ON Semiconductor LM2594 User Manual

LM2594

0.5 A, Step-Down Switching Regulator

The LM2594 regulator is monolithic integrated circuit ideally suited for easy and convenient design of a step−down switching regulator (buck converter). It is capable of driving a 0.5 A load with excellent line and load regulation. This device is available in adjustable output version. It is internally compensated to minimize the number of external components to simplify the power supply design.

Since LM2594 converter is a switch−mode power supply, its efficiency is significantly higher in comparison with popular three−terminal linear regulators, especially with higher input voltages. The LM2594 operates at a switching frequency of 150 kHz thus allowing smaller sized filter components than what would be needed with lower frequency switching regulators. Available in a standard 8−Lead PDIP and 8−Lead Surface Mount packages.

The other features include a guaranteed $4% tolerance on output voltage within specified input voltages and output load conditions, and $15% on the oscillator frequency. External shutdown is included, featuring 50 mA (typical) standby current. Self protection features include switch cycle−by−cycle current limit for the output switch, as well as thermal shutdown for complete protection under fault conditions.

SOIC−8 D SUFFIX CASE 751

PDIP−8 N SUFFIX CASE 626

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MARKING

DIAGRAMS

LM2594

AYWW

2594−ADJ

AWL

YYWW

Features

• Adjustable Output Voltage Range 1.23 V − 37 V

• Guaranteed 0.5 A Output Load Current

• Wide Input Voltage Range up to 40 V

• 150 kHz Fixed Frequency Internal Oscillator

• TTL Shutdown Capability

• Low Power Standby Mode, typ 50 mA

• Thermal Shutdown and Current Limit Protection

• Internal Loop Compensation

• Moisture Sensitivity Level (MSL) Equals 1

• These are Pb−Free Devices

Applications

• Simple High−Efficiency Step−Down (Buck) Regulator

• Efficient Pre−Regulator for Linear Regulators

• On−Card Switching Regulators

• Positive to Negative Converter (Buck−Boost)

• Negative Step−Up Converters

• Power Supply for Battery Chargers

A = Assembly Location WL = Wafer Lot YY = Year WW = Work Week G or G = Pb−Free Package

PIN CONNECTIONS

SOIC−8

(Top View)

PDIP−8

(Top View)

OUTPUT

GND

ON/OFF

OUTPUT

GND

ON/OFF

January, 2009 − Rev. 0

ORDERING INFORMATION

See detailed ordering and shipping information in the package dimensions section on page 23 of this data sheet.

1 Publication Order Number:

LM2594/D

LM2594

12 V Unregulated

DC Input

CIN = 68 mF

Feedback

LM2594

ON/OFF

GND

Output

100 mH

1N5817

OUT

220 mF

R1 = 1 kW

R2 = 3k

OUT

= 5 V; I

load

= 0.5 A

Figure 1. Typical Application

0.5

Figure 2. Representative Block Diagram

PIN FUNCTION DESCRIPTION

Pin No. Symbol Description (Refer to Figure 1)

1 − 3 NC Not Connected

4 FB This pin is the direct input of the error amplifier and the resistor network R2, R1 is connected externally to

5 ON/OFF Allows the switching regulator circuit to be shut down using logic levels, thus dropping the total input supply

6 GND Circuit ground pin. See the information about the printed circuit board layout.

7 +V

8 OUTPUT Emitter of the internal switch. The saturation voltage Vsat of the output switch is typically 1 V. It should be

allow programming of the output voltage.

current to approximately 50 mA. The threshold voltage is typical. 1.6 V. Applying a voltage above this value (up to VIN) shuts the regulator off. If the voltage applied to this pin is lower than 1.6 V or if this pin is left open, the regulator will be in the “on” condition.

Positive input supply for LM2594 step−down switching regulator. In order to minimize voltage transients and

to supply the switching currents needed by the regulator, a suitable input bypass capacitor must be present (CIN in Figure 1)

kept in mind that PCB area connected to this pin should be kept to a minimum in order to minimize coupling to sensitive circuitry

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LM2594

MAXIMUM RATINGS

Symbol Rating Value Unit

ON/OFF ON/OFF Pin Input Voltage −0.3 V ≤ V ≤ +V

out

stg

−

− Lead Temperature (Soldering, 10 seconds) 260 °C

Stresses exceeding Maximum Ratings may damage the device. Maximum Ratings are stress ratings only. Functional operation above the Recommended Operating Conditions is not implied. Extended exposure to stresses above the Recommended Operating Conditions may affect device reliability.

Maximum Supply Voltage 45 V

Output Voltage to Ground (Steady−State) −1.0 V

Power Dissipation

8−Lead DIP Internally Limited W

Thermal Resistance, Junction−to−Ambient 100 °C/W

Thermal Resistance, Junction−to−Case 5.0 °C/W

Power Dissipation

8−Lead Surface Mount Internally Limited W

Thermal Resistance, Junction−to−Ambient 175 °C/W

Storage Temperature Range −65 to +150 °C

Minimum ESD Rating (Human Body Model: C = 100 pF, R = 1.5 kW)

Maximum Junction Temperature 150 °C

2.0 kV

OPERATING RATINGS (Operating Ratings indicate conditions for which the device is intended to be functional, but do not guarantee

specific performance limits. For guaranteed specifications and test conditions, see the Electrical Characteristics table)

Symbol Rating Value Unit

Operating Temperature Range −40 to +125 °C

Supply Voltage 4.5 V to 40 V V

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LM2594

SYSTEM PARAMETERS

ELECTRICAL CHARACTERISTICS Specifications with standard type face are for T

over full Operating Temperature Range −40°C to +125°C

Characteristics Symbol Min Typ Max Unit

LM2594 (Note 1, Test Circuit Figure 16)

= 12 V, I

Load

= 0.5 A, V

= 0.1 A, V

Load

= 5.0 V, ) V

out

≤ 0.5 A, V

Load

= 5.0 V) η − 80 %

out

= 5.0 V) V

out

Characteristics Symbol Min Typ Max Unit

= 5.0 V) I

out

= 0.5 A, Notes 3 and 4) V

out

FB_nom

Feedback Voltage (V

Feedback Voltage (8.0 V ≤ Vin ≤ 40 V, 0.1 A ≤ I

Efficiency (V

Feedback Bias Current (V

Oscillator Frequency (Note 2) f

Saturation Voltage (I

Max Duty Cycle “ON” (Note 4) DC 95 %

Current Limit (Peak Current, Notes 3 and 4) I

Output Leakage Current (Notes 5 and 6) Output = 0 V Output = −1.0 V

Quiescent Current (Note 5) I

Standby Quiescent Current (ON/OFF Pin = 5.0 V (“OFF”)) (Note 6)

ON/OFF PIN LOGIC INPUT

Threshold Voltage 1.6 V

= 0 V (Regulator OFF) V

out

= Nominal Output Voltage (Regulator ON) V

out

ON/OFF Pin Input Current

ON/OFF Pin = 5.0 V (Regulator OFF) I

ON/OFF Pin = 0 V (regulator ON) I

1. External components such as the catch diode, inductor, input and output capacitors can affect switching regulator system performance. When the LM2594 is used as shown in the Figure 16 test circuit, system performance will be as shown in system parameters section.

2. The oscillator frequency reduces to approximately 30 kHz in the event of an output short or an overload which causes the regulated output voltage to drop approximately 40% from the nominal output voltage. This self protection feature lowers the average dissipation of the IC by lowering the minimum duty cycle from 5% down to approximately 2%.

3. No diode, inductor or capacitor connected to output (Pin 8) sourcing the current.

4. Feedback (Pin 4) removed from output and connected to 0 V.

5. Feedback (Pin 4) removed from output and connected to +12 V to force the output transistor “off”.

6. Vin = 40 V.

= 25°C, and those with boldface type apply

1.23 V

1.193

1.18

135

120

0.7

0.65

25 100

150 165

1.0 1.2

1.0 1.6

0.5

osc

sat

stby

2.2

2.4

− 15 30

− 0.01 5.0

5.0 10 mA

50 200

1.267

1.28

200

180

1.4

1.8

2.0 30

250

1.0

0.8

kHz

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LM2594

TYPICAL PERFORMANCE CHARACTERISTICS (Circuit of Figure 16)

1.0

Vin = 20 V

0.8

= 100 mA

Load

0.6

Normalized at TJ = 25°C

0.4

0.2

-0.2

-0.4

, OUTPUT VOLTAGE CHANGE (%)

-0.6

out

-0.8

-1.0

7550250−25−50

100

TJ, JUNCTION TEMPERATURE (°C)

Figure 3. Normalized Output Voltage

2.0

= 500 mA

1.5

Load

1.0

= 100 mA

Load

0.5

INPUT - OUTPUT DIFFERENTIAL (V)

L = 100 mH R_ind = 30 mW

−50 −25 0 25 60 75 100 125

TJ, JUNCTION TEMPERATURE (°C)

Figure 5. Dropout Voltage Figure 6. Current Limit

−0.2

, OUTPUT VOLTAGE CHANGE (%)

−0.4

out

−0.6

125

, OUTPUT CURRENT (A)

1.4 I

= 100 mA

Load

1.2 T

= 25°C

1.0

0.8

0.6

= 5 V

out

0.4

0.2

0 5.0 10 15 20 25 30 35 40

Vin, INPUT VOLTAGE (V)

Figure 4. Line Regulation

1.3

1.2

1.1

1.0

0.9

0.8

0.7

0.6

−50 −25 0 25 60 75 100 12

TJ, JUNCTION TEMPERATURE (°C)

Vin = 12 V

= 5 V

out

Measured at GND Pin TJ = 25°C

= 500 mA

Load

, QUIESCENT CURRENT (mA)

Load

= 100 mA

0 5 10 15 20 25 30 35 40

Vin, INPUT VOLTAGE (V)

Figure 7. Quiescent Current Figure 8. Standby Quiescent Current

160

140

μA)

120

100

, STANDBY QUIESCENT CURRENT (

stby

−50 −25 0 25 60 75 100 12

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ON/OFF

= 5.0 V

Vin = 40 V

Vin = 12 V

TJ, JUNCTION TEMPERATURE (°C)

LM2594

(

)

(

)

TYPICAL PERFORMANCE CHARACTERISTICS (Circuit of Figure 16)

1.3

1.2

V E

1.1

1.0

LTA

−40°C

0.9

N V

0.8 25°C

0.7

ATURATI ,

sat

125°C

0.6

0.5

0.4

0.3

0 0.1 0.2 0.3 0.4 0.5

SWITCH CURRENT (A)

1.0

0.0

−1.0

−2.0

−3.0

−4.0

−5.0

−6.0

NORMALIZED FREQUENCY (%)

−7.0

−8.0

−9.0

−50 −25 0 25 50 75 100 125

TJ, JUNCTION TEMPERATURE (°C)

Figure 9. Switch Saturation Voltage Figure 10. Switching Frequency

5.0

4.5

4.0

3.5

3.0

LTA

2.5

2.0

, INPUT V

1.5

1.0

0.5

-50

Figure 11. Minimum Supply Operating Voltage Figure 12. Feedback Pin Current

' 1.23 V

out

= 100 mA

Load

TJ, JUNCTION TEMPERATURE (°C)

1251007550250-25

, FEEDBACK PIN CURRENT (nA)

100

-20

-40

-60

-80

-100

1251007550250-25-50

TJ, JUNCTION TEMPERATURE (°C)

EFFICIENCY (%)

0455403530252010 15

12 V, 500 mA

5 V, 500 mA

3.3 V, 500 mA

VIN, INPUT VOLTAGE (V)

Figure 13. Efficiency

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LM2594

TYPICAL PERFORMANCE CHARACTERISTICS (Circuit of Figure 16)

10 V

0.8 A

0.4 A

0.8 A

0.4 A

Figure 14. Switching Waveforms Figure 15. Load Transient Response

= 5 V

out

A: Output Pin Voltage, 10 V/div B: Switch Current, 0.4 A/div C: Inductor Current, 0.4 A/div, AC−Coupled D: Output Ripple Voltage, 50 mV/div, AC−Coupled

Horizontal Time Base: 2.0 ms/div

Output

Voltage

Change

- 100 mV

Load

Current

100 mV

0.5 A

0.1 A

100 ms/div2 ms/div

8.5 V - 40 V Unregulated

DC Input

100 mF

Adjustable Output Voltage Versions

Feedback

LM2594

Output

56ON/OFFGND

+ V

ref

= 1.23 V, R1

ref

out

1.0 )

1.0Ǔ

ref

out

R2 + R1ǒ

Where V between 1.0 k and 5.0 k

Figure 16. Typical Test Circuit

100 mH

D1 1N5822

out

220 mF

out

5.0 V/0.5 A

Load

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LM2594

PCB LAYOUT GUIDELINES

As in any switching regulator, the layout of the printed circuit board is very important. Rapidly switching currents associated with wiring inductance, stray capacitance and parasitic inductance of the printed circuit board traces can generate voltage transients which can generate electromagnetic interferences (EMI) and affect the desired operation. As indicated in the Figure 16, to minimize inductance and ground loops, the length of the leads indicated by heavy lines should be kept as short as possible.

For best results, single−point grounding (as indicated) or ground plane construction should be used.

DESIGN PROCEDURE

Buck Converter Basics

The LM2594 is a “Buck” or Step−Down Converter which is the most elementary forward−mode converter. Its basic schematic can be seen in Figure 17.

The operation of this regulator topology has two distinct time periods. The first one occurs when the series switch is on, the input voltage is connected to the input of the inductor.

The output of the inductor is the output voltage, and the rectifier (or catch diode) is reverse biased. During this period, since there is a constant voltage source connected across the inductor, the inductor current begins to linearly ramp upwards, as described by the following equation:

L(on)

VIN* V

OUT

During this “on” period, energy is stored within the core material in the form of magnetic flux. If the inductor is properly designed, there is sufficient energy stored to carry the requirements of the load during the “off” period.

Power Switch

On the other hand, the PCB area connected to the Pin 2 (emitter of the internal switch) of the LM2594 should be kept to a minimum in order to minimize coupling to sensitive circuitry.

Another sensitive part of the circuit is the feedback. It is important to keep the sensitive feedback wiring short. To assure this, physically locate the programming resistors near to the regulator, when using the adjustable version of the LM2594 regulator.

This period ends when the power switch is once again turned on. Regulation of the converter is accomplished by varying the duty cycle of the power switch. It is possible to describe the duty cycle as follows:

d +

, where T is the period of switching.

For the buck converter with ideal components, the duty cycle can also be described as:

out

d +

Figure 18 shows the buck converter, idealized waveforms of the catch diode voltage and the inductor current.

on(SW)

Power Switch

Off

Diode VoltageInductor Current

VD(FWD)

Power

Switch

Power Switch

Off

Power

Switch

Figure 17. Basic Buck Converter

out

Load

The next period is the “off” period of the power switch. When the power switch turns off, the voltage across the inductor reverses its polarity and is clamped at one diode voltage drop below ground by the catch diode. The current now flows through the catch diode thus maintaining the load current loop. This removes the stored energy from the inductor. The inductor current during this time is:

L(off)

OUT

* V

off

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min

Diode Diode

Figure 18. Buck Converter Idealized Waveforms

Power Switch

Power

Switch

Load

Time

(AV)

Time

LM2594

PROCEDURE (ADJUSTABLE OUTPUT VERSION: LM2594)

Procedure Example

Given Parameters:

= Regulated Output Voltage

out

= Maximum DC Input Voltage

in(max)

Load(max)

1. Programming Output Voltage

To select the right programming resistor R1 and R2 value (see Figure 1) use the following formula:

Resistor R1 can be between 1.0 k and 5.0 kW. (For best temperature coefficient and stability with time, use 1% metal film resistors).

= Maximum Load Current

+ V

ref

1.0 )

out

R2 + R1

R2 R1

out

where V

* 1.0

ref

= 1.23 V

ref

Given Parameters:

= 5.0 V

out

= 12 V

in(max)

Load(max)

1. Programming Output Voltage (selecting R1 and R2) Select R1 and R2:

= 0.5 A

+ 1.23ǒ1.0 )

out

R2 + R1

R2 = 3.0 kW, choose a 3.0k metal film resistor.

ref

* 1.0Ǔ+

Select R1 = 1.0 kW

1.23 V

* 1.0

2. Input Capacitor Selection (Cin)

To prevent large voltage transients from appearing at the input and for stable operation of the converter, an aluminium or tantalum electrolytic bypass capacitor is needed between the input pin +V located close to the IC using short leads. This capacitor should have a low ESR (Equivalent Series Resistance) value.

For additional information see input capacitor section in the “Application Information” section of this data sheet.

3. Catch Diode Selection (D1)

A. Since the diode maximum peak current exceeds the

regulator maximum load current the catch diode current rating must be at least 1.2 times greater than the maximum load current. For a robust design, the diode should have a current rating equal to the maximum current limit of the LM2594 to be able to withstand a continuous output short.

B. The reverse voltage rating of the diode should be at least

1.25 times the maximum input voltage.

and ground pin GND This capacitor should be

2. Input Capacitor Selection (Cin)

A 68 mF, 50 V aluminium electrolytic capacitor located near

the input and ground pin provides sufficient bypassing.

3. Catch Diode Selection (D1) A. For this example, a 1.0 A current rating is adequate.

B. For Vin = 12 V use a 20 V 1N5817 Schottky diode or

any suggested fast recovery diode in the Table 2.

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