ST LM350 User Manual

Three-terminal 3 A adjustable voltage regulators

Features

■ Guaranteed 3 A output current

■ Adjustable output down to 1.2 V

■ Line regulation typically 0.005 %/V

■ Load regulation typically 0.1 %

■ Guaranteed thermal regulation

■ Current limit constant with temperature

■ Standard 3-lead transistor package

LM350

TO-3

Table 1. Device summary

Order codes

TO-3 Temperature range

LM350K 0 to 125 °C

February 2008 Rev 2 1/14

www.st.com

14

Contents LM350

Contents

1 Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2 Pin configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3 Maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

4 Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

5 Typical performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

6 Application hints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

6.1 External capacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

6.2 Load regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

6.3 Protection diodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

7 Application circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

8 Package mechanical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

9 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

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LM350 Diagram

1 Diagram

Figure 1. Schematic diagram

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Pin configuration LM350

2 Pin configuration

Figure 2. Pin connections (bottom view)

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LM350 Maximum ratings

3 Maximum ratings

Table 2. Absolute maximum ratings

Symbol Parameter Value Unit

P

- VOInput-output voltage differential 35 V

V

I

T

STG

T

LEAD

T

OP

Power dissipation Internally limited

D

Storage temperature range -65 to 150 °C

lead temperature (Soldering, 10 seconds) 300 °C

Operating junction temperature range 0 to 125 °C

Note: Absolute maximum ratings are those values beyond which damage to the device may occur.

Functional operation under these condition is not implied

Table 3. Thermal data

Symbol Parameter Value Unit

R

R

thJC

thJA

Thermal resistance junction-case 1.5 °C/W

Thermal resistance junction-ambient 35 °C/W

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Electrical characteristics LM350

4 Electrical characteristics

Table 4. Electrical characteristics (VI -VO = 5V, IO = 1.5 A. Although power dissipation is internally

limited, these specifications apply to power dissipation up to 30 W, unless otherwise
specified)

Symbol Parameter Test conditions Min. Typ. Max. Unit

K

Line regulation

VI

K

Load regulation

VO

Thermal regulation Pulse = 20 ms 0.002 0.02 %/W

I

ΔI

V

K

K

I

O(MIN)

I

O(MAX)

V

R

K

1. Regulation is measured at constant junction temperature. Changes in output voltage due to heating effects are taken into

Adjustment pin current 50 100 µA

ADJ

Adjustment pin current

ADJ

change

Reference voltage

REF

Line regulation

K

VI

Load regulation

VO

Temperature stability TJ = T

VT

Minimum load current VI - VO ≤ 35 V 3.5 10 mA

Current limit VI - VO ≤ 10 V

RMS output noise

NO

(% of V

O

Ripple rejection ratio

VF

Long term stability Ta = 125°C 0.3 1 %

VH

account separately by thermal rejection.

(1)

(1)

(1)

(1)

Ta = 25°C, VI - VO = 3 to 35 V 0.005 0.03 %/V

≤ 5 V 5 25 mV

V

Ta = 25°C

= 10 mA to 3 A

I

O

= 10 mA to 3 A, VI - VO = 3 to 35 V 0.2 5 µA

I

L

- VO = 3 to 35 V, IO = 10 mA to 3 A

V

I

P ≤ 30 W

O

V

≥ 5 V 0.1 0.5 %

O

1.19 1.24 1.29 V

VI - VO = 3 to 35 V 0.02 0.05 %/V

V

≤ 5 V 20 70 mV

IO = 10 mA to 3 A

to T

MIN

MAX

O

V

≥ 5 V 0.3 1.5 %

O

1%

DC 3 4.5

A

- VO = 30 V 1

V

I

= 25°C, f = 10 Hz to 10 kHz 0.001 %

T

)

a

V

= 10 V, f = 120 Hz 65

O

= 10 µF 66 86

C

ADJ

dB

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LM350 Typical performance

5 Typical performance

Figure 3. 1.2 V to 25 V adjustable regulator

Δ Needed if device is far from filter capacitors.

* Optional-improves transient response. Output capacitors in the range of 1 µF to 100 µF of aluminium or

tantalum electrolytic are commonly used to provide improved output impedance and rejection of transients

= 1.25 V (1 + R2/R1)

** V

O

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Application hints LM350

6 Application hints

In operation, the LM350 develops a nominal 1.25 V reference voltage, V
output and adjustment terminal. The reference voltage is impressed across program resistor
R

and, since the voltage is constant, a constant current I1 then flows through the output set

1

resistor R

V

= V

O

Figure 4. Circuit

, giving an output voltage of:

2

(1+ R2 / R1) + I

(REF)

ADJ

x R

, between the

(REF)

.

2

Since the 50 µA current from the adjustment terminal represents an error term, the LM350
was designed to minimize I

ADJ

this, all quiescent operating current is returned to the output establishing a minimum load
current requirement. If there is insufficient load on the output, the output will rise.

6.1 External capacitors

An input bypass capacitor is recommended. A 0.1 µF disc or 1 µF solid tantalum on the input
is suitable input by passing for almost all applications. The device is more sensitive to the
absence of input bypassing when adjustment or output capacitors are used by the above
values will eliminate the possibility of problems.

The adjustment terminal can be bypassed to ground on the LM350 to improve ripple
rejection. This bypass capacitor prevents ripple form being amplified as the output voltage is
increased. With a 10 µF bypass capacitor 75 dB ripple rejection is obtainable at any output
level. Increases over 20 µF do not appreciably improve the ripple rejection at frequencies
above 120 Hz. If the bypass capacitor is used, it is sometimes necessary to include
protection diodes to prevent the capacitor from discharging through internal low current
paths and damaging the device.

In general, the best type of capacitors to use are solid tantalum. Solid tantalum capacitors
have low impedance even at high frequencies. Depending upon capacitor construction, it
takes about 25 µF in aluminium electrolytic to equal 1 µF solid tantalum at high frequencies.
Ceramic capacitors are also good at high frequencies, but some types have a large

and make it very constant with line and load changes. To do

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LM350 Application hints

decrease in capacitance at frequencies around 0.5 MHz. For this reason, 0.01 µF disc may
seem to work better than a 0.1 µF disc as a bypass.

Although the LM350 is stable with no output capacitors, like any feedback circuit, certain
values of external capacitance can cause excessive ringing. This occurs with values
between 500 pF and 5000 pF. A 1 µF solid tantalum (or 25 µF aluminium electrolytic) on the
output swamps this effect and insures stability.

6.2 Load regulation

The LM350 is capable of providing extremely good load regulation but a few precautions are
needed to obtain maximum performance. The current set resistor connected between the
adjustment terminal and the output terminal (usually 240 Ω) should be tied directly to the
output of the regulator rather than near the load. This eliminates line drops from appearing
effectively in series with the reference and degrading regulation. For example, a 15 V
regulator with 0.05 Ω resistance between the regulator and load will have a load regulation
due to line resistance of 0.05 Ω x I
effective line resistance will be 0.05 Ω (1 + R

Figure 5 shows the effect of resistance between the regulator and 140 Ω set resistor. With

the TO-3 package, it is easy to minimize the resistance from the case to the set resistor, by
using 2 separate leads to the case. The ground of R
load to provide remote ground sensing and improve load regulation.

. If the set resistor is connected near the load the

L

) or in this case, 11.5 times worse.

2/R1

can be returned near the ground of the

2

6.3 Protection diodes

When external capacitors are used with any IC regulator it is sometimes necessary to add
protection diodes to prevent the capacitors from discharging through low current points into
the regulator. Most 20 µF capacitors have low enough internal series resistance to deliver
20 A spikes when shorted. Although the surge is short, there is enough energy to damage
parts of the IC.

When an output capacitor is connected to a regulator and the input is shorted, the output
capacitor will discharge into the output of the regulator. The discharge current depends on
the value of the capacitor, the output voltage of the regulator, and the rate of decrease of V
In the LM350 this discharge path is through a large junction that is able to sustain 25 A
surge with no problem. This is not true of other types of positive regulators. For output
capacitors of 100 µF or less at output of 15 V or less, there is no need to use diodes.

The bypass capacitor on the adjustment terminal can discharge through a low current
junction. Discharge occurs when either the input or output is shorted. Internal to the LM350
is a 50 Ω resistor which limits the peak discharge current. No protection is needed for output
voltages of 25 V or less and 10 µF capacitance. Figure 6 shows an LM350 with protection
diodes included for use with outputs greater than 25 V and high values of output
capacitance.

.

I

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Application circuits LM350

7 Application circuits

Figure 5. Regulator with line resistance in output lead

Figure 6. Regulator with protection diodes

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LM350 Package mechanical data

8 Package mechanical data

In order to meet environmental requirements, ST offers these devices in ECOPACK®
packages. These packages have a lead-free second level interconnect. The category of
second Level Interconnect is marked on the package and on the inner box label, in
compliance with JEDEC Standard JESD97. The maximum ratings related to soldering
conditions are also marked on the inner box label. ECOPACK is an ST trademark.
ECOPACK specifications are available at: www.st.com.

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Package mechanical data LM350

TO-3 mechanical data

Dim.

A 11.85 0.466

B0.96 1.05 1.10 0.037 0.041 0.043

C 1.70 0.066

D 8.7 0.342

E 20.0 0.787

G 10.9 0.429

N 16.9 0.665

P 26.2 1.031

R 3.88 4.09 0.152 0.161

U 39.5 1.555

V 30.10 1.185

Min. Typ. Max. Min. Typ. Max.

mm. inch.

P

A

G

U

V

N

O

B

D

C

E

R

P003C/C

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LM350 Revision history

9 Revision history

Table 5. Document revision history

Date Revision Changes

29-Sep-2006 1

11-Feb-2008 2 Added: Table 1 on page 1.

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LM350

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