LM2621MMX - NATIONAL SEMICONDUCTOR

LM2621

LM2621 Low Input Voltage, Step-Up DC-DC Converter

Literature Number: SNVS033C

LM2621

Low Input Voltage, Step-Up DC-DC Converter

General Description

The LM2621 is a high efficiency, step-up DC-DC switching

regulator for battery-powered and low input voltage systems.

It accepts an input voltage between 1.2V and 14V and

converts it into a regulated output voltage. The output volt-

age can be adjusted between 1.24V and 14V. It has an

internal 0.17ΩN-Channel MOSFET power switch. Efficien-

cies up to 90% are achievable using the LM2621.

The high switching frequency (adjustable up to 2MHz) of the

LM2621 allows for tiny surface mount inductors and capaci-

tors. Because of the unique constant-duty-cycle gated oscil-

lator topology very high efficiencies are realized over a wide

load range. The supply current is reduced to 80µA because

of the BiCMOS process technology. In the shutdown mode,

the supply current is less than 2.5µA.

The LM2621 is available in a Mini-SO-8 package. This pack-

age uses half the board area of a standard 8-pin SO and has

a height of just 1.09 mm.

Features

nSmall Mini-SO8 Package (Half the Footprint of Standard

8-Pin SO Package)

n1.09 mm Package Height

nUp to 2 MHz Switching Frequency

n1.2V to 14V Input Voltage

n1.24V - 14V Adjustable Output Voltage

nUp to 1A Load Current

n0.17 ΩInternal MOSFET

nUp to 90% Regulator Efficiency

n80 µA Typical Operating Current

n<2.5µA Guaranteed Supply Current In Shutdown

Applications

nPDAs, Cellular Phones

n2-Cell and 3-Cell Battery-Operated Equipment

nPCMCIA Cards, Memory Cards

nFlash Memory Programming

nTFT/LCD Applications

n3.3V to 5.0V Conversion

nGPS Devices

nTwo-Way Pagers

nPalmtop Computers

nHand-Held Instruments

Typical Application Circuit

10093412

March 2005

LM2621 Low Input Voltage, Step-Up DC-DC Converter

Connection Diagram

Mini SO-8 (MM) Package

10093418

Top View

Ordering Information

Order Number Package Type NSC Package

Drawing

Package

Marking Supplied As

LM2621MMX Mini SO-8 MUA08A S06A 3000 Units on Tape and Reel

LM2621MM Mini SO-8 MUA08A S06A 1000 Units on Tape and Reel

LM2621

www.national.com 2

Absolute Maximum Ratings (Note 1)

If Military/Aerospace specified devices are required,

please contact the National Semiconductor Sales Office/

Distributors for availability and specifications.

SW Pin Voltage −0.5 V to 14.5V

BOOT, V

, EN and FB Pins −0.5V to 10V

FREQ Pin 100µA

(Note 2) 240˚C/W

Jmax

(Note 2) 150˚C

Storage Temperature Range −65˚C to +150˚C

Lead Temp. (Soldering, 5 sec) 260˚C

Power Dissipation (T

=25˚C)

(Note 2)

500mW

ESD Rating (Note 3) 2kV

Operating Conditions (Note 1)

Pin 2.5V to 5V

FB, EN Pins 0 to V

BOOT Pin 0 to 10V

Ambient Temperature (T

) −40˚C to +85˚C

Electrical Characteristics

Limits in standard typeface are for T

= 25˚C, and limits in boldface type apply over the full operating temperature range of

−40˚C to +85˚C. Unless otherwise specified: V

OUT

= 3.3V.

Symbol Parameter Condition Typ Min Max Units

IN_ST

Minimum Start-Up Supply

Voltage (Note 4)

LOAD

= 0mA 1.1 1.2 V

IN_OP

Minimum Operating Supply

Voltage (once started)

LOAD

= 0mA 0.65 V

FB Pin Voltage 1.24 1.2028 1.2772 V

OUT_MAX

Maximum Output Voltage 14 V

HYST

Hysteresis Voltage (Note 7) 30 45 mV

ηEfficiency V

= 3.6V; V

OUT

= 5V; I

LOAD

500mA 87

= 2.5V; V

OUT

= 3.3V; I

LOAD

= 200mA

D Switch Duty Cycle 70 60 80 %

Operating Quiescent Current

(Note 6)

FB Pin >1.3V; EN Pin at V

80 110 µA

Shutdown Quiescent Current

(Note 7)

, BOOT and SW Pins at

5.0V; EN Pin <200mV

0.01 2.5 µA

Switch Peak Current Limit 2.85 A

DS_ON

MOSFET Switch On

Resistance

0.17 Ω

Enable Section

EN_LO

EN Pin Voltage Low (Note 8) 0.15V

EN_HI

EN Pin Voltage High (Note 8) 0.7V

Note 1: Absolute maximum ratings indicate limits beyond which damage to the device may occur. Electrical specifications do not apply when operating the device

outside of its rated operating conditions.

Note 2: The maximum power dissipation must be derated at elevated temperatures and is dictated by Tjmax (maximum junction temperature), θJA (junction to

ambient thermal resistance), and TA(ambient temperature). The maximum allowable power dissipation at any temperature is Pdmax =(T

jmax -T

A)/ θJA or the number

given in the Absolute Maximum Ratings, whichever is lower.

Note 3: The human body model is a 100 pF capacitor discharged through a 1.5 kΩresistor into each pin. For Pin 8 (SW) the ESD rating is 1.5 kV.

Note 4: Output in regulation, VOUT =V

OUT (NOMINAL)±5%

Note 5: This is the hysteresis value of the internal comparator used for the gated-oscillator control scheme.

Note 6: This is the current into the VDD pin.

Note 7: This is the total current into pins VDD, BOOT, SW and FREQ.

Note 8: When the EN pin is below VEN_LO, the regulator is shut down; when it is above VEN_HI, the regulator is operating.

LM2621

www.national.com3

Pin Description

Pin Name Function

1 PGND Power Ground

2 EN Active-Low Shutdown Input

3 FREQ Frequency Adjust. An external resistor connected between this pin and Pin 6 (V

) sets the switching

frequency of the LM2621.

4 FB Output Voltage Feedback

5 SGND Signal Ground

Power Supply for Internal Circuitry

7 BOOT Bootstrap Supply for the Gate Drive of Internal MOSFET Power Switch

8 SW Drain of the Internal MOSFET Power Switch

Typical Performance Characteristics

Efficiency vs Load Current

OUT

= 5.0V

Efficiency vs Load Current

OUT

= 3.3V

10093401 10093402

vs Temperature

10093403 10093404

LM2621

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Typical Performance Characteristics (Continued)

vs Temperature I

vs V

10093405 10093406

vs V

IN_ST

vs Load Current

OUT

= 3.3V

10093407 10093408

Switching Frequency vs R

Peak Inductor Current vs

Load Current

10093409 10093410

LM2621

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Typical Performance Characteristics (Continued)

Maximum Load Current vs

Input Voltage

10093411

Detailed Description

OPERATING PRINCIPLE

The LM2621 is designed to provide step-up DC-DC voltage

regulation in battery-powered and low-input voltage sys-

tems. It combines a step-up switching regulator, N-channel

power MOSFET, built-in current limit, thermal limit, and volt-

age reference in a single 8-pin MSOP package . The switch-

ing DC-DC regulator boosts an input voltage between 1.2V

and 14V to a regulated output voltage between 1.24V and

14V. The LM2621 starts from a low 1.1V input and remains

operational down to 0.65V.

This device is optimized for use in cellular phones and other

applications requiring a small size, low profile, as well as low

quiescent current for maximum battery life during stand-by

and shutdown. A high-efficiency gated-oscillator topology

offers an output of up to 1A.

Additional features include a built-in peak switch current

limit, and thermal protection circuitry.

10093414

FIGURE 1. Functional Diagram

LM2621

www.national.com 6

Detailed Description (Continued)

GATED OSCILLATOR CONTROL SCHEME

A unique gated oscillator control scheme enables the

LM2621 to have an ultra-low quiescent current and provides

a high efficiency over a wide load range. The switching

frequency of the internal oscillator is programmable using an

external resistor and can be set between 300 kHz and 2

MHz.

This control scheme uses a hysteresis window to regulate

the output voltage. When the output voltage is below the

upper threshold of the window, the LM2621 switches con-

tinuously with a fixed duty cycle of 70% at the switching

frequency selected by the user. During the first part of each

switching cycle, the internal N-channel MOSFET switch is

turned on. This causes the current to ramp up in the inductor

and store energy. During the second part of each switching

cycle, the MOSFET is turned off. The voltage across the

inductor reverses and forces current through the diode to the

output filter capacitor and the load. Thus when the LM2621

switches continuously, the output voltage starts to ramp up.

When the output voltage hits the upper threshold of the

window, the LM2621 stops switching completely. This

causes the output voltage to droop because the energy

stored in the output capacitor is depleted by the load. When

the output voltage hits the lower threshold of the hysteresis

window, the LM2621 starts switching continuously again

causing the output voltage to ramp up towards the upper

threshold. Figure 2 shows the switch voltage and output

voltage waveforms.

Because of this type of control scheme, the quiescent cur-

rent is inherently very low. At light loads the gated oscillator

control scheme offers a much higher efficiency compared to

the conventional PWM control scheme.

LOW VOLTAGE START-UP

The LM2621 can start-up from input voltages as low as 1.1V.

On start-up, the control circuitry switches the N-channel

MOSFET continuously at 70% duty cycle until the output

voltage reaches 2.5V. After this output voltage is reached,

the normal step-up regulator feedback and gated oscillator

control scheme take over. Once the device is in regulation it

can operate down to a 0.65V input, since the internal power

for the IC can be boot-strapped from the output using the

pin.

SHUTDOWN

The LM2621 features a shutdown mode that reduces the

quiescent current to less than a guaranteed 2.5µA over

temperature. This extends the life of the battery in battery

powered applications. During shutdown, all feedback and

control circuitry is turned off. The regulator’s output voltage

drops to one diode drop below the input voltage. Entry into

the shutdown mode is controlled by the active-low logic input

pin EN (Pin 2). When the logic input to this pin pulled below

0.15V

, the device goes into shutdown mode. The logic

input to this pin should be above 0.7V

for the device to

work in normal step-up mode.

OUTPUT VOLTAGE RIPPLE FREQUENCY

A major component of the output voltage ripple is due to the

hysteresis used in the gated oscillator control scheme. The

frequency of this voltage ripple is proportional to the load

current. The frequency of this ripple does not necessitate the

use of larger inductors and capacitors however, since the

size of these components is determined by the switching

frequency of the oscillator which can be set upto 2MHz using

an external resistor.

INTERNAL CURRENT LIMIT AND THERMAL

PROTECTION

An internal cycle-by-cycle current limit serves as a protection

feature. This is set high enough (2.85A typical, approxi-

mately 4A maximum) so as not to come into effect during

normal operating conditions. An internal thermal protection

circuitry disables the MOSFET power switch when the junc-

tion temperature (T

) exceeds about 160˚C. The switch is

re-enabled when T

drops below approximately 135˚C.

10093415

FIGURE 2. Typical Step-Up Regulator Waveforms

LM2621

www.national.com7

Design Procedure

SETTING THE OUTPUT VOLTAGE

The output voltage of the step-up regulator can be set be-

tween 1.24V and 14V by connecting a feedback resistive

divider made of R

and R

. The resistor values are se-

lected as follows:

/[(V

OUT

/ 1.24) −1]

A value of 150kΩis suggested for R

. Then, R

can be

selected using the above equation. A 39pF capacitor (C

)

connected across R

helps in feeding back most of the AC

ripple at V

OUT

to the FB pin. This helps reduce the peak-to-

peak output voltage ripple as well as improve the efficiency

of the step-up regulator, because a set hysteresis of 30mV at

the FB pin is used for the gated oscillator control scheme.

BOOTSTRAPPING

When the output voltage (V

OUT

) is between 2.5V and 5.0V a

bootstrapped operation is suggested. This is achieved by

connecting the V

pin (Pin 6) to V

OUT

. However if the V

OUT

is outside this range, the V

pin should be connected to a

voltage source whose range is between 2.5V and 5V. This

can be the input voltage (V

), V

OUT

stepped down using a

linear regulator, or a different voltage source available in the

system. This is referred to as non-bootstrapped operation.

The maximum acceptable voltage at the BOOT pin (Pin 7) is

10V.

SETTING THE SWITCHING FREQUENCY

The switching frequency of the oscillator is selected by

choosing an external resistor (R

) connected between

FREQ and V

pins. See the graph titled „Switching Fre-

quency vs R

“ in the Typical Operating Characteristics

section of the datasheet for choosing the R

value to

achieve the desired switching frequency. A high switching

frequency allows the use of very small surface mount induc-

tors and capacitors and results in a very small solution size.

A switching frequency between 300kHz and 2MHz is recom-

mended.

INDUCTOR SELECTION

The LM2621’s high switching frequency enables the use of a

small surface mount inductor. A 6.8µH shielded inductor is

suggested. The inductor should have a saturation current

rating higher than the peak current it will experience during

circuit operation (see graph titled „Peak Inductor Current vs.

Load Current“ in the Typical Performance Characteristics

section). Less than 100mΩESR is suggested for high effi-

ciency.

Open-core inductors cause flux linkage with circuit compo-

nents and interfere with the normal operation of the circuit.

They should be avoided. For high efficiency, choose an

inductor with a high frequency core material, such as ferrite,

to reduce the core losses. To minimize radiated noise, use a

toroid, pot core or shielded core inductor. The inductor

should be connected to the SW pin as close to the IC as

possible. See Table 1 for a list of the inductor manufacturers.

OUTPUT DIODE SELECTION

A Schottky diode should be used for the output diode. The

forward current rating of the diode should be higher than the

load current, and the reverse voltage rating must be higher

than the output voltage. Do not use ordinary rectifier diodes,

since slow switching speeds and long recovery times cause

the efficiency and the load regulation to suffer. Table 1 shows

a list of the diode manufacturers.

INPUT AND OUTPUT FILTER CAPACITORS SELECTION

Tantalum chip capacitors are recommended for the input and

output filter capacitors. A 22µF capacitor is suggested for the

input filter capacitor. It should have a DC working voltage

rating higher than the maximum input voltage. A 68µF tan-

talum capacitor is suggested for the output capacitor. The

DC working voltage rating should be greater than the output

voltage. Very high ESR values (>3Ω) should be avoided.

Table 1 shows a list of the capacitor manufacturers.

TABLE 1. Suggested Manufacturers List

Inductors Capacitors Diodes

Coilcraft

Tel: (800) 322-2645

Fax: (708) 639-1469

Sprague/ Vishay

Tel: (207) 324-4140

Fax: (207) 324-7223

Motorola

Tel: (800) 521-6274

Fax: (602) 244-6609

Coiltronics

Tel: (407) 241-7876

Fax: (407) 241-9339

Kemet

Tel: (864) 963-6300

Fax: (864) 963-6521

International Rectifier (IR)

Tel: (310) 322-3331

Fax: (310) 322-3332

Pulse Engineering

Tel: (619) 674-8100

Fax: (619) 674-8262

Nichicon

Tel: (847) 843-7500

Fax: (847) 843-2798

General Semiconductor

Tel: (516) 847-3222

Fax: (516) 847-3150

PC BOARD LAYOUT

High switching frequencies and high peak currents make a

proper layout of the PC board an important part of design.

Poor design can cause excessive EMI and ground-bounce,

both of which can cause malfunction and loss of regulation

by corrupting voltage feedback signal and injecting noise

into the control section.

Power components - such as the inductor, input and output

filter capacitors, and output diode - should be placed as

close to the regulator IC as possible, and their traces should

be kept short, direct and wide. The ground pins of the input

and output filter capacitors and the PGND and SGND pins of

LM2621 should be connected using short, direct and wide

traces. The voltage feedback network (R

, and C

)

should be kept very close to the FB pin. Noisy traces, such

as from the SW pin, should be kept away from the FB and

pins. The traces that run between V

out

and the FB pin of

the IC should be kept away from the inductor flux. Always

provide sufficient copper area to dissipate the heat due to

power loss in the circuitry and prevent the thermal protection

circuitry in the IC from shutting the IC down.

LM2621

www.national.com 8

Application examples

EXAMPLE 1. 5V/0.5A Step-Up Regulator

10093412

U1 National LM2621MM

C1 Vishay/Sprague 595D226X06R3B2T, Tantalum

C2 Vishay/Sprague 595D686X0010C2T, Tantalum

D1 Motorola MBRS140T3

L Coilcraft DT1608C-682

EXAMPLE 2. 2mm Tall 5V/0.2A Step-Up Regulator for Low Profile Applications

10093417

U1 National LM2621MM

C1 Vishay/Sprague 592D156X06R3B2T, Tantalum

C2 Vishay/Sprague 592D336X06R3C2T, Tantalum

D1 Motorola MBRS140T3

L Vishay/Dale ILS-3825-03

LM2621

www.national.com9

EXAMPLE 3. 3.3V/0.5A SEPIC Regulator

10093422

U1 National LM2621MM

C1 Vishay/Sprague 595D226X06R3B2T, Tantalum

C2 Vishay/Sprague 595D686X0010C2T, Tantalum

D1 Motorola MBRS140T3

L1, L2 Coilcraft DT1608C-682

Vishay/Vitramon VJ1210Y105M , Ceramic

LM2621

www.national.com 10

Physical Dimensions inches (millimeters)

unless otherwise noted

8-Lead Mini SO-8 (MM)

NS Package Number MUA08A

For Order Numbers, refer to the table in the "Ordering Information" section of this document.

National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves

the right at any time without notice to change said circuitry and specifications.

For the most current product information visit us at www.national.com.

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properly used in accordance with instructions for use

provided in the labeling, can be reasonably expected to result

in a significant injury to the user.

2. A critical component is any component of a life support

device or system whose failure to perform can be reasonably

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www.national.com

LM2621 Low Input Voltage, Step-Up DC-DC Converter

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