LMP8646MKE [TI]

Precision Current Limiter; 精密电流限制器


型号：	LMP8646MKE
厂家：	TEXAS INSTRUMENTS
描述：	Precision Current Limiter 精密电流限制器限制器
文件：	总23页 (文件大小：492K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

February 1, 2012

LMP8646

Precision Current Limiter

General Description

The LMP8646 is a precision current limiter used to improve

the current limit accuracy of any switching or linear regulator

with an available feedback node.

The LMP8646 accepts input signals with a common mode

voltage ranging from -2V to 76V. It has a variable gain which

is used to adjust the sense current. The gain is configured

with a single external resistor, R_G, providing a high level of

flexibility and accuracy up to 2%. The adjustable bandwidth,

which allows the device to be used with a variety of applica-

tions, is configurable with a single external capacitor in par-

allel with R_G. In addition, the output is buffered in order to

provide a low output impedance.

Features

Provides circuit protection and current limiting

■

Single supply operation

-2V to +76V common mode voltage range

Variable gain set by external resistor

Adjustable bandwidth set by external capacitor

Buffered output

3% output accuracy achievable at V_SENSE= 100 mV

Key Specifications

Supply voltage range

Output current (source)

Gain accuracy

Transconductance

Offset

Quiescent current

Input bias

PSRR

2.7V to 12V

0 to 5 mA

2.0% (max)

200 µA/V

±1 mV (max)

380 µA

■

The LMP8646 is an ideal choice for industrial, automotive,

telecommunications, and consumer applications where cir-

cuit protection and improved precision systems are required.

The LMP8646 is available in a 6-pin TSOT package and can

operate at temperature range of −40°C to 125°C.

■

12 µA (typ)

85 dB

■

Applications

■

CMRR

Temperature range

6-Pin TSOT Package

95 dB

■

High-side and low-side current limit

■

−40°C to 125°C

■

Circuit fault protection

■

Battery and supercap charging

LED constant current drive

Power management

■

Typical Application

30123534

LMP™ is a trademark of National Semiconductor Corporation.

301235 SNOSC63

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Ordering Information

Package

Part Number

LMP8646MK

LMP8646MKX

LMP8646MKE

Package Marking

Transport Media

1k Units Tape and Reel

3k Units Tape and Reel

250 Units Tape and Reel

NSC Drawing

6-Pin TSOT

AK7A

MK06A

Connection Diagram

6-Pin TSOT

30123502

Top View

Pin Descriptions

Name

V_OUT

V^-

Description

Single-Ended Output Voltage

Negative Supply Voltage. This pin should be connected to ground.

+IN

-IN

Positive Input

Negative Input

R_G

External Gain Resistor. An external capacitance (C_G) may be added in parallel

with R_Gto limit the bandwidth.

V⁺

Positive Supply Voltage

Block Diagram

30123530

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Voltage at R_Gpin

13.2V

V^-to V⁺

Absolute Maximum Ratings (Note 1)

Voltage at OUT pin

If Military/Aerospace specified devices are required,

please contact the Texas Instruments Sales Office/

Distributors for availability and specifications.

Storage Temperature Range

Junction Temperature (Note 3)

For soldering specifications,

-65°C to 150°C

150°C

ESD Tolerance (Note 2)

Human Body Model

see product folder at www.national.com and

www.national.com/ms/MS/MS-SOLDERING.pdf

For input pins: +IN and -IN

For all other pins

Machine Model

±4000V

±2000V

200V

Operating Ratings (Note 1)

Supply Voltage (V_S= V⁺- V⁻)

Temperature Range (Note 3)

Package Thermal Resistance(Note 3)

TSOT-6

Charge device model

1250

2.7V to 12V

-40°C to 125°C

Supply Voltage (V_S= V⁺- V⁻)

Differential voltage +IN- (-IN)

Voltage at pins +IN, -IN

13.2V

96°C/W

-6V to 80V

2.7V Electrical Characteristics (Note 4)

Unless otherwise specified, all limits guaranteed for at T_A= 25°C, V_S=(V⁺– V^-) = (2.7V - 0V) = 2.7 V, −2V < V_CM< 76V, R_G=

25kΩ, R_L= 10 kΩ. Boldface limits apply at the temperature extremes.

Min

Typ

Max

Symbol

V_OFFSET

Parameter

Condition

Units

(Note 6) (Note 5) (Note 6)

Input Offset Voltage

V_CM= 2.1V

-1

-1.7

1.7

TCV_OS

Input Offset Voltage Drift (Note 7,

Note 9)

V_CM= 2.1V

μV/°C

μA

I_B

Input Bias Current (Note 10)

Input Voltage Noise (Note 9)

e_ni

120

f > 10 kHz, R_G= 5 kΩ

V_CM= 12V, R_G= 5 kΩ

nV/

V/V

µA/V

V_SENSE

Gain A_V

Max Input Sense Voltage (Note 9)

600

100

Adjustable Gain Setting (Note 9) V_CM= 12V

Transconductance = 1/R_IN

V_CM= 2.1V

200

-2

-3.4

3.4

V_CM= 2.1V

Accuracy

−40°C to 125°C, V_CM=2.1V

V_CM= 2.1V, 2.7V < V⁺< 12V,

2.1V < V_CM< 76V

ppm /°C

Gm drift (Note 9)

140

PSRR

CMRR

Power Supply Rejection Ratio

Common Mode Rejection Ratio

-2V <V_CM< 2.1V,

I_S

Slew Rate (Note 8, Note 9)

V_CM= 5V, C_G= 4 pF, V_SENSEfrom 25 mV

to 175 mV, C_L= 30 pF, R_L= 1MΩ

V_CM= 2.1V

0.5

V/µs

Supply Current

380

610

807

V_CM= −2V

2000

2500

2700

V_OUT

Maximum Output Voltage

Minimum Output Voltage

Maximum Output Voltage

Minimum Output Voltage

Output current (Note 9)

1.1

1.6

V_CM= 2.1V, R_G= 500 kΩ

V_CM= 2.1V

VS = V_CM= 3.3V, R_G= 500 kΩ

Sourcing, V_OUT= 600mV, R_G= 150kΩ

I_OUT

C_LOAD

Max Output Capacitance Load

(Note 9)

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5V Electrical Characteristics (Note 4)

Unless otherwise specified, all limits guaranteed for at T_A= 25°C, V_S=V⁺-V^-, V⁺= 5V, V⁻= 0V, −2V < V_CM< 76V, R_g= 25kΩ, R_L=

10 kΩ. Boldface limits apply at the temperature extremes.

Min

Typ

Max

Symbol

V_OFFSET

Parameter

Condition

Units

(Note 6) (Note 5) (Note 6)

Input Offset Voltage

V_CM= 2.1V

-1

-1.7

1.7

TCV_OS

Input Offset Voltage Drift (Note 7, V_CM= 2.1V

Note 9)

μV/°C

μA

I_B

Input Bias Current (Note 10)

Input Voltage Noise (Note 9)

V_CM= 2.1V

12.5

120

e_ni

f > 10 kHz, R_G= 5 kΩ

V_CM= 12V, R_G= 5 kΩ

nV/

V_SENSE(MAX)Max Input Sense Voltage (Note 9)

600

Gain A_V

Adjustable Gain Setting (Note 9) V_CM= 12V

100

V/V

µA/V

Transconductance = 1/R_IN

Accuracy

V_CM= 2.1V

200

-2

-3.4

3.4

Gm drift (Note 9)

−40°C to 125°C, V_CM= 2.1V

V_CM= 2.1V, 2.7V < V⁺< 12V,

2.1V <V_CM< 76V

140

ppm /°C

PSRR

CMRR

Power Supply Rejection Ratio

Common Mode Rejection Ratio

-2V < V_CM< 2.1V

I_S

Slew Rate(Note 8, Note 9)

V_CM= 5V, C_G= 4 pF, V_SENSEfrom 100 mV

to 500 mV, C_L= 30 pF, R_L= 1MΩ

V_CM= 2.1V

0.5

V/µs

Supply Current

450

660

939

V_CM= −2V

2100

2800

3030

V_OUT

Maximum Output Voltage

Minimum Output Voltage

Output current (Note 9)

3.3

V_CM=5V, R_G= 500 kΩ

V_CM=2.1V

I_OUT

Sourcing, V_OUT= 1.65V, R_G= 150kΩ

C_LOAD

Max Output Capacitance Load

(Note 9)

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12V Electrical Characteristics (Note 4)

Unless otherwise specified, all limits guaranteed for at T_A= 25°C, V_S=V⁺-V^-, V⁺= 12V, V⁻= 0V, −2V < V_CM< 76V, R_g= 25kΩ, R_L

= 10 kΩ. Boldface limits apply at the temperature extremes.

Min

Typ

Max

Symbol

V_OFFSET

Parameter

Condition

Units

(Note 6) (Note 5) (Note 6)

Input Offset Voltage

V_CM= 2.1V

-1

-1.7

1.7

TCV_OS

Input Offset Voltage Drift (Note 7, V_CM= 2.1V

Note 9)

μV/°C

μA

I_B

Input Bias Current (Note 10)

Input Voltage Noise (Note 9)

V_CM= 2.1V

e_ni

120

f > 10 kHz, R_G= 5 kΩ

V_CM=12V, R_G= 5 kΩ

nV/

V_SENSE(MAX)Max Input Sense Voltage (Note 9)

600

Gain A_V

Adjustable Gain Setting (Note 9) V_CM= 12V

100

V/V

µA/V

Transconductance = 1/R_IN

Accuracy

V_CM= 2.1V

200

-2

-3.4

3.4

Gm drift (Note 9)

−40°C to 125°C, V_CM=2.1V

V_CM= 2.1V, 2.7V <V⁺< 12V,

2.1V <V_CM< 76V

140

ppm /°C

PSRR

CMRR

Power Supply Rejection Ratio

Common Mode Rejection Ratio

–2V <V_CM< 2.1V

I_S

Slew Rate (Note 8, Note 9)

V_CM= 5V, C_G= 4 pF, V_SENSEfrom 100 mV

to 500 mV, C_L= 30 pF, R_L=1MΩ

V_CM= 2.1V

0.6

V/µs

Supply Current

555

845

1123

V_CM= −2V

2200

2900

3110

V_OUT

Maximum Output Voltage

Minimum Output Voltage

Output current (Note 9)

V_CM= 12V, R_G= 500kΩ,

V_CM= 2.1V

I_OUT

Sourcing, V_OUT= 5.25V, R_G= 150kΩ

C_LOAD

Max Output Capacitance Load

(Note 9)

Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device

is intended to be functional, but specific performance is not guaranteed. For guaranteed specifications and the test conditions, see the Electrical Characteristics

Tables.

Note 2: Human Body Model, applicable std. MIL-STD-883, Method 3015.7. Machine Model, applicable std. JESD22-A115-A (ESD MM std. of JEDEC) Field-

Induced Charge-Device Model, applicable std. JESD22-C101-C (ESD FICDM std. of JEDEC).

Note 3: The maximum power dissipation must be derated at elevated temperatures and is dictated by T_J(MAX), θ_JA, and the ambient temperature, T_A. The maximum

allowable power dissipation P_DMAX= (T_J(MAX)- T_A)/ θ_JAor the number given in Absolute Maximum Ratings, whichever is lower.

Note 4: Electrical Table values apply only for factory testing conditions at the temperature indicated. Factory testing conditions result in very limited self-heating

of the device such that T_J= T_A. No guarantee of parametric performance is indicated in the electrical tables under conditions of internal self-heating where T_J

T_A.

Note 5: Typical values represent the most likely parametric norm at the time of characterization. Actual typical values may vary over time and will also depend

on the application and configuration. The typical values are not tested and are not guaranteed on shipped production material.

Note 6: All limits are guaranteed by testing, design, or statistical analysis.

Note 7: Offset voltage temperature drift is determined by dividing the change in V_OSat the temperature extremes by the total temperature change.

Note 8: The number specified is the average of rising and falling slew rates and measured at 90% to 10%.

Note 9: This parameter is guaranteed by design and/or characterization and is not tested in production.

Note 10: Positive Bias Current corresponds to current flowing into the device.

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Typical Performance Characteristics Unless otherwise specified: T_A= 25°C, V_S=V⁺-V^-, V_SENSE= +IN -

(-IN), R_L= 10 kΩ.

Supply Curent vs. Supply Voltage for V_CM= 2V

Supply Current vs. V_CM

2400

2184

1968

1752

3500

3150

2800

2450

2100

1750

1400

1050

700

12V

-40°C VCM = 2V

1536

25°C

125°C

1320

1104

888

-40°C VCM = -2V

25°C

125°C

672

350

456

240

10 11 12 13

-3 -1

11 13

VS (V)

VCM (V)

30123562

30123564

AC PSRR vs. Frequency

AC CMRR vs. Frequency

30123513

30123512

CMRR vs. High V_CM

Gain vs. Frequency (BW = 1kHz)

-105

-108

-111

-114

-117

-120

-123

-126

-129

-132

-135

Vs = 5V

Vs = 12V

-3

-10

-17

-24

-31

-38

-45

Rg = 50kΩ

Rg = 25kΩ

Rg = 10kΩ

100

10k

100k

40 44 48 52 56 60 64 68 72 76

VCM (V)

FREQUENCY (Hz)

30123536

30123596

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Gain vs. Frequency (BW = 35kHz)

Gain Accuracy vs. V_CM

0.240

0.192

0.144

0.096

0.048

0.000

-0.048

-0.096

-0.144

-0.192

-0.240

Vs = 2.7V

Vs = 3.3V

-8

-18

Rg = 50kΩ

Rg = 25kΩ

Rg = 10kΩ

-28

100

10k

100k

-2

14 22 30 38 46 54 62 70 78

VCM (V)

FREQUENCY (Hz)

30123537

30123578

Gain Accuracy vs. V_CM

V_OUTvs. V_SENSE

0.240

4.0

3.6

3.2

2.8

2.4

2.0

1.6

1.2

0.8

0.4

0.0

RG = 10kΩ

RG = 25kΩ

RG = 50kΩ

0.192

0.144

0.096

0.048

0.000

-0.048

-0.096

-0.144

-0.192

-0.240

Vs = 5V

Vs = 12V

-2

18 28 38 48 58 68 78

VCM (V)

0.1

0.2

0.3

0.4

0.5

0.6

VSENSE (V)

30123579

30123561

V_{OUT_MAX}vs. Gain at Vs = 2.7V

V_{OUT_MAX}vs. Gain at Vs = 5.0V

4.0

3.6

3.2

2.8

2.4

2.0

1.6

1.2

0.8

0.4

0.0

Vcm = 0V

1.3

Vcm = 0V

Vcm = 5V

Vcm = 12V

Vcm = 5V, 12V

1.2

1.1

1.0

0.9

0.8

0.7

0.6

0.5

0.4

10 12 14

GAIN

30123573

30123574

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V_{OUT_MAX}vs. Gain at Vs = 12V

V_{OUT_MAX}vs. V_Sat V_CM= -2V

1.80

1.74

1.68

1.62

1.56

1.50

1.44

1.38

1.32

1.26

1.20

VCM = 0V

VCM = 5V

VCM = 12V

10 12 14

GAIN

VS (V)

30123575

30123576

V_{OUT_MAX}vs. V_Sat V_CM= 2.1V

Large Step Response at BW = 1kHz

2.2

2.1

2.0

1.9

1.8

1.7

1.6

1.5

1.4

1.3

1.2

VSENSE

Rg = 50kΩ

Rg = 25kΩ

Rg = 10kΩ

10 12 14

TIME (0.5 ms/DIV)

VS (V)

30123543

30123577

Large Step Response at BW = 35 kHz

Small Step Response at BW = 1 kHz

VSENSE

Rg = 50kΩ

Rg = 25kΩ

Rg = 10kΩ

VSENSE

Rg = 50kΩ

Rg = 25kΩ

Rg = 10kΩ

TIME (20 μs/DIV)

TIME (500 μs/DIV)

30123544

30123545

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Small Step Response at BW = 35 kHz

Settling Time (Rise) at 1kHz

VSENSE

Rg = 50kΩ

Rg = 25kΩ

Rg = 10kΩ

Rg = 50kΩ

Rg = 25kΩ

Rg = 10kΩ

TIME (20 μs/DIV)

TIME (100 μs/DIV)

30123546

30123548

30123550

30123547

Settling Time (Fall) at 1kHz

Settling Time (Rise) at 35kHz

VSENSE

Rg = 50kΩ

Rg = 25kΩ

Rg = 10kΩ

Rg = 50kΩ

Rg = 25kΩ

Rg = 10kΩ

TIME (100 μs/DIV)

TIME (5 μs/DIV)

30123549

Settling Time (Fall) at 35kHz

Common Mode Step Response (Rise) at 35 kHz

VOUT

VCM

VSENSE

Rg = 50kΩ

Rg = 25kΩ

Rg = 10kΩ

TIME (5 μs/DIV)

TIME (0.2 ms/DIV)

30123551

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Common Mode Step Response (Fall) at 35 kHz

VOUT

VCM

TIME (0.2 ms/DIV)

30123552

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FUNCTIONAL DESCRIPTION

MAXIMUM OUTPUT VOLTAGE, V_{OUT_MAX}

GENERAL

The maximum output voltage, V_{OUT_MAX}, depends on the sup-

ply voltage, V_S= V⁺- V^-, and on the common mode voltage,

V_CM= (+IN + -IN) / 2.

The LMP8646 is a single supply precision current limiter with

variable gain selected through an external resistor (R_G) and

a variable bandwidth selected through an external capacitor

(C_G) in parallel with R_G. Its common-mode of operation is -2V

to +76V, and the LMP8646 has an buffered output to provide

a low output impedance. More details of the LMP8646's func-

tional description can be seen in the following subsections.

The following subsections show three cases to calculate for

V_{OUT_MAX}

Case 1: −2V < V_CM< 1.8V, and V_S> 2.7V

If V_S≥ 5 V,

then V_{OUT_MAX}= 1.3V.

Else if Vs = 2.7V,

THEORY OF OPERATION

As seen from Figure 1, the sense current flowing through

R_SENSEdevelops a voltage drop equal to V_SENSE. The high

impedance inputs of the amplifier does not conduct this cur-

rent and the high open loop gain of the sense amplifier forces

its non-inverting input to the same voltage as the inverting

input. In this way the voltage drop across R_INmatches

V_SENSE. The current I_INflowing through R_INhas the following

equation:

then V_{OUT_MAX}= 1.1V.

Case 2: 1.8V < V_CM< V_S, and V_S> 3.3V

In this case, V_Xis a fixed value that depends on the supply

voltage. V_Xhas the following values:

If V_S= 12V, then V_X= 10V.

Else if V_S= 5V, then V_X= 3.3V .

Else if V_S= 2.7V, then V_X= 1.1V.

ꢀꢀ

I_IN= V_SENSE/ R_IN= R_SENSE*I_SENSE/R_IN

where R_IN= 1/Gm = 1/(200 µA/V) = 5 kOhm

If V_X≤ (V_CM- V_SENSE- 0.25) ,

then V_{OUT_MAX}= V_X.

I_INflows entirely across the external gain resistor R_Gto de-

velop a voltage drop equal to:

Else,

V_RG= I_IN*R_G= (V_SENSE/R_IN) *R_G= [(R_SENSE*I_SENSE) / R_IN]*R_G

V_{OUT_MAX}= (V_CM- V_SENSE- 0.25).

For example, if V_CM= 4V, V_S= 5V (and thus V_X= 3.3V),

V_SENSE= 0.1 V, then V_{OUT_MAX}= 3.3V because 3.3V ≤ (4 -

0.1 - 0.25).

This voltage is buffered and showed at the output with a very

low impedance allowing a very easy interface of the LMP8646

with the feedback of many voltage regulators. This output

voltage has the following equation:

Case 3: V_CM> V_S, and V_S> 2.7V

If V_S= 12V, then V_{OUT_MAX}= 10V.

Else if V_S= 5V, then V_{OUT_MAX}= 3.3V .

Else if V_S= 2.7V, then V_{OUT_MAX}= 1.1V.

V_OUT= V_RG= [(R_SENSE*I_SENSE) / R_IN]*R_G

V_OUT= V_SENSE* R_G/R_IN

V_OUT= V_SENSE* R_G/(5 kOhm)

V_OUT= V_SENSE* Gain, where Gain = R_G/R_IN

30123503

FIGURE 1. Current monitor

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APPLICATIONS INFORMATION

OUTPUT ACCURACY

SELECTION OF THE SENSE RESISTOR, R_SENSE

The accuracy of the current measurement also depends on

the value of the shunt resistor R_SENSE. Its value depends on

the application and is a compromise between small-signal

accuracy and maximum permissible voltage loss in the load

line.

The output accuracy is the device error contributed by the

LMP8646 based on its offset and gain errors. The LMP8646

output accuracy has the following equations:

R_SENSEis directly proportional to V_SENSEthrough the equation

R_SENSE= (V_SENSE) / (I_SENSE). If V_SENSEis small, then there is

a smaller voltage loss in the load line, but the output accuracy

is worse because the LMP8646 offset error will contribute

more. Therefore, high values of R_SENSEprovide better output

accuracy by minimizing the effects of offset, while low values

of R_SENSEminimize the voltage loss in the load line. For most

applications, best performance is obtained with an R_SENSE

value that provides a V_SENSEof 100 mV to 200 mV.

30123538

R_SENSEConsideration for System Error

The output accuracy described in the previous section talks

about the error contributed just by the LMP8646. The system

error, however, consists of the errors contributed by the

LMP8646 as well as other external resistors such as R_SENSE

and R_G. Let's rewrite the output accuracy equation for the

system error assuming that R_SENSEis non-ideal and R_Gis

ideal. This equation can be seen as:

FIGURE 2. Output Accuracy Equations

For example, assume V_SENSE= 100 mV, R_G= 10 kOhm, and

it is known that V_OFFSET= 1 mV and Gm_Accuracy = 2%

(Electrical Characteristics Table), then the output accuracy

can be calculated as:

30123554

30123539

FIGURE 5. System Error Equation Assuming R_SENSEis

Non-ideal and R_Gis Ideal

FIGURE 3. Output Accuracy Example

In fact, as V_SENSEdecreases, the output accuracy worsens as

seen in Figure 4. These equations provide a valuable tool to

estimate how the LMP8646 affects the overall system perfor-

mance. Knowing this information allows the system designer

to pick the appropriate external resistances (R_SENSEand R_G)

to adjust for the tolerable system error. Examples of this tol-

erable system error can be seen in the next sections.

Continuing from the previous output accuracy example, we

can calculate for the system error assuming that R_SENSE= 100

mOhm (with 1% tolerance), I_SENSE= 1A, and R_G= 10 kOhm.

From the Electrical Characteristics Table, it is also known that

V_OFFSET= 1 mV and Gm_Accuracy = 2%.

10.0

9.2

8.4

7.6

6.8

6.0

5.2

4.4

3.6

2.8

30123555

FIGURE 6. System Error Example Assuming R_SENSEis

Non-ideal and R_Gis Ideal

Because an R_SENSEtolerance will increase the system error,

we recommend selecting an R_SENSEresistor with low toler-

ance.

2.0

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

VSENSE (V)

30123570

FIGURE 4. Output Accuracy vs. V_SENSE

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SELECTION OF THE GAIN RESISTOR, R_G

Case 3: V_CM> V_S, and V_S> 3.3V

For the LMP8646, the gain is selected through an external

resistor connected to the R_Gpin. The voltage at this R_Gpin is

500

equal to V_OUT, which has the equation V_OUT= V_RG= V_SENSE

R_G/(5 kOhm).

VS = 3.3V

VS = 5.0V

VS = 12.0V

400

300

200

100

In fact, R_Gmust be chosen such that the V_OUTdoes not ex-

ceed its maximum ratings (V_{OUT_MAX}) as described in the

MAXIMUM OUTPUT VOLTAGE, V_{OUT_MAX}section. Using

this V_{OUT_MAX}and the equation R_{G_MAX}= (V_{OUT_MAX}

5kOhm) / (V_SENSE), a plot of R_{G_MAX}vs. V_SENSEcan be seen

for three cases below. Use these plots to help select the ap-

propriate R_Gvalue so that V_SENSEand V_OUTstay within the

recommended operating ratings. Since these plots are for

R_{G_MAX}, all of the combinations of R_Gbelow the curve are

allowed.

Case 1: −2V < V_CM< 1.8V, and V_S> 3.3V

0.0

0.1

0.2

0.3

0.4

0.5

VSENSE (V)

500

30123560

VS = 3.3V

FIGURE 9. Allowed R_Gfor CASE 3

VS = 5.0V or 12.0V

400

300

200

100

R_GConsideration for System Error

The previous section discussed the system error assuming

that R_SENSEis non-ideal and R_Gis ideal. This section expands

the system error equation by assuming that both R_SENSEand

R_Gare non-ideal. This system error equation can be rewritten

as:

0.0

0.1

0.2

0.3

0.4

0.5

VSENSE (V)

30123558

FIGURE 7. Allowed R_Gfor CASE 1

30123556

Case 2: 1.8V < V_CM< V_S, and V_S> 3.3V

FIGURE 10. System Error Equation Assuming R_SENSEand

R_Gare Non-ideal

500

Continuing from the previous system error equation, we can

recalculate for the system error assuming that R_Ghas a 1%

tolerance.

VS = 3.3V @ VCM = 2V

VS = 5.0V @ VCM = 2.5V

VS = 12V @ VCM = 6V

400

300

200

100

30123557

FIGURE 11. System Error Example Assuming R_SENSEand

R_Gare Non-ideal

0.0

0.1

0.2

0.3

0.4

0.5

VSENSE (V)

30123559

Because an R_Gtolerance will increase the system error, we

recommend selecting an R_Gresistor with low tolerance.

FIGURE 8. Allowed R_Gfor CASE 2

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APPLICATION #1: CURRENT LIMITER WITH A CAPACITIVE LOAD

30123531

FIGURE 12. SuperCap Application with LM3102

Regulator

Step 3: Choose the gain resistor, R_G, for LMP8646

R_Gis chosen from the limited sense current. As stated,

V_OUT(R_SENSEI_LIMIT(R_G5kOhm). Since

V_OUT= V_FB= 0.8V, the limited sense current is 1.5A, and

R_SENSEis 55 mOhm, R_Gcan be calculated as:

A supercap application requires a very high capacitive load to

)

be charged. This example assumes the output capacitor is 5F

with a limited sense current at 1.5A. The LM3102 will provide

the current to charge the supercap, and the LMP8646 will

monitor this current to make sure it does not exceed the de-

sired 1.5A value.

R_G= (V_OUT* 5 kOhm) / (R_SENSE* I_LIMIT

)

R_G= (0.8 * 5 kOhm) / (55 mOhm* 1.5A) = 50 kOhm

(approximate)

This is done by connecting the LMP8646 output to the feed-

back pin of the LM3102, as shown in Figure 12. This feedback

voltage at the FB pin is compared to a 0.8V internal reference.

Any voltage above this 0.8V means the output current is

above the desired value of 1.5A, and the LM3102 will reduce

its output current to maintain the desired 0.8V at the FB pin.

Step 4: Choose the Bandwidth Capacitance, C_G.

The product of C_Gand R_Gdetermines the bandwidth for the

LMP8646. Refer to the Typical Performance Characteristics

plots to see the range for the LMP8646 bandwidth and gain.

Since each application is very unique, the LMP8646 band-

width capacitance, C_G, needs to be adjusted to fit the appro-

priate application.

The following steps show the design procedures for this su-

percap application. In summary, the steps consist of selecting

the components for the voltage regulator, integrating the

LMP8646 and selecting the proper values for its gain, band-

width, and output resistor, and adjusting these components

to yield the desired performance.

Bench data has been collected for the supercap application

with the LM3102 regulator, and we found that this application

works best for a bandwidth of 500 Hz to 3 kHz. Operating

outside of this recommended bandwidth range might create

an undesirable load current ringing. We recommend choosing

a bandwidth that is in the middle of this range and using the

equation C_G= 1/(2*pi*R_G*Bandwidth) to find C_G. For exam-

ple, if the bandwidth is 1.75 kHz and R_Gis 50 kOhm, then

C_Gis approximately 1.8 nF. After this selection, capture the

plot for l_LIMITand adjust C_Guntil a desired load current plot is

obtained.

Step 1: Choose the components for the Regulator.

Refer to the LM3102 evaluation board application note

(AN-1646) to select the appropriate components for the

LM3102 voltage regulator.

Step 2: Choose the sense resistor, R_SENSE

R_SENSEsets the voltage V_SENSEbetween +IN and -IN and has

the following equation:

Step 5: Calculate the Output Accuracy and Tolerable Sys-

tem Error

Since the LMP8646 is a precision current limiter, the output

current accuracy is extremely important. This accuracy is af-

fected by the system error contributed by the LMP8646 device

R_SENSE= V_OUT/ [(I_LIMIT) * (R_G/ 5kOhm)]

In general, R_SENSEdepends on the output voltage, limit cur-

rent, and gain. Refer to section SELECTION OF THE SENSE

RESISTOR, R_SENSEto choose the appropriate R_SENSEvalue;

this example uses 55 mOhm.

www.ti.com

error and other errors contributed by external resistances,

such as R_SENSEand R_G.

Next, use the formula below to calculate for ROUT:

In this application, V_SENSE= I_LIMIT* R_SENSE= 1.5A * 55 mOhm

= 0.0825V, and R_G= 50 kOhm. From the Electrical Charac-

teristics Table, it is known that V_OFFSET= 1 mV and Gm_Ac-

curacy = 2%. Using the equations shown in Figure 2, the

output accuracy can be calculated as 3.24%.

30123533

After figuring out the LMP8646 output accuracy, choose a

tolerable system error or the output current accuracy that is

bigger than the LMP8646 output accuracy. This tolerable sys-

tem error will be labeled as I_ERROR, and it has the equation

I_ERROR= (I_MAX- I_LIMIT)/I_MAX(%). In this example, we will

choose an I_ERRORof 5%, which will be used to calculate for

ROUT shown in the next step.

FIGURE 13. ROUT Equation

For example, assume the minimum LM3102 output voltage,

V_{O_REG_MIN}, is 0.6V, then ROUT can be calculated as ROUT

= [1.575A * 55 mOhm * (49.9k / 5k) - 0.8] / [ (0.8 / 2k) - (0.6 -

0.8) / 10k] = 153.6 Ohm.

Populate ROUT with a resistor that is as close as possible to

153.6 Ohm (this application uses 160 Ohm). If the limited

sense current has a gain error and is not 1.5A at any point in

time, then adjust this ROUT value to obtain the desired limit

current.

Step 6: Choose the output resistor, ROUT

At startup, the capacitor is not charged yet and thus the output

voltage of the LM3102 is very small. Therefore, at startup, the

output current is at its maximum (I_MAX). When the output volt-

age is at its nominal, then the output current will settle to the

desired limited value. Because a large current error is not de-

sired, ROUT needs to be chosen to stabilize the loop with

minimal initial startup current error. Follow the equations and

example below to choose the appropriate value for ROUT to

minimize this initial error.

We recommend that the value for ROUT is at least 50 Ohm.

Step 7: Adjusting Components

Capture the output current and output voltage plots and adjust

the components as necessary. The most common compo-

nents to adjust are C_Gto decrease the current ripple and

ROUT to get a low current error. An example output current

and voltage plot can be seen in Figure 14 .

As discussed in step 4, the allowable I_ERRORis 5%, where

I_ERROR= (I_MAX- I_LIMIT)/I_MAX(%). Therefore, the maximum al-

lowable current is calculated as: I_MAX= I_LIMIT(1+ I_ERROR) =

1.5A * (1 + 5/100) = 1.575 A.

Vo_load

I_limit

I_max

I_limit

Vo_reg_min

-10

TIME (s)

30123540

FIGURE 14. SuperCap Application with LM3102 Regulator Plot

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APPLICATION #2: CURRENT LIMITER WITH A RESISTIVE LOAD

30123532

FIGURE 15. Resistive Load Application with LMZ12003 Regulator

This subsection describes the design process for a resistive

load application with the LMZ12003 voltage regulator as seen

in Figure 15. To see the current limiting capability of the

LMP8646, the open-loop current must be greater than the

close-loop current. An open-loop occurs when the LMP8646

output is not connected the LMZ12003’s feedback pin. For

this example, we will let the open-loop current to be 1.5A and

the close-loop current, I_LIMIT, to be 1A.

Step 4: Choose the Bandwidth Capacitance, C_G.

The product of C_Gand R_Gdetermines the bandwidth for the

LMP8646. Refer to the Typical Performance Characteristics

plots to see the range for the LMP8646 bandwidth and gain.

Since each application is very unique, the LMP8646 band-

width capacitance, C_G, needs to be adjusted to fit the appro-

priate application.

Bench data has been collected for this resistive load applica-

tion with the LMZ12003 regulator, and we found that this

application works best for a bandwidth of 2 kHz to 30 kHz.

Operating anything less than this recommended bandwidth

might prevent the LMP8646 from quickly limiting the current.

We recommend choosing a bandwidth that is in the middle of

this range and using the equation: C_G= 1/(2*pi*R_G*Band-

width) to find C_G(this example uses a C_Gvalue of 0.1nF). After

this selection, capture the load current plot and adjust C_Guntil

a desired output current plot is obtained.

Step 1: Choose the components for the Regulator.

Refer to the LMZ12003 application note (AN-2031) to select

the appropriate components for the LMZ12003.

Step 2: Choose the sense resistor, R_SENSE

R_SENSEsets the voltage V_SENSEbetween +IN and -IN and has

the following equation:

R_SENSE= V_OUT/ [(I_LIMIT) * (R_G/ 5kOhm)]

Step 5: Choose the output resistor, ROUT, for the

LMP8646

In general, R_SENSEdepends on the output voltage, limit cur-

rent, and gain. Refer to section SELECTION OF THE SENSE

RESISTOR, R_SENSEto choose the appropriate R_SENSEvalue;

this example uses 50 mOhm.

ROUT plays a very small role in the overall system perfor-

mance for the resistive load application. ROUT was important

in the supercap application because it affects the initial cur-

rent error. Because current is directly proportional to voltage

for a resistive load, the output current is not large at startup.

The bigger the ROUT, the longer it takes for the output voltage

to reach its final value. We recommend that the value for

ROUT is at least 50 Ohm, which is the chosen value for this

example.

Step 3: Choose the gain resistor, R_G, for LMP8646

R_Gis chosen from I_LIMIT. As stated, V_OUT= (R_SENSE* I_LIMIT) *

(R_G/ 5kOhm). Since V_OUT= V_FB= 0.8V, I_LIMIT= 1A, and

R_SENSE= 50 mOhm , R_Gcan be calculated as:

R_G= (V_OUT* 5 kOhm) / (R_SENSE* I_LIMIT

)

R_G= (0.8 * 5 kOhm) / (50 mOhm* 1A) = 80 kOhm

Step 6: Adjusting Components

Capture the output current and output voltage plots and adjust

the components as necessary. The most common compo-

www.ti.com

nent to adjust is C_Gfor the bandwidth. An example of the

output current and voltage plot can be seen in Figure 16.

2.20

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

I_limit

1.98

Vclose_loop

1.76

1.54

1.32

1.10

0.88

0.66

0.44

0.22

0.00

-0.030 -0.018 -0.006 0.006 0.018 0.030

TIME (s)

30123541

FIGURE 16. Plot for the Resistive Load Application with LMZ12003 Regulator Plot

www.ti.com

APPLICATION #3: CURRENT LIMITER WITH A LOW-DROPOUT REGULATOR AND RESISTIVE LOAD

30123535

FIGURE 17. Resistive Load Application with LP38501 Regulator

This next example is the same as the last example, except

that the regulator is now a low-dropout regulator, the

LP38501, as seen in Figure 17. For this example, we will let

the open-loop current to be 1.25A and the close-loop current,

I_LIMIT, to be 1A.

Since each application is very unique, the LMP8646 band-

width capacitance, C_G, needs to be adjusted to fit the appro-

priate application.

Bench data has been collected for this resistive load applica-

tion with the LP38501 regulator, and we found that this appli-

cation works best for a bandwidth of 50 Hz to 300 Hz.

Operating anything larger than this recommended bandwidth

might prevent the LMP8646 from quickly limiting the current.

We recommend choosing a bandwidth that is in the middle of

this range and using the equation: C_G= 1/(2*pi*R_G*Band-

width) to find C_G(this example uses a C_Gvalue of 10 nF).

After this selection, capture the plot for I_SENSEand adjust C_G

until a desired sense current plot is obtained.

Step 1: Choose the components for the Regulator.

Refer to the LP38501 application note (AN-1830) to select the

appropriate components for the LP38501.

Step 2: Choose the sense resistor, R_SENSE

R_SENSEsets the voltage V_SENSEbetween +IN and -IN and has

the following equation:

R_SENSE= V_OUT/ [(I_LIMIT) * (R_G/ 5kOhm)]

Step 5: Choose the output resistor, ROUT, for the

LMP8646

In general, R_SENSEdepends on the output voltage, limit cur-

rent, and gain. Refer to section SELECTION OF THE SENSE

RESISTOR, R_SENSEto choose the appropriate R_SENSEvalue;

this example uses 58 mOhm.

ROUT plays a very small role in the overall system perfor-

mance for the resistive load application. ROUT was important

in the supercap application because it affects the initial cur-

rent error. Because current is directly proportional to voltage

for a resistive load, the output current is not large at startup.

The bigger the ROUT, the longer it takes for the output voltage

to reach its final value. We recommend that the value for

ROUT is at least 50 Ohm, which is the value we used for this

example.

Step 3: Choose the gain resistor, R_G, for LMP8646

R_Gis chosen from I_LIMIT. As stated, V_OUT= (R_SENSE* I_LIMIT) *

(R_G/ 5kOhm). Since V_OUT= ADJ = 0.6V, I_LIMIT= 1A, and

R_SENSE= 58 mOhm , R_Gcan be calculated as:

R_G= (V_OUT* 5 kOhm) / (R_SENSE* I_LIMIT

)

Step 6: Adjusting Components

R_G= (0.6 * 5 kOhm) / (58 mOhm* 1A) = 51.7 kOhm

Capture the output current and output voltage plots and adjust

the components as necessary. The most common compo-

nent to adjust is C_Gfor the bandwidth. An example plot of the

output current and voltage can be seen in Figure 18.

Step 4: Choose the Bandwidth Capacitance, C_G.

The product of C_Gand R_Gdetermines the bandwidth for the

LMP8646. Refer to the Typical Performance Characteristics

plots to see the range for the LMP8646 bandwidth and gain.

www.ti.com

2.5

2.0

1.5

1.0

0.5

0.0

2.5

2.0

1.5

1.0

0.5

0.0

Vclose_loop

I_limit

-10 10 30 50 70 90 110 130 150 170

TIME (ms)

30123542

FIGURE 18. Plot for the Resistive Load Application with the LP38501 LDO Regulator

www.ti.com

Physical Dimensions inches (millimeters) unless otherwise noted

TSOT-6

NS Package Number MK06A

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Notes

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Notes

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LMP8646MKE [TI]

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