TI TPS54618

TPS54618
www.ti.com
SLVSAE9 – NOVEMBER 2010
2.95-V to 6-V Input, 6-A Output, 2-MHz, Synchronous Step-Down
Switcher With Integrated FETs ( SWIFT™)
Check for Samples: TPS54618
FEATURES
DESCRIPTION
•
The TPS54618 device is a full featured 6 V, 6 A,
synchronous step down current mode converter with
two integrated MOSFETs.
1
2
•
•
•
•
•
•
•
•
Two 12-mΩ (typical) MOSFETs for High
Efficiency at 6-A Loads
300kHz to 2MHz Switching Frequency
0.8 V ± 1% Voltage Reference Over
Temperature (-40°C to 150°C)
Synchronizes to External Clock
Adjustable Slow Start/Sequencing
UV and OV Power Good Output
–40°C to 150°C Operating Junction
Temperature Range
Thermally Enhanced 3mm × 3mm 16-pin QFN
Pin Compatible to TPS54418
APPLICATIONS
•
•
•
Low-Voltage, High-Density Power Systems
Point of Load Regulation for High Performance
DSPs, FPGAs, ASICs and Microprocessors
Broadband, Networking and Optical
Communications Infrastructure
SIMPLIFIED SCHEMATIC
The TPS54618 provides accurate regulation for a
variety of loads with an accurate ±1% Voltage
Reference (VREF) over temperature.
Efficiency is maximized through the integrated 12mΩ
MOSFETs and 515mA typical supply current. Using
the enable pin, shutdown supply current is reduced to
5.5µA by entering a shutdown mode.
Under voltage lockout is internally set at 2.6 V, but
can be increased by programming the threshold with
a resistor network on the enable pin. The output
voltage startup ramp is controlled by the slow start
pin. An open drain power good signal indicates the
output is within 93% to 107% of its nominal voltage.
Frequency fold back and thermal shutdown protects
the device during an over-current condition.
vertical spacer
vertical spacer
VIN
The TPS54618 is supported in the SwitcherPro™
Software Tool at www.ti.com/switcherpro.
CBOOT
VIN
BOOT
CI
The TPS54618 enables small designs by integrating
the MOSFETs, implementing current mode control to
reduce external component count, reducing inductor
size by enabling up to 2 MHz switching frequency,
and minimizing the IC footprint with a small 3mm x
3mm thermally enhanced QFN package.
R4
TPS54618
EN
LO
VOUT
For more SWIFTTM documentation, see the TI
website at www.ti.com/swift.
PH
CO
R5
100
3 Vin
R1
PWRGD
95
GND
AGND
POWERPAD
C ss
RT
R3
C1
R2
85
Efficiency - %
SS/TR
RT /CLK
COMP
5 Vin
90
VSENSE
80
75
70
65
60
fs = 500kHz
55
50
Vout = 1.8V
0
1
2
3
4
IO - Output Current - A
5
6
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.
SWIFT, SwitcherPro are trademarks of Texas Instruments.
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.
Copyright © 2010, Texas Instruments Incorporated
TPS54618
SLVSAE9 – NOVEMBER 2010
www.ti.com
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.
ORDERING INFORMATION (1)
(1)
TJ
PACKAGE
PART NUMBER
–40°C to 150°C
3 × 3 mm QFN
TPS54618RTE
For the most current package and ordering information, see the Package Option Addendum at the end
of this document, or see the TI website at www.ti.com
ABSOLUTE MAXIMUM RATINGS (1)
UNIT
VIN
–0.3 to 7
EN
–0.3 to 3.3
BOOT
Input voltage
PH + 7
VSENSE
–0.3 to 3
COMP
–0.3 to 3
PWRGD
–0.3 to 7
SS/TR
–0.3 to 3
RT/CLK
–0.3 to 3.3
BOOT-PH
Output voltage
Source current
Sink current
7
PH
–0.6 to 7
PH 10 ns Transient
–2 to 10
100
RT/CLK
100
COMP
100
µA
PWRGD
10
mA
SS/TR
100
µA
2
kV
Electrostatic discharge (Charged Devic Model) QSS 009-147 (JESD22-C101B.01)
(1)
(2)
2
V
EN
Electrostatic discharge (Human Body Model) QSS 009-105 (JESD22-A114A) (2)
Temperature
V
µA
500
V
Tj
–40 to 150
°C
Tstg
–65 to 150
°C
Stresses beyond those listed under absolute maximum ratings may cause permanent damage to the device. These are stress ratings
only, and functional operation of the device at these or any other conditions beyond those indicated under ELECTRICAL
SPECIFICATIONS is not implied. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability.
The human body model is a 100-pF capacitor discharged through a 1.5-kΩ resistor into each pin. The machine model is a 200-pF
capacitor discharged directly into each pin.
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SLVSAE9 – NOVEMBER 2010
THERMAL INFORMATION
TPS54618
THERMAL METRIC (1) (2) (3)
(RTE)
UNITS
(QFN-16) PINS
Junction-to-ambient thermal resistance
qJA
56.4
(4)
qJA
Junction-to-ambient thermal resistance
yJT
Junction-to-top characterization parameter
0.9
yJB
Junction-to-board characterization parameter
22.2
qJC(top)
Junction-to-case(top) thermal resistance
28.7
qJC(bottom)
Junction-to-case(bottom) thermal resistance
12.5
qJB
Junction-to-board thermal resistance
22.7
(1)
(2)
(3)
(4)
35.5
°C/W
For more information about traditional and new thermal metrics, see the IC Package Thermal Metrics application report, SPRA953.
Maximum power dissipation may be limited by overcurrent protection
Power rating at a specific ambient temperature TA should be determined with a junction temperature of 150°C. This is the point where
distortion starts to substantially increase. Thermal management of the PCB should strive to keep the junction temperature at or below
150°C for best performance and long-term reliability. See power dissipation estimate in application section of this data sheet for more
information.
Test boards conditions:
(a) 2 inches x 2 inches, 4 layers, thickness: 0.062 inch
(b) 2 oz. copper traces located on the top of the PCB
(c) 2 oz. copper ground planes on the 2 internal layers and bottom layer
(d) 4 thermal vias (10mil) located under the device package
ELECTRICAL CHARACTERISTICS
TJ = –40°C to 150°C, VIN = 2.95 to 6 V (unless otherwise noted)
DESCRIPTION
CONDITIONS
MIN
TYP
MAX
UNIT
SUPPLY VOLTAGE (VIN PIN)
Operating input voltage
Internal under voltage lockout threshold
2.95
6.0
VIN UVLO STOP
2.28
2.5
VIN UVLO START
2.45
2.6
V
V
Shutdown supply current
EN = 0 V, 25°C, 2.95 V ≤ VIN ≤ 6 V
5.5
15
mA
Quiescent Current - Iq
VSENSE = 0.9 V, VIN = 5 V, 25°C, RT = 400 kΩ
515
650
mA
Rising
1.25
Falling
1.18
Enable threshold + 50 mV
–3.5
Enable threshold – 50 mV
–1.9
ENABLE AND UVLO (EN PIN)
Enable threshold
Input current
V
mA
VOLTAGE REFERENCE (VSENSE PIN)
Voltage Reference
2.95 V ≤ VIN ≤ 6 V, –40°C <TJ < 150°C
0.791
0.799
0.807
BOOT-PH = 5 V
12
25
BOOT-PH = 2.95 V
16
33
VIN = 5 V
13
25
VIN = 2.95 V
17
33
V
MOSFET
High side switch resistance
Low side switch resistance
mΩ
mΩ
ERROR AMPLIFIER
Input current
2
nA
Error amplifier transconductance (gm)
–2 mA < I(COMP) < 2 mA, V(COMP) = 1 V
245
mmhos
Error amplifier transconductance (gm) during
slow start
–2 mA < I(COMP) < 2 mA, V(COMP) = 1 V,
Vsense = 0.4 V
79
mmhos
Error amplifier source/sink
V(COMP) = 1 V, 100 mV overdrive
±20
mA
25
A/V
COMP to Iswitch gm
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ELECTRICAL CHARACTERISTICS (continued)
TJ = –40°C to 150°C, VIN = 2.95 to 6 V (unless otherwise noted)
DESCRIPTION
CONDITIONS
MIN
TYP
MAX
VIN = 6 V, 25°C < TJ < 150°C
7.46
10.6
15.3
VIN = 2.95 V, 25°C < TJ < 150°C
7.68
10.2
13.5
UNIT
CURRENT LIMIT
Current limit threshold
A
THERMAL SHUTDOWN
Thermal Shutdown
Hysteresis
168
°C
20
°C
TIMING RESISTOR AND EXTERNAL CLOCK (RT/CLK PIN)
Switching frequency range using RT mode
Switching frequency
200
Rt = 400 kΩ
Switching frequency range using CLK mode
500
300
Minimum CLK pulse width
RT/CLK voltage
400
2000
kHz
600
kHz
2000
kHz
75
R(RT/CLK) = 400 kΩ
ns
0.5
RT/CLK high threshold
1.6
RT/CLK low threshold
0.4
V
2.2
V
0.6
V
RT/CLK falling edge to PH rising edge delay
Measure at 500 kHz with RT resistor in series
90
ns
PLL lock in time
Measure at 500 kHz
42
ms
Measured at 50% points on PH, IOUT = 3 A
75
Measured at 50% points on PH, VIN = 6 V,
IOUT = 0 A
120
PH (PH PIN)
Minimum On time
Minimum Off time
Rise Time
Fall Time
Prior to skipping off pulses, BOOT-PH = 2.95 V,
IOUT = 3 A
VIN = 6 V, 6 A
ns
60
ns
2.25
V/ns
2
BOOT (BOOT PIN)
BOOT Charge Resistance
VIN = 5 V
16
Ω
BOOT-PH UVLO
VIN = 2.95 V
2.1
V
SLOW START AND TRACKING (SS/TR PIN)
Charge Current
V(SS/TR) = 0.4 V
2
mA
SS/TR to VSENSE matching
V(SS/TR) = 0.4 V
54
mV
SS/TR to reference crossover
98% normal
1.1
V
SS/TR discharge voltage (Overload)
VSENSE = 0 V
61
mV
SS/TR discharge current (Overload)
VSENSE = 0 V, V(SS/TR) = 0.4 V
350
µA
SS discharge current (UVLO, EN, Thermal
fault)
VIN = 5 V, V(SS) = 0.5 V
1.9
mA
VSENSE falling (Fault)
91
POWER GOOD (PWRGD PIN)
VSENSE threshold
VSENSE rising (Good)
93
VSENSE rising (Fault)
109
VSENSE falling (Good)
107
% Vre
Hysteresis
VSENSE falling
2
% Vref
Output high leakage
VSENSE = VREF, V(PWRGD) = 5.5 V
7
nA
On resistance
Output low
I(PWRGD) = 3 mA
Minimum VIN for valid output
V(PWRGD) < 0.5 V at 100 mA
4
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56
100
Ω
0.2
0.3
V
0.65
1.5
V
Copyright © 2010, Texas Instruments Incorporated
Product Folder Link(s): TPS54618
TPS54618
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SLVSAE9 – NOVEMBER 2010
DEVICE INFORMATION
PIN CONFIGURATION
VIN
1
VIN
2
GND
GND
VIN
EN
PWRGD
BOOT
QFN16
RTE PACKAGE
(TOP VIEW)
16
15
14
13
12
PH
11
PH
3
10
PH
4
9
PowerPAD
(17)
8
RT/CLK
7
COMP
6
VSENSE
AGND
5
SS/TR
PIN FUNCTIONS
PIN
NAME
DESCRIPTION
NO.
AGND
5
Analog Ground should be electrically connected to GND close to the device.
BOOT
13
A bootstrap capacitor is required between BOOT and PH. If the voltage on this capacitor is below the minimum
required by the BOOT UVLO, the output is forced to switch off until the capacitor is refreshed.
COMP
7
Error amplifier output, and input to the output switch current comparator. Connect frequency compensation
components to this pin.
EN
15
Enable pin, internal pull-up current source. Pull below 1.2 V to disable. Float to enable. Can be used to set the
on/off threshold (adjust UVLO) with two additional resistors.
3, 4
Power Ground. This pin should be electrically connected directly to the power pad under the IC.
GND
PH
10, 11,
12
The source of the internal high side power MOSFET, and drain of the internal low side (synchronous) rectifier
MOSFET.
PowerPAD
17
GND pin should be connected to the exposed power pad for proper operation. This power pad should be
connected to any internal PCB ground plane using multiple vias for good thermal performance.
PWRGD
14
An open drain output; asserts low if output voltage is low due to thermal shutdown, overcurrent,
over/under-voltage or EN shut down.
RT/CLK
8
Resistor Timing or External Clock input pin.
SS/TR
9
Slow start and tracking. An external capacitor connected to this pin sets the output voltage rise time.
This pin can also be used for tracking.
VIN
VSENSE
1, 2, 16
6
Input supply voltage, 2.95 V to 6 V.
Inverting node of the transconductance (gm) error amplifier.
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FUNCTIONAL BLOCK DIAGRAM
PWRGD
EN
VIN
i1
Shutdown
93%
ihys
Thermal
Shutdown
Enable
Comparator
Logic
UVLO
Shutdown
Shutdown
Logic
109%
Enable
Threshold
Boot
Charge
Voltage
Reference
Boot
UVLO
Minimum
COMP Clamp
ERROR
AMPLIFIER
PWM
Comparator
VSENSE
SS/TR
Current
Sense
BOOT
Logic and PWM
Latch
Shutdown
Logic
S
COMP
Slope
Compensation
PH
Frequency
Shift
Overload
Recovery
Maximum
Clamp
Oscillator
with PLL
GND
TPS54618RTE Block Diagram
AGND
6
POWERPAD
RT/CLK
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SLVSAE9 – NOVEMBER 2010
TYPICAL CHARACTERISTICS CURVES
FREQUENCY vs TEMPERATURE
0.025
525
520
0.023
High Side Rdson Vin = 3.3 V
0.021
Low Side Rdson Vin = 3.3 V
0.019
0.017
0.015
0.013
High Side Rdson Vin = 5 V
0.011
Low Side Rdson Vin = 5 V
0.009
RT = 400 kW,
Vin = 5 V
515
fs - Switching Frequency - kHz
RDSON - Static Drain-Source On-State Resistance - W
HIGH SIDE AND LOW SIDE RDS(ON) vs TEMPERATURE
510
505
500
495
490
485
480
0.007
0.005
-50
-25
0
25
50
75
100
TJ - Junction Temperature - °C
125
475
-50
150
-25
0
25
50
75
100
TJ - Junction Temperature - °C
Figure 1.
HIGH SIDE CURRENT LIMIT vs TEMPERATURE
VOLTAGE REFERENCE vs TEMPERATURE
0.807
Vin = 3.3 V
Vin = 6 V
11.5
0.805
11
0.803
Vref - Voltage Reference - V
High Side Switching Current - A
150
Figure 2.
12
10.5
Vin = 2.95 V
10
9.5
9
8.5
0.801
0.799
0.797
0.795
0.793
8
25
50
75
100
TJ - Junction Temperature - °C
125
150
0.791
-50
-25
0
25
50
75
100
TJ - Junction Temperature - °C
Figure 3.
125
150
Figure 4.
SWITCHING FREQUENCY vs
RT RESISTANCE LOW FREQUENCY RANGE
SWITCHING FREQUENCY vs VSENSE
100
2000
1800
Vsense Falling
Nominal Switching Frequency - %
fs - Switching Frequency - kHz
125
1600
1400
1200
1000
800
75
Vsense Rising
50
25
400
200
80
180
280
380
480
580
RT - Resistance - kW
680
780
880
980
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Vsense - V
Figure 5.
Figure 6.
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TYPICAL CHARACTERISTICS CURVES (continued)
TRANSCONDUCTANCE (SLOW START) vs
JUNCTION TEMPERATURE
TRANSCONDUCTANCE vs TEMPERATURE
105
310
Vin = 3.3 V
100
Vin = 3.3 V
EA - Transconductance - mA/V
EA - Transconductance - mA/V
290
270
250
230
210
95
90
85
80
75
70
65
190
60
170
-50
-25
0
25
50
75
100
TJ - Junction Temperature - °C
125
55
-50
150
-25
0
25
50
75
100
TJ - Junction Temperature - °C
Figure 7.
125
150
Figure 8.
EN PIN VOLTAGE vs TEMPERATURE
EN PIN CURRENT vs TEMPERATURE
1.3
-3
1.29
-3.1
Vin = 5 V,
Ven = Threshold +50 mV
1.28
Vin = 3.3 V, rising
1.27
-3.2
EN - Pin Current - mA
EN - Threshold - V
1.26
1.25
1.24
1.23
1.22
1.21
1.2
Vin = 3.3 V, falling
1.19
-3.3
-3.4
-3.5
-3.6
-3.7
1.18
-3.8
1.17
-3.9
1.16
1.15
-50
-25
0
25
50
75
100
TJ - Junction Temperature - °C
125
-4
-50
150
-25
0
Figure 9.
125
150
Figure 10.
EN PIN CURRENT vs TEMPERATURE
CHARGE CURRENT vs TEMPERATURE
-1
-1.2
25
50
75
100
TJ - Junction Temperature - °C
-1.4
Vin = 5 V
Vin = 5 V,
Ven = Threshold -50 mV
-1.6
Iss/tr - Charge Current - mA
EN - Pin Current - mA
-1.4
-1.6
-1.8
-2
-2.2
-2.4
-1.8
-2
-2.2
-2.4
-2.6
-2.6
-2.8
-2.8
-3
-50
-25
0
25
50
75
100
TJ - Junction Temperature - °C
125
150
-3
-50
-30
Figure 11.
8
-10
10
30
50
70
90
TJ - Junction Temperature - °C
110
130
150
Figure 12.
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TYPICAL CHARACTERISTICS CURVES (continued)
INPUT VOLTAGE vs TEMPERATURE
SHUTDOWN SUPPLY CURRENT vs TEMPERATURE
2.8
8
2.7
7
Shutdown Supply Current - mA
Vin = 3.3 V
VI - Input Voltage - V
2.6
UVLO Stop Switching
2.5
2.4
2.3
UVLO Start Switching
2.2
6
5
4
3
2
1
2.1
2
-50
-25
0
25
50
75
100
TJ - Junction Temperature - °C
125
0
-50
150
-25
0
Figure 13.
125
150
Figure 14.
SHUTDOWN SUPPLY CURRENT vs INPUT VOLTAGE
VIN SUPPLY CURRENT vs JUNCTION TEMPERATURE
800
8
TJ = 25°C
Vin = 3.3 V
7
700
6
Ivin - Supply Current - mA
Shutdown Supply Current - mA
25
50
75
100
TJ - Junction Temperature - °C
5
4
3
2
600
500
400
300
1
0
3
3.5
4
4.5
5
VI - Input Voltage - V
5.5
200
-50
6
-25
0
Figure 15.
125
150
Figure 16.
VIN SUPPLY CURRENT vs INPUT VOLTAGE
PWRGD THRESHOLD vs TEMPERATURE
800
110
TJ = 25°C
108
700
106
PWRGD Threshold - % Vref
Ivin - Supply Current - mA
25
50
75
100
TJ - Junction Temperature - °C
600
500
400
Vsense Falling
104
Vsense Rising, Vin = 5 V
102
100
98
96
Vsense Rising
Vsense Falling
94
92
300
90
200
3
3.5
4
4.5
5
VI - Input Voltage - V
5.5
6
88
-50
-25
Figure 17.
0
25
50
75
100
TJ - Junction Temperature - °C
125
150
Figure 18.
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TYPICAL CHARACTERISTICS CURVES (continued)
SS/TR to VSENSE OFFSET vs TEMPERATURE
100
100
Vin = 3.3 V
90
Vin = 5 V,
SS = 0.4 V
90
80
80
SSTR - Vsense Offset - mV
RDSON - Static Drain-Source On-State Resistance - W
PWRGD ON-RESISTANCE vs TEMPERATURE
70
60
50
40
30
70
60
50
40
30
20
20
10
10
0
-50
-25
0
25
50
75
100
TJ - Junction Temperature - °C
125
0
-50
150
-25
0
25
50
75
100
TJ - Junction Temperature - °C
Figure 19.
EFFICIENCY vs LOAD CURRENT
EFFICIENCY vs LOAD CURRENT
100
95
95
90
90
Vout = 3.3 V
Vout = 1.8 V
Vout = 1.8 V
85
Vout = 1.05 V
Efficiency - %
Efficiency - %
85
80
75
70
65
Vout = 1.05 V
80
75
70
65
Vin = 5 V,
fs = 1 MHz,
TJ = 25°C
60
55
0
1
Vin = 3.5 V,
fs = 1 MHz,
TJ = 25°C
60
55
2
3
4
5
50
6
1
0
4
Figure 21.
Figure 22.
5
6
EFFICIENCY vs LOAD CURRENT
100
95
95
90
90
Vout = 3.3 V
Vout = 1.8 V
Vout = 1.8 V
85
Efficiency - %
85
Efficiency - %
3
Output Current - A
EFFICIENCY vs LOAD CURRENT
Vout = 1.05 V
80
75
70
65
Vout = 1.05 V
80
75
70
65
Vin = 5 V,
fs = 500 kHz,
TJ = 25°C
60
55
10
2
Output Current - A
100
50
150
Figure 20.
100
50
125
0
1
Vin = 3.5 V,
fs = 500 kHz,
TJ = 25°C
60
55
2
3
4
5
6
50
0
1
2
3
Output Current - A
Output Current - A
Figure 23.
Figure 24.
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5
6
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OVERVIEW
The TPS54618 is a 6-V, 6-A, synchronous step-down (buck) converter with two integrated n-channel MOSFETs.
To improve performance during line and load transients the device implements a constant frequency, peak
current mode control which reduces output capacitance and simplifies external frequency compensation design.
The wide switching frequency of 200 kHz to 2000 kHz allows for efficiency and size optimization when selecting
the output filter components. The switching frequency is adjusted using a resistor to ground on the RT/CLK pin.
The device has an internal phase lock loop (PLL) on the RT/CLK pin that is used to synchronize the power
switch turn on to a falling edge of an external system clock.
The TPS54618 has a typical default start up voltage of 2.45 V. The EN pin has an internal pull-up current source
that can be used to adjust the input voltage under voltage lockout (UVLO) with two external resistors. In addition,
the pull up current provides a default condition when the EN pin is floating for the device to operate. The total
operating current for the TPS54618 is typically 515 mA when not switching and under no load. When the device
is disabled, the supply current is less than 5.5 mA.
The integrated 12 mΩ MOSFETs allow for high efficiency power supply designs with continuous output currents
up to 6 amperes.
The TPS54618 reduces the external component count by integrating the boot recharge diode. The bias voltage
for the integrated high side MOSFET is supplied by a capacitor between the BOOT and PH pins. The boot
capacitor voltage is monitored by an UVLO circuit and turns off the high side MOSFET when the voltage falls
below a preset threshold. This BOOT circuit allows the TPS54618 to operate approaching 100%. The output
voltage can be stepped down to as low as the 0.799 V reference.
The TPS54618 has a power good comparator (PWRGD) with 2% hysteresis.
The TPS54618 minimizes excessive output overvoltage transients by taking advantage of the overvoltage power
good comparator. When the regulated output voltage is greater than 109% of the nominal voltage, the
overvoltage comparator is activated, and the high side MOSFET is turned off and masked from turning on until
the output voltage is lower than 107%.
The SS/TR (slow start/tracking) pin is used to minimize inrush currents or provide power supply sequencing
during power up. A small value capacitor should be coupled to the pin for slow start. The SS/TR pin is
discharged before the output power up to ensure a repeatable restart after an overtemperature fault, UVLO fault
or disabled condition.
The use of a frequency fold-back circuit reduces the switching frequency during startup and over current fault
conditions to help limit the inductor current.
DETAILED DESCRIPTION
FIXED FREQUENCY PWM CONTROL
The TPS54618 uses an adjustable fixed frequency, peak current mode control. The output voltage is compared
through external resistors on the VSENSE pin to an internal voltage reference by an error amplifier which drives
the COMP pin. An internal oscillator initiates the turn on of the high side power switch. The error amplifier output
is compared to the high side power switch current. When the power switch current reaches the COMP voltage
level the high side power switch is turned off and the low side power switch is turned on. The COMP pin voltage
increases and decreases as the output current increases and decreases. The device implements a current limit
by clamping the COMP pin voltage to a maximum level and also implements a minimum clamp for improved
transient response performance.
SLOPE COMPENSATION AND OUTPUT CURRENT
The TPS54618 adds a compensating ramp to the switch current signal. This slope compensation prevents
sub-harmonic oscillations as duty cycle increases. The available peak inductor current remains constant over the
full duty cycle range.
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BOOTSTRAP VOLTAGE (BOOT) AND LOW DROPOUT OPERATION
The TPS54618 has an integrated boot regulator and requires a small ceramic capacitor between the BOOT and
PH pin to provide the gate drive voltage for the high side MOSFET. The value of the ceramic capacitor should be
0.1 mF. A ceramic capacitor with an X7R or X5R grade dielectric with a voltage rating of 10 V or higher is
recommended because of the stable characteristics over temperature and voltage.
To improve drop out, the TPS54618 is designed to operate at 100% duty cycle as long as the BOOT to PH pin
voltage is greater than 2.2 V. The high side MOSFET is turned off using an UVLO circuit, allowing for the low
side MOSFET to conduct when the voltage from BOOT to PH drops below 2.2 V. Since the supply current
sourced from the BOOT pin is very low, the high side MOSFET can remain on for more switching cycles than are
required to refresh the capacitor, thus the effective duty cycle of the switching regulator is very high.
ERROR AMPLIFIER
The TPS54618 has a transconductance amplifier. The error amplifier compares the VSENSE voltage to the lower
of the SS/TR pin voltage or the internal 0.799 V voltage reference. The transconductance of the error amplifier is
245mA/V during normal operation. When the voltage of VSENSE pin is below 0.799 V and the device is
regulating using the SS/TR voltage, the gm is typically greater than 79 mA/V, but less than 245 mA/V. The
frequency compensation components are placed between the COMP pin and ground.
VOLTAGE REFERENCE
The voltage reference system produces a precise ±1% voltage reference over temperature by scaling the output
of a temperature-stable bandgap circuit. The bandgap and scaling circuits produce 0.799 V at the non-inverting
input of the error amplifier.
ADJUSTING THE OUTPUT VOLTAGE
The output voltage is set with a resistor divider from the output node to the VSENSE pin. It is recommended to
use divider resistors with 1% tolerance or better. Start with a 100 kΩ for the R1 resistor and use the Equation 1
to calculate R2. To improve efficiency at very light loads consider using larger value resistors. If the values are
too high the regulator is more susceptible to noise and voltage errors from the VSENSE input current are
noticeable.
vertical spacer
vertical spacer
æ
ö
0.799 V
R2 = R1 ´ ç
÷
è VO - 0.799 V ø
(1)
TPS54618
VO
R1
VSENSE
–
R2
0.799 V
+
Figure 25. Voltage Divider Circuit
ENABLE AND ADJUSTING UNDER-VOLTAGE LOCKOUT
The TPS54618 is disabled when the VIN pin voltage falls below 2.28 V. If an application requires a higher
under-voltage lockout (UVLO), use the EN pin as shown in Figure 26 to adjust the input voltage UVLO by using
two external resistors. It is recommended to use the EN resistors to set the UVLO falling threshold (VSTOP) above
12
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2.6V. The rising threshold (VSTART) should be set to provide enough hysteresis to allow for any input supply
variations. The EN pin has an internal pull-up current source that provides the default condition of the TPS54618
operating when the EN pin floats. Once the EN pin voltage exceeds 1.25 V, an additional 1.6 mA of hysteresis is
added. When the EN pin is pulled below 1.18 V, the 1.6 mA is removed. This additional current facilitates input
voltage hysteresis.
TPS54618
i hys
VIN
1.6 mA
i1
R1
1.9 mA
R2
EN
+
–
Figure 26. Adjustable Under Voltage Lock Out
æV
ö
VSTART ç ENFALLING ÷ - VSTOP
è VENRISING ø
R1 =
æ V
ö
Ip ç1 - ENFALLING ÷ + Ih
VENRISING ø
è
(2)
vertical spacer
R2 =
VSTOP
R1´ VENFALLING
- VENFALLING + R1(Ip + Ih )
(3)
Where R1 and R2 are in ohms, Ih = 1.6uA, Ip = 1.9uA, VENRISING = 1.25V, VENFALLING = 1.18V
SLOW START / TRACKING PIN
The TPS54618 regulates to the lower of the SS/TR pin and the internal reference voltage. A capacitor on the
SS/TR pin to ground implements a slow start time. The TPS54618 has an internal pull-up current source of 2 mA
which charges the external slow start capacitor. Equation 4 calculates the required slow start capacitor value
where Tss is the desired slow start time in ms, Iss is the internal slow start charging current of 2 mA, and Vref is
the internal voltage reference of 0.799 V.
vertical spacer
Tss(mS) ´ Iss(mA)
Css(nF) =
Vref(V)
(4)
If during normal operation, the VIN goes below the UVLO, EN pin pulled below 1.2 V, or a thermal shutdown
event occurs, the TPS54618 stops switching. When the VIN goes above UVLO, EN is released or pulled high, or
a thermal shutdown is exited, then SS/TR is discharged to below 40 mV before reinitiating a powering up
sequence. The VSENSE voltage will follow the SS/TR pin voltage with a 54mV offset up to 85% of the internal
voltage reference. When the SS/TR voltage is greater than 85% on the internal reference voltage the offset
increases as the effective system reference transitions from the SS/TR voltage to the internal voltage reference.
SEQUENCING
Many of the common power supply sequencing methods can be implemented using the SS/TR, EN and PWRGD
pins. The sequential method can be implemented using an open drain or collector output of a power on reset pin
of another device. Figure 27 shows the sequential method. The power good is coupled to the EN pin on the
TPS54618 which enables the second power supply once the primary supply reaches regulation.
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Ratio-metric start up can be accomplished by connecting the SS/TR pins together. The regulator outputs ramp
up and reach regulation at the same time. When calculating the slow start time the pull up current source must
be doubled in Equation 4. The ratio metric method is illustrated in Figure 29.
TPS54618
PWRGD1
EN1
EN2
EN1
SS1
SS2
EN2
PWRGD2
VO1
VO2
Figure 27. Sequential Start-Up Sequence
Figure 28. Sequential Startup using EN and
PWRGD
TPS54618
EN1
SS/TR1
EN
PWRGD1
SS
TPS54618
EN2
VO1
VO2
SS/TR2
PWRGD2
Figure 29. Schematic for Ratio-metric Start-Up
Sequence
vertical spacer
Figure 30. Ratio-metric Startup
Ratio-metric and simultaneous power supply sequencing can be implemented by connecting the resistor network
of R1 and R2 shown in Figure 31 to the output of the power supply that needs to be tracked or another voltage
reference source. Using Equation 5 and Equation 6, the tracking resistors can be calculated to initiate the Vout2
slightly before, after or at the same time as Vout1. Equation 7 is the voltage difference between Vout1 and
Vout2. The ΔV variable is zero volts for simultaneous sequencing. To minimize the effect of the inherent SS/TR
to VSENSE offset (Vssoffset) in the slow start circuit and the offset created by the pullup current source (Iss) and
tracking resistors, the Vssoffset and Iss are included as variables in the equations. To design a ratio-metric start
up in which the Vout2 voltage is slightly greater than the Vout1 voltage when Vout2 reaches regulation, use a
negative number in Equation 5 through Equation 7 for ΔV. Equation 7 will result in a positive number for
applications which the Vout2 is slightly lower than Vout1 when Vout2 regulation is achieved. Since the SS/TR pin
must be pulled below 40mV before starting after an EN, UVLO or thermal shutdown fault, careful selection of the
14
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tracking resistors is needed to ensure the device will restart after a fault. Make sure the calculated R1 value from
Equation 5 is greater than the value calculated in Equation 8 to ensure the device can recover from a fault. As
the SS/TR voltage becomes more than 85% of the nominal reference voltage the Vssoffset becomes larger as
the slow start circuits gradually handoff the regulation reference to the internal voltage reference. The SS/TR pin
voltage needs to be greater than 1.1 V for a complete handoff to the internal voltage reference as shown in
Figure 30.
vertical spacer
R1 =
Vout2 + D V
Vssoffset
´
Vref
Iss
(5)
vertical spacer
R2 =
Vref ´ R1
Vout2 + DV - Vref
(6)
vertical spacer
DV = Vout1 - Vout2
(7)
vertical spacer
R1 > 2930 ´ Vout1- 145 ´ DV
(8)
vertical spacer
TPS54618
EN1
VOUT1
EN1
SS/TR1
PWRGD1
SS2
Vout1
TPS54618
EN2
VOUT 2
Vout2
R1
SS/TR2
R2
PWRGD2
Figure 31. Ratio-metric and Simultaneous Startup
Sequence
Figure 32. Ratio-metric Start-Up using Coupled
SS/TR Pins
CONSTANT SWITCHING FREQUENCY and TIMING RESISTOR (RT/CLK Pin)
The switching frequency of the TPS54618 is adjustable over a wide range from 300 kHz to 2000 kHz by placing
a maximum of 700 kΩ and minimum of 85 kΩ, respectively, on the RT/CLK pin. An internal amplifier holds this
pin at a fixed voltage when using an external resistor to ground to set the switching frequency. The RT/CLK is
typically 0.5 V. To determine the timing resistance for a given switching frequency, use the curve in Figure 5 or
Equation 9.
247530
RT (kW) =
Fsw(kHz)1.0533
(9)
vertical spacer
Fsw(kHz) =
131904
RT(kW)0.9492
(10)
To reduce the solution size one would typically set the switching frequency as high as possible, but tradeoffs of
the efficiency, maximum input voltage and minimum controllable on time should be considered.
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The minimum controllable on time is typically 75 ns at full current load and 120 ns at no load, and limits the
maximum operating input voltage or output voltage.
OVERCURRENT PROTECTION
The TPS54618 implements a cycle by cycle current limit. During each switching cycle the high side switch
current is compared to the voltage on the COMP pin. When the instantaneous switch current intersects the
COMP voltage, the high side switch is turned off. During overcurrent conditions that pull the output voltage low,
the error amplifier responds by driving the COMP pin high, increasing the switch current. The error amplifier
output is clamped internally. This clamp functions as a switch current limit.
FREQUENCY SHIFT
To operate at high switching frequencies and provide protection during overcurrent conditions, the TPS54618
implements a frequency shift. If frequency shift was not implemented, during an overcurrent condition the low
side MOSFET may not be turned off long enough to reduce the current in the inductor, causing a current
runaway. With frequency shift, during an overcurrent condition the switching frequency is reduced from 100%,
then 50%, then 25%, as the voltage decreases from 0.799 to 0 volts on VSENSE pin to allow the low side
MOSFET to be off long enough to decrease the current in the inductor. During start-up, the switching frequency
increases as the voltage on VSENSE increases from 0 to 0.799 volts. See Figure 6 for details.
REVERSE OVERCURRENT PROTECTION
The TPS54618 implements low side current protection by detecting the voltage across the low side MOSFET.
When the converter sinks current through its low side FET, the control circuit turns off the low side MOSFET if
the reverse current is typically more than 4.5 A. By implementing this additional protection scheme, the converter
is able to protect itself from excessive current during power cycling and start-up into pre-biased outputs.
SYNCHRONIZE USING THE RT/CLK PIN
The RT/CLK pin is used to synchronize the converter to an external system clock. See Figure 33. To implement
the synchronization feature in a system, connect a square wave to the RT/CLK pin with an on time of at least
75ns. If the pin is pulled above the PLL upper threshold, a mode change occurs and the pin becomes a
synchronization input. The internal amplifier is disabled and the pin is a high impedance clock input to the
internal PLL. If clocking edges stop, the internal amplifier is re-enabled and the mode returns to the frequency set
by the resistor. The square wave amplitude at this pin must transition lower than 0.6 V and higher than 1.6 V
typically. The synchronization frequency range is 300 kHz to 2000 kHz. The rising edge of the PH is
synchronized to the falling edge of RT/CLK pin.
TPS54618
SYNC Clock
PLL
RT/CLK
Clock
Source
RT
PH
Figure 33. Synchronizing to a System Clock
16
Figure 34. Plot of Synchronizing to System Clock
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POWER GOOD (PWRGD PIN)
The PWRGD pin output is an open drain MOSFET. The output is pulled low when the VSENSE voltage enters
the fault condition by falling below 91% or rising above 109% of the nominal internal reference voltage. There is
a 2% hysteresis on the threshold voltage, so when the VSENSE voltage rises to the good condition above 93%
or falls below 107% of the internal voltage reference the PWRGD output MOSFET is turned off. It is
recommended to use a pull-up resistor between the values of 1kΩ and 100kΩ to a voltage source that is 6 V or
less. The PWRGD is in a valid state once the VIN input voltage is greater than 1.5 V.
OVERVOLTAGE TRANSIENT PROTECTION
The TPS54618 incorporates an overvoltage transient protection (OVTP) circuit to minimize voltage overshoot
when recovering from output fault conditions or strong unload transients. The OVTP feature minimizes the output
overshoot by implementing a circuit to compare the VSENSE pin voltage to the OVTP threshold which is 109%
of the internal voltage reference. If the VSENSE pin voltage is greater than the OVTP threshold, the high side
MOSFET is disabled preventing current from flowing to the output and minimizing output overshoot. When the
VSENSE voltage drops lower than the OVTP threshold the high side MOSFET is allowed to turn on the next
clock cycle.
THERMAL SHUTDOWN
The device implements an internal thermal shutdown to protect itself if the junction temperature exceeds 168°C.
The thermal shutdown forces the device to stop switching when the junction temperature exceeds the thermal
trip threshold. Once the die temperature decreases below 150°C, the device reinitiates the power up sequence
by discharging the SS pin to below 40 mV. The thermal shutdown hysteresis is 20°C.
SMALL SIGNAL MODEL FOR LOOP RESPONSE
Figure 35 shows an equivalent model for the TPS54618 control loop which can be modeled in a circuit simulation
program to check frequency response and dynamic load response. The error amplifier is a transconductance
amplifier with a gm of 245 mA/V. The error amplifier can be modeled using an ideal voltage controlled current
source. The resistor R0 and capacitor Co model the open loop gain and frequency response of the amplifier. The
1-mV AC voltage source between the nodes a and b effectively breaks the control loop for the frequency
response measurements. Plotting a/c shows the small signal response of the frequency compensation. Plotting
a/b shows the small signal response of the overall loop. The dynamic loop response can be checked by
replacing the RL with a current source with the appropriate load step amplitude and step rate in a time domain
analysis.
PH
VO
Power Stage
25 A/V
a
b
R1
c
RL
COMP
R3
C2
RESR
C1
C0
R0
0.799 V
VSENSE
gm
245 µA/V
COUT
R2
Figure 35. Small Signal Model for Loop Response
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SIMPLE SMALL SIGNAL MODEL FOR PEAK CURRENT MODE CONTROL
Figure 35 is a simple small signal model that can be used to understand how to design the frequency
compensation. The TPS54618 power stage can be approximated to a voltage controlled current source (duty
cycle modulator) supplying current to the output capacitor and load resistor. The control to output transfer
function is shown in Equation 11 and consists of a dc gain, one dominant pole and one ESR zero. The quotient
of the change in switch current and the change in COMP pin voltage (node c in Figure 35) is the power stage
transconductance. The gm for the TPS54618 is 25 A/V. The low frequency gain of the power stage frequency
response is the product of the transconductance and the load resistance as shown in Equation 12. As the load
current increases and decreases, the low frequency gain decreases and increases, respectively. This variation
with load may seem problematic at first glance, but the dominant pole moves with load current [see Equation 13].
The combined effect is highlighted by the dashed line in the right half of Figure 36. As the load current
decreases, the gain increases and the pole frequency lowers, keeping the 0-dB crossover frequency the same
for the varying load conditions which makes it easier to design the frequency compensation.
vertical spacer
vertical spacer
VO
Adc
VC
RESR
fp
RL
gmps
COUT
fz
Figure 36. Simple Small Signal Model and Frequency Response for Peak Current Mode Control
æ
ç 1+
vo
è 2p
= Adc ´
vc
æ
ç 1+
è 2p
ö
s
÷
× ¦z ø
ö
s
÷
× ¦p ø
(11)
Adc = gmps ´ RL
¦p =
C OUT
(12)
1
´ RL ´ 2p
(13)
1
´ RESR ´ 2p
(14)
vertical spacer
¦z =
COUT
SMALL SIGNAL MODEL FOR FREQUENCY COMPENSATION
The TPS54618 uses a transconductance amplifier for the error amplifier and readily supports two of the
commonly used frequency compensation circuits. The compensation circuits are shown in Figure 37. The Type 2
circuits are most likely implemented in high bandwidth power supply designs using low ESR output capacitors. In
Type 2A, one additional high frequency pole is added to attenuate high frequency noise.
18
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VO
R1
VSENSE
COMP
gmea
R2
Vref
RO
CO
5pF
Type 2A
R3
C2
Type 2B
R3
C1
C1
Figure 37. Types of Frequency Compensation
The design guidelines for TPS54618 loop compensation are as follows:
1. The modulator pole, fpmod, and the esr zero, fz1 must be calculated using Equation 15 and Equation 16.
Derating the output capacitor (COUT) may be needed if the output voltage is a high percentage of the
capacitor rating. Use the capacitor manufacturer information to derate the capacitor value. Use Equation 17
and Equation 18 to estimate a starting point for the crossover frequency, fc. Equation 17 is the geometric
mean of the modulator pole and the esr zero and Equation 18 is the mean of modulator pole and the
switching frequency. Use the lower value of Equation 17 or Equation 18 as the maximum crossover
frequency.
¦ p m od =
Iout m ax
2 p ´ Vout ´ Cout
(15)
vertical spacer
¦ z m od =
1
2 p ´ Resr ´ Cout
(16)
vertical spacer
¦C =
¦p mod ´ ¦ z mod
(17)
vertical spacer
¦C =
¦p mod ´
¦ sw
2
(18)
vertical spacer
2. R3 can be determined by
2p × ¦ c ´ Vo ´ COUT
R3 =
gmea ´ Vref ´ gmps
(19)
vertical spacer
Where is the gmea amplifier gain (245 mA/V), gmps is the power stage gain (25 A/V).
¦p =
3. Place a compensation zero at the dominant pole
R ´ COUT
C1 = L
R3
1
C OUT ´ R L ´ 2 p . C1 can be determined by
(20)
vertical spacer
4. C2 is optional. It can be used to cancel the zero from Co’s ESR.
Resr ´ COUT
C2 =
R3
(21)
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APPLICATION INFORMATION
DESIGN GUIDE – STEP-BY-STEP DESIGN PROCEDURE
This example details the design of a high frequency switching regulator design using ceramic output capacitors.
This design is available as the HPA606 evaluation module (EVM). A few parameters must be known in order to
start the design process. These parameters are typically determined on the system level. For this example, we
start with the following known parameters:
Output Voltage
1.8 V
Transient Response 1.5 A to 4.5 A load step
ΔVout = 4%
Maximum Output Current
6A
Input Voltage
3 V to 6V, 5 V nominal
Output Voltage Ripple
< 30 mV p-p
Switching Frequency (Fsw)
1000 kHz
SELECTING THE SWITCHING FREQUENCY
The first step is to decide on a switching frequency for the regulator. Typically, you want to choose the highest
switching frequency possible since this produces the smallest solution size. The high switching frequency allows
for lower valued inductors and smaller output capacitors compared to a power supply that switches at a lower
frequency. However, the highest switching frequency causes extra switching losses, which hurt the converter’s
performance. The converter is capable of running from 300 kHz to 2 MHz. Unless a small solution size is an
ultimate goal, a moderate switching frequency of 1MHz is selected to achieve both a small solution size and a
high efficiency operation. Using Equation 9, R4 is calculated to be 180 kΩ. A standard 1% 182 kΩ value was
chosen in the design.
Figure 38. High Frequency, 1.8 V Output Power Supply Design with Adjusted UVLO
OUTPUT INDUCTOR SELECTION
The inductor selected works for the entire TPS54618 input voltage range. To calculate the value of the output
inductor, use Equation 22. KIND is a coefficient that represents the amount of inductor ripple current relative to the
maximum output current. The inductor ripple current is filtered by the output capacitor. Therefore, choosing high
inductor ripple currents impacts the selection of the output capacitor since the output capacitor must have a
ripple current rating equal to or greater than the inductor ripple current. In general, the inductor ripple value is at
the discretion of the designer; however, KIND is normally from 0.1 to 0.3 for the majority of applications.
For this design example, use KIND = 0.3 and the inductor value is calculated to be 0.7 mH. For this design, a
nearest standard value was chosen: 0.75 mH. For the output filter inductor, it is important that the RMS current
and saturation current ratings not be exceeded. The RMS and peak inductor current can be found from
Equation 24 and Equation 25.
For this design, the RMS inductor current is 6.01 A and the peak inductor current is 6.84 A. The chosen inductor
is a Toko FDV0630-R75M. It has a saturation current rating 0f 10 A and a RMS current rating of 8.9 A.
20
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The current flowing through the inductor is the inductor ripple current plus the output current. During power up,
faults or transient load conditions, the inductor current can increase above the calculated peak inductor current
level calculated above. In transient conditions, the inductor current can increase up to the switch current limit of
the device. For this reason, the most conservative approach is to specify an inductor with a saturation current
rating equal to or greater than the switch current limit rather than the peak inductor current.
Vinmax - Vout
Vout
´
L1 =
Io ´ Kind
Vinmax ´ ¦ sw
(22)
vertical spacer
Iripple =
Vinmax - Vout
Vout
´
L1
Vinmax ´ ¦ sw
(23)
vertical spacer
ILrms =
Io 2 +
æ Vo ´ (Vinmax - Vo) ö
1
´ ç
÷
12
è Vinmax ´ L1 ´ ¦ sw ø
2
(24)
vertical spacer
ILpeak = Iout +
Iripple
2
(25)
OUTPUT CAPACITOR
There are three primary considerations for selecting the value of the output capacitor. The output capacitor
determines the modulator pole, the output voltage ripple, and how the regulator responds to a large change in
load current. The output capacitance needs to be selected based on the more stringent of these three criteria.
The desired response to a large change in the load current is the first criteria. The output capacitor needs to
supply the load with current when the regulator can not. This situation would occur if there are desired hold-up
times for the regulator where the output capacitor must hold the output voltage above a certain level for a
specified amount of time after the input power is removed. The regulator is temporarily not able to supply
sufficient output current if there is a large, fast increase in the current needs of the load such as transitioning
from no load to a full load. The regulator usually needs two or more clock cycles for the control loop to see the
change in load current and output voltage and adjust the duty cycle to react to the change. The output capacitor
must be sized to supply the extra current to the load until the control loop responds to the load change. The
output capacitance must be large enough to supply the difference in current for 2 clock cycles while only allowing
a tolerable amount of droop in the output voltage. Equation 26 shows the minimum output capacitance necessary
to accomplish this.
For this example, the transient load response is specified as a 3% change in Vout for a load step from 1.5 A
(25% load) to 4.5 A (75% load). For this example, ΔIout = 4.5-1.5 = 3.0A and ΔVout= 0.04 × 1.8 = 0.072 V.
Using these numbers gives a minimum capacitance of 83 mF. This value does not take the ESR of the output
capacitor into account in the output voltage change. For ceramic capacitors, the ESR is usually small enough to
ignore in this calculation.
Equation 27 calculates the minimum output capacitance needed to meet the output voltage ripple specification.
Where fsw is the switching frequency, Vripple is the maximum allowable output voltage ripple, and Iripple is the
inductor ripple current. In this case, the maximum output voltage ripple is 30 mV. Under this requirement,
Equation 27 yields 7 uF.
vertical spacer
2 ´ DIout
Co >
¦ sw ´ DVout
(26)
vertical spacer
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Co >
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1
´
8 ´ ¦ sw
1
Voripple
Iripple
Where ΔIout is the change in output current, fsw is the regulators switching frequency and ΔVout is the
allowable change in the output voltage.
(27)
vertical spacer
Equation 28 calculates the maximum ESR an output capacitor can have to meet the output voltage ripple
specification. Equation 28 indicates the ESR should be less than 18 mΩ. In this case, the ESR of the ceramic
capacitor is much less than 18 mΩ.
Additional capacitance de-ratings for aging, temperature and DC bias should be factored in which increases this
minimum value. For this example, five 22 mF 10 V X5R ceramic capacitors with 3 mΩ of ESR are used. The
estimated capacitance after derating by a factor 0.75 is 82.5 µF.
Capacitors generally have limits to the amount of ripple current they can handle without failing or producing
excess heat. An output capacitor that can support the inductor ripple current must be specified. Some capacitor
data sheets specify the RMS (Root Mean Square) value of the maximum ripple current. Equation 29 can be used
to calculate the RMS ripple current the output capacitor needs to support. For this application, Equation 29 yields
520 mA.
Voripple
Resr <
Iripple
(28)
vertical spacer
Icorm s =
Vout ´ (Vinm ax - Vout)
12 ´ Vinm ax ´ L1 ´ ¦ sw
(29)
INPUT CAPACITOR
The TPS54618 requires a high quality ceramic, type X5R or X7R, input decoupling capacitor of at least 10 mF of
effective capacitance and in some applications a bulk capacitance. The effective capacitance includes any DC
bias effects. The voltage rating of the input capacitor must be greater than the maximum input voltage. The
capacitor must also have a ripple current rating greater than the maximum input current ripple of the TPS54618.
The input ripple current can be calculated using Equation 30.
The value of a ceramic capacitor varies significantly over temperature and the amount of DC bias applied to the
capacitor. The capacitance variations due to temperature can be minimized by selecting a dielectric material that
is stable over temperature. X5R and X7R ceramic dielectrics are usually selected for power regulator capacitors
because they have a high capacitance to volume ratio and are fairly stable over temperature. The output
capacitor must also be selected with the DC bias taken into account. The capacitance value of a capacitor
decreases as the DC bias across a capacitor increases.
For this example design, a ceramic capacitor with at least a 10 V voltage rating is required to support the
maximum input voltage. For this example, two 10 mF and one 0.1 mF 10 V capacitors in parallel have been
selected. The input capacitance value determines the input ripple voltage of the regulator. The input voltage
ripple can be calculated using Equation 31. Using the design example values, Ioutmax=6 A, Cin=20 mF, Fsw=1
MHz, yields an input voltage ripple of 149 mV and a rms input ripple current of 2.94 A.
Icirms = Iout ´
Vout
´
Vinmin
(Vinmin
- Vout )
Vinmin
vertical spacer
Ioutmax ´ 0.25
DVin =
Cin ´ ¦ sw
(30)
(31)
SLOW START CAPACITOR
The slow start capacitor determines the minimum amount of time it takes for the output voltage to reach its
22
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nominal programmed value during power up. This is useful if a load requires a controlled voltage slew rate. This
is also used if the output capacitance is very large and would require large amounts of current to quickly charge
the capacitor to the output voltage level. The large currents necessary to charge the capacitor may make the
TPS54618 reach the current limit or excessive current draw from the input power supply may cause the input
voltage rail to sag. Limiting the output voltage slew rate solves both of these problems.
The slow start capacitor value can be calculated using Equation 32. For the example circuit, the slow start time is
not too critical since the output capacitor value is 110 mF which does not require much current to charge to 1.8 V.
The example circuit has the slow start time set to an arbitrary value of 4ms which requires a 10 nF capacitor. In
TPS54618, Iss is 2.2 mA and Vref is 0.799 V.
Tss(ms) ´ Iss(mA)
Css(nF) =
Vref(V)
(32)
BOOTSTRAP CAPACITOR SELECTION
A 0.1 mF ceramic capacitor must be connected between the BOOT to PH pin for proper operation. It is
recommended to use a ceramic capacitor with X5R or better grade dielectric. The capacitor should have 10 V or
higher voltage rating.
OUTPUT VOLTAGE AND FEEDBACK RESISTORS SELECTION
For the example design, 100 kΩ was selected for R6. Using Equation 33, R7 is calculated as 80 kΩ. The nearest
standard 1% resistor is 80.6 kΩ.
Vref
R7 =
R6
Vo - Vref
(33)
Due to the internal design of the TPS54618, there is a minimum output voltage limit for any given input voltage.
The output voltage can never be lower than the internal voltage reference of 0.799 V. Above 0.799 V, the output
voltage may be limited by the minimum controllable on time. The minimum output voltage in this case is given by
Equation 34
Voutmin = Ontimemin ´ Fsmax ´ (Vinmax - loutmin ´ RDSmin ) - Ioutmin ´ (RL + RDSmin )
Where:
Voutmin = minimum achievable output voltage
Ontimemin = minimum controllable on-time (75 ns typical. 120 ns no load)
Fsmax = maximum switching frequency including tolerance
Vinmax = maximum input voltage
Ioutmin = minimum load current
RDSmin = minimum high-side MOSFET on resistance (See Electrical Characteristics)
RL = series resistance of output inductor
(34)
There is also a maximum achievable output voltage which is limited by the minimum off time. The maximum
output voltage is given by Equation 35
æ Offtimemax ö
æ tdead ö
Voutmax = Vin ´ ç 1 ÷ - Ioutmax ´ (RDSmax + RI) - (0.7 - Ioutmax ´ RDSmax )´ ç ts ÷
ts
è
ø
è
ø
Where:
Voutmax = maximum achievable output voltage
Vin = minimum input voltage
Offtimemax = maximum off time (90 ns typical for adequate margin)
ts = 1/Fs
Ioutmax = maximum current
RDSmax = maximum high-side MOSFET on resistance (See Electrical Characteristics)
RI = DCR of the inductor
tdead = dead time (60 ns)
(35)
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COMPENSATION
There are several industry techniques used to compensate DC/DC regulators. The method presented here is
easy to calculate and yields high phase margins. For most conditions, the regulator has a phase margin between
60 and 90 degrees. The method presented here ignores the effects of the slope compensation that is internal to
the TPS54618. Since the slope compensation is ignored, the actual cross over frequency is usually lower than
the cross over frequency used in the calculations. Use SwitcherPro software for a more accurate design.
To get started, the modulator pole, fpmod, and the esr zero, fz1 must be calculated using Equation 36 and
Equation 37. For Cout, the derated capacitance value is 82.5 µF. Use Equation 38 and Equation 39 to estimate a
starting point for the crossover frequency, fc. For the example design, fpmod is 6.43 kHz and fzmod is 643 kHz.
Equation 38 is the geometric mean of the modulator pole and the esr zero and Equation 39 is the mean of
modulator pole and the switching frequency. Equation 38 yields 64.3 kHz and Equation 39 gives 56.7 kHz. The
lower value of Equation 38 or Equation 39 is the maximum recommended crossover frequency. For this example,
a lower fc value of 40 kHz is specified. Next, the compensation components are calculated. A resistor in series
with a capacitor is used to create a compensating zero. A capacitor in parallel to these two components forms
the compensating pole (if needed).
¦ p m od =
Iout m ax
2 p ´ Vout ´ Cout
(36)
1
2 p ´ Resr ´ Cout
(37)
vertical spacer
¦ z m od =
vertical spacer
¦C =
¦p mod ´ ¦ z mod
(38)
vertical spacer
¦C =
¦p mod ´
¦ sw
2
(39)
vertical spacer
The compensation design takes the following steps:
1. Set up the anticipated cross-over frequency. Use Equation 40 to calculate the compensation network’s
resistor value. In this example, the anticipated cross-over frequency (fc) is 40 kHz. The power stage gain
(gmps) is 25 A/V and the error amplifier gain (gmea) is 245 mA/V.
2p × ¦ c ´ Vo ´ Co
R3 =
Gm ´ Vref ´ VIgm
(40)
2. Place compensation zero at the pole formed by the load resistor and the output capacitor. The compensation
network’s capacitor can be calculated from Equation 41.
Ro ´ Co
C4 =
R3
(41)
3. An additional pole can be added to attenuate high frequency noise. In this application, it is not necessary to
add it.
From the procedures above, the compensation network includes a 7.50 kΩ resistor and a 3300 pF capacitor.
24
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APPLICATION CURVES
EFFICIENCY
vs
LOAD CURRENT
EFFICIENCY
vs
LOAD CURRENT
100
100
90
90
70
Efficiency - %
70
Efficiency - %
Vin = 3.3 V
80
Vin = 5 V
Vin = 3 V
80
60
50
40
60
Vin = 5 V
50
40
30
30
20
20
10
10
0
0
1
2
3
4
5
6
0
0.01
Figure 39.
Iout = 2 A / div (1.5 A to 4.5 A load step)
1
10
Figure 40.
TRANSIENT RESPONSE, 1.5 A STEP
Vout = 50 mV / div (ac coupled)
0.1
Output Current - A
Output Current - A
POWER UP VOUT, VIN
Vin = 2 V / div
Vout = 1 V / div
PWRGD = 2 V / div
Time = 2 msec / div
Time = 200 usec / div
Figure 41.
Figure 42.
POWER UP VOUT, EN
OUTPUT RIPPLE, 3 A
Vout = 10 mV / div (ac coupled)
EN = 2 V / div
PH = 2 V / div
Vout = 1 V / div
PWRGD = 2 V / div
Time = 500 nsec / div
Time = 2 msec / div
Figure 43.
Figure 44.
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CLOSED LOOP RESPONSE, VIN (5 V), 3 A
Gain - dB
Vin = 100 mV / div (ac coupled)
PH = 2 V / div
60
180
50
150
40
120
30
90
20
60
10
30
0
0
–10
–30
–20
–60
–30
–90
–40
–120
–50
–60
100
Time = 500 nsec / div
Gain
Phase
1000
Figure 45.
–150
10k
Frequency - Hz
–180
1M
100k
Figure 46.
LOAD REGULATION
vs
LOAD CURRENT
REGULATION
vs
INPUT VOLTAGE
0.4
0.4
0.3
0.3
Iout = 3 A
Vin = 5 V
Output Voltage Deviation - %
Output Voltage Deviation - %
Phase - Degrees
INPUT RIPPLE, 3 A
0.2
0.1
Vin = 3.3 V
0
-0.1
-0.2
0.2
0.1
0
-0.1
-0.2
-0.3
-0.3
-0.4
-0.4
0
1
2
3
4
5
6
3
3.5
4
4.5
5
5.5
6
Input Voltage-V
Output Current - A
Figure 47.
Figure 48.
POWER DISSIPATION ESTIMATE
The following formulas show how to estimate the IC power dissipation under continuous conduction mode (CCM)
operation. The power dissipation of the IC (Ptot) includes conduction loss (Pcon), dead time loss (Pd), switching
loss (Psw), gate drive loss (Pgd) and supply current loss (Pq).
Pcon = Io2 × RDS_on_Temp
Pd = ƒsw × Io × 0.7 × 40 × 10–9
Psw = 1/2 × Vin × Io × ƒsw× 13 × 10–9
Pgd = 2 × Vin × ƒsw× 10 × 10–9
Pq = Vin × 515 × 10–6
Where:
IO is the output current (A).
RDS_on_Temp is the on-resistance of the high-side MOSFET with given temperature (Ω).
Vin is the input voltage (V).
ƒsw is the switching frequency (Hz).
26
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So
Ptot = Pcon + Pd + Psw + Pgd + Pq
For given TA,
TJ = TA + Rth × Ptot
For given TJMAX = 150°C
TAmax = TJ max – Rth × Ptot
Where:
Ptot is the total device power dissipation (W).
TA is the ambient temperature (°C).
TJ is the junction temperature (°C).
Rth is the thermal resistance of the package (°C/W).
TJMAX is maximum junction temperature (°C).
TAMAX is maximum ambient temperature (°C).
There are additional power losses in the regulator circuit due to the inductor AC and DC losses and trace
resistance that impact the overall efficiency of the regulator.
LAYOUT
Layout is a critical portion of good power supply design. There are several signal paths that conduct fast
changing currents or voltages that can interact with stray inductance or parasitic capacitance to generate noise
or degrade the power supplies performance. Care should be taken to minimize the loop area formed by the
bypass capacitor connections and the VIN pins. See Figure 49 for a PCB layout example. The GND pins and
AGND pin should be tied directly to the power pad under the IC. The power pad should be connected to any
internal PCB ground planes using multiple vias directly under the IC. Additional vias can be used to connect the
top side ground area to the internal planes near the input and output capacitors. For operation at full rated load,
the top side ground area along with any additional internal ground planes must provide adequate heat dissipating
area.
Locate the input bypass capacitor as close to the IC as possible. The PH pin should be routed to the output
inductor. Since the PH connection is the switching node, the output inductor should be located very close to the
PH pins, and the area of the PCB conductor minimized to prevent excessive capacitive coupling. The boot
capacitor must also be located close to the device. The sensitive analog ground connections for the feedback
voltage divider, compensation components, slow start capacitor and frequency set resistor should be connected
to a separate analog ground trace as shown. The RT/CLK pin is particularly sensitive to noise so the RT resistor
should be located as close as possible to the IC and routed with minimal lengths of trace. The additional external
components can be placed approximately as shown. It may be possible to obtain acceptable performance with
alternate PCB layouts, however this layout has been shown to produce good results and is meant as a guideline.
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VIA to
Ground
Plane
UVLO SET
RESISTORS
VIN
INPUT
BYPASS
CAPACITOR
BOOT
PWRGD
EN
VIN
VIN
BOOT
CAPACITOR
VIN
OUTPUT
INDUCTOR
PH
VIN
PH
EXPOSED
POWERPAD
AREA
GND
PH
GND
VOUT
OUTPUT
FILTER
CAPACITOR
PH
SLOW START
CAPACITOR
RT/CLK
COMP
VSENSE
AGND
SS
FEEDBACK
RESISTORS
ANALOG
GROUND
TRACE
FREQUENCY
SET
RESISTOR
TOPSIDE
GROUND
AREA
COMPENSATION
NETWORK
VIA to Ground Plane
Figure 49. PCB Layout Example
28
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PACKAGE OPTION ADDENDUM
www.ti.com
27-Nov-2010
PACKAGING INFORMATION
Orderable Device
Status
(1)
Package Type Package
Drawing
Pins
Package Qty
Eco Plan
(2)
Lead/
Ball Finish
MSL Peak Temp
(3)
Samples
(Requires Login)
TPS54618RTER
ACTIVE
WQFN
RTE
16
3000
Green (RoHS
& no Sb/Br)
CU NIPDAU Level-2-260C-1 YEAR
Purchase Samples
TPS54618RTET
ACTIVE
WQFN
RTE
16
250
Green (RoHS
& no Sb/Br)
CU NIPDAU Level-2-260C-1 YEAR
Request Free Samples
(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)
Eco Plan - The planned eco-friendly classification: Pb-Free (RoHS), Pb-Free (RoHS Exempt), or Green (RoHS & no Sb/Br) - please check http://www.ti.com/productcontent for the latest availability
information and additional product content details.
TBD: The Pb-Free/Green conversion plan has not been defined.
Pb-Free (RoHS): TI's terms "Lead-Free" or "Pb-Free" mean semiconductor products that are compatible with the current RoHS requirements for all 6 substances, including the requirement that
lead not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, TI Pb-Free products are suitable for use in specified lead-free processes.
Pb-Free (RoHS Exempt): This component has a RoHS exemption for either 1) lead-based flip-chip solder bumps used between the die and package, or 2) lead-based die adhesive used between
the die and leadframe. The component is otherwise considered Pb-Free (RoHS compatible) as defined above.
Green (RoHS & no Sb/Br): TI defines "Green" to mean Pb-Free (RoHS compatible), and free of Bromine (Br) and Antimony (Sb) based flame retardants (Br or Sb do not exceed 0.1% by weight
in homogeneous material)
(3)
MSL, Peak Temp. -- The Moisture Sensitivity Level rating according to the JEDEC industry standard classifications, and peak solder temperature.
Important Information and Disclaimer:The information provided on this page represents TI's knowledge and belief as of the date that it is provided. TI bases its knowledge and belief on information
provided by third parties, and makes no representation or warranty as to the accuracy of such information. Efforts are underway to better integrate information from third parties. TI has taken and
continues to take reasonable steps to provide representative and accurate information but may not have conducted destructive testing or chemical analysis on incoming materials and chemicals.
TI and TI suppliers consider certain information to be proprietary, and thus CAS numbers and other limited information may not be available for release.
In no event shall TI's liability arising out of such information exceed the total purchase price of the TI part(s) at issue in this document sold by TI to Customer on an annual basis.
Addendum-Page 1
PACKAGE MATERIALS INFORMATION
www.ti.com
1-Dec-2010
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
TPS54618RTER
WQFN
RTE
16
3000
330.0
12.4
3.3
3.3
1.1
8.0
12.0
Q2
TPS54618RTET
WQFN
RTE
16
250
180.0
12.4
3.3
3.3
1.1
8.0
12.0
Q2
Pack Materials-Page 1
PACKAGE MATERIALS INFORMATION
www.ti.com
1-Dec-2010
*All dimensions are nominal
Device
Package Type
Package Drawing
Pins
SPQ
Length (mm)
Width (mm)
Height (mm)
TPS54618RTER
WQFN
RTE
16
3000
346.0
346.0
29.0
TPS54618RTET
WQFN
RTE
16
250
190.5
212.7
31.8
Pack Materials-Page 2
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