average current provided by the bridge battery, and

channel power transistor at LX (typically 0.5V). A larger

reduces the discharge current, thereby extending the

discharge time. The same is true for decreasing the

output voltage or output current. For example, choose

1V for each cell, Table 2 summarizes some common

values.

Step 2: To avoid saturation, choose an inductor (L) with

Step 1. Use low series resistance (

mize efficiency. In this example, a 15µH inductor is

used. See Table 4 for a list of component suppliers.

The “edge-of-continuous” DC-DC algorithm causes the

inductor value to fall out of the peak current equation.

Therefore, the exact inductor value chosen is not criti-

cal to the design. However, the switching frequency is

inversely proportional to inductance, so trade-offs of

switching losses versus physical inductor size can be

made by adjusting the inductor value.

The maximum practical switching frequency is 400kHz.

quency that the counter increments/decrements.

For instance, using a charge time of 16 hours and a dis-

(Consult battery manufacturers’ specifications for stan-

dard charging information, which generally compen-

sates for battery inefficiencies.)

Step 4: Using the peak current calculated in Step 1,

Step 5: Resistors R1, R2, and R3 set the DC-DC con-

verter’s output voltage and the low-battery comparator

trip value. The sum of R1, R2, and R3 must be less than

2M

Ω, to minimize leakage errors. Choose resistor R1 =

750k

Ω for the example. Calculate R2 and R3 as follows:

Bridge-Battery Backup Controllers

for Notebooks

10

______________________________________________________________________________________

(V)

(V)

AVERAGE

(mA)

MINIMUM

DISCHARGE TIME

(MINUTES)

6

4

300

150

20

6

2

600

300

10

5

2

500

250

12

4.5

2

450

225

13.2

6

3

400

200

15

5

3

333

167

18

4.5

3

300

150

20

5

4

250

125

24

Table 2. Summary of Common Values for

Designing with the MAX1612/MAX1613

Note:

Table 3. Component List

INDUCTORS

CAPACITORS

RECTIFIERS

BATTERY

Sumida CD43

or CD54 series

Sprague 595D

series, AVX

TPS series

Motorola

MBR0530,

NIEC

EC10QS03L

Sanyo

N-50AAA

SUPPLIER

PHONE

FAX

AVX

USA: 207-287-5111

USA: 207-283-1941

Motorola

USA: 408-749-0510

800-521-6274

—

NIEC

USA: 805-867-2555

Japan: 81-3-3494-7411

USA: 805-867-2556

Japan: 81-3-3494-7414

Sumida

USA: 708-956-0666

Japan: 81-3-3607-5111

USA: 708-956-0702

Japan: 81-3-3607-5144

Table 4. Component Suppliers

Sanyo

USA: 619-661-6835

Japan: 81-7-2070-6306

USA: 619-661-1055

Japan: 81-7-2070-1174

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V

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PEAK

BBATT

RDSON

OUT

BBATT

D

OUT

RDSON

D

=

−

−

−

−

−

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