AIC1630CN - Analog Integrations Corporation

AN97-002

May 1997

Two-Cell, Step-Up Converter Design for

Portable Applications

James Liu

Introduction

In recent years, the markets for portable electrical

devices, such as the electronic dictionary, palmtop

computers, notebook PCs, PDAs and cellular

phones has grown rapidly. All of these devices use

batteries as the source of power. Due to their

limitations of small size and light weight and due to

increasingly stringent environmental requirements

in product development, the use of small numbers,

small size, long-lasting batteries has become a

trend.

The AIC1630/1631/1633 are very suitable for the

application of using 2~4 batteries. This article will

focus on the application of AIC1631-5 for 2

batteries in the following description.

A set of two alkaline, NiMH, or NiCd battery cells

accompanied by a switching converter with 3V,

3.3V, or 5V output voltage is frequently used as

the power supply source for portable electronic

products. Using two battery cells is usually a result

of compromise between size and battery

operating life. Besides the batteries’ own stored

energy, two other factors also influence batteries’

operating life: the product’s power consumption

and conversion efficiency of the converter. In

order to choose the appropriate switching

converter for an electronic product, therefore, 5

key factors need to be considered:

(1) The current capacity and regulation of the

output should meet the product’s demands.

(2) High conversion efficiency.

(3) Low power consumption.

(4) Squeeze out as much of the battery energy

as possible.

(5) Small size and light weight.

The first factor is absolutely necessary, and the

importance of the other four differs in priority

depending on the character of the product. For

products, which operate relatively high current

for extended time periods, conversion efficiency

is paramount. For products operate with low

current for most of the time, the current

consumption of the converter itself must also be

low. With regards to size and weight, an 8-pin IC,

which does not need a heatsink, with a small

inductor, and minimum number of external parts,

would be an ideal choice. In the following, we use

the AIC1631 DC/DC converter as an example for

several different special products using two

battery cells to demonstrate how the AIC1631

can be used to make an ideal power source.

(+5V)

+VOUT

+VIN

1N5819

CGND

LBO

LBI

VOUT

SGND

VIN

ILIM/SD

100mF

47mH*

*Sumida RCH-108

+100mF

33K

10mF

VIN

AIC1631-5

Fig. 1 A High-Efficiency 2-Cell to 5V Converter

AN97-002

Load Current (mA)

0 100 200 300 400 500 600

VIN=1.8V

VIN=2V

VIN=2.2V

VIN=2.5V

VIN=3V

VIN=3.3V

Efficiency (%)

Fig. 2 Conversion Efficiency for the 5V

Converter

Conversion Efficiency Considerations

Conversion efficiency is the main consideration for

those devices which consume above 2mA most of

the time. Fig. 1 shows a high efficiency converter

circuit suitable for producing 5V output from two

battery cells. The components shown in Fig. 1 are

easily obtained low cost common parts. If high-

quality parts (such as MMP core inductors) are

used, efficiencies of 90% and above can be

reached. However, if good output power quality is

desired, such as in Fig. 2 or better, at least two

points need to be observed:

(1) A good power plan and a correct layout for

the circuit board is a must. The AIC1631

evaluation boards are available for your

reference.

(2) One must have the correct inductor values.

The equivalent series resistance of the

inductor must be low and the core must not

be saturated under any operation condition.

The above example uses the Sumida RCH-

108 47mH inductor, whose coil resistance

(RDC) is 0.14W.

When converting 2 alkaline battery cells to 5V, an

inductor of 40-150mH is suggested. For NiCd

batteries, a 20-100mH inductor is suggested.

Inductors of less inductance can output a larger

current. However, too low inductance would result

in serious efficiency loss due to inductor core

saturation. The most reliable method, therefore, is

to test for the best conditions using a range of

suggested values. The lower coil resistance (RDC)

be chosen, the better efficiency obtained. For

products consuming about 20mA, the RDC should

be under 1W. For products with current

consumption at or above 50mA, the RDC should be

below 0.5W. The larger the consumption current,

the smaller the RDC required.

Low Power Consumption Products

The quiescent current of the AIC1631-5 in an no

load state is about 200mA. For special energy-

conscious products, some circuit configurations

are presented here to illustrate how the AIC1631

can be used:

(1) If this DC/DC converter is not required to

maintain output voltage, you may simply pull

pin 1 (shutdown pin) to ground by using a

control signal. The AIC1631 will stop

functioning and the power consumption drop

to less than 10mA. Normal operation can be

recovered by pulling pin 1 high. Generally

speaking, this is ideal for systems with

backup lithium batteries. Fig. 3 is an example

circuit. While using the lowest possible

amount of power, the circuit makes it possible

to retain data in memory even when changing

batteries.

AN97-002

47mH

100mF

+10mF

33K

SDN*

VIN

1N5819

(+5V) VOUT

AIC 1631-5

1N5819

VOUT

LBI

LBO

CGND

SGND

VIN

ILIM/SO

** +

* SDN=VIN or VOUT to Operate

SDN=GND to Shutdown

** Backup Lithium Battery

Fig. 3 Step-Up converter with Shutdown Control

(2) If the output voltage requires the AIC1631,

one could add a few external components to

make an oscillator to generate control signals

for the AIC1631 to perform intermittent

operations. This is called “Burst Mode

Operation”, and is shown in Fig. 4(a). If CTL

shown in Fig. 4(a) is connected to VIN or VOUT,

the circuit will function normally with the

characteristics similar to the circuit in Fig.1. If

CTL is connected to GND, the circuit will

enter Burst Mode with an output voltage of 4V,

and the circuit itself consumes very low

power, as shown in Fig. 4(b). To change the

output voltage in the burst mode, simply

change the divider ratio of R3 and R4 in Fig.

4(a). Note that the voltage of normal

operation will never be exceeded.

0.01mF

2N3904

1N4148

R2 R3

150K

47mH

+100mF

10mF

VIN

CTL*

VOUT

1N5819

AIC1631-5

VOUT

LBI

LBO

CGND

SGND

VIN

ILIM/SO

* CTL =GND in Burst Mode

CTL=VIN or VOUT in Normal Mode

(a)

Load Current (mA)

Total Supply Current (mA)

0 50 100 150 200 250 300

200

400

600

800

VIN=1.8V

VIN=2.5V

VIN=2V

VIN=3V

VIN=3.3V

(b)

Fig. 4. Low Supply Current, Burst Mode Step-Up

Converter

(a) Application Circuit

(b) Total supply Current vs. Load Current

(3) To get the most out of the batteries‘ energy,

the DC/DC converter must be able to work at

low battery voltage. The circuit shown in Fig.

5 still deliver 30mA at 5V with input voltage at

1.6V. The conversion efficiency, however, is

somewhat lower than circuit in Fig. 1.

These examples shown above use the

AIC1631-5 to produce a 5V output. The

AIC1631 and AIC1631-3 can be used similarly

to produce 3.3V and 3V, respectively.

AN97-002

47mH

+10mF

(+5V)

+VOUT

+VIN AIC1631-5

1N5819

VOUT

LBI

LBO

CGND

SGND

VIN

ILIM/SO

33K

100mF

Fig. 5 1 Cell to 5V Step-Up Converter

2~4 Cells to 5V Low Noise Power Supply

The AIC1631-5 can be used to convert 2-4

Alkaline cells to 5 volts. The low dropout linear

regulator is composed of the low battery detector

(LBI and LBO pins), resistors R1, R2, R3, R4, and

a PNP transistor 2SA1244, while the other

components perform the job of a step-up

switching regulator. The 1N4148 raises the output

voltage of the switch regulator by 0.4V, allowing

the linear low dropout regulator to filter out ripple

noise. This results in a 7% loss in conversion

efficiency, a necessary price to pay for a clean

power source. The lower the output voltage, the

higher the conversion efficiency loss due to this

0.4V voltage drop. For 3.3V (the AIC1631) and 3V

(the AIC1631-3) output voltages, it creates a

conversion efficiency loss around 10%.

2SA1244

1N4148

1N5819 C4

47mF

22mF

47mF

10mF

62K

R220K

1.2K

40mH

100mH

VOUT

(5V)

VIN

SGND CGND

SW LBO

VIN LBI

ILIM/SD VOUT

AIC1631-5

Fig. 6 2~4 Cells to 5V Low Noise Power Supply

Components Selection

A few guidelines can be followed when using the

LBI and LBO pins of the AIC1631 (also the

AIC1633 and the AIC1630) to form a low dropout

linear regulator. One can see from Fig. 6, that

output voltage is:

V(R R )

R1. 22 VOUT

The usable range of values for R4 is very large.

Ordinarily, 1MW is a good choice, R3 must match

with the PNP transistor. Since the current gain of

the PNP transistor is usually only between 5 and

30 when VCE=0.4V, the output current capability

will be inadequate if the R3 value is too large. On

the other hand, since the LB loop can adjust

automatically, it will be acceptable if R3 value is a

little smaller. However, if we use the right value for

R3, in addition to its normal function, it can also

AN97-002

perform current limiting function. Normally, when

output current is below 300mA and the PNP

transistor is selected according to the following

guidelines, the R3 value would be between 1KW

and 5KW.

The first rule in selecting a PNP transistor is that

the rated current should be high enough.

Secondly, current gain b of the PNP must be

maintained at greater than 10 (the greater the

value the better) when VCE=0.4V and IC is equal to

or larger than the required output current. The

transistor 2SA1244 in Fig. 6 is a good example.

Since under some circumstance, the use of a

capacitor with too large capacitance value or too

low ESR value (such as OSCON capacitors) can

easily shifts the dominant pole of the linear

regulator and causes an oscillation situation. An

electrolytic or a tantalum capacitor with

capacitance lower than 100mF is suggested for the

capacitor C3.

Due to the limited bandwidth of the linear regulator

and the ESR effect of C3, high frequency spikes

and noises may pass the linear regulator and

appear at the output terminal. L1 and C4 are

therefore added at the output node to form a p

filter to filter out high frequency noises and obtain

a truly clean power source.

PCB Layout Hints

With a correct PCB layout, the output voltage

noise of the circuit, shown in Fig. 1 should be

below 20mV (Peak to Peak). Under most

circumstances, the noise cannot even be seen.

However, sometimes one can see ringing of about

10MHz on the oscilloscope. This is referred to as

ground noise. Ground noise is conducted by the

PCB ground traces and closely related to the

layout of the ground plane. How do we distinguish

ground noise? Simply connect the ground end of

the oscilloscope probe to different spots of the

ground plane. If the noise‘s magnitude or shape

changes significantly, then it is ground noise. In

practical applications, we only need to apply a

0.1mF ceramic capacitor across the power input

terminals of the analog circuit, as shown in Fig. 7.

ANALOG

CIRCUIT

GND

VDD

VOUT

VIN

GND

+AIC1631

CIRCUIT 0.1mF

Fig. 7 Ground Noise Reduction by Using a

0.1mF Ceramic Capacitor