A1

A3

C3

SW

FB

EN

DC/DC

GND

LM3673TL-

1.8

C1

B2

C2

10 PF

C1

4.7 PF

2.2 PH

LP3991TL-

1.5

EN

LDO

GND

C4

4.7 PF

B2

A1

A2

B1

Li-Ion

2.7 ± 4.3V

1.5V

Ferrite

Bead

1.8V

LP3991

www.ti.com

SNVS296G – DECEMBER 2006 – REVISED JANUARY 2011

some capacitor sizes (e.g. 0402) may not be suitable in the actual application. Ceramic capacitors have the

lowest ESR values, thus making them best for eliminating high frequency noise. The ESR of a typical 4.7µF

ceramic capacitor is in the range of 20m

Ω to 40mΩ, which easily meets the ESR requirement for stability for the

LP3991. The temperature performance of ceramic capacitors varies by type. Capacitor type X7R is specified with

a tolerance of ±15% over the temperature range -55°C to +125°C. The X5R has a similar tolerance over the

reduced temperature range of -55°C to +85°C. Some large value ceramic capacitors (4.7µF) are manufactured

with Z5U or Y5V temperature characteristics, which can result in the capacitance dropping by more than 50% as

the temperature varies from 25°C to 85°C. Therefore X7R or X5R types are recommended in applications where

the temperature will change significantly above or below 25°C.

Tantalum capacitors are less desirable than ceramic for use as output capacitors because they are more

expensive when comparing equivalent capacitance and voltage ratings in the 1µF to 4.7µF range. Another

important consideration is that tantalum capacitors have higher ESR values than equivalent size ceramics. This

means that while it may be possible to find a tantalum capacitor with an ESR value within the stable range, it

would have to be larger in capacitance (which means bigger and more costly) than a ceramic capacitor with the

same ESR value. It should also be noted that the ESR of a typical tantalum will increase about 2:1 as the

temperature goes from 25°C down to -40°C, so some guard band must be allowed.

ENABLE CONTROL

enabled the regulator output is off and the device typically consumes 2nA.

regulator output permanently on.

micro SMD MOUNTING

The micro SMD package requires specific mounting techniques which are detailed in the National Semiconductor

Application Note (AN-1112). Referring to the section Surface Mount Technology (SMT) Assenbly Considerations,

it should be noted that the pad style which must be used with the 4 pin package is NSMD (non-solder mask

defined) type.

For best results during assembly, alignment ordinals on the PCB may be used to facilitate placement of the micro

SMD device.

micro SMD LIGHT SENSITIVITY

Exposing the micro SMD device to direct sunlight may cause mis-operation of the device. Light sources such as

halogen lamps can affect the electrical performance if brought near to the device.

The wavelengths which have most detrimental effect are reds and infra-reds, which means that fluorescent

lighting, used inside most buildings will have little effect on performance.

Figure 3. LP3991 Used as a Post DC/DC regulator

Copyright © 2006–2011, Texas Instruments Incorporated

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