LMP7312MAEVAL/NOPB - NATIONAL SEMICONDUCTOR

LM3691

LM3691 High Accuracy, Miniature 1A, Step-Down DC-DC Converter for Portable

Applications

Literature Number: SNVS506H

LM3691

September 27, 2011

High Accuracy, Miniature 1A, Step-Down DC-DC Converter

for Portable Applications

General Description

The LM3691 step-down DC-DC converter is optimized for

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

3 cell NiMH/NiCd batteries. It provides up to 1A load current,

over an input voltage range from 2.3V to 5.5V. There are sev-

eral different fixed voltage output options available.

LM3691 has a mode-control pin that allows the user to select

Forced PWM mode or ECO mode that changes modes be-

tween gated PWM mode and PWM automatically depending

on the load. In ECO, LM3691 offers superior efficiency and

very low Iq under light load conditions. ECO mode extends the

battery life through reduction of the quiescent current during

light load conditions and system standby.

The LM3691 is available in a 6-bump micro SMD package.

Only three external surface-mount components, a 1 μH in-

ductor, a 4.7 μF input capacitor and a 4.7 μF output capacitor,

are required.

Features

■VOUT = 0.75V to 3.3V

■±1% DC output voltage precision

■2.3V ≤ VIN ≤ 5.5V

■4 MHz switching frequency

■64 μA (typ.) quiescent current in ECO mode

■1A maximum load capability

■Automatic ECO/PWM mode switching

■Mode Pin to select ECO/Forced PWM mode

■1 μH inductor, 4.7 μF input capacitor (0603(1608) case

size) and 4.7 μF output capacitor (0603(1608) case size)

■Current overload and thermal shutdown protections

■Only three tiny surface-mount external components

required (solution size less than 15 mm2)

Applications

■Mobile Phones

■Hand-Held Radios

■MP3 players

■Portable Hard Disk Drives

Typical Application Circuit

30013430

FIGURE 1. Typical Application Circuit

Efficiency vs. Output Current

(VOUT = 1.8V, ECO Mode)

30013466

LM3691 High Accuracy, Miniature 1A, Step-Down DC-DC Converter for Portable Applications

Connection Diagram and Package Mark Information

30013406

FIGURE 2. 6-Bump Thin Micro SMD Package, Large Bump

NS Package Number TLA06LCA

Note: The actual physical placement of the package marking will vary from part to part. The package marking “X” designates the

date code; “V” is an NSC internal code for die traceability. Both will vary in production.

Pin Descriptions

Pin Micro SMD Name Description

A1 EN

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

when >1.2V.

Do not leave this pin floating.

B1 MODE

Mode Pin: Mode = 1, Forced PWM

Mode = 0, ECO

Do not leave this pin floating.

C1 FB Feedback analog input. Connect directly to the output filter capacitor. (Figure 1)

A2 VIN Power supply input. Connect to the input filter capacitor. (Figure 1)

B2 SW Switching node connection to the internal PFET switch and NFET synchronous rectifier.

C2 GND Ground pin.

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LM3691

Ordering Information

Voltage Option V Order Number 6-bump Micro

SMD Package Marking Supplied As

0.75 LM3691TL-0.75 V 250 units, Tape-and-Reel

LM3691TLX-0.75 V 3000 units, Tape-and-Reel

0.85* LM3691TL-0.85 TBD 250 units, Tape-and-Reel

LM3691TLX-0.85 TBD 3000 units, Tape-and-Reel

0.9* LM3691TL-0.9 TBD 250 units, Tape-and-Reel

LM3691TLX-0.9 TBD 3000 units, Tape-and-Reel

1.0 LM3691TL-1.0 F 250 units, Tape-and-Reel

LM3691TLX-1.0 F 3000 units, Tape-and-Reel

1.1* LM3691TL-1.1 TBD 250 units, Tape-and-Reel

LM3691TLX-1.1 TBD 3000 units, Tape-and-Reel

1.2 LM3691TL-1.2 X 250 units, Tape-and-Reel

LM3691TLX-1.2 X 3000 units, Tape-and-Reel

1.3* LM3691TL-1.3 TBD 250 units, Tape-and-Reel

LM3691TLX-1.3 TBD 3000 units, Tape-and-Reel

1.375* LM3691TL-1.375 U 250 units, Tape-and-Reel

LM3691TLX-1.375 U 3000 units, Tape-and-Reel

1.5 LM3691TL-1.5 Y 250 units, Tape-and-Reel

LM3691TLX–1.5 Y 3000 units, Tape-and-Reel

1.6* LM3691TL-1.6 TBD 250 units, Tape-and-Reel

LM3691TLX-1.6 TBD 3000 units, Tape-and-Reel

1.8 LM3691TL-1.8 Z 250 units, Tape-and-Reel

LM3691TLX-1.8 Z 3000 units, Tape-and-Reel

2.5 LM3691TL-2.5 8 250 units, Tape-and-Reel

LM3691TLX-2.5 8 3000 units, Tape-and-Reel

3.3 LM3691TL-3.3 T 250 units, Tape-and-Reel

LM3691TLX-3.3 T 3000 units, Tape-and-Reel

* If any of the voltage options other than the released voltages are required, please contact the National Semiconductor Sales Office/Distributors for availability.

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LM3691

Absolute Maximum Ratings (Note 1)

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/Distributors

for availability and specifications.

VIN Pin to GND −0.2V to 6.0V

EN, MODE, FB, SW pins (GND−0.2V) to

VIN + 0.2V

Junction Temperature (TJ-MAX) +150°C

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

Continuous Power Dissipation

(Note 3)

Internally Limited

Maximum Lead Temperature

(Soldering, 10 sec.)

260°C

ESD Rating (Note 4)

Human Body Model 2 kV

Machine Model 200V

Operating Ratings (Note 1, Note 2)

Input Voltage Range 2.3V to 5.5V

Recommended Load Current 0 mA to 1000 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 6)

(micro SMD)

85°C/W

Electrical Characteristics (Note 2, Note 7, Note 8) Limits in standard typeface are for TA = 25°C. Limits in

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

specifications apply to the LM3691 open loop Typical Application Circuit with VIN = EN = 3.6V.

Symbol Parameter Condition Min Typ Max Units

VFB Feedback Voltage PWM Mode. No load VOUT = 1.1V to 3.3V -1 +1 %

PWM Mode. No load VOUT = 0.75V to 1.0V -10 +10 mV

ISHDN Shutdown Supply Current EN = 0V 0.03 1µA

IQ_ECO ECO Mode IqECO Mode 64 80 µA

IQ_PWM PWM Mode IqPWM Mode 490 600 µA

RDSON (P) Pin-Pin Resistance for PFET VIN = VGS = 3.6V, IO = 200 mA 160 250 mΩ

RDSON (N) Pin-Pin Resistance for NFET VIN = VGS = 3.6V, IO = −200 mA 115 180 mΩ

ILIM Switch Peak Current Limit Open loop 1250 1500 1700 mA

VIH Logic High Input 1.2 V

VIL Logic Low Input 0.4 V

IEN,MODE Input Current 0.01 1µA

FSW Switching Frequency PWM Mode 3.6 44.4 MHz

VON UVLO threshold (Note 10) VIN rising 2.2 2.29 V

VIN falling 2.1 V

TSTARTUP Start Time (Note 9) 70 145 300 µs

Note 1: 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 2: All voltages are with respect to the potential at the GND pin.

Note 3: 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 4: 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 5: 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). Due to the pulsed nature of testing the part, the temp in the Electrical Characteristic table is specified as TA = TJ.

Note 6: 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.

Note 7: 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 8: 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 9: Not tested in production, guaranteed by design.

Note 10: The UVLO rising threshold minus the falling threshold is always positive.

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LM3691

Block Diagram

30013431

FIGURE 3. Simplified Functional Diagram

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LM3691

Typical Performance Characteristics LM3691TL Typical Application Circuit (page 1), VIN = 3.6V, VOUT

= 1.8V, TA = 25°, L = 1.0 μH, 2520, (LQM2HP1R0), CIN = COUT = 4.7 μF, 0603(1608), 6.3V, (C1608X5R0J475K) unless otherwise

noted.

Quiescent Supply current vs. Supply Voltage

No Switching (ECO Mode)

30013455

Quiescent Supply current vs. Supply Voltage

No Switching (PWM Mode)

30013456

Shutdown Current vs. Temp

(VOUT = 1.8V)

30013457

Switching Frequency vs. Temp

(VOUT = 1.8V, PWM Mode)

30013458

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LM3691

Output Voltage vs. Supply Voltage

(VOUT = 0.75V)

30013459

Output Voltage vs. Supply Voltage

(VOUT = 1.8V)

30013460

Output Voltage vs. Output Current

(VOUT = 0.75V)

30013461

Output Voltage vs. Output Current

(VOUT = 1.8V)

30013462

Input Current vs. Output Current

(VOUT = 0.75V)

30013463

Input Current vs. Output Current

(VOUT = 1.8V)

30013464

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LM3691

Efficiency vs. Output Current

(VOUT = 0.75V, ECO Mode)

30013465

Efficiency vs. Output Current

(VOUT = 1.8V, ECO Mode)

30013466

Efficiency vs. Output Current

(VOUT = 2.5V, ECO Mode)

30013443

Efficiency vs. Output Current

(VOUT = 0.75V, FPWM Mode)

30013467

Efficiency vs. Output Current

(VOUT = 1.8V, FPWM Mode)

30013468

Efficiency vs. Output Current

(VOUT = 2.5V, FPWM Mode)

300134a0

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LM3691

Load Current Threshold vs. Supply Voltage

(VOUT = 0.75V, ECO Mode to PWM Mode)

30013469

Load Current Threshold vs. Supply Voltage

(VOUT = 1.8V, ECO Mode to PWM Mode)

30013470

Output Voltage Ripple vs. Supply Voltage

(VOUT = 0.75V)

30013471

Output Voltage Ripple vs. Supply Voltage

(VOUT = 1.8V)

30013472

Closed Loop Current Limit vs. Temperature

(VOUT = 0.75V)

30013473

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LM3691

Closed Loop Current Limit vs. Temperature

(VOUT = 1.8V)

30013474

Line Transient Reponse

(VOUT = 0.75V, PWM Mode)

30013475

Line Transient Reponse

(VOUT = 1.8V, PWM Mode)

30013478

Load Transient Reponse

(VOUT = 0.75V, ECO Mode 1mA to 25 mA)

30013479

Load Transient Reponse

(VOUT = 0.75V, ECO Mode 25 mA to 1mA)

30013480

Load Transient Reponse

(VOUT = 1.8V, ECO Mode 1mA to 25 mA)

30013481

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LM3691

Load Transient Reponse

(VOUT = 1.8V, ECO Mode 25 mA to 1mA)

30013482

Load Transient Reponse

(VOUT = 0.75V, ECO Mode to PWM Mode)

30013483

Load Transient Reponse

(VOUT = 0.75V, PWM Mode to ECO Mode)

30013484

Load Transient Reponse

(VOUT = 1.8V, ECO Mode to PWM Mode)

30013485

Load Transient Reponse

(VOUT = 2.5V, ECO Mode to PWM Mode)

30013452

Load Transient Reponse

(VOUT = 2.5V, ECO Mode to PWM Mode)

30013453

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LM3691

Load Transient Reponse

(VOUT = 1.8V, FPWM Mode)

30013490

Load Transient Reponse

(VOUT = 0.75V, PWM Mode)

30013491

Load Transient Reponse

(VOUT = 1.8V, PWM Mode)

30013492

Load Transient Reponse

(VOUT = 2.5V, PWM Mode)

30013446

Start Up into ECO Mode

(VOUT = 0.75V, ROUT = 750Ω)

30013495

Start Up into PWM Mode

(VOUT = 0.75V, ROUT = 2.5Ω)

30013496

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LM3691

Start Up into ECO Mode

(VOUT = 1.8V, ROUT = 1.8 kΩ)

30013493

Start Up into PWM Mode

(VOUT = 1.8V, ROUT = 6Ω)

30013494

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LM3691

Operation Description

DEVICE INFORMATION

The LM3691, a high-efficiency, step-down DC-DC switching

buck converter, delivers a constant voltage from either a sin-

gle Li-Ion or three cell NiMH/NiCd battery to portable devices

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

tecture with synchronous rectification, the LM3691 has the

ability to deliver up to 1000 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), ECO, and shut-

down. The device operates in PWM mode at load currents of

approximately 50 mA (typ.) or higher. Lighter output current

loads cause the device to automatically switch into ECO

mode for reduced current consumption and a longer battery

life. Shutdown mode turns off the device, offering the lowest

current consumption (ISHUTDOWN = 0.03 µA typ.). Additional

features include soft-start, under voltage protection, current

overload protection, and thermal shutdown protection. As

shown in Figure 1, only three external power components are

required for implementation.

CIRCUIT OPERATION

The LM3691 operates as follows. During the first portion of

each switching cycle, the control block in the LM3691 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 cur-

rent from ground through the NFET to the output filter capac-

itor 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 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

rectangular wave formed by the switch and synchronous rec-

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

and output filter capacitor. The output voltage is equal to the

average 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 excellent load and line regulation.

The DC gain of the power stage is proportional to the input

voltage. To eliminate this dependence, feed forward inversely

proportional to the input voltage is introduced. While in PWM

mode, the output voltage is regulated by switching at a con-

stant 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 inductor 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.

30013497

FIGURE 4. Typical PWM Operation

Internal Synchronous Rectification

While in PWM mode, the LM3691 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 LM3691 to protect itself and

external components during overload conditions. PWM mode

implements current limit using an internal comparator that

trips at 1500 mA (typ). If the output is shorted to ground and

output voltage becomes lower than 0.3V (typ.), the device

enters a timed current limit mode where the switching fre-

quency will be one fourth, and NFET synchronous rectifier is

disabled, thereby preventing excess current and thermal run-

away.

ECO OPERATION

Setting mode pin low places the LM3691 in Auto mode. By

doing so the part switches from ECO (ECOnomy) state to

FPWM (Forced Pulse Width Modulation) state based on out-

put load current. At light loads (less than 50 mA), the converter

enters ECO mode. In this mode the part operates with low Iq.

During ECO operation, the converter positions the output

voltage slightly higher (+30 mV typ.) than the nominal output

voltage in FPWM operation. Because the reference is set

higher, the output voltage increases to reach the target volt-

age when the part goes from sleep state to switching state.

Once this voltage is reached the converter enters sleep mode,

thereby reducing switching losses and improving light load

efficiency. The output voltage ripple is slightly higher in ECO

mode (30 mV peak–peak ripple typ.).

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LM3691

30013498

FIGURE 5. Typical ECO Operation

FORCED PWM MODE

Setting Mode pin high (>1.2V) places the LM3691 in Forced

PWM. The part is in forced PWM regardless of the load.

SHUTDOWN MODE

Setting the EN input pin low (<0.4V) places the LM3691 in

shutdown mode. During shutdown the PFET switch, NFET

switch, reference, control and bias circuitry of the LM3691 are

turned off. Setting EN high (>1.2V) enables normal operation.

When turning on the device with EN soft-start is activated. EN

pin should be set low to turn off the LM3691 during system

power up and under-voltage conditions when the supply is

less than 2.3V. Do not leave the EN pin floating.

SOFT-START

The LM3691 has a soft-start circuit that limits in-rush current

during startup. Output voltage increase rate is 30 mV/µsec (at

VOUT = 1.8V typ.) during soft-start.

THERMAL SHUTDOWN PROTECTION

The LM3691 has a thermal overload protection function that

operates to protect itself from short-term misuse and overload

conditions. When the junction temperature exceeds around

150°C, the device inhibits operation. Both the PFET and the

NFET are turned off. When the temperature drops below 130°

C, normal operation resumes. Prolonged operation in thermal

overload conditions may damage the device and is consid-

ered bad practice.

OVER-TEMPERATURE MAXIMUM LOAD

RECOMMENDATIONS

VIN Maximum Load

2.5V to 5.5V 1000 mA

2.3V to 2.5V 650 mA

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LM3691

Application Information

INDUCTOR SELECTION

DC bias current characteristics of inductors must be consid-

ered. Different manufacturers follow different saturation cur-

rent rating specifications, so attention must be given to

details. DC bias curves should be requested from them as

part of the inductor selection process.

Minimum value of inductance to guarantee good perfor-

mance is 0.5 µH at 1.5A (ILIM typ.) bias current over the

ambient temp range. The inductor’s DC resistance should

be less than 0.1Ω for good efficiency at high current condition.

The inductor AC loss (resistance) also affects conversion ef-

ficiency. Higher Q factor at switching frequency usually gives

better efficiency at light load to middle load.

Table 1 lists suggested inductors and suppliers

INPUT CAPACITOR SELECTION

A ceramic input capacitor of 4.7 µF, 6.3V/10V is sufficient for

most applications. Place the input capacitor as close as pos-

sible to the VIN pin and GND pin of the device. A larger value

or higher voltage rating may be used to improve input voltage

filtering. Use X7R, X5R or B types; do not use Y5V or F. DC

bias characteristics of ceramic capacitors must be considered

when selecting case sizes like 0402. Minimum input capac-

itance to guarantee good performance is 2.2 µF at maxi-

mum input voltage DC bias including tolerances and over

ambient temp range.

The input filter capacitor supplies current to the PFET (high-

side) switch in the first half of each cycle and reduces 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 current. Se-

lect an input filter capacitor with sufficient ripple current rating.

The input current ripple can be calculated as:

OUTPUT CAPACITOR SELECTION

Use a 4.7μF, 6.3V ceramic capacitor, X7R, X5R or B types;

do not use Y5V or F. DC bias voltage characteristics of ce-

ramic capacitors must be considered. DC bias characteristics

vary from manufacturer to manufacturer, and DC bias curves

should be requested from them as part of the capacitor se-

lection process. The output filter capacitor smooths out cur-

rent 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. Minimum output capacitance to guaran-

tee good performance is 2.2 µF at the output voltage DC

bias including tolerances and over ambient temp range.

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

charging of the output capacitor and also due to its RESR and

can be calculated as:

Voltage peak-to-peak ripple due to capacitance =

Voltage peak-to-peak ripple due to ESR =

VPP-ESR = (2 * IRIPPLE) * RESR

Because these two components are out of phase the rms val-

ue can be used to get an approximate value of peak-to-peak

ripple.

Voltage peak-to-peak ripple, root mean squared =

Note that the output voltage ripple is dependent on the current

ripple and the equivalent series resistance of the output ca-

pacitor (RESR). The RESR is frequency dependent (as well as

temperature dependent); make sure the value used for cal-

culations is at the switching frequency of the part.

Table 2 lists suggested capacitors and suppliers.

TABLE 1. Suggested Inductors and Their Suppliers

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

LQM2HPN1R0MG0 Murata 2.5 x 2.0 x 1.0 55

MLP2520S1R0L TDK 2.5 x 2.0 x 1.0 60

KSLI252010BG1R0 HItachi Metals 2.5 x 2.0 x 1.0 80

MIPSZ2012D1R0 FDK 2.0 x 1.25 x 1.0 90

TABLE 2. Suggested Capacitors and Their Suppliers

Model Type Vendor Voltage Rating (V) Case Size

Inch (mm)

4.7 µF for CIN and COUT

C1608X5R0J475K Ceramic TDK 6.3 0603 (1608)

C1608X5R1A475K Ceramic TDK 10.0 0603 (1608)

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LM3691

PRINTED CIRCUIT 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. In particular parasitic

inductance from extra-long PCB trace lengths can cause ad-

ditional noise voltages through L*di/dt that adversely affect

the DC-DC converter IC circuitry. Good layout for the LM3691

can be implemented by following a few simple design rules.

1. Place the 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 radiated

noise.

2. Place the capacitors and inductor close to the LM3691.

Place the CIN capacitor as close as possible to the VIN

and GND pads. Place the COUT capacitor as close as

possible to the VOUT and GND connections.

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 buck 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 buck 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 buck and filter capacitors

together using generous component-side copper fill as a

pseudo-ground plane. 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 buck 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 by resistive losses across the

traces. Even 1mm of fine trace creates parasitic

inductance that can undesirably affect performance from

increased L*di/dt noise voltages.

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 buck 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.

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) type.

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 6-bump package used for LM3691 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 cor-

ners. Initially, the trace to each pad should be 7 mil wide, for

a section approximately 7 mil long or longer, as a thermal re-

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

width. The important criteria is symmetry. This ensures the

solder bumps on the LM3691 re-flow evenly and that the de-

vice solders level to the board. In particular, special attention

must be paid to the pads for bumps A2 and C2, because GND

and VIN are typically connected to large copper planes.

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 par-

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

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

edges.

17 www.national.com

LM3691

Physical Dimensions inches (millimeters) unless otherwise noted

6–bump Thin Micro SMD, Large Bump

NS Package Number TLA06LCA

X1 = 1.260 mm ± 0.030 mm

X2 = 1.565 mm ± 0.030 mm

X3 = 0.600 mm ± 0.075 mm

www.national.com 18

LM3691

19 www.national.com

LM3691

Notes

LM3691 High Accuracy, Miniature 1A, Step-Down DC-DC Converter for Portable Applications

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