LM3677LE-1.5/NOPB - National Semiconductor

October 28, 2008

LM3677

3MHz, 600mA Miniature Step-Down DC-DC Converter for

Ultra Low Voltage Circuits

General Description

The LM3677 step-down DC-DC converter is optimized for

powering ultra-low voltage circuits from a single Li-Ion cell

battery and input voltage rails from 2.7V to 5.5V. It provides

up to 600 mA load current over the entire input voltage range.

The LM3677 is configured to different fixed voltage output

options as well as an adjustable output voltage version range

from 1.2V to 3.3V.

The device offers superior features and performance for mo-

bile phones and similar portable applications with complex

power management systems. Automatic intelligent switching

between PWM low-noise and PFM low-current mode offers

improved system control. During PWM mode operation, the

device operates at a fixed frequency of 3 MHz (typ). PWM

mode drives loads from ~ 80 mA to 600 mA max. Hysteretic

PFM mode extends the battery life by reducing the quiescent

current to 16 µA (typ.) during light load and standby operation.

Internal synchronous rectification provides high efficiency. In

shutdown mode (Enable pin pulled down), the device turns

off and reduces battery consumption to 0.01 µA (typ.).

The LM3677 is available in a lead-free (NOPB) 5-bump micro

SMD package and 6-pin LLP package. A switching frequency

of 3 MHz (typ.) allows use of tiny surface-mount components.

Only three external surface-mount components, an inductor

and two ceramic capacitors, are required.

Features

■16 µA typical quiescent current

■600 mA maximum load capability

■3 MHz PWM fixed switching frequency (typ.)

■Automatic PFM/PWM mode switching

■Available in 5-bump micro SMD package and 6-pin LLP

package

■Internal synchronous rectification for high efficiency

■Internal soft start

■0.01 µA typical shutdown current

■Operates from a single Li-Ion cell battery

■Only three tiny surface-mount external components

required (solution size less than 20 mm2)

■Current overload and thermal shutdown protection

Applications

■Mobile Phones

■PDAs

■MP3 Players

■W-LAN

■Portable Instruments

■Digital Still Cameras

■Portable Hard Disk Drives

Typical Application Circuits

30008401

FIGURE 1. Typical Application Circuit

30008407

FIGURE 2. Typical Application Circuit - ADJ. Version

Efficiency vs. Output Current

(VOUT = 1.8V)

30008487

LM3677 3MHz, 600mA Miniature Step-Down DC-DC Converter for Ultra Low Voltage Circuits

Connection Diagram and Package Mark Information

5-Bump micro SMD Package NS Package Number TLA05FEA

30008444

FIGURE 3. 5 Bump Micro SMD Package

30008400

FIGURE 4. 6 Pin LLP Package

Pin Descriptions

Pin # Name Description

A1 1 VIN Power supply input. Connect to the input filter capacitor (Figure 1).

A3 6 GND Ground pin.

C1 3 EN Enable pin. The device is in shutdown mode when voltage to this pin is < 0.4V and

enabled when > 1.0V. Do not leave this pin floating.

C3 4 FB Feedback analog input. Connect directly to the output filter capacitor ( FIGURE 1).

B2 2, 5 SW Switching node connection to the internal PFET switch and NFET synchronous

rectifier.

Ordering Information

Order Number Spec Package Marking Supplied As

LM3677TL-1.2 (Note 1) NOPB 3250 units, Tape-and-Reel

LM3677TLX-1.2 (Note 1) NOPB 3000 units, Tape-and-Reel

LM3677TL-1.3 NOPB V250 units, Tape-and-Reel

LM3677TLX-1.3 NOPB 3000 units, Tape-and-Reel

LM3677TL-1.5 NOPB X250 units, Tape-and-Reel

LM3677TLX-1.5 NOPB 3000 units, Tape-and-Reel

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LM3677

Order Number Spec Package Marking Supplied As

LM3677TL-1.8 NOPB Y250 units, Tape-and-Reel

LM3677TLX-1.8 NOPB 3000 units, Tape-and-Reel

LM3677TL-1.875 NOPB 9250 units, Tape-and-Reel

LM3677TLX-1.875 NOPB 3000 units, Tape-and-Reel

LM3677TL-2.5 NOPB Z250 units, Tape-and-Reel

LM3677TLX-2.5 NOPB 3000 units, Tape-and-Reel

LM3677TL-ADJ NOPB 4250 units, Tape-and-Reel

LM3677TLX-ADJ NOPB 3000 units, Tape-and-Reel

LM3677LEE-1.2 NOPB

250 units, Tape-and-Reel

LM3677LE-1.2 NOPB 1000 units, Tape-and-Reel

LM3677LEX-1.2 NOPB 4500 units, Tape-and-Reel

LM3677LEE-1.5 NOPB

250 units, Tape-and-Reel

LM3677LE-1.5 NOPB 1000 units, Tape-and-Reel

LM3677LEX-1.5 NOPB 4500 units, Tape-and-Reel

LM3677LEE-1.8 NOPB

250 units, Tape-and-Reel

LM3677LE-1.8 NOPB 1000 units, Tape-and-Reel

LM3677LEX-1.8 NOPB 4500 units, Tape-and-Reel

LM3677LEE-1.82 NOPB

250 units, Tape-and-Reel

LM3677LE-1.82 NOPB 1000 units, Tape-and-Reel

LM3677LEX-1.82 NOPB 4500 units, Tape-and-Reel

Note 1: For output voltage 1.2V or lower, input voltage needs to be derated to the range of 2.7V to 5.0V in order to perform within specification.

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LM3677

Absolute Maximum Ratings (Note 2)

If Military/Aerospace specified devices are required,

please contact the National Semiconductor Sales Office/

Distributors for availability and specifications.

If Military/Aerospace specified devices are required, please

contact the National Semiconductor Sales Office/Distributor

for availability and specifications.

VIN Pin: Voltage to GND −0.2V to 6.0V

FB, SW, EN Pin: (GND−0.2V) to

(VIN + 0.2V)

Continuous Power Dissipation

(Note 4)

Internally Limited

Junction Temperature (TJ-MAX) +125°C

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

Maximum Lead Temperature

(Soldering, 10 sec.)

260°C

ESD Rating (Note 5)

Human Body Model: All Pins 2.0 kV

Machine Model: All Pins 200V

Operating Ratings (Note 2), (Note 3)

Input Voltage Range 2.7V to 5.5V

Recommended Load Current 0 mA to 600 mA

Junction Temperature (TJ) Range −30°C to +125°C

Ambient Temperature (TA) Range (Note

−30°C to +85°C

Thermal Properties

Junction-to-Ambient Thermal

Resistance (θJA) (Note 7)

85°C/W

Electrical Characteristics (Note 3), (Note 9), (Note 10) Limits in standard typeface are for TJ = TA = 25°C.

Limits in boldface type apply over the operating ambient temperature range (−30°C ≤ TA ≤ +85°C). Unless otherwise noted,

specifications apply to the LM3677 with VIN = EN = 3.6V.

Symbol Parameter Condition Min Typ Max Units

VIN Input Voltage (Note 11) 2.7 5.5 V

VFB

Feedback Voltage (TL) PWM mode -2.5 +2.5 %

Feedback Voltage (LE) -4.0 +4.0

VREF Internal Reference Voltage 0.5 V

ISHDN Shutdown Supply Current EN = 0V 0.01 1µA

IQDC Bias Current into VIN No load, device is not switching 16 35 µA

RDSON (P) Pin-Pin Resistance for PFET VIN= VGS= 3.6V, ISW= 100mA 350 450 mΩ

RDSON (N) Pin-Pin Resistance for NFET VIN= VGS= 3.6V, ISW= -100mA 150 250 mΩ

ILIM Switch Peak Current Limit Open Loop(Note 8) 1085 1220 1375 mA

VIH Logic High Input 1.0 V

VIL Logic Low Input 0.4 V

IEN Enable (EN) Input Current 0.01 1µA

FOSC Internal Oscillator Frequency PWM Mode 2.5 33.5 MHz

Note 2: Absolute Maximum Ratings indicate limits beyond which damage to the component may occur. Operating Ratings are conditions under which operation

of the device is guaranteed. Operating Ratings do not imply guaranteed performance limits. For guaranteed performance limits and associated test conditions,

see the Electrical Characteristics tables.

Note 3: All voltages are with respect to the potential at the GND pin.

Note 4: Internal thermal shutdown circuitry protects the device from permanent damage. Thermal shutdown engages at TJ= 150°C (typ.) and disengages at

TJ= 130°C (typ.).

Note 5: 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. MIL-STD-883 3015.7

Note 6: In Applications where high power dissipation and/or poor package resistance is present, the maximum ambient temperature may have to be derated.

Maximum ambient temperature (TA-MAX) is dependent on the maximum operating junction temperature (TJ-MAX), the maximum power dissipation of the device in

the application (PD-MAX) and the junction to ambient thermal resistance of the package (θJA) in the application, as given by the following equation: TA-MAX= TJ-MAX

− (θJAx PD-MAX). Refer to Dissipation rating table for PD-MAX values at different ambient temperatures.

Note 7: Junction to ambient thermal resistance is highly application and board layout dependent. In applications where high power dissipation exists, special care

must be given to thermal dissipation issues in board design. Value specified here 85 °C/W is based on measurement results using a 4 layer board as per JEDEC

standards.

Note 8: Refer to datasheet curves for closed loop data and its variation with regards to supply voltage and temperature. Electrical Characteristic table reflects

open loop data (FB=0V and current drawn from SW pin ramped up until cycle by cycle current limit is activated). Closed loop current limit is the peak inductor

current measured in the application circuit by increasing output current until output voltage drops by 10%.

Note 9: Min and Max limits are guaranteed by design, test or statistical analysis. Typical numbers are not guaranteed, but do represent the most likely norm.

Note 10: The parameters in the electrical characteristic table are tested under open loop conditions at VIN= 3.6V unless otherwise specified. For performance

over the input voltage range and closed loop condition, refer to the datasheet curves.

Note 11: For output voltage 1.2V or lower, input voltage needs to be derated to the range of 2.7V to 5.0V in order to perform within specification.

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LM3677

Dissipation Rating Table

θJA TA≤ 25°C

Power Rating

TA= 60°C

Power Rating

TA= 85°C

Power Rating

85°C/W (4-layer board)

micro SMD

1178 mW 785 mW 470 mW

117°C/W (4-layer board)

LLP

855 mW 556 mW 342 mW

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LM3677

Block Diagram

30008418

FIGURE 5. Simplified Functional Diagram

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LM3677

Typical Performance Characteristics

LM3677, Circuit of Figure 1, VIN = 3.6V, VOUT = 1.8V, TA = 25°C, unless otherwise noted.

Quiescent Supply Current vs. Supply Voltage

(Switching)

30008481

Shutdown Current vs. Temp

30008482

Switching Frequency vs. Temperature

30008483

RDS(ON) vs. Temperature

30008451

Open/Closed Loop Current Limit

vs. Temperature

30008449

Output Voltage vs. Supply Voltage

(VOUT = 1.8V)

30008484

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LM3677

Output Voltage vs. Supply Voltage

(VOUT = 2.5V)

30008438

Output Voltage vs. Temperature

(VOUT = 1.3V)

30008468

Output Voltage vs. Temperature

(VOUT = 1.8V)

30008485

Output Voltage vs. Temperature

(VOUT = 2.5V)

30008469

Output Voltage vs. Output Current

(VOUT = 1.8V)

30008486

Output Voltage vs. Output Current

(VOUT = 2.5V)

30008437

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LM3677

Efficiency vs. Output Current

(VOUT = 1.3V)

30008441

Efficiency vs. Output Current

(VOUT = 1.8V)

30008487

Efficiency vs. Output Current

(VOUT = 2.5V)

30008432

Output Current vs. Input Voltage at Mode Change Point

(VOUT = 1.3V)

30008435

Output Current vs. Input Voltage at Mode Change Point

(VOUT = 1.8V)

30008488

Output Current vs. Input Voltage at Mode Change Point

(VOUT = 2.5V)

30008436

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LM3677

Line Transient Response

VOUT = 1.3V (PWM Mode)

30008433

Line Transient Response

VOUT = 1.8V (PWM Mode)

30008477

Line Transient Response

VOUT = 1.8V (PWM Mode)

30008478

Line Transient Response

VOUT = 2.5V (PWM Mode)

30008439

Load Transient Response (VOUT = 1.3V)

(PFM Mode 1mA to 50mA)

30008493

Load Transient Response (VOUT = 1.3V)

(PFM Mode 50mA to 1mA)

30008494

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LM3677

Load Transient Response (VOUT = 1.8V)

(PFM Mode 1mA to 50mA)

30008473

Load Transient Response (VOUT = 1.8V)

(PFM Mode 50mA to 1mA)

30008474

Load Transient Response (VOUT = 2.5V)

(PFM Mode 1mA to 50mA)

30008498

Load Transient Response (VOUT = 2.5V)

(PFM Mode 50mA to 1mA)

30008430

Mode Change by Load Transients

VOUT = 1.3V (PFM to PWM)

30008495

Mode Change by Load Transients

VOUT = 1.3V (PWM to PFM)

30008496

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LM3677

Mode Change by Load Transients

VOUT = 1.8V (PFM to PWM)

30008475

Mode Change by Load Transients

VOUT = 1.8V (PWM to PFM)

30008476

Load Transient Response

VOUT = 1.3V (PWM Mode)

30008497

Load Transient Response

VOUT = 1.8V (PWM Mode)

30008472

Load Transient Response

VOUT = 2.5V (PWM Mode)

30008431

Start Up into PWM Mode

VOUT = 1.3V (Output Current= 300mA)

30008491

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LM3677

Start Up into PFM Mode

VOUT = 1.3V (Output Current= 1mA)

30008492

Start Up into PWM Mode

VOUT = 1.8V (Output Current= 300mA)

30008470

Start Up into PFM Mode

VOUT = 1.8V (Output Current= 1mA)

30008471

Start Up into PWM Mode

VOUT = 2.5V (Output Current= 300mA)

30008489

Start Up into PFM Mode

VOUT = 2.5V (Output Current= 1mA)

30008490

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LM3677

Operation Description

DEVICE INFORMATION

The LM3677, a high-efficiency step-down DC-DC switching

buck converter, delivers a constant voltage from a single Li-

Ion battery and input voltage rails from 2.7V to 5.5V to devices

such as cell phones and PDAs. Using a voltage-mode archi-

tecture with synchronous rectification, the LM3677 has the

ability to deliver up to 600 mA depending on the input voltage

and output voltage, ambient temperature, and the inductor

chosen.

There are three modes of operation depending on the current

required: PWM (Pulse Width Modulation), PFM (Pulse Fre-

quency Modulation), and shutdown. The device operates in

PWM mode at load current of approximately 80 mA or higher,

having a voltage precision of ±2.5% with 90% efficiency or

better. Lighter load current causes the device to automatically

switch into PFM mode for reduced current consumption (IQ =

16 µA typ.) and a longer battery life. Shutdown mode turns off

the device, offering the lowest current consumption

(ISHUTDOWN = 0.01 µA (typ.).

Additional features include soft-start, under voltage protec-

tion, current overload protection, and thermal shutdown pro-

tection. As shown in Figure 1, only three external power

components are required for implementation.

The part uses an internal reference voltage of 0.5V. It is rec-

ommended to keep the part in shutdown until the input voltage

exceeds 2.7V.

CIRCUIT OPERATION

The LM3677 operates as follows. During the first portion of

each switching cycle, the control block in the LM3677 turns

on the internal PFET switch. This allows current to flow from

the input through the inductor to the output filter capacitor and

load. The inductor limits the current to a ramp with a slope of

(VIN–VOUT)/L, by storing energy in a magnetic field.

During the second portion of each cycle, the controller turns

the PFET switch off, blocking current flow from the input, and

then turns the NFET synchronous rectifier on. The inductor

draws current from ground through the NFET to the output

filter capacitor and load, which ramps the inductor current

down with a slope of - VOUT/L.

The output filter stores charge when the inductor current is

high, and releases it when inductor current is low, smoothing

the voltage across the load.

The output voltage is regulated by modulating the PFET

switch-on time to control the average current sent to the load.

The effect is identical to sending a duty-cycle modulated rect-

angular wave formed by the switch and synchronous rectifier

at the SW pin to a low-pass filter formed by the inductor and

output filter capacitor. The output voltage is equal to the av-

erage voltage at the SW pin.

PWM OPERATION

During PWM operation, the converter operates as a voltage-

mode controller with input voltage feed forward. This allows

the converter to achieve good load and line regulation. The

DC gain of the power stage is proportional to the input voltage.

To eliminate this dependence, feed forward inversely propor-

tional to the input voltage is introduced.

While in PWM mode, the output voltage is regulated by

switching at a constant frequency and then modulating the

energy per cycle to control power to the load. At the beginning

of each clock cycle the PFET switch is turned on and the in-

ductor current ramps up until the comparator trips and the

control logic turns off the switch. The current limit comparator

can also turn off the switch in case the current limit of the

PFET is exceeded. Then the NFET switch is turned on and

the inductor current ramps down. The next cycle is initiated

by the clock turning off the NFET and turning on the PFET.

30008480

FIGURE 6. Typical PWM Operation

Internal Synchronous Rectification

While in PWM mode, the LM3677 uses an internal NFET as

a synchronous rectifier to reduce rectifier forward voltage

drop and associated power loss. Synchronous rectification

provides a significant improvement in efficiency whenever the

output voltage is relatively low compared to the voltage drop

across an ordinary rectifier diode.

Current Limiting

A current limit feature allows the LM3677 to protect itself and

external components during overload conditions. PWM mode

implements current limiting using an internal comparator that

trips at 1220 mA (typ.). If the output is shorted to ground the

device enters a timed current limit mode where the NFET is

turned on for a longer duration until the inductor current falls

below a low threshold, ensuring inductor current has more

time to decay, thereby preventing runaway.

PFM OPERATION

At very light loads, the converter enters PFM mode and op-

erates with reduced switching frequency and supply current

to maintain high efficiency.

The part will automatically transition into PFM mode when ei-

ther of the following conditions occurs for a duration of 32 or

more clock cycles:

A. The NFET current reaches zero.

B. The peak PMOS switch current drops below the IMODE

level, (Typically IMODE < 75 mA + VIN/55Ω ).

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LM3677

30008479

FIGURE 7. Typical PFM Operation

During PFM operation, the converter positions the output volt-

age slightly higher than the nominal output voltage during

PWM operation allowing additional headroom for voltage

drop during a load transient from light to heavy load. The PFM

comparators sense the output voltage via the feedback pin

and control the switching of the output FETs such that the

output voltage ramps between ~0.2% and ~1.8% above the

nominal PWM output voltage. If the output voltage is below

the ‘high’ PFM comparator threshold, the PMOS power switch

is turned on. It remains on until the output voltage reaches the

‘high’ PFM threshold or the peak current exceeds the IPFM

level set for PFM mode. The typical peak current in PFM mode

is: IPFM = 112 mA + VIN/20Ω .

Once the PMOS power switch is turned off, the NMOS power

switch is turned on until the inductor current ramps to zero.

When the NMOS zero-current condition is detected, the

NMOS power switch is turned off. If the output voltage is be-

low the ‘high’ PFM comparator threshold (see Figure 8), the

PMOS switch is again turned on and the cycle is repeated

until the output reaches the desired level. Once the output

reaches the ‘high’ PFM threshold, the NMOS switch is turned

on briefly to ramp the inductor current to zero, and then both

output switches are turned off and the part enters an ex-

tremely low-power mode. Quiescent supply current during

this ‘sleep’ mode is 16 µA (typ.), which allows the part to

achieve high efficiencies under extremely light load condi-

tions.

If the load current should increase during PFM mode (Figure

8) causing the output voltage to fall below the ‘low2’ PFM

threshold, the part will automatically transition into fixed-fre-

quency PWM mode. When VIN =2.7V the part transitions from

PWM to PFM mode at ~ 35 mA output current and from PFM

to PWM mode at ~ 95 mA , when VIN=3.6V, PWM to PFM

transition occurs at ~ 42 mA and PFM to PWM transition oc-

curs at ~ 115 mA, when VIN =4.5V, PWM to PFM transition

occurs at ~ 60 mA and PFM to PWM transition occurs at ~

135 mA.

30008403

FIGURE 8. Operation in PFM Mode and Transfer to PWM Mode

SHUTDOWN MODE

Setting the EN input pin low (<0.4V) places the LM3677 in

shutdown mode. During shutdown the PFET switch, NFET

switch, reference, control and bias circuitry of the LM3677 are

turned off. Setting EN high (>1.0V) enables normal operation.

It is recommended to set EN pin low to turn off the LM3677

during system power up and undervoltage conditions when

the supply is less than 2.7V. Do not leave the EN pin floating.

SOFT START

The LM3677 has a soft-start circuit that limits in-rush current

during start-up. During start-up the switch current limit is in-

creased in steps. Soft start is activated only if EN goes from

logic low to logic high after VIN reaches 2.7V. Soft start is im-

plemented by increasing switch current limit in steps of 200

mA, 400 mA, 600 mA and 1220 mA (typical switch current

limit). The start-up time thereby depends on the output ca-

pacitor and load current demanded at start-up. Typical start-

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LM3677

up times with a 10 µF output capacitor and 300 mA load is

300 µs and with 1 mA load is 200 µs.

Application Information

INDUCTOR SELECTION

There are two main considerations when choosing an induc-

tor: the inductor should not saturate, and the inductor current

ripple should be small enough to achieve the desired output

voltage ripple. Different saturation current rating specifica-

tions are followed by different manufacturers so attention

must be given to details. Saturation current ratings are typi-

cally specified at 25°C. However, ratings at the maximum

ambient temperature of application should be requested form

the manufacturer. The minimum value of inductance to

guarantee good performance is 0.7 µH at ILIM (typ.) DC

current over the ambient temperature range. Shielded in-

ductors radiate less noise and should be preferred.

There are two methods to choose the inductor saturation cur-

rent rating.

Method 1:

The saturation current is greater than the sum of the maxi-

mum load current and the worst case average to peak induc-

tor current. This can be written as

•IRIPPLE: average to peak inductor current

•IOUTMAX: maximum load current (600 mA)

•VIN: maximum input voltage in application

•L : min inductor value including worst case tolerances

(30% drop can be considered for method 1)

•f : minimum switching frequency (2.5 MHz)

•VOUT: output voltage

Method 2:

A more conservative and recommended approach is to

choose an inductor that has saturation current rating greater

than the max current limit of 1375 mA.

A 1.0 µH inductor with a saturation current rating of at least

1375 mA is recommended for most applications. The

inductor’s resistance should be less than 0.15Ω for good ef-

ficiency. Table 1 lists suggested inductors and suppliers. For

low-cost applications, an unshielded bobbin inductor could be

considered. For noise critical applications, a toroidal or shield-

ed-bobbin inductor should be used. A good practice is to lay

out the board with overlapping footprints of both types for de-

sign flexibility. This allows substitution of a low-noise shielded

inductor in the event that noise from low-cost bobbin models

is unacceptable.

INPUT CAPACITOR SELECTION

A ceramic input capacitor of 4.7 µF, 6.3V is sufficient for most

applications. Place the input capacitor as close as possible to

the VIN pin of the device. A larger value may be used for im-

proved input voltage filtering. Use X7R or X5R types; do not

use Y5V. DC bias characteristics of ceramic capacitors must

be considered when selecting case sizes like 0603 and 0805.

The minimum input capacitance to guarantee good per-

formance is 2.2 µF at 3V DC bias; 1.5 µF at 5V DC bias

including tolerances and over ambient temperature

range. The input filter capacitor supplies current to the PFET

switch of the LM3677 in the first half of each cycle and re-

duces voltage ripple imposed on the input power source. A

ceramic capacitor’s low ESR provides the best noise filtering

of the input voltage spikes due to this rapidly changing cur-

rent. Select a capacitor with sufficient ripple current rating.

The input current ripple can be calculated as:

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LM3677

TABLE 1. Suggested Inductors and Their Suppliers

Model Vendor Dimensions LxWxH(mm) D.C.R (max)

MIPSA2520D 1R0 FDK 2.5 x 2.0 x 1.2 100 mΩ

LQM2HP 1R0 Murata 2.5 x 2.0 x 0.95 100 mΩ

BRL2518T1R0M Taiyo Yuden 2.5x 1.8 x 1.2 80 mΩ

OUTPUT CAPACITOR SELECTION

A ceramic output capacitor of 10 µF, 6.3V is sufficient for most

applications. Use X7R or X5R types; do not use Y5V. DC bias

characteristics of ceramic capacitors must be considered

when selecting case sizes like 0603 and 0805. DC bias char-

acteristics vary from manufacturer to manufacturer and dc

bias curves should be requested from them as part of the ca-

pacitor selection process.

The minimum output capacitance to guarantee good per-

formance is 5.75 µF at 2.5V DC bias including tolerances

and over ambient temperature range. The output filter ca-

pacitor smooths out current flow from the inductor to the load,

helps maintain a steady output voltage during transient load

changes and reduces output voltage ripple. These capacitors

must be selected with sufficient capacitance and sufficiently

low ESR to perform these functions.

The output voltage ripple is caused by the charging and dis-

charging of the output capacitor and by the RESR and can be

calculated as:

Voltage peak-to-peak ripple due to capacitance can be ex-

pressed as follows

Voltage peak-to-peak ripple due to ESR can be expressed as

follows

VPP-ESR = (2 * IRIPPLE) * RESR

Because these two components are out of phase the rms (root

mean squared) value can be used to get an approximate val-

ue of peak-to-peak ripple.

Voltage peak-to-peak ripple, rms can be expressed as follow:

Note that the output voltage ripple is dependent on the induc-

tor current ripple and the equivalent series resistance of the

output capacitor (RESR).

The RESR is frequency dependent (as well as temperature

dependent); make sure the value used for calculations is at

the switching frequency of the part.

TABLE 2. Suggested Capacitors and Their Suppliers

Model Type Vendor Voltage Rating Case Size

Inch (mm)

4.7 µF for CIN

C1608X5R0J475 Ceramic, X5R TDK 6.3V 0603 (1608)

C2012X5R0J475 Ceramic, X5R TDK 6.3V 0805 (2012)

GRM21BR60J475 Ceramic, X5R muRata 6.3V 0805 (2012)

JMK212BJ475 Ceramic, X5R Taiyo-Yuden 6.3V 0805 (2012)

10 µF for COUT

C1608X5R0J106 Ceramic, X5R TDK 6.3V 0603 (1608)

C2012X5R0J106 Ceramic, X5R TDK 6.3V 0805 (2012)

GRM21BR60J106 Ceramic, X5R muRata 6.3V 0805 (2012)

JMK212BJ106 Ceramic, X5R Taiyo-Yuden 6.3V 0805 (2012)

17 www.national.com

LM3677

OUTPUT VOLTAGE SELECTION FOR LM3677-ADJ

The output voltage of the adjustable parts can be pro-

grammed through the resistor network connected from VOUT

to FB to GND. The resistor and FB to GND (R2) should be 200

kΩ to keep the current drawn through this network well below

16 µA quiescent current level (PFM mode) but large enough

that it is not susceptible to noise. If R2 is 200 kΩ, and given

the VFB is 0.5V, then the current through the resistor feedback

network will be 2.5 µA. The output voltage of the adjustable

parts ranges from 1.2V and 3.3V. The output voltage formula

is:

30008406

VOUT: output voltage (V)

VFB: feedback voltage (0.5V typical)

R1: feedback resistor from VOUT to FB (Ω)

R2: feedback resistor from to FB to GND (Ω)

For the fixed output voltage parts the feedback resistors are

internal and R1 is 0Ω.

The bypass capacitors C1 and C2 (labeled C3 and C4 on

Evaluation Board) in parallel with the feedback resistors are

chosen for increased stability. Below are the formulas for C1

and C2:

30008404

30008405

TABLE 3. LM3677–ADJ Configurations for Various VOUT (Circuit of Figure 2)

VOUT (V) R1(kΩ ) R2(kΩ ) C1(pF) C2(pF) L (µH) CIN (µF) COUT (µF)

1.2 280 200 8.2 none 1.0 4.7 10

1.3 320 200 8.2 none 1.0 4.7 10

1.5 357 178 6.8 none 1.0 4.7 10

1.6 442 200 5.6 none 1.0 4.7 10

1.8 464 178 5.6 none 1.0 4.7 10

2.5 402 100 6.0 none 1.0 4.7 10

2.8 464 100 5.6 24 1.0 4.7 10

3.3 562 100 5.6 24 1.0 4.7 10

www.national.com 18

LM3677

Micro SMD Package Assembly And

Use

Use of the micro SMD package requires specialized board

layout, precision mounting and careful re-flow techniques, as

detailed in National Semiconductor Application Note 1112.

Refer to the section "Surface Mount Technology (SMD) As-

sembly Considerations". For best results in assembly, align-

ment ordinals on the PC board should be used to facilitate

placement of the device. The pad style used with micro SMD

package must be the NSMD (non-solder mask defined typ.).

This means that the solder-mask opening is larger than the

pad size. This prevents a lip that otherwise forms if the solder-

mask and pad overlap, from holding the device off the surface

of the board and interfering with mounting. See Application

Note 1112 for specific instructions how to do this. The 5-bump

package used for LM3677 has 300–micron solder balls and

requires 10.82 mils pads for mounting on the circuit board.

The trace to each pad should enter the pad with a 90° entry

angle to prevent debris from being caught in deep corners.

Initially, the trace to each pad should be 7 mil wide, for a sec-

tion approximately 7 mil long or longer, as a thermal relief.

Then each trace should neck up or down to its optimal width.

The important criteria is symmetry. This ensures the solder

bumps on the LM3677 re-flow evenly and that the device sol-

ders level to the board. In particular, special attention must be

paid to the pads for bumps A1 and A3, because GND and

VIN are typically connected to large copper planes, inade-

quate thermal relief can result in late or inadequate re-flow of

these bumps.

The micro SMD package is optimized for the smallest possi-

ble size in applications with red or infrared opaque cases.

Because the micro SMD package lacks the plastic encapsu-

lation characteristic of larger devices, it is vulnerable to light.

Backside metallization and/or epoxy coating, along with front-

side shading by the printed circuit board, reduce this sensi-

tivity. However, the package has exposed die edges. In

particular, micro SMD devices are sensitive to light, in the red

and infrared range, shining on the package’s exposed die

edges.

BOARD LAYOUT CONSIDERATIONS

PC board layout is an important part of DC-DC converter de-

sign. Poor board layout can disrupt the performance of a DC-

DC converter and surrounding circuitry by contributing to EMI,

ground bounce, and resistive voltage loss in the traces. These

can send erroneous signals to the DC-DC converter IC, re-

sulting in poor regulation or instability. Poor layout can also

result in re-flow problems leading to poor solder joints be-

tween the micro SMD package and board pads. Poor solder

joints can result in erratic or degraded performance.

30008454

FIGURE 9. Board Layout Design Rules for the LM3677

Good layout for the LM3677 can be implemented by following

a few simple design rules, as illustrated in .

1. Place the LM3677 on 10.82 mil pads. As a thermal relief,

connect to each pad with a 7 mil wide, approximately 7

mil long trace, and then incrementally increase each

trace to its optimal width. The important criterion is

symmetry to ensure the solder bumps on the re-flow

evenly (see Micro SMD Package Assembly and Use).

2. Place the LM3677, inductor and filter capacitors close

together and make the traces short. The traces between

these components carry relatively high switching

currents and act as antennas. Following this rule reduces

19 www.national.com

LM3677

radiated noise. Special care must be given to place the

input filter capacitor very close to the VIN and GND pin.

3. Arrange the components so that the switching current

loops curl in the same direction. During the first half of

each cycle, current flows from the input filter capacitor,

through the LM3677 and inductor to the output filter

capacitor and back through ground, forming a current

loop. In the second half of each cycle, current is pulled

up from ground, through the LM3677 by the inductor, to

the output filter capacitor and then back through ground,

forming a second current loop. Routing these loops so

the current curls in the same direction prevents magnetic

field reversal between the two half-cycles and reduces

radiated noise.

4. Connect the ground pins of the LM3677, and filter

capacitors together using generous component-side

copper fill as a pseudo-ground plane. Then connect this

to the ground-plane (if one is used) with several vias. This

reduces ground-plane noise by preventing the switching

currents from circulating through the ground plane. It also

reduces ground bounce at the LM3677 by giving it a low-

impedance ground connection.

5. Use wide traces between the power components and for

power connections to the DC-DC converter circuit. This

reduces voltage errors caused by resistive losses across

the traces

6. Route noise sensitive traces such as the voltage

feedback path away from noisy traces between the

power components. The voltage feedback trace must

remain close to the LM3677 circuit and should be routed

directly from FB to VOUT at the output capacitor and

should be routed opposite to noise components. This

reduces EMI radiated onto the DC-DC converter’s own

voltage feedback trace.

7. Place noise sensitive circuitry, such as radio IF blocks,

away from the DC-DC converter, CMOS digital blocks

and other noisy circuitry. Interference with noise-

sensitive circuitry in the system can be reduced through

distance.

In mobile phones, for example, a common practice is to place

the DC-DC converter on one corner of the board, arrange the

CMOS digital circuitry around it (since this also generates

noise), and then place sensitive preamplifiers and IF stages

on the diagonally opposing corner. Often, the sensitive cir-

cuitry is shielded with a metal pan and power to it is post-

regulated to reduce conducted noise, using low-dropout

linear regulators.

www.national.com 20

LM3677

Physical Dimensions inches (millimeters) unless otherwise noted

5-Bump (Large) Micro SMD Package, 0.5 mmPitch

NS Package Number TLA05FEA

The dimensions for X1, X2, and X3 are as given:

X1 = 1.107 mm +/- 0.030 mm

X2 = 1.488 mm +/- 0.030 mm

X3 = 0.600 mm +/- 0.075 mm

21 www.national.com

LM3677

6-pin LLP Package, 0.5 mm Pitch

NS Package Number LEB06A

The dimensions for A, B, and C are as given:

A = 2.0 mm +/- 0.1 mm

B = 1.5 mm +/- 0.1 mm

C = 0.60 mm +/- 0.06 mm

www.national.com 22

LM3677

Notes

23 www.national.com

LM3677

Notes

LM3677 3MHz, 600mA Miniature Step-Down DC-DC Converter for Ultra Low Voltage Circuits

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