LMR70503 [TI]

SIMPLE SWITCHERÂ® Buck-Boost Converter For Negative Output Voltage in Î¼SMD; SIMPLE SWITCHERÂ®降压 - 升压转换器，负输出电压Î¼SMD


型号：	LMR70503
厂家：	TEXAS INSTRUMENTS
描述：	SIMPLE SWITCHERÂ® Buck-Boost Converter For Negative Output Voltage in Î¼SMD SIMPLE SWITCHERÂ®降压 - 升压转换器，负输出电压Î¼SMD 转换器升压转换器
文件：	总19页 (文件大小：2434K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

LMR70503

SIMPLE SWITCHER® Buck-Boost Converter For Negative Output

Voltage in µSMD

Small Output Voltage Ripple

●

Features

●

WEBENCH® Enabled

Tiny 8-Bump Thin Micro SMD Package: 0.84 mm × 1.615

mm × 0.6 mm

Performance Benefits

2.8 V to 5.5 V Input Voltage Range

Adjustable Output Voltage: -0.9 V to -5.5 V

320 mA Switch Current Limit

●

Easy To Use

●

Tiny Overall Solution Size Reduces System Cost

500 kHz Minimum Switching Frequency

Ground Referred Enable Input

Under Voltage Lock Out (UVLO)

No External Compensation

Applications

General Purpose Negative Voltage Supply

●

Negative Rail / Bias Supply For Op-amp And Data

Converters

LCD Biasing

Internal Soft Start

●

1 µA Shutdown Supply Current

System Performance

Efficiency, V_OUT= -5.0 V

Efficiency, V_OUT= -2.5 V

VIN = 2.8V

VIN = 3.3V

VIN = 4.0V

VIN = 5.0V

VIN = 5.5V

VIN = 2.8V

VIN = 3.3V

VIN = 4.0V

VIN = 5.0V

VIN = 5.5V

10 20 30 40 50 60 70 80 90 100

LOAD (mA)

120 150 180

LOAD (mA)

30184975

30184979

Typical Application Circuit

30184901

PRODUCTION DATA information is current as of

publication date. Products conform to specifications per

the terms of the Texas Instruments standard warranty.

Production processing does not necessarily include

testing of all parameters.

301849 SNVS850

LMR70503

Ordering Information

Order Number

Supplied As

Package Type

Package Drawing

Package Marking

LMR70503TM NOPB

250 Units on Tape and Reel

Thin Micro SMD

TMP08BAA

LMR70503TMX NOPB 3000 Units on Tape and Reel

Connection Diagrams

30184902

30184903

LMR70503 Bump Locations - Top View

LMR70503 Package Marking - Top View

(Diamond Denotes Bump A1)

Pin Descriptions

Pin Number

Name

Description

VREF

Reference voltage output; connect to the bottom feedback resistor.

Active high enable input for the device. Enable voltage level is referred to GND. Device must be enabled

only with the presence of valid VIN (2.8 V to 5.5 V). The peak of the Enable input voltage must always

lower than VIN voltage.

C1, C2

GND

Analog ground for internal bias circuitry.

Switch node pin, connected to the internal high side MOSFET. The cathode of the external Schottky

diode must be connected as close as possible to this pin, in order to reduce inductance in the

discontinuous current path.

FB is connected to VOUT and VREF through two feedback resistors. It is compared to GND to regulate

the output voltage.

Output voltage. The anode of the external Schottky diode and output filter capacitor(s) should be

connected to this pin.

VOUT

VIN

Power supply input pin, connected to the internal high side MOSFET and powers the internal circuity.

LMR70503

Absolute Maximum Ratings (Note 1)

If Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/ Distributors for

availability and specifications.

VIN to GND

VOUT to GND

SW to GND

EN to GND

-0.5 V to 6.0 V

-6.5 V to 0.5 V

-6.5 V to VIN +0.2 V

-0.5 V to VIN

-0.5V to 5.5V

±2 kV

FB to GND

ESD Rating(Note 2)

Junction Temperature

Storage Temperature Range

150 °C

-65 °C to 150 °C

For Soldering Specs see:

http://www.ti.com/lit/an/snoa549c/

snoa549c.pdf

Operating Ratings

Input Voltage Range (V_IN)

2.8 V to 5.5 V

-0.9 V to -5.5 V

-40°C to 125°C

Output Voltage Range (V_OUT

)

Junction Temperature Range (T_J)

Electrical Characteristics Specifications with standard typeface are for T_J= 25°C only; limits in bold face type

apply over the operating junction temperature (T_J) range of -40 °C to +125 °C. Typical values represent the most likely parametric

norm at T_J= 25°C, and are provided for reference purposes only. V_IN= 3.3 V, V_OUT= -5.0 V, V_EN= 1.8 V, unless otherwise indicated

in the conditions column.

Min

(Note 3)

Typ

Max

Symbol

V_REF

Parameter

Conditions

Units

(Note 4) (Note 3)

Reference Voltage

Shutdown Current

R_REF=100 kΩ to GND

1.166

1.19

0.01

1.214

EN = 0 V

V_IN= 5.5 V

I_SD

µA

EN = 1.8 V, V_IN= 5.5 V,

No Switching

I_Q

Quiescent Current

245

2.55

0.13

300

2.7

µA

V_INUnder Voltage Lock Out Threshold -

UVLO_RISE

UVLO_HYS

Rising

V_INUnder Voltage Lock Out Hysteresis

Band

0.1

V_EN-RISE

V_EN-HYS

I_EN

V_IN= 5.5 V

EN Input Voltage Rising Threshold

EN Input Voltage Threshold Hysteresis

Enable Current

1.05

0.15

1.2

kHz

I_FB

FB pin current

F_SW-MIN

T_ON-MIN

R_DSON

I_PEAK-CL

TSD_TH-HIGH

TSD_HYS

Minimum Switching frequency

Minimum High Side Switch On Time

Switch On State Resistance

Switch Peak Current limit(Note 5)

Thermal Shutdown Threshold - Rising

Thermal Shutdown Hysteresis Band

400

270

500

Load = 0 A

V_IN= 2.8V

1.1

320

165

Ω

°C

370

Junction Temperature

Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur, including inoperability and degradation of device reliability and/

or performance. Functional operation of the device and/or non-degradation at the Absolute Maximum Ratings or other conditions beyond those indicated in the

recommended Operating Ratings is not implied. The recommended Operating Ratings indicate conditions at which the device is functional and should not be

operated beyond such conditions.

Note 2: ESD using the human body model which is a 100 pF capacitor discharged through a 1.5 kΩ resistor into each pin. Test method is per JESD22–A114.

Note 3: Min and Max limits are 100% production tested at an ambient temperature (T_A) of 25 °C. Limits over the operating temperature range are guaranteed

through correlation using Statistical Quality Control (SQC) methods. Limits are used to calculate Average Outgoing Quality Level (AOQL).

Note 4: Typical specifications represent the most likely parametric norm at 25°C operation.

Note 5: The switch peak current limit is internally trimmed. The actual peak current limit observed on the applications are dependant on the input voltage V_IN

inductance value L and junction temperature T_J.

LMR70503

Typical Performance Characteristics Unless otherwise specified, the following conditions apply: V_IN

3.3 V, V_OUT= -5.0 V, V_EN= 1.8 V, C_IN= 10 µF 6.3 V X5R ceramic capacitor; C_OUT= 2 × 22 µF 6.3 V X5R ceramic capacitor; L =

6.8 µH (VLS2012ET-6R8M); T_AMBIENT= 25 °C.

Efficiency, V_OUT= -5.0 V

Output Regulation, V_OUT= -5.0 V

5.10

5.08

5.06

5.04

5.02

5.00

4.99

4.96

4.95

4.92

4.90

VIN = 2.8V

VIN = 3.3V

VIN = 4.0V

VIN = 5.0V

VIN = 5.5V

VIN = 3.3V

VIN = 4.0V

VIN = 5.0V

VIN = 5.5V

10 20 30 40 50 60 70 80 90 100

LOAD (mA)

10 20 30 40 50 60 70 80 90 100

LOAD (mA)

30184975

30184976

Efficiency, V_OUT= -3.3 V

Output Regulation, V_OUT= -3.3 V

3.40

3.38

3.36

3.34

3.32

3.30

3.28

3.26

3.24

3.22

3.20

VIN = 2.8V

VIN = 3.3V

VIN = 4.0V

VIN = 5.0V

VIN = 5.5V

VIN = 3.3V

VIN = 4.0V

VIN = 5.0V

VIN = 5.5V

80 100 120 140

20 40 60 80 100 120 140

LOAD (mA)

30184977

30184978

Efficiency, V_OUT= -2.5 V

Output Regulation, V_OUT= -2.5 V

2.60

2.58

2.56

2.54

2.52

2.50

2.48

2.46

2.44

2.42

2.40

VIN = 2.8V

VIN = 3.3V

VIN = 4.0V

VIN = 5.0V

VIN = 5.5V

VIN = 3.3V

VIN = 4.0V

VIN = 5.0V

VIN = 5.5V

120 150 180

LOAD (mA)

30184979

30184980

LMR70503

Efficiency, V_OUT= -1.5 V

Output Regulation, V_OUT= -1.5 V

1.60

1.58

1.56

1.54

1.52

1.50

1.48

1.46

1.44

1.42

1.40

VIN = 2.8V

VIN = 3.3V

VIN = 4.0V

VIN = 5.0V

VIN = 5.5V

VIN = 3.3V

VIN = 4.0V

VIN = 5.0V

VIN = 5.5V

90 120 150 180 210

LOAD (mA)

30 60 90 120 150 180 210

LOAD (mA)

30184981

30184982

Efficiency, V_OUT= -0.9 V

Output Regulation, V_OUT= -0.9 V

1.00

0.98

0.96

0.94

0.92

0.90

0.88

0.86

0.84

0.82

0.80

VIN = 2.8V

VIN = 3.3V

VIN = 4.0V

VIN = 5.0V

VIN = 5.5V

VIN = 3.3V

VIN = 4.0V

VIN = 5.0V

VIN = 5.5V

100

150

200

250

100

150

200

250

LOAD (mA)

30184983

30184984

Maximum Load Current

Minimum Switching Frequency

600

250

200

150

100

580

560

540

520

500

480

460

VOUT = -5V

VOUT = -3.3V

VOUT = -2.5V

VOUT = -1.5V

VOUT = -0.9V

Temp = -40°C

Temp = 25°C

Temp = 125°C

2.8 3.2 3.6 4.0 4.4 4.8 5.2

VIN (V)

2.5

3.0

3.5

4.0

VIN (V)

4.5

5.0

5.5

30184971

30184989

LMR70503

No Load Supply Current

Rds-on

3.0

2.5

2.0

1.5

1.0

0.5

0.0

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

Vin = 2.8V

Vin = 4.0V

Vin = 5.5V

2.8 3.2 3.6 4.0 4.4 4.8 5.2

VIN (V)

-40 -20

20 40 60 80 100 120 140

TEMPERATURE (°C)

30184924

30184987

Enable Thresholds

Soft Start Time (No Load)

800

700

600

500

400

300

200

100

Vout = -5.0V

Vout = -3.3V

Vout = -2.5V

Vout = -1.5V

Vout = -0.9V

1.1

1.0

0.9

0.8

0.7

0.6

0.5

0.4

Rising TH -40°C

Falling TH -40°C

Rising TH 25°C

Falling TH 25°C

Rising TH 125°C

Falling TH 125°C

2.5

3.0

3.5

4.0

VIN (V)

4.5

5.0

5.5

2.5

3.0

3.5

4.0

VIN (V)

4.5

5.0

5.5

30184988

30184986

Soft Start Delay Time

(From EN Rising Edge)

Soft Off Time, V_OUT= -5.5 V

(No Load, From EN Falling Edge)

160

140

120

100

800

700

600

500

400

300

Temp = -40°C

Temp = 25°C

Temp = 125°C

VIN = 2.8 V

VIN = 3.0 V

VIN = 4.0 V

VIN = 5.0 V

2.5

3.0

3.5

4.0

VIN (V)

4.5

5.0

5.5

10 20 30 40 50 60 70 80 90

TEMP (°C)

30184985

30184930

LMR70503

Soft Start And Soft Off Waveform

V_IN= 5.0 V, V_OUT= -5.0 V, No Load

Soft Start And Soft Off Waveform

V_IN= 5.0 V, V_OUT= -5.0 V, Load = 50 Ω

30184951

30184952

Typical Switching Waveform

V_IN= 5.0 V, V_OUT= -5.0 V, No Load

Typical Switching Waveform

V_IN= 5.0 V, V_OUT= -5.0 V, I_OUT= 70 mA

30184955

30184954

Load Transient, V_IN= 4.0 V, V_OUT= -5.5 V

Load steps between 2 mA and 50 mA

Short Circuit Waveform

V_IN= 5.0 V, V_OUT= -5.5 V

30184912

30184911

LMR70503

Block Diagram

30184904

General Description

The LMR70503 is a buck-boost converter with adjustable negative output voltage in a tiny 8-bump thin micro SMD package. Its

unique control method is designed to provide fast transient response, low output noise, high efficiency, and tight regulation in the

smallest possible PCB area. The LMR70503 has built in soft start, peak current limit, minimum switching frequency, and Under

Voltage Lock Out (UVLO), with no external compensation required. For ease of use, the Enable pin is referred to the IC ground,

instead of the lowest potential of the IC: the negative output voltage.

Operating Description

The LMR70503 integrates an inverting buck-boost controller and a high-side MOSFET in one tiny 8-bump thin micro SMD package.

A simplified buck-boost converter schematic is shown in Figure 1.

30184940

FIGURE 1. Buck Boost Converter

The LMR70503 controller incorporates a unique peak current mode control method with a minimum switching frequency limit. The

integrated switch is turned off when its current crosses the peak current limit, while it is turned on when the magnitude of V_OUT

droops below a threshold. When the switch is off, the inductor current goes through the diode and charges the output capacitor(s).

With fixed peak current limit, the switching frequency decreases with decreased load current. At light load, the switching frequency

will decrease to the audible frequency range, which is not acceptable in many applications. The LMR70503 is designed to operate

with peak current mode control and limit the switching frequency to 500 kHz (typical) minimum, to avoid audible frequency inter-

ference. At light load, when the switching frequency drops to the minimum, the inductor current limit is reduce instead of frequency

to maintain regulation. The LMR70503 also incorporates an internal dummy load to compensate for the extra charges in the min-

imum ON-time (T_ON-MIN) condition. More details on the LMR70503 operation are described in the later sessions. Typical switching

waveforms in discontinuous conduction mode (DCM) and continuous conduction mode (CCM), including the inductor current, the

switch node voltage and the output voltage ripple (absolute value), are shown in Figure 2.

LMR70503

30184943

FIGURE 2. Typical Waveforms In Buck Boost Converter

Figure 3 illustrates the switching frequency, the peak current limit, the output voltage and the dummy load with different load current.

More details on each operation mode will be described later.

1. No load to very light load: high side switch is turned on for T_ON-MIN; switching frequency is limited at the minimum switching

frequency; and the dummy load is turned on.

2. Light load: switching frequency is limited at the minimum switching frequency, peak current limit increases with increased load

current; and the dummy load is off.

3. Heavy load: peak current equals the maximum peak current limit; switching frequency increases with increased load current;

and the dummy load is off.

30184944

FIGURE 3. The LMR70503 Operation Modes vs. Load Current

Minimum Switching Frequency Operation

In a typical peak current mode controlled DC-DC converter, the peak current limit is constant and the switching frequency decreases

when load current reduces. To maintain low noise operation and avoid audio frequency interference, the minimum switching fre-

quency of the LMR70503 is limited at 500 kHz typically. At heavy load, the peak current limit remains constant and the switching

frequency varies with the load to regulate the output voltage. With reduced loading, the absolute output voltage is going to be

charged higher than regulation if the switching frequency cannot decrease accordingly. Therefore, to regulate the output voltage

with minimum frequency at light load, the peak current limit is reduced, in proportional to the output voltage offset.

In this mode, as shown in Figure 3, the switching frequency is fixed to the minimum switching frequency, the peak inductor current

increases with load current, and the output voltage magnitude has a small offset above regulation.

LMR70503

Minimum ON-Time and Dummy Load

When load current is near zero, the peak inductor current can not reduce further due to T_ON-MINof the high side switch. Under such

conditions, an internal dummy load is turned on by sensing excessive output voltage offset, which removes the extra charge from

the output capacitor(s). In this condition, the switching frequency is fixed to the minimum value. The peak inductor current value is

at its minimum value, as shown in Figure 3. The dummy load current is zero when the LMR70503 operates with on time higher

than T_ON-MIN

The minimum peak inductor current is determined by

I_PEAK-MIN= T_ON-MIN× V_IN/ L

where V_INis the supply voltage and L is the inductance value. The peak inductor current is higher with higher V_IN. The inductor

current falling slew rate is determined by

SR_FALLING= (|V_OUT| + V_F) / L

where |V_OUT| is the absolute value of the output voltage and V_Fis the forward voltage drop of the power diode. At lower |V_OUT|, it

takes longer time to discharge the inductor current to zero. Therefore, there is more energy to charge the output capacitor(s). The

output voltage will have more offset at higher V_INand lower V_OUT. The dummy load current is a function of the FB voltage: the more

the offset at the FB node, the higher the dummy load current, as shown in Figure 4.

VIN=5.5V -40°C

VIN=5.5V 25°C

VIN=5.5V 125°C

VIN=2.8V -40°C

VIN=2.8V 25°C

VIN=2.8V 125°C

-50

-40

-30

-20

-10

FB VOLTAGE (mV)

30184972

FIGURE 4. Dummy Load Current vs. FB Voltage

Constant Peak Current Operation

If the load current increases in the minimum switching frequency mode, the peak current limit will reach the maximum peak current

limit (I_PEAK-MAX). After this point, the LMR70503 behaves as a constant peak current converter with frequency modulation. The

transition load level between the constant frequency mode and the constant peak current mode varies with V_IN, V_OUTand L.

The I_PEAK-MAXis trimmed to 320 mA in the LMR70503. Due to propagation delays in the comparator and gate drive, the measured

peak inductor current will be higher than the trimmed value. The additional offset on the maximum peak current is proportional to

the inductor current rising slope: V_IN/ L, approximately. For a typical inductor, the inductance will reduce at hot temperature.

Therefore, I_PEAK-MAXis the highest with 5.5 V input voltage at hot temperature.

In the constant peak current operation mode, the switching frequency will increase with the increased load current, until the high

side switch off time equals the minimum off-time (T_OFF-MIN) limit. If the load keeps increasing when the switch operates with T_OFF-

_MIN, V_OUTwill drop out of regulation due to loading limits of buck-boost type of converters. The maximum loading capability is higher

with higher V_IN, larger L, lower V_OUT, and less losses in the converter. Figure 5 shows the measured maximum load current mea-

sured with the typical BOM shown in Table 1. To increase the maximum loading capability with given V_INand V_OUT, one can choose

a higher inductance value and a diode with lower forward voltage drop V_F.

LMR70503

250

200

150

100

VOUT = -5V

VOUT = -3.3V

VOUT = -2.5V

VOUT = -1.5V

VOUT = -0.9V

2.8 3.2 3.6 4.0 4.4 4.8 5.2

VIN (V)

30184971

FIGURE 5. LMR70503 Loading Capability vs. V_IN, L = 6.8 µH

The built-in T_OFF-MINtime is a function of both V_INand V_OUT, as shown in Figure 6.

900

800

700

600

500

400

300

VOUT = -0.9V

200

100

VOUT = -1.5V

VOUT = -2.5V

VOUT = -3.3V

VOUT = -5.0V

2.8 3.2 3.6 4.0 4.4 4.8 5.2

VIN (V)

30184974

FIGURE 6. Minimum Off Time vs. V_INat room temperature

Enable And UVLO

The LMR70503 features an enable (EN) pin and associated comparator to allow the user to easily sequence the LMR70503 from

an external voltage rail, or to manually set the input UVLO threshold. Enable threshold levels are referred to the LMR70503 ground,

instead of the lowest potential: the negative output voltage. Enable turning on (rising) and turning off (falling) thresholds are shown

in Figure 7.

LMR70503

1.1

1.0

0.9

0.8

0.7

0.6

0.5

0.4

Rising TH -40°C

Falling TH -40°C

Rising TH 25°C

Falling TH 25°C

Rising TH 125°C

Falling TH 125°C

2.5

3.0

3.5

4.0

VIN (V)

4.5

5.0

5.5

30184988

FIGURE 7. Enable Rising And Falling Thresholds vs. V_IN

It is important to ensure that a valid input voltage (2.8 V ≤ V_IN≤ 5.5 V) is present on the VIN pin before the EN input is asserted.

Also, as stated in the Absolute Maximum Ratings section of this data sheet, the voltage on the EN pin must always be less than

V_IN. This applies to both static and dynamic operation, and during start up and shut down sequences. If these precautions are not

followed, an internal test mode may be activated; possibly damaging the regulator. The EN input must not be left floating. A resistor

divider can be added from VIN to EN if an external enable signal is not available.

An input under voltage lock-out (UVLO) circuit prevents the regulator from turning on when the input voltage is not great enough

to properly bias the internal circuitry. The typical UVLO rising threshold is 2.55 V and typical hysteresis band is 0.13 V.

Soft Start And Soft Off

The LMR70503 begins to operate when EN goes high with the presence of valid V_IN, or V_INswings below UVLO level and back up

with the presence of valid EN voltage. The soft start action is inherent with the maximum peak current limit and minimum off time.

During start up, the inductor current rises to the maximum peak current limit, then the high-side switch is turned off for T_OFF-MINand

the output capacitor(s) is charged during this time. Then the high-side turns on to repeat the cycle. After the output voltage is

charged to the regulation level, the LMR70503 will operate in steady state. The soft start time will be longer with more output

capacitance, and / or lower supply voltage V_IN, and / or more loading during start up. Figure 8 shows soft start vs V_INwith L= 6.8

µH and no load. Soft-start is reset any time the part is shut down or a thermal shutdown event occurs.

800

Vout = -5.0V

Vout = -3.3V

700

600

500

400

300

200

100

Vout = -2.5V

Vout = -1.5V

Vout = -0.9V

2.5

3.0

3.5

4.0

VIN (V)

4.5

5.0

5.5

30184986

FIGURE 8. Soft Start Time (No Load) vs. V_IN

The LMR70503 will shutdown when EN pin voltage goes below the falling threshold, or V_INgoes below UVLO falling threshold.

When shutdown, the LMR70503 incorporates an output voltage discharge feature to bring the output voltage to zero volts, regard-

less of the load current. When the EN input is taken below its lower threshold, an internal MOSFET turns on and discharges the

output capacitors. Typical soft off times (from EN falling edge to 10% of Vout ) over V_INand temperature are shown in Figure 9.

Figure 10 shows the typical off time from 90% to 10% of Vout.

LMR70503

800

700

600

500

400

300

VIN = 2.8 V

VIN = 3.0 V

VIN = 4.0 V

VIN = 5.0 V

10 20 30 40 50 60 70 80 90

TEMP (°C)

30184930

FIGURE 9. Soft Off Time (EN Falling Edge To 10% Vout) vs. Temperature, V_OUT= -5.5 V, No Load

800

700

600

500

400

VIN = 2.8 V

VIN = 4.0 V

VIN = 5.0 V

300

10 20 30 40 50 60 70 80 90

TEMPERATURE (°C)

30184931

FIGURE 10. Soft Off Time (90% To 10% Vout) vs. Temperature, V_OUT= -5.5 V, No Load

Short Circuit Protection

Peak current mode control has inherent short circuit protection. The protection level is the maximum inductor current limit level. It

varies with V_INand temperature due to propagation delays. The minimum off-time limits the current going through the inductor

during a short circuit condition.

Over-Temperature Protection

Internal thermal shutdown (TSD) circuitry protects the LMR70503 should the maximum junction temperature be exceeded. This

protection is activated at 165 °C (typical), with the result that the regulator will shutdown until the junction temperature drops below

155 °C (typical). Of course the LMR70503 must not be operated continuously above 125 °C.

Design Guide

Output Voltage Setting

The output voltage of the LMR70503 is programmable by the voltage divider resistors. The reference voltage is typically 1.19 V.

To avoid overloading the V_REFcircuity, the resistor R_Ttied between V_REFand FB is recommended to be between 20 kΩ and 100

kΩ. With a selected R_T, R_Btied between V_OUTand FB can be found by

R_B= R_T* |V_OUT| / V_REF

A feed-forward capacitor C_FFcan be used between V_OUTand FB nodes to improve transient performance. 10 pF C0G, NP0 type

of capacitor is recommended in LMR70503 applications.

LMR70503

Input Capacitor And Output Capacitor Selection

The input capacitor selection is based on both input voltage ripple and RMS current. Good quality input capacitors are necessary

to limit the ripple voltage at the VIN pin while supplying most of the regulator current during switch on-time. Low ESR ceramic

capacitors are preferred. A minimum value of 10μF at 6.3 V, is required at the input of the LMR70503. Larger values of input

capacitance are desirable to reduce voltage ripple and noise on the input supply.

The output capacitor is responsible for filtering the output voltage and suppling load current during transients and during the power

diode off-time. Best performance is achieved with ceramic capacitors. For most applications, a minimum value of 22 μF, 6.3 V

capacitor is required at the output of the LMR70503. The percentage of ripple coupled to the FB node can be found by

RIPPLE PERCENTAGE = V_REF/ ( |V_OUT| + V_REF

)

where |V_OUT| is the magnitude of the output voltage and V_REFis the reference voltage. With lower magnitude V_OUT, a higher

percentage of output voltage ripple is coupled to the FB node. Output voltage ripple is also coupled to the FB node via the feed-

forward capacitor C_FF. Excessive ripple at the FB node may trigger peak current limit modulation causing unstable operation. Higher

output capacitance is needed at lower magnitude output voltage. For V_OUT= -0.9 V, a minimum of 44 μF, 6.3 V capacitor is required.

Avoid using too much capacitance at C_FF

A capacitor between V_INand V_OUTalso can be used to provide high frequency bypass. This capacitor is equivalent to the output

capacitors in the small signal model. It also reduces the output voltage ripple if sufficiency capacitance is used. The voltage rating

for this capacitor should be higher than V_IN+ |V_OUT|.

All ceramic capacitors have large voltage coefficients, in addition to normal tolerances and temperature coefficients. To help mitigate

these effects, multiple capacitors can be used in parallel to bring the minimum capacitance up to the desired value. This may also

help with RMS current constraints by sharing the current among several capacitors. With the LMR70503, ceramic capacitors rated

at 6.3 V, or higher, are suitable for all input and output voltage combinations. Many times it is desirable to use an electrolytic

capacitor on the input, in parallel with the ceramics. The moderate ESR of this capacitor can help to damp any ringing on the input

supply caused by long power leads. This method can also help to reduce voltage spikes that may exceed the maximum input

voltage rating of the LMR70503.

Power Inductor Selection

The power inductor selection is critical to the operation of the LMR70503. It affects the efficiency, the operation mode transition

point, the maximum loading capability and the size / cost of the solution. A 4.7 μH or 6.8 μH inductor is recommended for most

LMR70503 applications. The maximum loading capability is reduced with smaller inductance value. The no load V_OUToffset is

higher at low V_OUTwith smaller inductance value, due to higher peak current with the same T_ON-MIN. Higher inductance value

usually comes with higher DCR with the same size and cost. Higher DCR will reduce the efficiency especially at heavy load.

The inductor must be rated above the maximum peak current limit to prevent saturation. Good design practice requires that the

inductor rating be adequate for the maximum I_PEAK-MAXover V_INand temperature, plus some safety margin. If the inductor is not

rated for the maximum expected current, saturation at high current may cause damage to the LMR70503 and/or the power diode.

The DCR of the inductor should be as small as possible with given size / cost constrains to achieve optimal efficiency.

Power Diode Selection

A Schottky type power diode is required for all LMR70503 applications. The parameters of interests include the reverse voltage

rating, the DC current rating, the repetitive peak current rating, the forward voltage drop, the reverse leakage current and the

parasitic capacitance. In a buck-boost, this diode sees a reverse voltage of :

V_R-DIODE= |V_OUT| + V_IN

The reverse breakdown voltage rating of the diode should be selected for this value, plus safety margin. A good rule of thumb is

to select a diode with a reverse voltage rating of 1.3 times this maximum. Select a diode with a DC current rating at least equal to

the maximum load current that will be seen in the application and the repetitive peak current rating higher than I_PEAK-MAXover V_IN

and temperature. The forward voltage drop of the power diode is a big part of the power loss in a buck-boost converter. It is preferred

to be as low as possible. The reverse leakage current and the parasitic capacitance are also part of the power losses in the converter,

but usually less pronounced than the forward voltage drop loss. Pay attention to the temperature coefficients of all the parameters.

Some of them may vary greatly over temperature and may adversely affect the efficiency over temperature.

PC Board Layout Guidelines

Board layout is critical for the proper operation of switching power converters. Switch mode converters are very fast switching

devices. In such cases, the rapid increase of current combined with the parasitic trace inductance generates unwanted L·di/dt noise

spikes. The magnitude of this noise tends to increase as the output current increases. This noise may turn into electromagnetic

interference (EMI) and can also cause problems in device performance. Therefore, care must be taken in layout to minimize the

effect of this switching noise. The most important layout rule is to keep the AC current loops as small as possible.

Figure 1 shows the current flow in a buck-boost converter. The two dotted arrows indicate the current paths when the high side

switch is on and when the power diode is on, respectively. The components and traces that contain discontinuous currents are

critical in PCB layout design, since discontinuous currents contain high di/dt and high frequency noise. The components that carry

critical discontinuous currents include the input capacitor(s), the high side switch, the power diode and the output capacitor(s).

These components need to be placed as close as possible to each other and the traces between them must be made as short and

wide as possible: place the input capacitor(s) as close as possible to the VIN pin of the LMR70503; place the cathode of the diode

as close as possible to the SW pin; the anode of the diode should be as close as possible to the output capacitor(s); the GND end

of the output capacitor(s) should be as close as possible to that of the input capacitor(s). Doing so will yield a small loop area,

reducing the loop inductance and EMI.

LMR70503

The feedback resistors R_Band R_Tshould be placed as close as possible to the FB pin. Since FB is a high impedance node, noise

is likely be coupled to the FB node if the trace is long. The traces from V_OUTto the resistor divider and from the divider to the FB

pin should be far away from the discontinuous current path. It is recommended to use 4-layer board with ground plane as an internal

layer, route the discontinuous current path on the top layer and the feedback path on the other side of the ground plane. Then the

feedback path will be shielded from switching noise.

To avoid functional problems due to layout, review the PCB layout example in . It is also recommended to use 1oz copper boards

or heavier to help reducing the parasitic inductances of board traces.

PCB Layout Example

30184946

FIGURE 11. PCB Layout Example (top layer and top overlay)

LMR70503

LMR70503 Typical Application Circuit

30184945

LMR70503 Application Circuit Bill of Materials

V_IN= 2.8 V to 5.5 V, V_OUThas options of -0.9 V, -1.5 V, -2.5 V, -3.3 V and -5.0 V. Optimized for minimum solution size.

TABLE 1. Bill of Materials

Designator

Description

Case Size

Manufacturer

Manufacturer P/N

Inverting Buck-Boost

8-bump thin micro SMD Texas Instruments

LMR70503TM NOPB

Ceramic 10 µF 10 V X5R

0603

C_IN

0603

0402

TDK

C1608X5R1A106M

C1608X5R0J226M

Ceramic 22 µF 6.3 V X5R

0603

C_OUT1, C_OUT2

C_ff

TDK

CAP CER 10PF 50V 5%

NP0 0402

Murata

GRM1555C1H100JZ01D

Schottky 30 V 500 mA

SOD882

NXP Semi

TDK

PMEG3005EL

2.0*2.0*1.2mm

VLS2012ET-6R8M

6.8 µH, 0.76 A 362 mΩ

RES, 100k ohm, 1%,

0.063W, 0402

R_T

0402

Vishay Dale

CRCW0402100KFKED

0402

Vishay Dale

CRCW0402422KFKED

CRCW0402274KFKED

CRCW0402210KFKED

CRCW0402127KFKED

CRCW040275K0FKED

422 kΩ For Vout = -5.0V

274 kΩ For Vout = -3.3V

210 kΩ For Vout = -2.5V

127 kΩ For Vout = -1.5V

75 kΩ For Vout = -0.9V

R_B*

RES, 20k ohm, 5%,

0.063W, 0402

R_EN1, R_EN2

0402

Vishay Dale

CRCW040220K0JNED

* R_Bis represented by R1 in Figure 11.

LMR70503

Physical Dimensions inches (millimeters) unless otherwise noted

8-Bump Thin Micro SMD Package

Package Number TMP08BAA

X1 = 0.84 ± 0.03 mm, X2 = 1.615 ± 0.03 mm, X3 = 0.6 ± 0.075 mm

Notes

Incorporated

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LMR70503 [TI]

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