis
V
35
mV
300
ns
MINIMUM OFF-TIME
Minimum off-time
VFB = 0 V
REGULATION AND OV COMPARATORS
FB reference threshold
Internal reference, trip point for switch = ON
FB overvoltage threshold
Trip point for switch = OFF
FB bias current
2.445
2.5
2.55
V
2.875
V
100
nA
165
°C
25
°C
THERMAL SHUTDOWN
TSD
Thermal shutdown temperature
Thermal shutdown hysteresis
(1)
All limits are ensured. All electrical characteristics having room temperature limits are tested during production with TA = TJ = 25°C. All
minimum and maximum limits are ensured by correlating the electrical characteristics to process and temperature variations and
applying statistical process control.
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6.6 Typical Characteristics
Circuit of Figure 10
Figure 1. Efficiency vs Load Current and VIN
Figure 2. VCC vs VIN
CURRENT LIMIT OFF TIME (Ps)
35
30
25
20
15
RCL = 500k
300k
10
100k
5
50k
0
0
0.5
1.0
1.5
2.0
2.5
VFB (V)
6
Figure 3. On-Time vs Input Voltage and RT
Figure 4. Current Limit OFF-Time vs VFB and RCL
Figure 5. Maximum Frequency vs VOUT and VIN
Figure 6. ICC Current vs Applied VCC Voltage
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7 Detailed Description
7.1 Overview
The SM72485 step-down switching regulator features all the functions required to implement a low-cost, efficient,
buck-bias power converter. This high-voltage regulator contains a 100-V, N-channel buck switch that is easy to
implement and is provided in the VSSOP and the thermally enhanced WSON package. The regulator is based
on a control scheme using an on-time inversely proportional to VIN. The control scheme requires no loop
compensation. Current limit is implemented with forced off-time, which is inversely proportional to VOUT. This
scheme ensures short-circuit control while providing minimum foldback.
The SM72485 can be applied in numerous applications to efficiently regulate down higher voltages. This
regulator is well suited for high voltage PV panel junction boxes, 48-V telecom and the new 42-V automotive
power bus ranges. Features include: thermal shutdown, VCC undervoltage lockout, gate drive undervoltage
lockout, maximum duty cycle limit timer, intelligent current limit off-timer, and a precharge switch.
7.2 Functional Block Diagram
SM72485
7 V BIAS
REGULATOR
6 V to 95 V
Input
VIN
C5
C1
V IN
SENSE
Q2
GND
BYPASS
SWITCH
VCC
UVLO
THERMAL
SHUTDOWN
VCC
C3
RT
ON TIMER
START
0 .7 V
RT
FINISH
RT / SD
BST
SHUTDOWN
OVER-VOLTAGE
COMPARATOR
START
GD
UVLO
300ns MIN
Vin
SD
2 . 875 V
FINISH
SSET Q
FB
R CL
VOUT
D1
RCLRQ
REGULATION
COMPARATOR
RTN
SW
SHIFT
FB
R CL
L1
LEVEL
2.5A
RCL
C4
DRIVER
OFF TIMER
PRE CHARGE
FINISH
START
CURRENT LIMIT
OFF TIMER
0.3A
BUCK
SWITCH
CURRENT
SENSE
R FB 2
R FB 1
R3
C2
Copyright © 2016, Texas Instruments Incorporated
7.3 Feature Description
7.3.1 Control Circuit Overview
The SM72485 is a buck DC-DC regulator that uses a control scheme in which the on-time varies inversely with
line voltage (VIN). Control is based on a comparator and the on-time one-shot, with the output voltage feedback
(FB) compared to an internal reference (2.5 V). If the FB level is below the reference the buck switch is turned on
for a fixed time determined by the line voltage and a programming resistor (RT). Following the ON period, the
switch remains off for at least the minimum off-timer period of 300 ns. If FB is still below the reference at that
time, the switch turns on again for another on-time period. This continues until regulation is achieved.
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Feature Description (continued)
The SM72485 operates in discontinuous conduction mode at light load currents, and continuous conduction
mode at heavy load current. In discontinuous conduction mode, current through the output inductor starts at zero
and ramps up to a peak during the on-time, then ramps back to zero before the end of the off-time. The next ontime period starts when the voltage at FB falls below the internal reference, until then the inductor current
remains zero. In this mode the operating frequency is lower than in continuous conduction mode, and varies with
load current. Therefore at light loads the conversion efficiency is maintained, because the switching losses
reduce with the reduction in load and frequency. The discontinuous operating frequency can be calculated in
Equation 1.
F
VOUT 2 u Lu 1.04 u 1020
RL u (RT )2
where
•
RL = the load resistance
(1)
In continuous conduction mode, current flows continuously through the inductor and never ramps down to zero.
In this mode the operating frequency is greater than the discontinuous mode frequency and remains relatively
constant with load and line variations. The approximate continuous mode operating frequency can be calculated
in Equation 2.
VOUT
F
1.385 u 10 10 u RT
(2)
The output voltage (VOUT) is programmed by two external resistors as shown in the Functional Block Diagram.
The regulation point can be calculated in Equation 3.
VOUT = 2.5 × (RFB1 + RFB2) / RFB1
(3)
The SM72485 regulates the output voltage based on ripple voltage at the feedback input, requiring a minimum
amount of ESR for the output capacitor C2. A minimum of 25 mV to 50 mV of ripple voltage at the feedback pin
(FB) is required for the SM72485. In cases where the capacitor ESR is too small, additional series resistance
may be required (R3 in the block diagram).
For applications where lower output voltage ripple is required the output can be taken directly from a low-ESR
output capacitor, as shown in Figure 7. However, R3 slightly degrades the load regulation.
L1
SW
RFB2
SM72485
R3
FB
VOUT2
RFB1
C2
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Figure 7. Low Ripple Output Configuration
8
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Feature Description (continued)
7.3.2 Current Limit
The SM72485 contains an intelligent current limit off-timer. If the current in the Buck switch exceeds 0.3 A, the
present cycle is immediately terminated, and a non-resetable off-timer is initiated. The length of off-time is
controlled by an external resistor (RCL) and the FB voltage (see Figure 4). When FB = 0 V, a maximum off-time is
required, and the time is preset to 35 µs. This condition occurs when the output is shorted, and during the initial
part of start-up. This amount of time ensures safe short-circuit operation up to the maximum input voltage of
95 V. In cases of overload where the FB voltage is above zero volts (not a short circuit) the current limit off-time
is less than 35 µs. Reducing the off-time during less severe overloads reduces the amount of foldback, recovery
time, and the start-up time. The off-time is calculated in Equation 4.
tOFF = 10–5 / (0.285 + (VFB / 6.35 × 10–6 × RCL))
(4)
The current-limit-sensing circuit is blanked for the first 50 ns to 70 ns of each on-time so it is not falsely tripped
by the current surge which occurs at turnon. The current surge is required by the re-circulating diode (D1) for its
turnoff recovery.
7.3.3 N-Channel Buck Switch and Driver
The SM72485 integrates an N-channel buck switch and associated floating high voltage gate driver. The gate
driver circuit works in conjunction with an external bootstrap capacitor and an internal high voltage diode. A 0.01µF ceramic capacitor (C4) connected between the BST pin and SW pin provides the voltage to the driver during
the on-time.
During each off-time, the SW pin is at approximately 0 V, and the bootstrap capacitor charges from VCC through
the internal diode. The minimum off-timer, set to 300 ns, ensures a minimum time each cycle to recharge the
bootstrap capacitor.
The internal precharge switch at the SW pin is turned on for ≊150 ns during the minimum off-time period,
ensuring sufficient voltage exists across the bootstrap capacitor for the on-time. This feature helps prevent
operating problems which can occur during very light load conditions, involving a long off-time, during which the
voltage across the bootstrap capacitor could otherwise reduce below the gate drive UVLO threshold. The
precharge switch also helps prevent start-up problems which can occur if the output voltage is precharged prior
to turnon. After current limit detection, the precharge switch is turned on for the entire duration of the forced offtime.
7.3.4 Thermal Protection
The SM72485 must be operated so the junction temperature does not exceed 125°C during normal operation. An
internal thermal shutdown circuit is provided to shutdown the SM72485 in the event of a higher than normal
junction temperature. When activated, typically at 165°C, the controller is forced into a low power reset state by
disabling the buck switch. This feature prevents catastrophic failures from accidental device overheating. When
the junction temperature reduces below 140°C (typical hysteresis = 25°C) normal operation is resumed.
7.4 Device Functional Modes
7.4.1 Start-Up Regulator (VCC)
The high voltage bias regulator is integrated within the SM72485. The input pin (VIN) can be connected directly
to line voltages between 6 V and 95 V, with transient capability to 100 V. Referring to the block diagram and the
graph of VCC vs VIN, when VIN is between 6 V and the bypass threshold (nominally 8.5 V), the bypass switch (Q2)
is on, and VCC tracks VIN within 100 mV to 150 mV. The bypass switch on-resistance is approximately 100 Ω,
with inherent current limiting at approximately 100 mA. When VIN is above the bypass threshold Q2 is turned off,
and VCC is regulated at 7 V. The VCC regulator output current is limited at approximately 9.2 mA. When the
SM72485 is shutdown using the RT/SD pin, the VCC bypass switch is shut off regardless of the voltage at VIN.
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Device Functional Modes (continued)
When VIN exceeds the bypass threshold, the time required for Q2 to shut off is approximately 2 µs to 3 µs. The
capacitor at VCC (C3) must be a minimum of 0.47 µF to prevent the voltage at VCC from rising above its
absolute maximum rating in response to a step input applied at VIN. C3 must be placed as close as possible to
the VCC and RTN pins. In applications with a relatively high input voltage, power dissipation in the bias regulator
is a concern. An auxiliary voltage of between 7.5 V and 14 V can be diode connected to the VCC pin to shut off
the VCC regulator, thereby reducing internal power dissipation. The current required into the VCC pin is shown in
Figure 6. Internally a diode connects VCC to VIN requiring that the auxiliary voltage be less than VIN.
The turnon sequence is shown in Figure 8. During the initial delay (t1) VCC ramps up at a rate determined by its
current limit and C3 while internal circuitry stabilizes. When VCC reaches the upper threshold of its undervoltage
lockout (UVLO, typically 5.3 V) the buckswitch is enabled. The inductor current increases to the current limit
threshold (ILIM) and during t2 VOUT increases as the output capacitor charges up. When VOUT reaches the
intended voltage the average inductor current decreases (t3) to the nominal load current (IO).
VIN
t1
7V
UVLO
V CC
Vin
SW Pin
0V
I LIM
Inductor
Current
IO
t2
t3
V OUT
Figure 8. Start-Up Sequence
7.4.2 Regulation Comparator
The feedback voltage at FB is compared to an internal 2.5-V reference. In normal operation (the output voltage is
regulated), an on-time period is initiated when the voltage at FB falls below 2.5 V. The buck switch remains on
for the on-time, causing the FB voltage to rise above 2.5 V. After the on-time period, the buck switch remains off
until the FB voltage again falls below 2.5 V. During start-up, the FB voltage is below 2.5 V at the end of each ontime, resulting in the minimum off-time of 300 ns. Bias current at the FB pin is nominally 100 nA.
7.4.3 Overvoltage Comparator
The feedback voltage at FB is compared to an internal 2.875-V reference. If the voltage at FB rises 2.875 V
above the on-time pulse is immediately terminated. This condition can occur if the input voltage, or the output
load, change suddenly. The buck switch does not turn on again until the voltage at FB falls below 2.5 V.
10
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Device Functional Modes (continued)
7.4.4 ON-Time Generator and Shutdown
The on-time for the SM72485 is determined by the RT resistor, and is inversely proportional to the input voltage
(VIN), resulting in a nearly constant frequency as VIN is varied over its range. The on-time equation for the
SM72485 is Equation 5.
tON = 1.385 × 10–10 × RT / VIN
(5)
RT must be selected for a minimum on-time (at maximum VIN) greater than 400 ns, for proper current limit
operation. This requirement limits the maximum frequency for each application, depending on VIN and VOUT.
The SM72485 can be remotely disabled by taking the RT/SD pin to ground. See Figure 9. The voltage at the
RT/SD pin is between 1.5 V and 3 V, depending on VIN and the value of the RT resistor.
Input
Voltage
VIN
SM72485
RT
RT/SD
STOP
RUN
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Figure 9. Shutdown Implementation
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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
The SM72485 is a step-down DC-to-DC regulator. It is typically used to convert a higher DC voltage to a lower
DC voltage with a maximum output current of 150 mA. The following design procedure can be used to select
components for the SM72485. This section presents a simplified discussion of the design process.
The final circuit is shown in Figure 10. The circuit was tested, and the resulting performance is shown in
Figure 14 and Figure 15.
8.2 Typical Application
12V - 90V
Input
VCC
VIN
8
C1
1.0 PF
7
C3
0.47 PF
C5
0.1 PF
BST
RT
2
309k
RT/SD
C4
0.01 PF
SM72485
6
L1
220 PH
10.0V
SW
VOUT
1
SHUTDOWN
D1
RCL
RFB2
3.01k
R3
RFB1
1.0k
C2
22 PF
3.3
3
RCL
FB
316k
RTN
5
4
GND
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Figure 10. SM72485 Example Circuit Diagram
8.2.1 Design Requirements
For this design example, use the parameters listed in Table 1 as the input parameters.
Table 1. Design Parameters
12
PARAMETER
EXAMPLE VALUE
Input voltage
12 V to 90 V
Output voltage
10 V
Minimum load current
100 mA
Maximum load current
150 mA
Feedback resistor ratio
3:1
Switching frequency
234 kHz
Inductor
200 µH
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8.2.2 Detailed Design Procedure
8.2.2.1 Selection of External Components
Here is a guide for determining the component values illustrated with a design example. See Functional Block
Diagram and Table 2 for more information. The following sections configure the SM72485 for:
• Input voltage (VIN): 12 V to 90 V
• Output voltage (VOUT1): 10 V
• Load current (for continuous conduction mode): 100 mA to 150 mA
Table 2. Bill of Materials
ITEM
DESCRIPTION
PART NUMBER
VALUE
C1
Ceramic capacitor
TDK C4532X7R2A105M
1 µF, 100 V
C2
Ceramic capacitor
TDK C4532X7R1E226M
22 µF, 25 V
C3
Ceramic capacitor
Kemet C1206C474K5RAC
0.47 µF, 50 V
C4
Ceramic capacitor
Kemet C1206C103K5RAC
0.01 µF, 50 V
C5
Ceramic capacitor
TDK C3216X7R2A104M
0.1 µF, 100 V
D1
Schottky power diode
Diodes Inc. DFLS1100
100 V, 1 A
COILTRONICS DR125-221-R, or
L1
Power inductor
RFB2
Resistor
RFB1
Resistor
Vishay CRCW12061001F
1 kΩ
R3
Resistor
Vishay CRCW12063R30F
3.3 Ω
RT
Resistor
Vishay CRCW12063093F
309 kΩ
TDK SLF10145T-221MR65
Vishay CRCW12063011F
220 µH
3.01 kΩ
RCL
Resistor
Vishay CRCW12063163F
316 kΩ
U1
Switching regulator
Texas Instruments SM72485
—
8.2.2.1.1 RFB1 and RFB2
VOUT = VFB × (RFB1 + RFB2) / RFB1, and because VFB = 2.5 V, the ratio of RFB2 to RFB1 calculates as 3:1. Standard
values of 3.01 kΩ and 1 kΩ are chosen. Other values could be used as long as the 3:1 ratio is maintained.
8.2.2.1.2 Fs and RT
The recommended operating frequency range for the SM72485 is 50 kHz to 1.1 MHz. Unless the application
requires a specific frequency, the choice of frequency is generally a compromise, because it affects the size of
L1 and C2, and the switching losses. The maximum allowed frequency, based on a minimum on-time of 400 ns,
is calculated from Equation 6.
FMAX = VOUT / (VINMAX × 400 ns)
(6)
For this exercise, FMAX = 277 kHz. From Equation 2, RT calculates to 260 kΩ. A standard value 309-kΩ resistor is
used to allow for tolerances in Equation 2, resulting in a frequency of 234 kHz.
8.2.2.1.3 L1
The main parameter affected by the inductor is the output current ripple amplitude. The choice of inductor value
therefore depends on both the minimum and maximum load currents, keeping in mind that the maximum ripple
current occurs at maximum VIN.
Minimum load current: To maintain continuous conduction at minimum Io (100 mA), the ripple amplitude (IOR)
must be less than 200 mAP–P so the lower peak of the waveform does not reach zero. L1 is calculated using
Equation 7.
VOUT × (VIN VOUT )
L1
IOR ×Fs × VIN
(7)
At VIN = 90 V, L1(min) calculates to 190 µH. The next larger standard value (220 µH) is chosen and with this
value IOR calculates to 173 mAP–P at VIN = 90 V, and 32 mAP–P at VIN = 12 V.
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Maximum load current: At a load current of 150 mA, the peak of the ripple waveform must not reach the
minimum ensured value of the SM72485’s current limit threshold (240 mA). Therefore the ripple amplitude must
be less than 180 mAP–P, which is already satisfied in the above calculation. With L1 = 220 µH, at maximum VIN
and IO, the peak of the ripple is 236 mA. While L1 must carry this peak current without saturating or exceeding its
temperature rating, it also must be capable of carrying the maximum specified value of the SM72485’s current
limit threshold (360 mA) without saturating, because the current limit is reached during start-up.
The DC resistance of the inductor must be as low as possible to minimize its power loss.
8.2.2.1.4 C3
The capacitor on the VCC output provides not only noise filtering and stability, but its primary purpose is to
prevent false triggering of the VCC UVLO at the buck switch on or off transitions. C3 must be no smaller than
0.47 µF.
8.2.2.1.5 C2 and R3
When selecting the output filter capacitor C2, the items to consider are ripple voltage due to its ESR, ripple
voltage due to its capacitance, and the nature of the load.
8.2.2.1.6 ESR and R3
A low ESR for C2 is generally desirable so as to minimize power losses and heating within the capacitor.
However, the regulator requires a minimum amount of ripple voltage at the feedback input for proper loop
operation. For the SM72485 the minimum ripple required at pin 5 is 25 mVP–P, requiring a minimum ripple at
VOUT of 100 mV. Because the minimum ripple current (at minimum VIN) is 32 mAP–P, the minimum ESR required
at VOUT is 100 mV / 32 mA = 3.12 Ω. Because quality capacitors for SMPS applications have an ESR
considerably less than this, R3 is inserted as shown in the Functional Block Diagram. R3’s value, along with C2’s
ESR, must result in at least 25-mVP–P ripple at pin 5. Generally, R3 is 0.5 to 4 Ω.
8.2.2.1.7 RCL
When current limit is detected, the minimum off-time set by this resistor must be greater than the maximum
normal off-time, which occurs at maximum input voltage. Using Equation 5, the minimum on-time is 476 ns,
yielding an off-time of 3.8 µs (at 234 kHz). Due to the 25% tolerance on the on-time, the off-time tolerance is also
25%, yielding a maximum off-time of 4.75 µs. Allowing for the response time of the current limit detection circuit
(350 ns) increases the maximum off-time to 5.1 µs. This is increased an additional 25% to 6.4 µs to allow for the
tolerances of Equation 4. Using Equation 4, RCL calculates to 310 kΩ at VFB = 2.5 V. A standard value 316-kΩ
resistor is used.
8.2.2.1.8 D1
The important parameters are reverse recovery time and forward voltage. The reverse recovery time determines
how long the reverse current surge lasts each time the buck switch is turned on. The forward voltage drop is
significant in the event the output is short-circuited as it is only this diode’s voltage which forces the inductor
current to reduce during the forced off-time. For this reason, a higher voltage is better, although that affects
efficiency. A good choice is a Schottky power diode, such as the DFLS1100. D1’s reverse voltage rating must be
at least as great as the maximum VIN, and its current rating be greater than the maximum current limit threshold
(360 mA).
8.2.2.1.9 C1
This capacitor’s purpose is to supply most of the switch current during the on-time, and limit the voltage ripple at
VIN, on the assumption that the voltage source feeding VIN has an output impedance greater than zero. At
maximum load current, when the buck switch turns on, the current into pin 8 suddenly increases to the lower
peak of the output current waveform, ramp up to the peak value, then drop to zero at turnoff. The average input
current during this on-time is the load current (150 mA). For a worst case calculation, C1 must supply this
average load current during the maximum on-time. To keep the input voltage ripple to less than 2 V (for this
exercise), C1 is calculated by Equation 8.
I× tON 0.15 A× 3.57Ps
C1 =
0.268 PF
'V
2.0 V
(8)
14
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Quality ceramic capacitors in this value have a low ESR which adds only a few millivolts to the ripple. It is the
capacitance which is dominant in this case. To allow for the capacitor’s tolerance, temperature effects, and
voltage effects, a 1-µF, 100-V, X7R capacitor is used.
8.2.2.1.10 C4
TI recommends a value of 0.01 µF for C4, as this is appropriate in the majority of applications. A high-quality
ceramic capacitor, with low ESR is recommended as C4 supplies the surge current to charge the buck switch
gate at turnon. A low ESR also ensures a quick recharge during each off-time. At minimum VIN, when the on-time
is at maximum, it is possible during start-up that the C4 does not fully recharge during each 300-ns off-time. The
circuit is not able to complete the start-up and achieve output regulation then. This can occur when the frequency
is intended to be low (for example, RT = 500 K). In this case, C4 must be increased so it can maintain sufficient
voltage across the buck switch driver during each on-time.
8.2.2.1.11 C5
This capacitor helps avoid supply voltage transients and ringing due to long lead inductance at VIN. A low ESR,
0.1-µF ceramic chip capacitor is recommended, placed close to the SM72485.
8.2.2.2 Low Output Ripple Configurations
For applications where low output ripple is required, the following sections can be used to reduce or nearly
eliminate the ripple.
8.2.2.2.1 Reduced Ripple Configuration
In Figure 11, Cff is added across RFB2 to AC-couple the ripple at VOUT directly to the FB pin. This allows the
ripple at VOUT to be reduced to a minimum of 25 mVp–p by reducing R3, because the ripple at VOUT is not
attenuated by the feedback resistors. The minimum value for Cff is determined from Equation 9.
3 × t ON (max)
Cff =
(RFB1 / / RFB 2 )
where
•
tON(max) is the maximum on-time which occurs at VIN(min)
(9)
The next larger standard value capacitor must be used for Cff.
L1
SW
VOUT
Cff
SM72485
RFB2
R3
FB
RFB1
C2
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Figure 11. Reduced Ripple Configuration
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8.2.2.2.2 Minimum Ripple Configuration
If the application requires a lower value of ripple (
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