TI1 LM5017 600ma constant on-time synchronous buck regulator Datasheet

LM5017
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LM5017 100V, 600mA Constant On-Time Synchronous Buck Regulator
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FEATURES
PACKAGES
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1
2
Wide 9V to 100V Input Range
Integrated 100V, High and Low Side Switches
No Schottky Required
Constant On-time Control
No Loop Compensation Required
Ultra-Fast Transient Response
Nearly Constant Operating Frequency
Intelligent Peak Current Limit
Adjustable Output Voltage from 1.225V
Precision 2% Feedback Reference
Frequency Adjustable to 1MHz
Adjustable Undervoltage Lockout (UVLO)
Remote Shutdown
Thermal Shutdown
DESCRIPTION
The LM5017 is a 100V, 600mA synchronous stepdown regulator with integrated high side and low side
MOSFETs. The constant-on-time (COT) control
scheme employed in the LM5017 requires no loop
compensation, provides excellent transient response,
and enables very low step-down ratios. The on-time
varies inversely with the input voltage resulting in
nearly constant frequency over the input voltage
range. A high voltage startup regulator provides bias
power for internal operation of the IC and for
integrated gate drivers.
A peak current limit circuit protects against overload
conditions. The undervoltage lockout (UVLO) circuit
allows the input undervoltage threshold and
hysteresis to be independently programmed. Other
protection features include thermal shutdown and
bias supply undervoltage lockout (VCC UVLO).
APPLICATIONS
•
•
•
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WSON-8
SO PowerPAD-8
Smart Power Meters
Telecommunication Systems
Automotive Electronics
Isolated Bias Supply
The LM5017 is available in WSON-8 and SO
PowerPAD-8 plastic packages.
Typical Application
LM5017
CIN
2
+
4
RUV2
RON
SD
3
BST
VIN
SW
RON
+
8
CBST
L1
VOUT
CVCC
VCC
UVLO
FB
RUV1
7
RTN
1
+
9V-100V
VIN
6
RFB2
5
RC
+
RFB1
COUT
1
2
Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of
Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.
All trademarks are the property of their respective owners.
PRODUCTION DATA information is current as of publication date.
Products conform to specifications per the terms of the Texas
Instruments standard warranty. Production processing does not
necessarily include testing of all parameters.
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Connection Diagram
RTN
1
VIN
2
UVLO
3
RON
4
SO
PowerPAD8
Exp Pad
8
SW
7
BST
6
VCC
5
FB
Figure 1. Top View (Connect Exposed Pad to RTN)
RTN
1
VIN
2
UVLO
3
RON
4
8 SW
WSON-8
Exp Pad
7 BST
6 VCC
5 FB
Figure 2. Top View (Connect Exposed Pad to RTN)
Pin Descriptions
Pin
Name
1
RTN
2
VIN
3
UVLO
4
Description
Application Information
Ground
Ground connection of the integrated circuit.
Input Voltage
Operating input range is 9V to 100V.
Input Pin of Undervoltage Comparator
Resistor divider from VIN to UVLO to GND programs the
undervoltage detection threshold. An internal current
source is enabled when UVLO is above 1.225V to
provide hysteresis. When UVLO pin is pulled below
0.66V externally, the parts goes in shutdown mode.
RON
On-Time Control
A resistor between this pin and VIN sets the switch ontime as a function of VIN. Minimum recommended ontime is 100ns at max input voltage.
5
FB
Feedback
This pin is connected to the inverting input of the internal
regulation comparator. The regulation level is 1.225V.
6
VCC
Output from the Internal High Voltage Series Pass
Regulator. Regulated at 7.6V
The internal VCC regulator provides bias supply for the
gate drivers and other internal circuitry. A 1.0μF
decoupling capacitor is recommended.
7
BST
Bootstrap Capacitor
An external capacitor is required between the BST and
SW pins (0.01μF ceramic). The BST pin capacitor is
charged by the VCC regulator through an internal diode
when the SW pin is low.
8
SW
Switching Node
Power switching node. Connect to the output inductor
and bootstrap capacitor.
EP
Exposed Pad
Exposed pad must be connected to RTN pin. Connect to
system ground plane on application board for reduced
thermal resistance.
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.
2
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Absolute Maximum Ratings
(1) (2)
VIN, UVLO to RTN
-0.3V to 100V
SW to RTN
-1.5V to VIN +0.3V
BST to VCC
100V
BST to SW
13V
RON to RTN
-0.3V to 100V
VCC to RTN
-0.3V to 13V
FB to RTN
-0.3V to 5V
ESD Rating (Human Body Model
Lead Temperature
(3)
2kV
(4)
200°C
Storage Temperature Range
(1)
(2)
(3)
(4)
-55°C to +150°C
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 ensured specifications and test conditions, see the Electrical Characteristics.
The RTN pin is the GND reference electrically connected to the substrate.
If Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/ Distributors for availability and
specifications.
The human body model is a 100pF capacitor discharged through a 1.5kΩ resistor into each pin.
For detailed information on soldering plastic SO PowerPAD package, refer to the SNOA549 available from Texas Instruments. Max
solder time not to exceed 4 seconds.
Operating Ratings
(1)
VIN Voltage
9V to 100V
−40°C to +125°C
Operating Junction Temperature
(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 ensured specifications and test conditions, see the Electrical Characteristics.
The RTN pin is the GND reference electrically connected to the substrate.
Thermal Characteristics (1)
WSON-8
SO PowerPAD-8
UNIT
θJA
Junction-to-ambient thermal resistance
41.3
41.1
°C/W
θJCbot
Junction-to-case (bottom) thermal resistance
3.2
2.4
°C/W
ΨJB
Junction-to-board thermal characteristic parameter
19.2
24.4
°C/W
θJB
Junction-to-board thermal resistance
19.1
30.6
°C/W
θJCtop
Junction-to-case (top) thermal resistance
34.7
37.3
°C/W
ΨJT
Junction-to-top thermal characteristic parameter
0.3
6.7
°C/W
(1)
The package thermal impedance is calculated in accordance with JESD 51.
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Electrical Characteristics
Specifications with standard typeface are for TJ = 25°C, and those with boldface type apply over full Operating Junction
Temperature range. VIN = 48V, unless otherwise stated. See (1).
Symbol
Parameter
Conditions
Min
Typ
Max
6.25
7.6
8.55
Units
VCC Supply
VCC Reg
VCC Regulator Output
VIN = 48V, ICC = 20mA
VCC Current Limit
VIN = 48V (2)
VCC Undervoltage Lockout Voltage
(VCC increasing)
26
V
mA
4.15
VCC Undervoltage Hysteresis
4.5
4.9
300
V
mV
VCC Drop Out Voltage
VIN = 9V, ICC = 20mA
2.3
V
IIN Operating Current
Non-Switching, FB = 3V
1.75
mA
IIN Shutdown Current
UVLO = 0V
50
225
µA
Buck Switch RDS(ON)
ITEST = 200mA, BST-SW =
7V
0.8
1.8
Ω
Synchronous RDS(ON)
ITEST = 200mA
0.45
1
Ω
Gate Drive UVLO
VBST − VSW Rising
3
3.6
Switch Characteristics
2.4
Gate Drive UVLO Hysteresis
260
V
mV
Current Limit
Current Limit Threshold
0.7
Current Limit Response Time
Time to Switch Off
OFF-Time Generator (Test 1)
OFF-Time Generator (Test 2)
1.02
1.3
A
150
ns
FB = 0.1V, VIN = 48V
12
µs
FB = 1.0V, VIN = 48V
2.5
µs
On-Time Generator
TON Test 1
VIN = 32V, RON = 100k
270
350
460
ns
TON Test 2
VIN = 48V, RON = 100k
188
250
336
ns
TON Test 3
VIN = 75V, RON = 250k
250
370
500
ns
TON Test 4
VIN = 10V, RON = 250k
1880
3200
4425
ns
Minimum Off-Time
Minimum Off-Timer
FB = 0V
144
ns
Regulation and Overvoltage Comparators
FB Regulation Level
Internal Reference Trip Point
for Switch ON
FB Overvoltage Threshold
Trip Point for Switch OFF
1.2
1.225
FB Bias Current
1.25
V
1.62
V
60
nA
Undervoltage Sensing Function
UV Threshold
UV Rising
1.19
1.225
1.26
V
UV Hysteresis Input Current
UV = 2.5V
-10
-20
-29
µA
Remote Shutdown Threshold
Voltage at UVLO Falling
0.32
0.66
V
110
mV
Thermal Shutdown Temperature
165
°C
Thermal Shutdown Hysteresis
20
°C
Remote Shutdown Hysteresis
Thermal Shutdown
Tsd
(1)
(2)
4
All limits are specified by design. All electrical characteristics having room temperature limits are tested during production at TA = 25°C.
All hot and cold limits are specified by correlating the electrical characteristics to process and temperature variations and applying
statistical process control.
VCC provides self bias for the internal gate drive and control circuits. Device thermal limitations limit external loading.
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Typical Performance Characteristics
Efficiency at 200kHz, 10V
VCC vs VIN
Figure 3.
Figure 4.
VCC vs ICC
ICC vs External VCC
Figure 5.
Figure 6.
TON vs VIN and RON
TOFF (ILIM) vs VFB and VIN
Figure 7.
Figure 8.
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Typical Performance Characteristics (continued)
IIN vs VIN (Operating, Non Switching)
IIN vs VIN (Shutdown)
Figure 9.
Figure 10.
Switching Frequency vs VIN
Figure 11.
6
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Block Diagram
LM5017
START-UP
REGULATOR
VIN
VCC
V UVLO
20 µA
4.5V
UVLO
THERMAL
SHUTDOWN
UVLO
1.225V
SD
VDD REG
BST
0.66V
SHUTDOWN
BG REF
VIN
DISABLE
ON/OFF
TIMERS
RON
1.225V
SW
COT CONTROL
LOGIC
FEEDBACK
FB
ILIM
COMPARATOR
OVER-VOLTAGE
1.62V
CURRENT
LIMIT
ONE-SHOT
RTN
+
VILIM
Figure 12. Functional Block Diagram
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Functional Description
The LM5017 step-down switching regulator features all the functions needed to implement a low cost, efficient,
buck converter capable of supplying up to 0.6A to the load. This high voltage regulator contains 100V, N-channel
buck and synchronous switches, is easy to implement, and is provided in thermally enhanced SO PowerPAD-8
and WSON-8 packages. The regulator operation is based on a constant on-time control scheme using an ontime inversely proportional to VIN. This control scheme does not require loop compensation. The current limit is
implemented with a forced off-time inversely proportional to VOUT. This scheme ensures short circuit protection
while providing minimum foldback. The simplified block diagram of the LM5017 is shown in Figure 12, Functional
Block Diagram.
The LM5017 can be applied in numerous applications to efficiently regulate down higher voltages. This regulator
is well suited for 48V telecom and 42V automotive power bus ranges. Protection features include: thermal
shutdown, Undervoltage Lockout (UVLO), minimum forced off-time, and an intelligent current limit.
Control Overview
The LM5017 buck regulator employs a control principle based on a comparator and a one-shot on-timer, with the
output voltage feedback (FB) compared to an internal reference (1.225V). If the FB voltage is below the
reference the internal buck switch is turned on for the one-shot timer period, which is a function of the input
voltage and the programming resistor (RON). Following the on-time the switch remains off until the FB voltage
falls below the reference, but never before the minimum off-time forced by the minimum off-time one-shot timer.
When the FB pin voltage falls below the reference and the minimum off-time one-shot period expires, the buck
switch is turned on for another on-time one-shot period. This will continue until regulation is achieved and the FB
voltage is approximately equal to 1.225V (typ).
In a synchronous buck converter, the low side (sync) FET is ‘on’ when the high side (buck) FET is ‘off’. The
inductor current ramps up when the high side switch is ‘on’ and ramps down when the high side switch is ‘off’.
There is no diode emulation feature in this IC, and therefore, the inductor current may ramp in the negative
direction at light load. This causes the converter to operate in continuous conduction mode (CCM) regardless of
the output loading. The operating frequency remains relatively constant with load and line variations. The
operating frequency can be calculated as follows:
VOUT
gsw = -10
10 x RON
The output voltage (VOUT) is set by two external resistors (RFB1, RFB2). The regulated output voltage is calculated
as follows:
RFB2 + RFB1
VOUT = 1.225V x
RFB1
RFB2 VOUT - 1.225V
=
1.225V
RFB1
This regulator regulates the output voltage based on ripple voltage at the feedback input, requiring a minimum
amount of ESR for the output capacitor (COUT). A minimum of 25mV of ripple voltage at the feedback pin (FB) is
required for the LM5017. In cases where the capacitor ESR is too small, additional series resistance may be
required (RC in Figure 13 Low Ripple Output Configuration).
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 13 Low Ripple Output Configuration. However, RC slightly degrades the
load regulation.
VCC Regulator
The LM5017 contains an internal high voltage linear regulator with a nominal output of 7.6V. The input pin (VIN)
can be connected directly to the line voltages up to 100V. The VCC regulator is internally current limited to 30mA.
The regulator sources current into the external capacitor at VCC. This regulator supplies current to internal circuit
blocks including the synchronous MOSFET driver and the logic circuits. When the voltage on the VCC pin
reaches the undervoltage lockout (VCC UVLO) threshold of 4.5V, the IC is enabled.
The VCC regulator contains an internal diode connection to the BST pin to replenish the charge in the gate drive
boot capacitor when SW pin is low.
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At high input voltages, the power dissipated in the high voltage regulator is significant and can limit the overall
achievable output power. As an example, with the input at 48V and switching at high frequency, the VCC
regulator may supply up to 7mA of current resulting in 48V x 7mA = 336mW of power dissipation. If the VCC
voltage is driven externally by an alternate voltage source, between 8V and 13V, the internal regulator is
disabled. This reduces the power dissipation in the IC.
L1
VOUT
SW
LM5017
RFB2
FB
RC
+
RFB1
COUT
VOUT
(low ripple)
Figure 13. Low Ripple Output Configuration
Regulation Comparator
The feedback voltage at FB is compared to an internal 1.225V reference. In normal operation, when the output
voltage is in regulation, an on-time period is initiated when the voltage at FB falls below 1.225V. The high side
switch will stay on for the on-time, causing the FB voltage to rise above 1.225V. After the on-time period, the
high side switch will stay off until the FB voltage again falls below 1.225V. During start-up, the FB voltage will be
below 1.225V at the end of each on-time, causing the high side switch to turn on immediately after the minimum
forced off-time of 144ns. The high side switch can be turned off before the on-time is over, if the peak current in
the inductor reaches the current limit threshold.
Overvoltage Comparator
The feedback voltage at FB is compared to an internal 1.62V reference. If the voltage at FB rises above 1.62V
the on-time pulse is immediately terminated. This condition can occur if the input voltage and/or the output load
changes suddenly. The high side switch will not turn on again until the voltage at FB falls below 1.225V.
On-Time Generator
The on-time for the LM5017 is determined by the RON 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
LM5017 is:
10-10 x RON
TON =
VIN
See figure “TON vs VIN and RON” in the section “ Performance Curves”. RON should be selected for a minimum ontime (at maximum VIN) greater than 100ns, for proper operation. This requirement limits the maximum switching
frequency for high VIN.
Current Limit
The LM5017 contains an intelligent current limit off-timer. If the current in the buck switch exceeds 1.02A the
present cycle is immediately terminated, and a non-resetable off-timer is initiated. The length of off-time is
controlled by the FB voltage and the input voltage VIN. As an example, when FB = 0V and VIN = 48V, the
maximum off-time is set to 16μs. 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 100V.
In cases of overload where the FB voltage is above zero volts (not a short circuit) the current limit off-time is
reduced. Reducing the off-time during less severe overloads reduces the amount of foldback, recovery time, and
start-up time. The off-time is calculated from the following equation:
0.07 x VIN
Ps
TOFF(ILIM) =
VFB + 0.2V
The current limit protection feature is peak limited. The maximum average output will be less than the peak.
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N-Channel Buck Switch and Driver
The LM5017 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
0.01uF ceramic capacitor connected between the BST pin and the SW pin provides the voltage to the driver
during the on-time. During each off-time, the SW pin is at approximately 0V, and the bootstrap capacitor charges
from VCC through the internal diode. The minimum off-timer, set to 144ns , ensures a minimum time each cycle to
recharge the bootstrap capacitor.
Synchronous Rectifier
The LM5017 provides an internal synchronous N-Channel MOSFET rectifier. This MOSFET provides a path for
the inductor current to flow when the high-side MOSFET is turned off.
The synchronous rectifier has no diode emulation mode, and is designed to keep the regulator in continuous
conduction mode even during light loads which would otherwise result in discontinuous operation.
Undervoltage Detector
The LM5017 contains a dual level undervoltage lockout (UVLO) circuit. When the UVLO pin voltage is below
0.66V, the controller is in a low current shutdown mode. When the UVLO pin voltage is greater than 0.66V but
less than 1.225V, the controller is in standby mode. In standby mode the VCC bias regulator is active while the
regulator output is disabled. When the VCC pin exceeds the VCC undervoltage threshold and the UVLO pin
voltage is greater than 1.225V, normal operation begins. An external set-point voltage divider from VIN to GND
can be used to set the minimum operating voltage of the regulator.
UVLO hysteresis is accomplished with an internal 20μA current source that is switched on or off into the
impedance of the set-point divider. When the UVLO threshold is exceeded, the current source is activated to
quickly raise the voltage at the UVLO pin. The hysteresis is equal to the value of this current times the resistance
RUV2.
UVLO
Mode
Description
<0.66V
VCC
Shutdown
VCC regulator disabled.
Switcher disabled.
0.66V – 1.225V
Standby
VCC regulator enabled
Switcher disabled.
VCC <4.5V
Standby
VCC regulator enabled.
Switcher disabled.
VCC >4.5V
Operating
VCC enabled.
Switcher enabled.
>1.225V
If the UVLO pin is wired directly to the VIN pin, the regulator will begin operation once the VCC undervoltage is
satisfied.
VIN
CIN
2
VIN
+
RUV2
LM5017
3
UVLO
RUV1
Figure 14. UVLO Resistor Setting
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Thermal Protection
The LM5017 should be operated so the junction temperature does not exceed 150°C during normal operation.
An internal Thermal Shutdown circuit is provided to protect the LM5017 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,
disabling the buck switch and the VCC regulator. This feature prevents catastrophic failures from accidental
device overheating. When the junction temperature reduces below 145°C (typical hysteresis = 20°C), the VCC
regulator is enabled, and normal operation is resumed.
APPLICATION INFORMATION
SELECTION OF EXTERNAL COMPONENTS
Selection of external components is illustrated through a design example. The design example specifications are
as follows:
Buck Converter Design Specifications
Input voltage range
12.5V to 95V
Output voltage
10V
Maximum Load current
500mA
Switching Frequency
200kHz
RFB1, RFB2:
VOUT = VFB x (RFB2/RFB1 + 1), and since VFB = 1.225V, the ratio of RFB2 to RFB1 calculates as 7:1. Standard
values of 6.98kΩ and 1.00kΩ are chosen. Other values could be used as long as the 7:1 ratio is maintained.
Frequency Selection:
At the minimum input voltage, the maximum switching frequency of LM5017 is restricted by the forced minimum
off-time (TOFF(MIN)) as given by:
1 - DMAX
1 - 10/12.5
=
= 1 MHz
gSW(MAX) =
200 ns
TOFF(MIN)
Similarly, at maximum input voltage, the maximum switching frequency of LM5017 is restricted by the minimum
TON as given by:
DMIN
10/95
gSW(MAX) =
=
= 1.05 MHz
TON(MIN) 100 ns
Resistor RON sets the nominal switching frequency based on the following equations:
VOUT
gSW =
K x RON
(1)
–10
where K = 1 x 10 . Operation at high switching frequency results in lower efficiency while providing the smallest
solution. For this example a conservative 200kHz was selected, resulting in RON = 504kΩ. Selecting a standard
value for RON = 499kΩ results in a nominal frequency of 202kHz.
Inductor Selection:
The minimum inductance is selected to limit the output ripple to 20 to 40 percent of the maximum load current. In
addition, the peak inductor current at maximum load should be smaller than the minimum current limit as given in
Electrical Characteristics table. The inductor current ripple is given by:
VIN - VOUT VOUT
x
ûIL =
VIN
L1 x gSW
The maximum ripple is observed at maximum input voltage. Substituting VIN = 95V and ΔIL = 40 percent x IOUT
(max) results in L1 = 198.4μH. The next higher standard value of 220μH is chosen. The peak-to-peak minimum
and maximum inductor current ripples of 35mA and 204mA are given at minimum and maximum input voltages
respectively. The peak inductor and switch current is given by
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ILI(peak) = IOUT +
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ûIL(MAX)
= 602 mA
2
which is smaller than the minimum current limit. The inductor should be able to withstand the maximum current
limit of 1.3A, which can be reached during startup and overload conditions.
LM5017
CIN
2
+
+
4
CBYP
RUV2
Shutdown
RON
3
BST
VIN
SW
RON
+
8
CBST
L1
VOUT
CVCC
VCC
UVLO
FB
RUV1
7
RTN
1
+
9V-100V
VIN
6
RC
RFB2
5
RFB1
+
COUT
Figure 15. Reference Schematic for Selection of External Components
Output Capacitor:
The output capacitor is selected to minimize the capacitive ripple across it. The maximum ripple is observed at
maximum input voltage and is given by:
ûIL
COUT =
8 x gsw x ûVripple
where ΔVripple is the voltage ripple across the capacitor. Substituting ΔVripple = 10mV gives COUT = 12.64μF. A
22μF standard value is selected. An X5R or X7R type capacitor with a voltage rating 16V or higher should be
selected.
Series Ripple Resistor RC:
The series resistor should be selected to produce sufficient ripple at the feedback node. The ripple produced by
RC is proportional to the inductor current ripple, and therefore RC should be chosen for minimum inductor current
ripple which occurs at minimum input voltage. The RC is calculated by the equation:
25 mV VOUT
x
RC >
ûIL(MIN) VREF
This gives an RC of greater than or equal to 5.15Ω. Selecting RC = 5.23Ω results in ~1V of maximum output
voltage ripple. For applications requiring lower output voltage ripple, Type II or Type III ripple injection circuits
should be used as described in the section “ Ripple Configuration”.
VCC and Bootstrap Capacitor:
The VCC capacitor provides charge to bootstrap capacitor as well as internal circuitry and low side gate driver.
The Bootstrap capacitor provides charge to high side gate driver. A good value for CVCC is 1μF. A good value for
CBST is 0.01μF.
Input Capacitor:
Input capacitor should be large enough to limit the input voltage ripple:
IOUT(MAX)
CIN >
8 x gSW x ûVIN
choosing a ΔVIN = 0.5V gives a minimum CIN = 1.24μF. A standard value of 2.2μF is selected. The input
capacitor should be rated for the maximum input voltage under all conditions. A 100V, X7R dielectric should be
selected for this design.
Input capacitor should be placed directly across VIN and RTN (pin 2 and 1) of the IC. If it is not possible to place
all of the input capacitor close to the IC, a 0.47μF capacitor should be placed near the IC to provide a bypass
path for the high frequency component of the switching current. This helps limit the switching noise.
12
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UVLO Resistors:
The UVLO resistors RFB1 and RFB2 set the UVLO threshold and hysteresis according to the following relationship:
VIN(HYS) = IHYS x RUV2
and
VIN (UVLO,rising) = 1.225V x (
RUV2
+ 1)
RUV1
where IHYS = 20μA. Setting UVLO hysteresis of 2.5V and UVLO rising threshold of 12V results in RUV1 = 14.53kΩ
and RUV2 = 125kΩ. Selecting standard value of RUV1 = 14kΩ and RUV2 = 125kΩ results in UVLO thresholds and
hysteresis of 12.4V and 2.5V respectively.
APPLICATION CIRCUIT: 12V TO 95V INPUT AND 10V, 500mA OUTPUT BUCK CONVERTER
The application schematic of a buck supply is shown in Figure 16 below. For output voltage (VOUT) above the
maximum regulation threshold of VCC (8.55V, see Electrical Characteristics), the VCC pin can be connected to
VOUT through a diode (D2), as shown below, for higher efficiency and lower power dissipation in the IC.
12V-95V
VIN
(TP1)
30177733
BST
2
C4
2.2 F
+
C5 +
R5
0.47 F 127 NŸ
GND
(TP2)
(TP4)
UVLO/SD
SW
(TP6)
LM5017
VIN
4
SW
RON
R3
499 NŸ 3
7 0.01 F
+
C1
8
R4
46.4 NŸ
UVLO
VCC
R7
14 NŸ
FB
EXP
220 H
L1
RTN
1
6
0Ÿ
R8
C6
C8
0.1 F
5
+
(TP3)
R2
0Ÿ
3300 pF
D2
U1
VOUT
R1
6.98 NŸ
+
R6
1 NŸ
C7
1 F
C9
22 F
GND
(TP5)
Figure 16. Final Schematic for 12V to 95V Input, and 10V, 500mA Output Buck Converter
ISOLATED DC-DC CONVERTER USING LM5017
An isolated supply using LM5017 is shown in Figure 17 below. Inductor (L) in a typical buck circuit is replaced
with a coupled inductor (X1). A diode (D1) is used to rectify the voltage on a secondary output. The nominal
voltage at the secondary output (VOUT2) is given by:
N
VOUT2 = VOUT1 x S - VF
NP
where VF is the forward voltage drop of D1, and NP, NS are the number of turns on the primary and secondary
of coupled inductor X1. For output voltage (VOUT1) above the maximum VCC (8.55V), the VCC pin can be diode
connected to VOUT1 for higher effiicency and low dissipation in the IC. See AN-2204 (SNVA611) for a complete
isolated bias design with LM5017.
VOUT2
D1
+
NS
LM5017
VIN
20V-100V
BST
CIN
+
X1
CBST
SW
VIN
COUT2
VOUT1
Rr NP
+
+
RON
RUV2
RON
Cac
RFB2
VCC
UVLO
COUT1
Cr
D2
RUV1
RTN
FB
+
CVCC
RFB1
Figure 17. Typical Isolated Application Schematic
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RIPPLE CONFIGURATION
LM5017 uses Constant-On-Time (COT) control scheme, in which the on-time is terminated by an on-timer, and
the off-time is terminated by the feedback voltage (VFB) falling below the reference voltage (VREF). Therefore, for
stable operation, the feedback voltage must decrease monotonically, in phase with the inductor current during
the off-time. Furthermore, this change in feedback voltage (VFB) during off-time must be large enough to
suppress any noise component present at the feedback node.
Table 1 shows three different methods for generating appropriate voltage ripple at the feedback node. Type 1
and Type 2 ripple circuits couple the ripple at the output of the converter to the feedback node (FB). The output
voltage ripple has two components:
1. Capacitive ripple caused by the inductor current ripple charging/discharging the output capacitor.
2. Resistive ripple caused by the inductor current ripple flowing through the ESR of the output capacitor.
The capacitive ripple is not in phase with the inductor current. As a result, the capacitive ripple does not
decrease monotonically during the off-time. The resistive ripple is in phase with the inductor current and
decreases monotonically during the off-time. The resistive ripple must exceed the capacitive ripple at the output
node (VOUT) for stable operation. If this condition is not satisfied unstable switching behavior is observed in COT
converters, with multiple on-time bursts in close succession followed by a long off-time.
Type 3 ripple method uses Rr and Cr and the switch node (SW) voltage to generate a triangular ramp. This
triangular ramp is ac coupled using Cac to the feedback node (FB). Since this circuit does not use the output
voltage ripple, it is ideally suited for applications where low output voltage ripple is required. See application note
AN-1481 (SNVA166) for more details for each ripple generation method.
Table 1.
Type 1
Lowest Cost Configuration
Type 2
Reduced Ripple Configuration
VOUT
Type 3
Minimum Ripple Configuration
VOUT
L1
VOUT
L1
L1
R FB2
Cac
R FB2
RC
To FB
C OUT
COUT
R FB2
GND
R FB1
GND
25 mV VOUT
x
ûIL(MIN) VREF
Cr
Cac
To FB
R FB1
RC >
Rr
RC
C OUT
To FB
R FB1
GND
C>
5
gsw (RFB2||RFB1)
25 mV
RC >
ûIL(MIN)
Cr = 3300 pF
Cac = 100 nF
(VIN(MIN) - VOUT) x TON
RrCr <
25 mV
SOFT START
A soft-start feature can be implemented to the LM5017 using an external circuit. As shown in Figure 18, the softstart circuit consists of one capacitor, C1, two resistors, R1 and R2, and a diode, D. During the initial start-up, the
VCC voltage is established prior to the VOUT voltage. D is thereby forward biased and the FB voltage is pulled up
above the reference voltage (1.225V). The switcher is disabled. With the charging of the capacitor C1, the voltage
at node B gradually decreases. Due to the action of the control circuit, VOUT will gradually rise to maintain the FB
voltage at the reference voltage. Once the voltage at node B is lower than the FB voltage, plus the voltage drop
of D, the soft-start is finished and D is reverse biased.
During the initial part of the start-up, the FB voltage can be approximated as follows. Please note that the effect
of R1 has been ignored to simplify the calculation:
RFB1 x RFB2
VFB = (VCC - VD) x
R2 x (RFB1 + RFB2) + RFB1 x RFB2
14
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To achieve the desired soft-start, the following design guidance is recommended:
(1) R2 is selected so that VFB is higher than 1.225V for a VCC of 4.5V, but is lower than 5V when VCC is 8.55V. If
an external VCC is used, VFB should not exceed 5V at maximum VCC.
(2) C1 is selected to achieve the desired start-up time that can be determined as follows:
RFB1 x RFB2
)
tS = C1 x (R2 +
RFB1 + RFB2
(3) R1 is used to maintain the node B voltage at zero after the soft-start is finished. A value larger than the
feedback resistor divider is preferred.
Based on the schematic shown in Figure 16, selecting C1=1uF, R2=1kΩ, R1=30kΩ results in a soft-start time of
about 2ms.
VCC
VOUT
C1
RFB2
R2
To FB
D
B
RFB1
R1
Figure 18. Soft-Start Circuit
LAYOUT RECOMMENDATION
A proper layout is essential for optimum performance of the circuit. In particular, the following guidelines should
be observed:
1. CIN: The loop consisting of input capacitor (CIN), VIN pin, and RTN pin carries switching currents. Therefore,
the input capacitor should be placed close to the IC, directly across VIN and RTN pins and the connections to
these two pins should be direct to minimize the loop area. In general it is not possible to accommodate all of
input capacitance near the IC. A good practice is to use a 0.1μF or 0.47μF capacitor directly across the VIN
and RTN pins close to the IC, and the remaining bulk capacitor as close as possible (Refer to Figure 19
Placement of Bypass Capacitors).
2. CVCC and CBST: The VCC and bootstrap (BST) bypass capacitors supply switching currents to the high and
low side gate drivers. These two capacitors should also be placed as close to the IC as possible, and the
connecting trace length and loop area should be minimized (See Figure 19 Placement of Bypass
Capacitors).
3. The Feedback trace carries the output voltage information and a small ripple component that is necessary for
proper operation of LM5017. Therefore, care should be taken while routing the feedback trace to avoid
coupling any noise to this pin. In particular, feedback trace should not run close to magnetic components, or
parallel to any other switching trace.
4. SW trace: The SW node switches rapidly between VIN and GND every cycle and is therefore a possible
source of noise. The SW node area should be minimized. In particular, the SW node should not be
inadvertently connected to a copper plane or pour.
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RTN
1
VIN
2
UVLO
3
RON
4
8
SW
7
BST
6
VCC
5
FB
CIN
SO
PowerPAD8
CVCC
Figure 19. Placement of Bypass Capacitors
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PACKAGE MATERIALS INFORMATION
www.ti.com
31-May-2013
TAPE AND REEL INFORMATION
*All dimensions are nominal
Device
Package Package Pins
Type Drawing
SPQ
Reel
Reel
A0
Diameter Width (mm)
(mm) W1 (mm)
B0
(mm)
K0
(mm)
P1
(mm)
W
Pin1
(mm) Quadrant
LM5017MRE/NOPB
SO
Power
PAD
DDA
8
250
178.0
12.4
6.5
5.4
2.0
8.0
12.0
Q1
LM5017MRX/NOPB
SO
Power
PAD
DDA
8
2500
330.0
12.4
6.5
5.4
2.0
8.0
12.0
Q1
LM5017SD/NOPB
WSON
NGU
8
1000
178.0
12.4
4.3
4.3
1.3
8.0
12.0
Q1
LM5017SDX/NOPB
WSON
NGU
8
4500
330.0
12.4
4.3
4.3
1.3
8.0
12.0
Q1
Pack Materials-Page 1
PACKAGE MATERIALS INFORMATION
www.ti.com
31-May-2013
*All dimensions are nominal
Device
Package Type
Package Drawing
Pins
SPQ
Length (mm)
Width (mm)
Height (mm)
LM5017MRE/NOPB
SO PowerPAD
DDA
LM5017MRX/NOPB
SO PowerPAD
DDA
8
250
213.0
191.0
55.0
8
2500
367.0
367.0
35.0
LM5017SD/NOPB
WSON
LM5017SDX/NOPB
WSON
NGU
8
1000
203.0
190.0
41.0
NGU
8
4500
367.0
367.0
35.0
Pack Materials-Page 2
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