MAX1612 Datasheet, PDF (10/12 Pages) Maxim Integrated Products – Bridge-Battery Backup Controllers for Notebooks

Bridge-Battery Backup Controllers
for Notebooks
where I_PEAK is the peak current, I_OUT is the load cur-
rent, V_BBATT is the bridge-battery voltage, V_D is the for-
ward drop across D1, V_OUT is the output voltage, I_IN is
average current provided by the bridge battery, and
V_RDS(ON) is the voltage drop across the internal N-
channel power transistor at LX (typically 0.5V). A larger
number of cells reduces the I_PEAK and, in effect,
reduces the discharge current, thereby extending the
discharge time. The same is true for decreasing the
output voltage or output current. For example, choose
the following values: I_OUT = 100mA, V_OUT = 5V, and
V_BBATT = 2V (two cells). Using the minimum voltage of
1V for each cell, Table 2 summarizes some common
values.
Step 2: To avoid saturation, choose an inductor (L) with
a peak current rating above the I_PEAK calculated in
Step 1. Use low series resistance (≤ 200mΩ), to opti-
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.
f
=
1
L(I_PEAK
)



(V_BBATT
− V_RDSON ) (V_OUT
(V_OUT − V_RDSON
− V_BBATT
− V_D )
−
V_D
)



where f is the switching frequency, V_OUT is the output
voltage, V_RDSON is the voltage across the internal MOS-
FET switch, V_D is the forward voltage of D1, I_PEAK is the
peak current, and V_BBATT is the bridge battery voltage.
The maximum practical switching frequency is 400kHz.
Step 3: Choose the charging (C_CC) and discharging
(C_CD) timing capacitors. These capacitors set the fre-
quency that the counter increments/decrements.
C_CC (nF) = 4.3 · expected charge time (in hours)
C_CD (nF) = 4.3 · expected discharge time (in hours)
For instance, using a charge time of 16 hours and a dis-
charge time of one hour, C_CC = 68nF and C_CD = 4.3nF.
(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,
calculate the series resistor (R_BBON) as follows:
R _BBON = (V _BBON · 42,000) / I_PEAK
where V _BBON = 2V (internally regulated).
Table 2. Summary of Common Values for
Designing with the MAX1612/MAX1613
V_OUT V_BBATT AVERAGE I_IN
(V)
(V) I_PEAK (mA) (mA)
6
2
5
2
4.5
2
6
3
5
3
4.5
3
6
4
5
4
600
300
500
250
450
225
400
200
333
167
300
150
300
150
250
125
MINIMUM
DISCHARGE TIME
(MINUTES)
10
12
13.2
15
18
20
20
24
Note: In this table, I_OUT = 100mA and battery capacity = 50mAh.
Table 3. Component List
INDUCTORS
Sumida CD43
or CD54 series
CAPACITORS RECTIFIERS BATTERY
Sprague 595D
series, AVX
TPS series
Motorola
MBR0530,
NIEC
EC10QS03L
Sanyo
N-50AAA
Table 4. Component Suppliers
SUPPLIER
PHONE
AVX
USA: 207-287-5111
Motorola
USA: 408-749-0510
800-521-6274
FAX
USA: 207-283-1941
—
NIEC
USA: 805-867-2555
USA: 805-867-2556
Japan: 81-3-3494-7411 Japan: 81-3-3494-7414
Sanyo
USA: 619-661-6835
USA: 619-661-1055
Japan: 81-7-2070-6306 Japan: 81-7-2070-1174
Sumida
USA: 708-956-0666
USA: 708-956-0702
Japan: 81-3-3607-5111 Japan: 81-3-3607-5144
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
2MΩ, to minimize leakage errors. Choose resistor R1 =
750kΩ for the example. Calculate R2 and R3 as follows:
R2 = [ V_OUT (R3) - 2 (R1) - 2 (R3) ] / (2 - V_OUT )
R3 = (R1 + R2) / [ (V_TRIP / 1.8) - 1]
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