LM2717MTXADJNOPB - NATIONAL SEMICONDUCTOR

LM2705

LM2705 Micropower Step-up DC/DC Converter with 150mA Peak Current Limit

Literature Number: SNVS191D

LM2705

June 17, 2010

Micropower Step-up DC/DC Converter with 150mA Peak

Current Limit

General Description

The LM2705 is a micropower step-up DC/DC in a small 5-lead

SOT-23 package. A current limited, fixed off-time control

scheme conserves operating current resulting in high effi-

ciency over a wide range of load conditions. The 21V switch

allows for output voltages as high as 20V. The low 400ns off-

time permits the use of tiny, low profile inductors and capac-

itors to minimize footprint and cost in space-conscious

portable applications. The LM2705 is ideal for LCD panels

requiring low current and high efficiency as well as white LED

applications for cellular phone back-lighting. The LM2705 can

drive up to 3 white LEDs from a single Li-Ion battery. The low

peak inductor current of the LM2705 makes it ideal for USB

applications.

Features

■150mA, 0.7Ω, internal switch

■Uses small surface mount components

■Adjustable output voltage up to 20V

■2.2V to 7V input range

■Input undervoltage lockout

■0.01µA shutdown current

■Small 5-Lead SOT-23 package

Applications

■LCD Bias Supplies

■White LED Back-Lighting

■Handheld Devices

■Digital Cameras

■Portable Applications

Typical Application Circuit

20039701

FIGURE 1. Typical 20V Application

LM2705 Micropower Step-up DC/DC Converter with 150mA Peak Current Limit

Connection Diagram

Top View

20039702

SOT23-5

TJmax = 125°C, θJA = 220°C/W (Note 2)

Ordering Information

Order Number Package Type NSC Package Drawing Top Mark Supplied As

LM2705MF-ADJ SOT23-5 MA05B S59B 1000 Units, Tape and Reel

LM2705MFX-ADJ SOT23-5 MA05B S59B 3000 Units, Tape and Reel

Pin Descriptions/Functions

Pin Name Function

1 SW Power Switch input.

2 GND Ground.

3 FB Output voltage feedback input.

4 SHDN Shutdown control input, active low.

5 VIN Analog and Power input.

SW(Pin 1): Switch Pin. This is the drain of the internal NMOS

power switch. Minimize the metal trace area connected to this

pin to minimize EMI.

GND(Pin 2): Ground Pin. Tie directly to ground plane.

FB(Pin 3): Feedback Pin. Set the output voltage by selecting

values for R1 and R2 using:

Connect the ground of the feedback network to an AGND

plane which should be tied directly to the GND pin.

SHDN(Pin 4): Shutdown Pin. The shutdown pin is an active

low control. Tie this pin above 1.1V to enable the device. Tie

this pin below 0.3V to turn off the device.

VIN(Pin 5): Input Supply Pin. Bypass this pin with a capacitor

as close to the device as possible.

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LM2705

Absolute Maximum Ratings (Note 1)

If Military/Aerospace specified devices are required,

please contact the National Semiconductor Sales Office/

Distributors for availability and specifications.

VIN 7.5V

SW Voltage 21V

FB Voltage 2V

SHDN Voltage 7.5V

Maximum Junction Temp. TJ

(Note 2)

150°C

Lead Temperature

(Soldering 10 sec.) 300°C

Vapor Phase

(60 sec.) 215°C

Infrared

(15 sec.) 220°C

ESD Ratings (Note 3)

Human Body Model

Machine Model (Note 4)

2kV

200V

Operating Conditions

Junction Temperature

(Note 5) −40°C to +125°C

Supply Voltage 2.2V to 7V

SW Voltage Max. 20.5V

Electrical Characteristics

Specifications in standard type face are for TJ = 25°C and those in boldface type apply over the full Operating Temperature

Range (TJ = −40°C to +125°C). Unless otherwise specified VIN =2.2V.

Symbol Parameter Conditions Min

(Note 5)

Typ

(Note 6)

Max

(Note 5)Units

IQDevice Disabled FB = 1.3V 40 70

µADevice Enabled FB = 1.2V 235 300

Shutdown SHDN = 0V 0.01 2.5

VFB FeedbackTrip Point 1.189 1.237 1.269 V

ICL Switch Current Limit 100 150 180 mA

IBFB Pin Bias Current FB = 1.23V (Note 7) 30 120 nA

VIN Input Voltage Range 2.2 7.0 V

RDSON Switch RDSON 0.7 1.6 Ω

TOFF Switch Off Time 400 ns

ISD SHDN Pin Current SHDN = VIN, TJ = 25°C 0 80

nASHDN = VIN, TJ = 125°C 15

SHDN = GND 0

ILSwitch Leakage Current VSW = 20V 0.05 5 µA

UVP Input Undervoltage Lockout ON/OFF Threshold 1.8 V

VFB

Hysteresis

Feedback Hysteresis 8 mV

SHDN

Threshold

SHDN low 0.7 0.3 V

SHDN High 1.1 0.7

θJA SOT23-5 Thermal Resistance 220 °C/W

Note 1: Absolute maximum ratings are limits beyond which damage to the device may occur. Operating Ratings are conditions for which the device is intended

to be functional, but device parameter specifications may not be guaranteed. For guaranteed specifications and test conditions, see the Electrical Characteristics.

Note 2: The maximum allowable power dissipation is a function of the maximum junction temperature, TJ(MAX), the junction-to-ambient thermal resistance,

θJA, and the ambient temperature, TA. See the Electrical Characteristics table for the thermal resistance. The maximum allowable power dissipation at any ambient

temperature is calculated using: PD (MAX) = (TJ(MAX) − TA)/θJA. Exceeding the maximum allowable power dissipation will cause excessive die temperature.

Note 3: The human body model is a 100 pF capacitor discharged through a 1.5 kΩ resistor into each pin. The machine model is a 200 pF capacitor discharged

directly into each pin.

Note 4: ESD susceptibility using the machine model is 150V for SW pin.

Note 5: All limits guaranteed at room temperature (standard typeface) and at temperature extremes (bold typeface). All room temperature limits are 100%

production tested or guaranteed through statistical analysis. All limits at temperature extremes are guaranteed via correlation using standard Statistical Quality

Control (SQC) methods. All limits are used to calculate Average Outgoing Quality Level (AOQL).

Note 6: Typical numbers are at 25°C and represent the most likely norm.

Note 7: Feedback current flows into the pin.

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LM2705

Typical Performance Characteristics

Enable Current vs VIN

(Part Switching)

20039705

Disable Current vs VIN

(Part Not Switching)

20039706

Efficiency vs Load Current

20039724

Efficiency vs Load Current

20039725

SHDN Threshold vs VIN

20039713

Switch Current Limit vs VIN

20039737

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LM2705

Switch RDSON vs VIN

20039715

FB Trip Point and FB Pin Current vs Temperature

20039723

Output Voltage vs Load Current

20039727

Off Time vs Temperature

20039741

Step Response

20039728

VOUT = 20V, VIN = 3.0V

1) Load, 0.5mA to 5mA to 0.5mA, DC

2) VOUT, 200mV/div, AC

3) IL, 100mA/div, DC

T = 100µs/div

Start-Up/Shutdown

20039729

VOUT = 20V, VIN = 3.0V

1) SHDN, 1V/div, DC

2) IL, 100mA/div, DC

3) VOUT, 10V/div, DC

T = 400µs/div

RL = 3.9kΩ

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LM2705

Operation

20039704

FIGURE 2. LM2705 Block Diagram

20039730

VOUT = 20V, VIN = 2.7V, IOUT = 2.5mA

1) VSW, 20V/div, DC

2) Inductor Current, 100mA/div, DC

3) VOUT, 200mV/div, AC

T = 10µs/div

FIGURE 3. Typical Switching Waveform

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LM2705

The LM2705 features a constant off-time control scheme.

Operation can be best understood by referring to Figure 2 and

Figure 3. Transistors Q1 and Q2 and resistors R3 and R4 of

Figure 2 form a bandgap reference used to control the output

voltage. When the voltage at the FB pin is less than 1.237V,

the Enable Comp in Figure 2 enables the device and the

NMOS switch is turned on pulling the SW pin to ground. When

the NMOS switch is on, current begins to flow through induc-

tor L while the load current is supplied by the output capacitor

COUT. Once the current in the inductor reaches the current

limit, the CL Comp trips and the 400ns One Shot turns off the

NMOS switch.The SW voltage will then rise to the output volt-

age plus a diode drop and the inductor current will begin to

decrease as shown in Figure 3. During this time the energy

stored in the inductor is transferred to COUT and the load. After

the 400ns off-time the NMOS switch is turned on and energy

is stored in the inductor again. This energy transfer from the

inductor to the output causes a stepping effect in the output

ripple as shown in Figure 3.

This cycle is continued until the voltage at FB reaches 1.237V.

When FB reaches this voltage, the enable comparator then

disables the device turning off the NMOS switch and reducing

the Iq of the device to 40uA. The load current is then supplied

solely by COUT indicated by the gradually decreasing slope at

the output as shown in Figure 3. When the FB pin drops

slightly below 1.237V, the enable comparator enables the de-

vice and begins the cycle described previously. The SHDN

pin can be used to turn off the LM2705 and reduce the Iq to

0.01µA. In shutdown mode the output voltage will be a diode

drop lower than the input voltage.

Application Information

INDUCTOR SELECTION - BOOST REGULATOR

The appropriate inductor for a given application is calculated

using the following equation:

where VD is the schottky diode voltage, ICL is the switch cur-

rent limit found in the Typical Performance Characteristics

section, and TOFF is the switch off time. When using this equa-

tion be sure to use the minimum input voltage for the appli-

cation, such as for battery powered applications. For the

LM2705 constant-off time control scheme, the NMOS power

switch is turned off when the current limit is reached. There is

approximately a 100ns delay from the time the current limit is

reached in the NMOS power switch and when the internal

logic actually turns off the switch. During this 100ns delay, the

peak inductor current will increase. This increase in inductor

current demands a larger saturation current rating for the in-

ductor. This saturation current can be approximated by the

following equation:

Choosing inductors with low ESR decrease power losses and

increase efficiency.

Care should be taken when choosing an inductor. For appli-

cations that require an input voltage that approaches the

output voltage, such as when converting a Li-Ion battery volt-

age to 5V, the 400ns off time may not be enough time to

discharge the energy in the inductor and transfer the energy

to the output capacitor and load. This can cause a ramping

effect in the inductor current waveform and an increased rip-

ple on the output voltage. Using a smaller inductor will cause

the IPK to increase and will increase the output voltage ripple

further.

For typical curves and evaluation purposes the DT1608C se-

ries inductors from Coilcraft were used. Other acceptable

inductors would include, but are not limited to, the SLF6020T

series from TDK, the NP05D series from Taiyo Yuden, the

CDRH4D18 series from Sumida, and the P1166 series from

Pulse.

INDUCTOR SELECTION - SEPIC REGULATOR

The following equation can be used to calculate the approxi-

mate inductor value for a SEPIC regulator:

The boost inductor, L1, can be smaller or larger but is gener-

ally chosen to be the same value as L2. See Figure 9and

Figure 10 for typical SEPIC applications.

DIODE SELECTION

To maintain high efficiency, the average current rating of the

schottky diode should be larger than the peak inductor cur-

rent, IPK. Schottky diodes with a low forward drop and fast

switching speeds are ideal for increasing efficiency in portable

applications. Choose a reverse breakdown of the schottky

diode larger than the output voltage.

CAPACITOR SELECTION

Choose low ESR capacitors for the output to minimize output

voltage ripple. Multilayer ceramic capacitors are the best

choice. For most applications, a 1µF ceramic capacitor is suf-

ficient. For some applications a reduction in output voltage

ripple can be achieved by increasing the output capacitor.

Output voltage ripple can further be reduced by adding a

4.7pF feed-forward capacitor in the feedback network placed

in parallel with RF1, see Figure 2.

Local bypassing for the input is needed on the LM2705. Mul-

tilayer ceramic capacitors are a good choice for this as well.

A 4.7µF capacitor is sufficient for most applications. For ad-

ditional bypassing, a 100nF ceramic capacitor can be used to

shunt high frequency ripple on the input.

LAYOUT CONSIDERATIONS

The input bypass capacitor CIN, as shown in Figure 1, must

be placed close to the IC. This will reduce copper trace re-

sistance which effects input voltage ripple of the IC. For

additional input voltage filtering, a 100nF bypass capacitor

can be placed in parallel with CIN to shunt any high frequency

noise to ground. The output capacitor, COUT, should also be

placed close to the IC. Any copper trace connections for the

Cout capacitor can increase the series resistance, which di-

rectly effects output voltage ripple. The feedback network,

resistors R1 and R2, should be kept close to the FB pin to

minimize copper trace connections that can inject noise into

the system. The ground connection for the feedback resistor

network should connect directly to an analog ground plane.

The analog ground plane should tie directly to the GND pin.

If no analog ground plane is available, the ground connection

for the feedback network should tie directly to the GND pin.

Trace connections made to the inductor and schottky diode

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LM2705

should be minimized to reduce power dissipation and in-

crease overall efficiency.

Copt / Ropt included

20039749

Copt / Ropt excluded

20039750

FIGURE 4. Output Ripple Voltage

20039709

20039742

FIGURE 5. 2 White LED Application and Efficiency

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LM2705

20039734

20039743

FIGURE 6. 3 White LED Application and Efficiency

20039735

FIGURE 7. Li-Ion 12V Application

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LM2705

20039736

FIGURE 8. 5V to 12V Application

20039739

FIGURE 9. 3.3V SEPIC Application

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LM2705

20039740

FIGURE 10. 5V SEPIC Application

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LM2705

Physical Dimensions inches (millimeters) unless otherwise noted

5-Lead Small Outline Package (M5)

For Ordering, Refer to Ordering Information Table

NS Package Number MA05B

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LM2705

Notes

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LM2705

Notes

LM2705 Micropower Step-up DC/DC Converter with 150mA Peak Current Limit

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