13
12
11
10
14
15
16
4
5
6
7
3
2
1+
+
98
VREF
Osc
Control
TL494
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Folder
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An IMPORTANT NOTICE at the end of this data sheet addresses availability, warranty, changes, use in safety-critical applications,
intellectual property matters and other important disclaimers. PRODUCTION DATA.
TL494
SLVS074H –JANUARY 1983–REVISED MARCH 2017
TL494 Pulse-Width-Modulation Control Circuits
1
1 Features
1• Complete PWM Power-Control Circuitry
• Uncommitted Outputs for 200-mA Sink or
Source Current
• Output Control Selects Single-Ended or
Push-Pull Operation
• Internal Circuitry Prohibits Double Pulse at
Either Output
• Variable Dead Time Provides Control Over
Total Range
• Internal Regulator Provides a Stable 5-V
Reference Supply With 5% Tolerance
• Circuit Architecture Allows Easy Synchronization
2 Applications
• Desktop PCs
• Microwave Ovens
• Power Supplies: AC/DC, Isolated,
With PFC, > 90 W
• Server PSUs
• Solar Micro-Inverters
• Washing Machines: Low-End and High-End
• E-Bikes
• Power Supplies: AC/DC, Isolated,
No PFC, < 90 W
• Power: Telecom/Server AC/DC Supplies:
Dual Controller: Analog
• Smoke Detectors
• Solar Power Inverters
3 Description
The TL494 device incorporates all the functions
required in the construction of a pulse-width-
modulation (PWM) control circuit on a single chip.
Designed primarily for power-supply control, this
device offers the flexibility to tailor the power-supply
control circuitry to a specific application.
The TL494 device contains two error amplifiers, an
on-chip adjustable oscillator, a dead-time control
(DTC) comparator, a pulse-steering control flip-flop, a
5-V, 5%-precision regulator, and output-control
circuits.
The error amplifiers exhibit a common-mode voltage
range from –0.3 V to VCC – 2 V. The dead-time
control comparator has a fixed offset that provides
approximately 5% dead time. The on-chip oscillator
can be bypassed by terminating RT to the reference
output and providing a sawtooth input to CT, or it can
drive the common circuits in synchronous multiple-rail
power supplies.
The uncommitted output transistors provide either
common-emitter or emitter-follower output capability.
The TL494 device provides for push-pull or single-
ended output operation, which can be selected
through the output-control function. The architecture
of this device prohibits the possibility of either output
being pulsed twice during push-pull operation.
The TL494C device is characterized for operation
from 0°C to 70°C. The TL494I device is characterized
for operation from –40°C to 85°C.
Device Information(1)
PART NUMBER PACKAGE (PIN) BODY SIZE
TL494
SOIC (16) 9.90 mm × 3.91 mm
PDIP (16) 19.30 mm × 6.35 mm
SOP (16) 10.30 mm × 5.30 mm
TSSOP (16) 5.00 mm × 4.40 mm
(1) For all available packages, see the orderable addendum at
the end of the data sheet.
4 Simplified Block Diagram
2
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Table of Contents
1 Features.................................................................. 1
2 Applications ........................................................... 1
3 Description............................................................. 1
4 Simplified Block Diagram ..................................... 1
5 Revision History..................................................... 2
6 Pin Configuration and Functions......................... 3
7 Specifications......................................................... 4
7.1 Absolute Maximum Ratings ...................................... 4
7.2 ESD Ratings.............................................................. 4
7.3 Recommended Operating Conditions....................... 4
7.4 Thermal Information.................................................. 4
7.5 Electrical Characteristics, Reference Section........... 5
7.6 Electrical Characteristics, Oscillator Section............. 5
7.7 Electrical Characteristics, Error-Amplifier Section .... 5
7.8 Electrical Characteristics, Output Section................. 6
7.9 Electrical Characteristics, Dead-Time Control
Section....................................................................... 6
7.10 Electrical Characteristics, PWM Comparator
Section....................................................................... 6
7.11 Electrical Characteristics, Total Device................... 6
7.12 Switching Characteristics........................................ 6
7.13 Typical Characteristics............................................ 7
8 Parameter Measurement Information .................. 8
9 Detailed Description............................................ 10
9.1 Overview................................................................. 10
9.2 Functional Block Diagram....................................... 10
9.3 Feature Description................................................. 10
9.4 Device Functional Modes........................................ 12
10 Application and Implementation........................ 13
10.1 Application Information.......................................... 13
10.2 Typical Application ............................................... 13
11 Power Supply Recommendations ..................... 20
12 Layout................................................................... 20
12.1 Layout Guidelines ................................................. 20
12.2 Layout Example .................................................... 21
13 Device and Documentation Support................. 21
13.1 Trademarks........................................................... 21
13.2 Electrostatic Discharge Caution............................ 21
13.3 Glossary................................................................ 21
14 Mechanical, Packaging, and Orderable
Information........................................................... 21
5 Revision History
Changes from Revision G (January 2015) to Revision H Page
• Updated package illustration.................................................................................................................................................. 1
• Corrected resistor polarity references in the Current-Limiting Amplifier section.................................................................. 15
• Updated Figure 12. .............................................................................................................................................................. 15
Changes from Revision F (January 2014) to Revision G Page
• Added Applications,Device Information table, Pin Functions table, ESD Ratings table, Thermal Information table, ,
Feature Description section, Device Functional Modes,Application and Implementation section, Power Supply
Recommendations section, Layout section, Device and Documentation Support section, and Mechanical,
Packaging, and Orderable Information section. ..................................................................................................................... 1
Changes from Revision E (February 2005) to Revision F Page
• Updated document to new TI data sheet format - no specification changes......................................................................... 1
• Removed Ordering Information table. .................................................................................................................................... 1
• Added ESD warning............................................................................................................................................................. 21
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
1IN+
1IN−
FEEDBACK
DTC
CT
RT
GND
C1
2IN+
2IN−
REF
OUTPUT CTRL
VCC
C2
E2
E1
D, DB, N, NS, OR PW PACKAGE
(TOP VIEW)
3
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6 Pin Configuration and Functions
Pin Functions
PIN TYPE DESCRIPTION
NAME NO.
1IN+ 1 I Noninverting input to error amplifier 1
1IN- 2 I Inverting input to error amplifier 1
2IN+ 16 I Noninverting input to error amplifier 2
2IN- 15 I Inverting input to error amplifier 2
C1 8 O Collector terminal of BJT output 1
C2 11 O Collector terminal of BJT output 2
CT 5 — Capacitor terminal used to set oscillator frequency
DTC 4 I Dead-time control comparator input
E1 9 O Emitter terminal of BJT output 1
E2 10 O Emitter terminal of BJT output 2
FEEDBACK 3 I Input pin for feedback
GND 7 — Ground
OUTPUT
CTRL 13 I Selects single-ended/parallel output or push-pull operation
REF 14 O 5-V reference regulator output
RT 6 — Resistor terminal used to set oscillator frequency
VCC 12 — Positive Supply
4
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(1) 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 Recommended Operating
Conditions is not implied. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability.
(2) All voltages are with respect to the network ground terminal.
7 Specifications
7.1 Absolute Maximum Ratings
over operating free-air temperature range (unless otherwise noted)(1)
MIN MAX UNIT
VCC Supply voltage(2) 41 V
VIAmplifier input voltage VCC + 0.3 V
VOCollector output voltage 41 V
IOCollector output current 250 mA
Lead temperature 1,6 mm (1/16 inch) from case for 10 seconds 260 °C
Tstg Storage temperature range –65 150 °C
7.2 ESD Ratings MAX UNIT
V(ESD) Electrostatic discharge Human body model (HBM), per ANSI/ESDA/JEDEC JS-001, all pins 500 V
Charged device model (CDM), per JEDEC specification JESD22-
C101, all pins 200
7.3 Recommended Operating Conditions MIN MAX UNIT
VCC Supply voltage 7 40 V
VIAmplifier input voltage –0.3 VCC – 2 V
VOCollector output voltage 40 V
Collector output current (each transistor) 200 mA
Current into feedback terminal 0.3 mA
fOSC Oscillator frequency 1 300 kHz
CTTiming capacitor 0.47 10000 nF
RTTiming resistor 1.8 500 kΩ
TAOperating free-air temperature TL494C 0 70 °C
TL494I –40 85
(1) Maximum power dissipation is a function of TJ(max), θJA, and TA. The maximum allowable power dissipation at any allowable ambient
temperature is PD= (TJ(max) – TA) / θJA. Operating at the absolute maximum TJof 150°C can affect reliability.
(2) The package thermal impedance is calculated in accordance with JESD 51-7.
7.4 Thermal Information
over operating free-air temperature range (unless otherwise noted)
PARAMETER TL494 UNIT
D DB N NS PW
RθJA Package thermal
impedance(1)(2) 73 82 67 64 108 °C/W
( )
N2
n
n 1
x X
N 1
=
-
s = -
å
5
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(1) For conditions shown as MIN or MAX, use the appropriate value specified under recommended operating conditions.
(2) All typical values, except for parameter changes with temperature, are at TA= 25°C.
(3) Duration of short circuit should not exceed one second.
7.5 Electrical Characteristics, Reference Section
over recommended operating free-air temperature range, VCC = 15 V, f = 10 kHz (unless otherwise noted)
PARAMETER TEST CONDITIONS(1) TL494C, TL494I UNIT
MIN TYP(2) MAX
Output voltage (REF) IO= 1 mA 4.75 5 5.25 V
Input regulation VCC = 7 V to 40 V 2 25 mV
Output regulation IO= 1 mA to 10 mA 1 15 mV
Output voltage change with temperature ΔTA= MIN to MAX 2 10 mV/V
Short-circuit output current(3) REF = 0 V 25 mA
(1) For conditions shown as MIN or MAX, use the appropriate value specified under recommended operating conditions.
(2) All typical values, except for parameter changes with temperature, are at TA= 25°C.
(3) Standard deviation is a measure of the statistical distribution about the mean as derived from the formula:
(4) Temperature coefficient of timing capacitor and timing resistor are not taken into account.
7.6 Electrical Characteristics, Oscillator Section
CT= 0.01 μF, RT= 12 kΩ(see Figure 5)
PARAMETER TEST CONDITIONS(1) TL494C, TL494I UNIT
MIN TYP(2) MAX
Frequency 10 kHz
Standard deviation of frequency(3) All values of VCC, CT, RT, and TAconstant 100 Hz/kHz
Frequency change with voltage VCC = 7 V to 40 V, TA= 25°C 1 Hz/kHz
Frequency change with temperature(4) ΔTA= MIN to MAX 10 Hz/kHz
(1) All typical values, except for parameter changes with temperature, are at TA= 25°C.
7.7 Electrical Characteristics, Error-Amplifier Section
See Figure 6
PARAMETER TEST CONDITIONS TL494C, TL494I UNIT
MIN TYP(1) MAX
Input offset voltage VO(FEEDBACK) = 2.5 V 2 10 mV
Input offset current VO(FEEDBACK) = 2.5 V 25 250 nA
Input bias current VO(FEEDBACK) = 2.5 V 0.2 1 μA
Common-mode input voltage range VCC = 7 V to 40 V –0.3 to VCC – 2 V
Open-loop voltage amplification ΔVO= 3 V, VO= 0.5 V to 3.5 V, RL= 2 kΩ70 95 dB
Unity-gain bandwidth VO= 0.5 V to 3.5 V, RL= 2 kΩ800 kHz
Common-mode rejection ratio ΔVO= 40 V, TA= 25°C 65 80 dB
Output sink current (FEEDBACK) VID = –15 mV to –5 V, V (FEEDBACK) = 0.7 V 0.3 0.7 mA
Output source current (FEEDBACK) VID = 15 mV to 5 V, V (FEEDBACK) = 3.5 V –2 mA
6
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(1) All typical values, except for temperature coefficient, are at TA= 25°C.
7.8 Electrical Characteristics, Output Section
PARAMETER TEST CONDITIONS MIN TYP(1) MAX UNIT
Collector off-state current VCE = 40 V, VCC = 40 V 2 100 μA
Emitter off-state current VCC = VC= 40 V, VE= 0 –100 μA
Collector-emitter saturation voltage Common emitter VE= 0, IC= 200 mA 1.1 1.3 V
Emitter follower VO(C1 or C2) = 15 V, IE= –200 mA 1.5 2.5
Output control input current VI= Vref 3.5 mA
(1) All typical values, except for temperature coefficient, are at TA= 25°C.
7.9 Electrical Characteristics, Dead-Time Control Section
See Figure 5 PARAMETER TEST CONDITIONS MIN TYP(1) MAX UNIT
Input bias current (DEAD-TIME CTRL) VI= 0 to 5.25 V –2 –10 μA
Maximum duty cycle, each output VI(DEAD-TIME CTRL) = 0, CT= 0.01 μF,
RT= 12 kΩ45% —
Input threshold voltage (DEAD-TIME CTRL) Zero duty cycle 3 3.3 V
Maximum duty cycle 0
(1) All typical values, except for temperature coefficient, are at TA= 25°C.
7.10 Electrical Characteristics, PWM Comparator Section
See Figure 5 PARAMETER TEST CONDITIONS MIN TYP(1) MAX UNIT
Input threshold voltage (FEEDBACK) Zero duty cyle 4 4.5 V
Input sink current (FEEDBACK) V (FEEDBACK) = 0.7 V 0.3 0.7 mA
(1) All typical values, except for temperature coefficient, are at TA= 25°C.
7.11 Electrical Characteristics, Total Device
PARAMETER TEST CONDITIONS MIN TYP(1) MAX UNIT
Standby supply current RT= Vref,
All other inputs and outputs open VCC = 15 V 6 10 mA
VCC = 40 V 9 15
Average supply current VI(DEAD-TIME CTRL) = 2 V, See Figure 5 7.5 mA
(1) All typical values, except for temperature coefficient, are at TA= 25°C.
7.12 Switching Characteristics
TA= 25°C PARAMETER TEST CONDITIONS MIN TYP(1) MAX UNIT
Rise time Common-emitter configuration, See Figure 7 100 200 ns
Fall time 25 100 ns
Rise time Emitter-follower configuration, See Figure 8 100 200 ns
Fall time 40 100 ns
4
3
2
1
00 10 20
VI− Input Voltage − (mV)
VO− Output V
oltage − (V)
Df = 1%
(1)
40
10
100
1 k 4 k 10 k 40 k 100 k 400 k 1 M
f − Oscillator Frequency and Frequency V
ariation − Hz
400
1 k
4 k
10 k
40 k
100 k
RT− T
iming Resistance − Ω
0.1 Fµ
−2%
−1%
0% 0.01 Fµ
0.001 Fµ
VCC = 15 V
TA= 25 C°
CT= 1 Fµ
10
0
100
20
1 10 100 1 M
A − Amplifier Voltage Amplification − dB
30
f − Frequency − Hz
1 k
VCC = 15 V
ΔVO= 3 V
TA= 25 C°
10 k
40
50
60
70
80
90
100 k
7
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7.13 Typical Characteristics
Frequency variation (Δf) is the change in oscillator frequency
that occurs over the full temperature range.
Figure 1. Oscillator Frequency and Frequency Variation
vs
Timing Resistance
xxx
xxx Figure 2. Amplifier Voltage Amplification
vs
Frequency
Figure 3. Error Amplifier Transfer Characteristics Figure 4. Error Amplifier Bode Plot
Test
Inputs
DTC
FEEDBACK
RT
CT
GND
50 kW
12 kW
0.01 mF
VCC
REF
OUTPUT
CTRL
E2
C2
E1
C1 Output 1
Output 2
150 W
2 W
150 W
2 W
VCC = 15 V
TEST CIRCUIT
1IN+
VCC
VCC
0 V
0 V
Voltage
at C1
Voltage
at C2
Voltage
at CT
DTC
FEEDBACK
0 V
0.7 V
0% MAX 0%
Threshold Voltage
Threshold Voltage
VOLTAGE WAVEFORMS
Duty Cycle
Error
Amplifiers
7
14
12
8
9
11
10
4
3
6
5
1
2
16
15
13
1IN−
2IN−
2IN+
8
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8 Parameter Measurement Information
Figure 5. Operational Test Circuit and Waveforms
Output
Each Output
Circuit
68 W
2 W
15 V
CL= 15 pF
(See Note A)
90%
10%
90%
10%
tf
tr
TEST CIRCUIT OUTPUT VOLTAGE WAVEFORM
NOTE A: CLincludes probe and jig capacitance.
Output
Each Output
Circuit
68 W
2 W
15 V
CL= 15 pF
(See Note A)
90%
10%
90%
10%
tftr
TEST CIRCUIT OUTPUT VOLTAGE WAVEFORM
NOTE A: CLincludes probe and jig capacitance.
+
−
+
−
VI
Vref
FEEDBACK
Amplifier Under Test
Other Amplifier
9
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Parameter Measurement Information (continued)
Figure 6. Amplifier Characteristics
Figure 7. Common-Emitter Configuration
Figure 8. Emitter-Follower Configuration
GND
VCC
Reference
Regulator
C1
Pulse-Steering
Flip-Flop
C1
1D
DTC
CT
RT
PWM
Comparator
+
−
Error Amplifier 1
≈0.1 V
Dead-Time Control
Comparator
Oscillator
OUTPUT CTRL
(see Function Table)
0.7 mA
E1
C2
E2
+
−
Error Amplifier 2
1IN+
1IN−
2IN+
2IN−
FEEDBACK
REF
6
5
4
1
2
16
15
3
13
8
9
11
10
12
14
7
Q1
Q2
≈0.7 V
10
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9 Detailed Description
9.1 Overview
The design of the TL494 not only incorporates the primary building blocks required to control a switching power
supply, but also addresses many basic problems and reduces the amount of additional circuitry required in the
total design. The TL494 is a fixed-frequency pulse-width-modulation (PWM) control circuit. Modulation of output
pulses is accomplished by comparing the sawtooth waveform created by the internal oscillator on the timing
capacitor (CT) to either of two control signals. The output stage is enabled during the time when the sawtooth
voltage is greater than the voltage control signals. As the control signal increases, the time during which the
sawtooth input is greater decreases; therefore, the output pulse duration decreases. A pulse-steering flip-flop
alternately directs the modulated pulse to each of the two output transistors. For more information on the
operation of the TL494, see the application notes located on ti.com.
9.2 Functional Block Diagram
9.3 Feature Description
9.3.1 5-V Reference Regulator
The TL494 internal 5-V reference regulator output is the REF pin. In addition to providing a stable reference, it
acts as a preregulator and establishes a stable supply from which the output-control logic, pulse-steering flip-flop,
oscillator, dead-time control comparator, and PWM comparator are powered. The regulator employs a band-gap
circuit as its primary reference to maintain thermal stability of less than 100-mV variation over the operating free-
air temperature range of 0°C to 70°C. Short-circuit protection is provided to protect the internal reference and
preregulator; 10 mA of load current is available for additional bias circuits. The reference is internally
programmed to an initial accuracy of ±5% and maintains a stability of less than 25-mV variation over an input
voltage range of 7 V to 40 V. For input voltages less than 7 V, the regulator saturates within 1 V of the input and
tracks it.
T T
1
f
2R C
=
´
T T
1
f
R C
=
´
OSC
T T
1
f
R C
=
´
T
CHARGE
3 V C
T
I
´
=
CHARGE
T
3 V
I
R
=
11
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Feature Description (continued)
9.3.2 Oscillator
The oscillator provides a positive sawtooth waveform to the dead-time and PWM comparators for comparison to
the various control signals.
The frequency of the oscillator is programmed by selecting timing components RTand CT. The oscillator charges
the external timing capacitor, CT, with a constant current, the value of which is determined by the external timing
resistor, RT. This produces a linear-ramp voltage waveform. When the voltage across CTreaches 3 V, the
oscillator circuit discharges it, and the charging cycle is reinitiated. The charging current is determined by the
formula:
(1)
The period of the sawtooth waveform is:
(2)
The frequency of the oscillator becomes:
(3)
However, the oscillator frequency is equal to the output frequency only for single-ended applications. For push-
pull applications, the output frequency is one-half the oscillator frequency.
Single-ended applications:
(4)
Push-pull applications:
(5)
9.3.3 Dead-time Control
The dead-time control input provides control of the minimum dead time (off time). The output of the comparator
inhibits switching transistors Q1 and Q2 when the voltage at the input is greater than the ramp voltage of the
oscillator. An internal offset of 110 mV ensures a minimum dead time of ∼3% with the dead-time control input
grounded. Applying a voltage to the dead-time control input can impose additional dead time. This provides a
linear control of the dead time from its minimum of 3% to 100% as the input voltage is varied from 0 V to 3.3 V,
respectively. With full-range control, the output can be controlled from external sources without disrupting the
error amplifiers. The dead-time control input is a relatively high-impedance input (II< 10 μA) and should be used
where additional control of the output duty cycle is required. However, for proper control, the input must be
terminated. An open circuit is an undefined condition.
9.3.4 Comparator
The comparator is biased from the 5-V reference regulator. This provides isolation from the input supply for
improved stability. The input of the comparator does not exhibit hysteresis, so protection against false triggering
near the threshold must be provided. The comparator has a response time of 400 ns from either of the control-
signal inputs to the output transistors, with only 100 mV of overdrive. This ensures positive control of the output
within one-half cycle for operation within the recommended 300-kHz range.
12
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Feature Description (continued)
9.3.5 Pulse-Width Modulation (PWM)
The comparator also provides modulation control of the output pulse width. For this, the ramp voltage across
timing capacitor CTis compared to the control signal present at the output of the error amplifiers. The timing
capacitor input incorporates a series diode that is omitted from the control signal input. This requires the control
signal (error amplifier output) to be ∼0.7 V greater than the voltage across CTto inhibit the output logic, and
ensures maximum duty cycle operation without requiring the control voltage to sink to a true ground potential.
The output pulse width varies from 97% of the period to 0 as the voltage present at the error amplifier output
varies from 0.5 V to 3.5 V, respectively.
9.3.6 Error Amplifiers
Both high-gain error amplifiers receive their bias from the VIsupply rail. This permits a common-mode input
voltage range from –0.3 V to 2 V less than VI. Both amplifiers behave characteristically of a single-ended single-
supply amplifier, in that each output is active high only. This allows each amplifier to pull up independently for a
decreasing output pulse-width demand. With both outputs ORed together at the inverting input node of the PWM
comparator, the amplifier demanding the minimum pulse out dominates. The amplifier outputs are biased low by
a current sink to provide maximum pulse width out when both amplifiers are biased off.
9.3.7 Output-Control Input
The output-control input determines whether the output transistors operate in parallel or push-pull. This input is
the supply source for the pulse-steering flip-flop. The output-control input is asynchronous and has direct control
over the output, independent of the oscillator or pulse-steering flip-flop. The input condition is intended to be a
fixed condition that is defined by the application. For parallel operation, the output-control input must be
grounded. This disables the pulse-steering flip-flop and inhibits its outputs. In this mode, the pulses seen at the
output of the dead-time control/PWM comparator are transmitted by both output transistors in parallel. For push-
pull operation, the output-control input must be connected to the internal 5-V reference regulator. Under this
condition, each of the output transistors is enabled, alternately, by the pulse-steering flip-flop.
9.3.8 Output Transistors
Two output transistors are available on the TL494. Both transistors are configured as open collector/open
emitter, and each is capable of sinking or sourcing up to 200 mA. The transistors have a saturation voltage of
less than 1.3 V in the common-emitter configuration and less than 2.5 V in the emitter-follower configuration. The
outputs are protected against excessive power dissipation to prevent damage, but do not employ sufficient
current limiting to allow them to be operated as current-source outputs.
9.4 Device Functional Modes
When the OUTPUT CTRL pin is tied to ground, the TL494 is operating in single-ended or parallel mode. When
the OUTPUT CTRL pin is tied to VREF, the TL494 is operating in normal push-pull operation.
VREF
R1
1 kW
Load
Control
Osc
NTE331 140 mH
VO
NTE6013
NTE153
TL494
R2
4 kW
5-V
REF
R11
100 W
R12
30 W
R10
270 W
1 2 3 4 6 7 8
910111213141516
RF
51 kW
RT
50 kW
R5
510 W
R7
9.1 kW
R3
5.1 kW
R4
5.1 kW
R6
1 kW
C2
2.5 mF
5-V
REF
5-V
REF
R13
0.1 W
CT
0.001 Fm
5
R8
5.1 k
R9
5.1 k
Q2
Q1
+ −
+ −
32-V
Input
13
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10 Application and Implementation
NOTE
Information in the following applications sections is not part of the TI component
specification, and TI does not warrant its accuracy or completeness. TI’s customers are
responsible for determining suitability of components for their purposes. Customers should
validate and test their design implementation to confirm system functionality.
10.1 Application Information
The following design example uses the TL494 to create a 5-V/10-A power supply. This application was taken
from application note SLVA001.
10.2 Typical Application
Figure 9. Switching and Control Sections
RT+1
fOSC CT
+1
(20 103) (0.001 10*6)+50 kW
OSC
T T
1
f
R C
=
´
O
RECTIFIER(AVG) O
I
V5 V
I I 10 A 1.6 A
V 32 V
» ´ » ´ =
RECTIFIER SECONDARY
V V 2 24 V 2 34 V= ´ = ´ =
20,000 μF
Bridge
Rectifiers
3 A/50 V
20,000 μF
0.3 W
120 V 24 V
3 A +32 V
+ +
14
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Typical Application (continued)
10.2.1 Design Requirements
• VI= 32 V
• VO=5V
• IO= 10 A
• fOSC = 20-kHz switching frequency
• VR= 20-mV peak-to-peak (VRIPPLE)
•ΔIL= 1.5-A inductor current change
10.2.2 Detailed Design Procedure
10.2.2.1 Input Power Source
The 32-V dc power source for this supply uses a 120-V input, 24-V output transformer rated at 75 VA. The 24-V
secondary winding feeds a full-wave bridge rectifier, followed by a current-limiting resistor (0.3 Ω) and two filter
capacitors (see Figure 10).
Figure 10. Input Power Source
The output current and voltage are determined by Equation 6 and Equation 7:
(6)
(7)
The 3-A/50-V full-wave bridge rectifier meets these calculated conditions. Figure 9 shows the switching and
control sections.
10.2.2.2 Control Circuits
10.2.2.2.1 Oscillator
Connecting an external capacitor and resistor to pins 5 and 6 controls the TL494 oscillator frequency. The
oscillator is set to operate at 20 kHz, using the component values calculated by Equation 8 and Equation 9:
(8)
Choose CT= 0.001 μF and calculate RT:
(9)
10.2.2.2.2 Error Amplifier
The error amplifier compares a sample of the 5-V output to the reference and adjusts the PWM to maintain a
constant output current (see Figure 11).
1V
R13 0.1
10 A
= = W
15
16
R1
1 NŸ
R2
4 NŸ
Load
+
TL494
14
VREF
TL494
R13
0.1 Ÿ
VO
L
SC O
I
I I 10.75 A
2
= + =
R8
5.1 kW
VO
VREF +
−
1
14
13 Error
Amplifier
2
3
TL494
R9
5.1 kW
R5
510 W
R7
51 kW
R3
5.1 kW
R4
5.1 kW
TL494
15
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Typical Application (continued)
Figure 11. Error-Amplifier Section
The TL494 internal 5-V reference is divided to 2.5 V by R3 and R4. The output-voltage error signal also is
divided to 2.5 V by R8 and R9. If the output must be regulated to exactly 5.0 V, a 10-kΩpotentiometer can be
used in place of R8 to provide an adjustment.
To increase the stability of the error-amplifier circuit, the output of the error amplifier is fed back to the inverting
input through RT, reducing the gain to 101.
10.2.2.2.3 Current-Limiting Amplifier
The power supply was designed for a 10-A load current and an ILswing of 1.5 A, therefore, the short-circuit
current should be:
(10)
The current-limiting circuit is shown in Figure 12.
Figure 12. Current-Limiting Circuit
Resistors R1 and R2 set the reference of approximately 1 V on the inverting input of the current-limiting amplifier.
Resistor R13, in series with the load, applies 1 V to the non-inverting terminal of the current-limiting amplifier
when the load current reaches 10 A. The output pulse width reduces accordingly. The value of R13 is calculated
in Equation 11.
(11)
10.2.2.2.4 Soft Start and Dead Time
To reduce stress on the switching transistors at the start-up time, the start-up surge that occurs as the output
filter capacitor charges must be reduced. The availability of the dead-time control makes implementation of a
soft-start circuit relatively simple (see Figure 13).
soft start time 50 s 50cycles
C2 2.5 F
R6 1 k
- m ´
= = = m
W
1 1
t 50 sper clock cycle
f 20kHz
= = = m
TL494
Pin 4 Voltage
PWM Output
Oscillator Ramp Voltage
ton
RT
C2
+5 V
0.1 V
Osc
R6
14
5
4
7
Oscillator Ramp
+
16
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Typical Application (continued)
Figure 13. Soft-Start Circuit
The soft-start circuit allows the pulse width at the output to increase slowly (see Figure 13) by applying a
negative slope waveform to the dead-time control input (pin 4).
Initially, capacitor C2 forces the dead-time control input to follow the 5-V regulator, which disables the outputs
(100% dead time). As the capacitor charges through R6, the output pulse width slowly increases until the control
loop takes command. With a resistor ratio of 1:10 for R6 and R7, the voltage at pin 4 after start-up is 0.1 × 5 V,
or 0.5 V.
The soft-start time generally is in the range of 25 to 100 clock cycles. If 50 clock cycles at a 20-kHz switching
rate is selected, the soft-start time is:
(12)
The value of the capacitor then is determined by:
(13)
This helps eliminate any false signals that might be created by the control circuit as power is applied.
L
3
O
I1.5 A
C3 94 F
8f V 8 20 10 0.1 V
D
= = = m
D´ ´ ´
O(ripple)
L
VV
ESR(max) 0.067
I 1.5 A
D
= = » W
D
R1
C1
D1
S1
L
VO
VI
17
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Typical Application (continued)
10.2.2.3 Inductor Calculations
The switching circuit used is shown in Figure 39.
Figure 14. Switching Circuit
The size of the inductor (L) required is:
d = duty cycle = VO/VI= 5 V/32 V = 0.156
f = 20 kHz (design objective)
ton = time on (S1 closed) = (1/f) × d = 7.8 μs
toff = time off (S1 open) = (1/f) – ton = 42.2 μs
L≉(VI– VO) × ton/ΔIL
≉[(32 V – 5 V) × 7.8 μs]/1.5 A
≉140.4 μH
10.2.2.4 Output Capacitance Calculations
Once the filter inductor has been calculated, the value of the output filter capacitor is calculated to meet the
output ripple requirements. An electrolytic capacitor can be modeled as a series connection of an inductance, a
resistance, and a capacitance. To provide good filtering, the ripple frequency must be far below the frequencies
at which the series inductance becomes important. So, the two components of interest are the capacitance and
the effective series resistance (ESR). The maximum ESR is calculated according to the relation between the
specified peak-to-peak ripple voltage and the peak-to-peak ripple current.
(14)
The minimum capacitance of C3 necessary to maintain the VOripple voltage at less than the 100-mV design
objective is calculated according to Equation 15:
(15)
A 220-mF, 60-V capacitor is selected because it has a maximum ESR of 0.074 Ωand a maximum ripple current
of 2.8 A.
10.2.2.5 Transistor Power-Switch Calculations
The transistor power switch was constructed with an NTE153 pnp drive transistor and an NTE331 npn output
transistor. These two power devices were connected in a pnp hybrid Darlington circuit configuration (see
Figure 15).
[ ]
I BE CE
B
V V (Q1) V (TL494) 32 (1.5 0.7)
R10 i 0.144
R10 207
- + - +
£ =
£ W
L
O
B
FE FE
I
I2
i 144mA
h (Q2) h (Q1)
+
³ ³
´
FE C
h (Q2) at I of 10.0 A 5=
FE C
h (Q1) at I of 3 A 15=
NTE331
TL494
Control
Q2
11 10 9
Q1
NTE153
32 V
8
R10
270 W
R12
30 W
R11
100 W
I
O
L
I 10.8 A
2
D
+ =
18
TL494
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Figure 15. Power-Switch Section
The hybrid Darlington circuit must be saturated at a maximum output current of IO+ΔIL/2 or 10.8 A. The
Darlington hFE at 10.8 A must be high enough not to exceed the 250-mA maximum output collector current of the
TL494. Based on published NTE153 and NTE331 specifications, the required power-switch minimum drive was
calculated by Equation 16 through Equation 18 to be 144 mA:
(16)
(17)
(18)
The value of R10 was calculated by:
(19)
Based on these calculations, the nearest standard resistor value of 220 Ωwas selected for R10. Resistors R11
and R12 permit the discharge of carriers in switching transistors when they are turned off.
The power supply described demonstrates the flexibility of the TL494 PWM control circuit. This power-supply
design demonstrates many of the power-supply control methods provided by the TL494, as well as the versatility
of the control circuit.
5
4
3
2
1
00 1 2 3 4 5 6 7
VI− Input Voltage − (V)
6
VREF − Reference V
oltage − (V)
19
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10.2.3 Application Curves for Output Characteristics
Figure 16. Reference Voltage vs Input Voltage
20
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11 Power Supply Recommendations
The TL494 is designed to operate from an input voltage supply range between 7 V and 40 V. This input supply
should be well regulated. If the input supply is located more than a few inches from the device, additional bulk
capacitance may be required in addition to the ceramic bypass capacitors. A tantalum capacitor with a value of
47 μF is a typical choice, however this may vary depending upon the output power being delivered.
12 Layout
12.1 Layout Guidelines
Always try to use a low EMI inductor with a ferrite type closed core. Some examples would be toroid and
encased E core inductors. Open core can be used if they have low EMI characteristics and are located a bit
more away from the low power traces and components. Make the poles perpendicular to the PCB as well if using
an open core. Stick cores usually emit the most unwanted noise.
12.1.1 Feedback Traces
Try to run the feedback trace as far from the inductor and noisy power traces as possible. You would also like
the feedback trace to be as direct as possible and somewhat thick. These two sometimes involve a trade-off, but
keeping it away from inductor EMI and other noise sources is the more critical of the two. Run the feedback trace
on the side of the PCB opposite of the inductor with a ground plane separating the two.
12.1.2 Input/Output Capacitors
When using a low value ceramic input filter capacitor, it should be located as close to the VCC pin of the IC as
possible. This will eliminate as much trace inductance effects as possible and give the internal IC rail a cleaner
voltage supply. Some designs require the use of a feed-forward capacitor connected from the output to the
feedback pin as well, usually for stability reasons. In this case it should also be positioned as close to the IC as
possible. Using surface mount capacitors also reduces lead length and lessens the chance of noise coupling into
the effective antenna created by through-hole components.
12.1.3 Compensation Components
External compensation components for stability should also be placed close to the IC. Surface mount
components are recommended here as well for the same reasons discussed for the filter capacitors. These
should not be located very close to the inductor either.
12.1.4 Traces and Ground Planes
Make all of the power (high current) traces as short, direct, and thick as possible. It is good practice on a
standard PCB board to make the traces an absolute minimum of 15 mils (0.381 mm) per Ampere. The inductor,
output capacitors, and output diode should be as close to each other possible. This helps reduce the EMI
radiated by the power traces due to the high switching currents through them. This will also reduce lead
inductance and resistance as well, which in turn reduces noise spikes, ringing, and resistive losses that produce
voltage errors. The grounds of the IC, input capacitors, output capacitors, and output diode (if applicable) should
be connected close together directly to a ground plane. It would also be a good idea to have a ground plane on
both sides of the PCB. This will reduce noise as well by reducing ground loop errors as well as by absorbing
more of the EMI radiated by the inductor. For multi-layer boards with more than two layers, a ground plane can
be used to separate the power plane (where the power traces and components are) and the signal plane (where
the feedback and compensation and components are) for improved performance. On multi-layer boards the use
of vias will be required to connect traces and different planes. It is good practice to use one standard via per 200
mA of current if the trace will need to conduct a significant amount of current from one plane to the other.
Arrange the components so that the switching current loops curl in the same direction. Due to the way switching
regulators operate, there are two power states. One state when the switch is on and one when the switch is off.
During each state there will be a current loop made by the power components that are currently conducting.
Place the power components so that during each of the two states the current loop is conducting in the same
direction. This prevents magnetic field reversal caused by the traces between the two half-cycles and reduces
radiated EMI.
GND
TL494
2 1IN±
3
4 DTC
CT
RT
7 GND
8 9E1
10E2
11C2
12VCC
13
14REF
152IN±
162IN+
FEEDBACK
1IN+
C1
OUTPUT
CTRL
1
VIA to Power Plane
Power or GND Plane
VIA to GND Plane
LEGEND
6
5
Output
VCC
21
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12.2 Layout Example
Figure 17. Operational Amplifier Board Layout for Noninverting Configuration
13 Device and Documentation Support
13.1 Trademarks
All trademarks are the property of their respective owners.
13.2 Electrostatic Discharge Caution
T