Datasheet LM2621MMX, LM2621MM Datasheet (NSC)

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

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

February 2000

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

0.17Ω N-Channel MOSFET power switch. Efficiencies 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 capaci­tors. Because of the unique constant-duty-cycle gated oscil­lator 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 pack­age uses half the board area of a standard 8-pin SO and has
a height of just 1.09 mm.

Features

n Small Mini-SO8 Package (Half the Footprint of Standard

8-Pin SO Package)

n 1.09 mm Package Height
n Up to 2 MHz Switching Frequency
n 1.2V to 14V Input Voltage
n 1.24V - 14V Adjustable Output Voltage
n Up to 1A Load Current
n 0.17 Ω Internal MOSFET
n Up to 90% Regulator Efficiency
n 80 µA Typical Operating Current

<

n

2.5µA Guaranteed Supply Current In Shutdown

Applications

n PDAs, Cellular Phones
n 2-Cell and 3-Cell Battery-Operated Equipment
n PCMCIA Cards, Memory Cards
n Flash Memory Programming
n TFT/LCD Applications
n 3.3V to 5.0V Conversion
n GPS Devices
n Two-Way Pagers
n Palmtop Computers
n Hand-Held Instruments

Typical Application Circuit

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© 2000 National Semiconductor Corporation DS100934 www.national.com

Connection Diagram

LM2621

Ordering Information

Mini SO-8 (MM) Package

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Top View

Order Number Package Type

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

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

NSC Package

Drawing

Package

Marking

Supplied As

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LM2621

Absolute Maximum Ratings (Note 1)

If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/

Power Dissipation (T

=25˚C)

A

500mW

(Note 2)
ESD Rating (Note 3) 2kV

Distributors for availability and specifications.

SW Pin Voltage −0.5 V to 14.5V
BOOT, V

, EN and FB Pins −0.5V to 10V

DD

FREQ Pin 100µA

θ

(Note 2) 240˚C/W

JA

T

(Note 2) 150˚C

Jmax

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

Operating Conditions (Note 1)

V

Pin 2.5V to 5V

DD

FB, EN Pins 0 to V
BOOT Pin 0 to 10V
Ambient Temperature (T

) −40˚C to +85˚C

A

DD

Lead Temp. (Soldering, 5 sec) 260˚C

Electrical Characteristics

Limits in standard typeface are for TJ= 25˚C, and limits in boldface type apply over the full operating temperature range of

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

Symbol Parameter Condition Typ Min Max Units

V

IN_ST

Minimum Start-Up Supply

I

LOAD

Voltage (Note 4)

V

IN_OP

Minimum Operating Supply

I

LOAD

Voltage (once started)

V

FB

V

OUT_MAX

V

HYST

η Efficiency V

FB Pin Voltage 1.24 1.2028 1.2772 V
Maximum Output Voltage 14 V
Hysteresis Voltage (Note 7) 30 45 mV

IN

= 500mA
V

IN

= 200mA
D Switch Duty Cycle 70 60 80 %
I

DD

Operating Quiescent Current

FB Pin>1.3V; EN Pin at V

(Note 6)

I

SD

I

CL

R

DS_ON

Shutdown Quiescent Current
(Note 7)

VDD, BOOT and SW Pins at

5.0V; EN Pin
Switch Peak Current Limit 2.85 A
MOSFET Switch On

Resistance

Enable Section

V

EN_LO

V

EN_HI

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 T
bient thermal resistance), and T
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 kΩ resistor into each pin. For Pin 8 (SW) the ESD rating is 1.5 kV.
Note 4: Output in regulation, V
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 V
Note 7: This is the total current into pins V
Note 8: When the EN pin is below V

EN Pin Voltage Low (Note 8) 0.15V
EN Pin Voltage High (Note 8) 0.7V

(ambient temperature). The maximum allowable power dissipation at any temperature is P

A

OUT=VOUT (NOMINAL

pin.

DD

EN_LO

)±5%

, BOOT, SW and FREQ.

DD

, the regulator is shut down; when it is above V

= 3.3V.

OUT

= 0mA 1.1 1.2 V

= 0mA 0.65 V

= 3.6V; V

= 2.5V; V

OUT

OUT

= 5V; I

= 3.3V; I

LOAD

LOAD

DD

87
87

80 110 µA

0.01 2.5 µA

<

200mV

0.17 Ω

DD

DD

(maximum junction temperature), θJA(junction to am-

jmax

, the regulator is operating.

EN_HI

dmax

=(T

)/ θJAor the number

jmax-TA

%

V
V

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Pin Description

LM2621

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

switching frequency of the LM2621.
4 FB Output Voltage Feedback
5 SGND Signal Ground
6V

DD

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

) sets the

DD

Efficiency vs Load Current
V

= 5.0V

OUT

IOPvs Temperature

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Efficiency vs Load Current
V

= 3.3V

OUT

ISDvs Temperature

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VFBvs Temperature

ISDvs V

DD

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IOPvs V

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DD

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V

vs Load Current

IN_ST

V

= 3.3V

OUT

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Switching Frequency vs R

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FQ

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Typical Performance Characteristics (Continued)

LM2621

Peak Inductor Current vs
Load Current

DS100934-10

Maximum Load Current vs
Input Voltage

Detailed Description

OPERATING PRINCIPLE

The LM2621 is designed to provide step-up DC-DC voltage
regulation in battery-powered and low-input voltage sys­tems. It combines a step-up switching regulator, N-channel
power MOSFET, built-in current limit, thermal limit, and volt­age reference in a single 8-pin MSOP package . The switch­ing 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.

DS100934-11

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.

FIGURE 1. Functional Diagram

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 fre-

DS100934-14

quency of the internal oscillator is programmable using an
external resistor and can be set between 300 kHz and 2
MHz.

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Detailed Description (Continued)

This control scheme uses a hysteresis window to regulate

LM2621

the output voltage. When the output voltage is below the up­per threshold of the window, the LM2621 switches continu­ously 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.

FIGURE 2. Typical Step-Up Regulator Waveforms

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
V

pin.

DD

SHUTDOWN

The LM2621 features a shutdown mode that reduces the
quiescent current to less than a guaranteed 2.5µAovertem­perature. 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
put to this pin should be above 0.7V

, the device goes into shutdown mode. The logic in-

DD

for the device to work

DD

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

DS100934-15

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, approxi­mately 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 junc­tion temperature (T
re-enabled when T

) exceeds about 160˚C. The switch is

J

drops below approximately 135˚C.

J

Design Procedure

SETTING THE OUTPUT VOLTAGE

The output voltage of the step-up regulator can be set be­tween 1.24V and 14V by connecting a feedback resistive di­vider made of R
as follows:

Avalue of 150kΩ is suggested for R
lected using the aboveequation.A39pF capacitor (C
nected across R
ripple at V
peak-to-peak output voltage ripple as well as improve the ef-

and RF2. The resistor values are selected

F1

R

F2=RF1

F1

to the FB pin. This helps reduce the

OUT

/[(V

/ 1.24) −1]

OUT

. Then, RF2can be se-

F1

helps in feeding back most of the AC

F1

) con-

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Design Procedure (Continued)

ficiency 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
bootstrapped operation is suggested. This is achieved by
connecting the V

pin (Pin 6) to V

DD

is outside this range, the VDDpin should be connected to a
voltage source whose range is between 2.5V and 5V. This
can be the input voltage (V
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
FREQ and V
quency vs R

pins. See the graph titled „Switching Fre-

DD

“ in the TypicalOperating Characteristics sec-

FQ

tion of the datasheet for choosing the R
the desired switching frequency. A high switching frequency
allows the use of very small surface mount inductors and ca­pacitors and results in a very small solution size. A switching
frequency between 300kHz and 2MHz is recommended.

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

) is between 2.5V and 5.0V a

OUT

. However if the V

OUT

), V

IN

stepped down using a

OUT

) connected between

FQ

value to achieve

FQ

OUT

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 100mΩ ESR is suggested for high effi­ciency.

Open-core inductors cause flux linkage with circuit compo­nents 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 efficiencyand 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. A68µF tantalum
capacitor is suggested for the output capacitor. The DC
working voltage rating should be greater than the output volt­age. Very high ESR values (

1

shows a list of the capacitor manufacturers.

>

3Ω) should be avoided.

Table

LM2621

TABLE 1. Suggested Manufacturers List

Inductors Capacitors Diodes

Coilcraft

Tel: [phone]
Fax: [phone]

Coiltronics

Tel: [phone]
Fax: [phone]

Pulse Engineering

Tel: [phone]
Fax: [phone]

Sprague/ Vishay

Tel: [phone]
Fax: [phone]

Kemet

Tel: [phone]
Fax: [phone]

Nichicon

Tel: [phone]
Fax: [phone]

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

Motorola

Tel: [phone]
Fax: [phone]

International Rectifier (IR)

Tel: [phone]
Fax: [phone]

General Semiconductor

Tel: [phone]
Fax: [phone]

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

F1,RF2

, and CF1)
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
V

pins. The traces that run between V

DD

and the FB pin of

out

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.

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APPLICATION EXAMPLES

LM2621

U1 National LM2621MM
C1 Vishay/Sprague 595D226X06R3B2T, Tantalum
C2 Vishay/Sprague 595D686X0010C2T, Tantalum
D1 Motorola MBRS140T3
L Coilcraft DT1608C-682

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

DS100934-12

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

U1 National LM2621MM
C1 Vishay/Sprague 592D156X06R3B2T, Tantalum
C2 Vishay/Sprague 592D336X06R3C2T, Tantalum
D1 Motorola MBRS140T3
L Vishay/Dale ILS-3825-03

DS100934-17

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APPLICATION EXAMPLES (Continued)

EXAMPLE 3. 3.3V/0.5A SEPIC Regulator

LM2621

U1 National LM2621MM
C1 Vishay/Sprague 595D226X06R3B2T, Tantalum
C2 Vishay/Sprague 595D686X0010C2T, Tantalum
D1 Motorola MBRS140T3
L1, L2 Coilcraft DT1608C-682
C

S

Vishay/Vitramon VJ1210Y105M , Ceramic

DS100934-22

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Physical Dimensions inches (millimeters) unless otherwise noted

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

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.

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