Rainbow Electronics MAX1621 - Manual (Page 17)

MAX1620/MAX1621
Digitally Adjustable LCD Bias Supplies
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COMPANY PART
Coilcraft
(847) 639-6400
DO1608
COMMENTS SIZE IN mm (H x W x L) µH RANGE
CD43 Up to 68µH 3.2 x 4 diameter
Up to 1mH 3.18 x 4.45 x 6.6
CD54 Up to 220µH 4.5 x 5.2 diameter
Sumida
USA (847) 956-0666
Japan 81-3-3607-5111
CDRH62B Up to 330µH
TDK
(847) 390-4373
DT1608 Up to 400µH 3.18 x 4.45 x 6.6 Shielded
3 x 6.2 x 6.2 Shielded
NLC565050 Up to 1mH 5 x 5 x 5.6
TPF0410 Up to 1mH 4 diameter x 10 L Leaded coil
Table 6. Inductor List
LCDON typically drives an external PNP transistor,
switching a positive V
OUT to the LCD. R7 limits the base
current in the PNP; R6 turns off the PNP when LCDON is
floating. R6 and R7 can be the same value. Choose R7
such that the minimum base current is greater than 1/50
of the collector current. For example, assume V
OUT,MIN
= 12.5V and I
LCD
= 10mA, then determine R7 as follows:
R7
≤
50 x (12.5 - 0.7) / 10mA = 59k
Ω
Remember that LCD voltage is the regulated output volt-
age minus the drop across the PNP switch. The drop
across the external transistor (typically 300mV) must be
accounted for.
If a PFET is preferred for the LCDON switch, R6 and R7
in Figure 4 may both be raised to 1M
Ω
or more to reduce
operating current. Be sure to choose a P FET with ade-
quate breakdown voltage. Since load current is typically
on the order of 10mA, an on-resistance of 10
Ω
or less is
usually adequate.
Choosing an Inductor
Practical inductor values range from 33µH to 1mH;
however, 100µH is a good choice for a wide range of
applications. Inductors with a ferrite core or equivalent
are recommended. The inductor’s current rating should
exceed the peak current as set by the k-factor and the
coil inductance; however, for most inductor types, the
coil’s specified current can be exceeded by 20% with
no impact on efficiency.
The peak current is set by the coil inductance as follows:
I PK = k-factor / L
and
If we assume that V BATT,MIN = 5.3V, V OUT,MAX =
25V, I OUT,MIN = 15mA, and a minimum k-factor of
16µs-V, then the required I PK is:
I PK = 2 x 15mA x 25 / 5.3 = 142mA
and
L
≤
= 16µs-V / 142mA = 113µH
The next-lowest practical inductor value is 100µH. Its
current rating must be:
24µs-V (maximum k-factor) / 100µH = 240mA
Table 5 summarizes the minimum inductance value
needed to provide various output currents at several
minimum input voltages. Table 6 lists some suitable coil
types and manufacturers, but is not intended to be a
complete list.
I
1
2
I V / V
OUT,MIN PK BATT,MIN OUT,MAX
= Ч Ч
1.8V 2.7V 3.6V 5.4V 7.2V 12V
IOUT
5mA 100µH 150µH 220µH 330µH 390µH 680µH
10mA 56µH 82µH 100µH 150µH 220µH 330µH
20mA 27µH 39µH 56µH 82µH 100µH 180µH
30mA 18µH 27µH 33µH 56µH 68µH 120µH
V
BATT,MIN
Table 5. Maximum Inductance vs. I
OUT
and V
BATT,MIN
(20V output)