MAX1620 Datasheet, PDF (17/20 Pages) Maxim Integrated Products – Digitally Adjustable LCD Bias Supplies

Digitally Adjustable LCD Bias Supplies
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 = 59kΩ
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 1MΩ 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 10Ω or less is
usually adequate.
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
I_OUT,MIN =
1
2
× I_PK × V_BATT,MIN / V_OUT,MAX
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:
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
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.
Table 5. Maximum Inductance vs. I_OUT and V_BATT,MIN (20V output)
V_BATT,MIN
1.8V
2.7V
3.6V
5.4V
7.2V
12V
5mA
100µH
150µH
220µH
330µH
390µH
680µH
IOUT
10mA
20mA
56µH
27µH
82µH
39µH
100µH
56µH
150µH
82µH
220µH
100µH
330µH
180µH
30mA
18µH
27µH
33µH
56µH
68µH
120µH
Table 6. Inductor List
COMPANY
PART
Sumida
USA [phone]
Japan 81-3-3607-5111
CD43
CD54
CDRH62B
Coilcraft
[phone]
DO1608
DT1608
TDK
[phone]
NLC565050
TPF0410
µH RANGE
Up to 68µH
Up to 220µH
Up to 330µH
Up to 1mH
Up to 400µH
Up to 1mH
Up to 1mH
SIZE IN mm (H x W x L)
3.2 x 4 diameter
4.5 x 5.2 diameter
3 x 6.2 x 6.2
3.18 x 4.45 x 6.6
3.18 x 4.45 x 6.6
5 x 5 x 5.6
4 diameter x 10 L
COMMENTS
Shielded
Shielded
Leaded coil
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