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MAX1620EEE+

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P20
MAX1620/MAX1621
Digitally Adjustable LCD Bias Supplies
16 ______________________________________________________________________________________
Setting the Maximum Output Voltage
(DAC Adjustment)
The DAC is adjustable from 0V to 1.5V in 32 steps, and
1LSB = 1.5V / 31. DAC adjustment of V
OUT
is provided
by adding R3 to the divider circuit (Figure 4). Be sure
that V
OUT,MAX
does not exceed the LCD panel rating.
For V
OUT,MAX
= 25V and V
OUT,MIN
= 12.5V, R3 is deter-
mined as follows:
R3 = R5 x (V
FB
) / (V
OUT,MAX
- V
OUT,MIN
)
= 2.2MΩ x (1.5) / (25 - 12.5) = 264kΩ
The general form for V
OUT
as a function of the DAC out-
put (V
DOUT
) is:
V
OUT
= V
OUT,MIN
+ (V
FB
- V
DOUT
) x R5 / R3
At power-up the DAC resets to mid-scale (10000), which
corresponds to V
DOUT
= 0.774V; therefore, the output
voltage after reset is as follows:
V
OUT,RESET
= V
OUT,MIN
+ (1.5 - 0.774) x R5 / R3
Note that for a positive output voltage, V
OUT
increases
as V
DOUT
decreases. V
OUT,MAX
corresponds to V
DOUT
= 0V, and V
OUT,MIN
corresponds to V
DOUT
= 1.5V.
For a negative output voltage, V
OUT
= V
OUT,MIN
+
(V
FB
- V
DOUT
) x R5 / R3. Assume V
OUT,MAX
= -25V and
V
OUT,MIN
= -12.5V; then determine R3 and V
OUT,RESET
as follows:
R3 = R5 x (V
FB
- V
DOUT,MAX
) / (V
OUT,MAX
- V
OUT,MIN
)
= 2.5MΩ x (0 - 1.5) / (-25 - -12.5) = 300kΩ
V
OUT,RESET
= -12.5 + (0 - 0.774) x (2.5M) /
(300k) = -18.95V
Note that for a negative output voltage, V
OUT
increases
as V
DOUT
increases. V
OUT,MAX
corresponds to V
DOUT
= 1.5V, and V
OUT,MIN
corresponds to V
DOUT
= 0V.
Potentiometer Adjustment
The output can be adjusted with a potentiometer instead
of the DAC. Choose R
POT
= 100kΩ, and connect it
between REF and GND. Connect R3 to the potentiome-
ter’s wiper, instead of to DOUT. The same design equa-
tions as above apply.
Controlling the LCD Using
POK and
LCDON
When voltage at POK is greater than 1V, the open-drain
LCDON output pulls low. LCDON withstands 27V; there-
fore, it can drive a PFET or PNP transistor to switch on
the MAX1620/MAX1621’s positive output. The following
represent three cases for using this feature:
1) As an off switch, to ensure that a positive boosted
output goes to 0V during shutdown. In this case,
connect POK to SHDN. Without this switch, the posi-
tive output falls to one diode-drop below the input
voltage (V
BATT
) in shutdown. LCDON is not needed
for negative outputs, which will fall to 0V in shut-
down anyway.
2) As an output sensing cutoff for positive outputs.
Connect POK to the feedback voltage divider to
sense the output voltage. The output is switched on
only when it reaches a set percentage of the set
voltage.
3) As an input sensing output cutoff for positive out-
puts. Connect POK to a voltage divider to sense the
input voltage. The output is switched on only when
the input reaches the set level (Figure 4).
To control the open-drain output LCDON by sensing
the input voltage, connect a resistor-divider (R1-R2,
Figure 4) from V
BATT
to POK. Choose R2 = 100k. For
example, if the minimum battery voltage is 5.3V, deter-
mine R1 as follows:
R1 = R2 x [(V
BATT
/ V
POK
) - 1]
= 100k x [(5.3 / 0.992) - 1] = 434kΩ
LCDON can also be controlled via software (MAX1621,
Table 4).
Table 4. MAX1621 LCDON Output
Truth Table
POK Pin
LCDON Output
LCDON Bit
<1V 0 Floating
<1V 1 Floating
>1V 1 ON, pulls low
>1V 0 Floating
MAX1620/MAX1621
Digitally Adjustable LCD Bias Supplies
______________________________________________________________________________________ 17
COMPANY PART
Coilcraft
(847) 639-6400
DO1608
COMMENTSSIZE 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 = 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.
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
=××
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)
MAX1620/MAX1621
Digitally Adjustable LCD Bias Supplies
18 ______________________________________________________________________________________
Diode Selection
The high maximum switching frequency of 300kHz
requires a high-speed rectifier. Schottky diodes, such as
the MBRS0540, are recommended. To maintain high effi-
ciency, the average current rating of the Schottky diode
must be greater than the peak switching current. Choose
a reverse breakdown voltage greater than the positive
output voltage or greater than the negative output volt-
age plus V
BATT
.
External Switching Transistor
Again, the high maximum switching frequency requires
a high-speed switching transistor to maintain efficiency.
Logic-level N-channel MOSFETs, such as the
MMFT3055VL, are recommended (N1). Choose a V
DS
rating greater than the positive output voltage or
greater than the negative output voltage plus V
BATT
.
To save cost in certain applications, a bipolar transistor
may be substituted for the MOSFET with a decrease in
efficiency. The conditions favoring substitution are limit-
ed input voltage range (V
DD
), low maximum battery
voltage (V
BATT
), and low output current. For example,
V
DD
= 3.0V to 3.6V, V
BATT,MAX
= 12V, and I
OUT
= 5mA
favors a bipolar transistor substitution to reduce cost.
To modify the Typical Operating Circuit (Figures 4 and
5) for a bipolar switching transistor, connect the collec-
tor to the inductor, the base to DLO, and the emitter to
PGND (Figure 10). Connect the base to DHI through a
series resistor to limit the base current. Choose the
resistor such that the minimum base current is greater
than 1/20 of the peak inductor current. For example,
assume V
DD,MIN
= 3V and I
PK
= 100mA; then R
S
≤ 20 x
(3 - 0.7) / 100mA = 460Ω.
Output Filter Capacitor
A 22µF, 35V, low-ESR, surface-mount tantalum output
capacitor is sufficient for most applications. Output rip-
ple voltage is dominated by the peak switch current
multiplied by the output capacitor’s effective series
resistance (ESR). 100mVp-p output ripple is a good tar-
get for the trade-off between cost and performance.
Capacitors smaller than 22µF may be used for light
loads and lower peak current. Surface-mount capaci-
tors are generally preferred because they lack the
inductance and resistance of their through-hole equiva-
lents. The AVX TPS series and the Sprague 593D and
595D series are good choices for low-ESR surface-
mount tantalum capacitors.
Moderate-performance aluminum-electrolytic or tanta-
lum capacitors can be successfully substituted in cost-
sensitive applications with low output current. Matsuo
and Nichicon provide suitable choices.
Input Bypass Capacitor
Two inputs, V
DD
and V
BATT
, require bypass capacitors.
Bypass V
DD
with a 0.1µF ceramic capacitor as close to
the IC as possible. The battery supplies high currents
to the inductor and requires local bulk bypassing close
to the inductor. A 22µF low-ESR surface-mount capaci-
tor is sufficient for most applications. Smaller capaci-
tors are acceptable if peak inductor current is low or
the battery’s internal impedance is low and the battery
is close to the inductor.
Charge-Pump Capacitor (Negative Output)
Possible negative output topologies are shown in
Figures 5 and 6. Overall efficiency for the negative out-
put configuration is less than for the positive output
circuit because of the extra components in the power-
transfer path. For efficient charge transfer, C4 must
have low ESR and should be smaller than the output
capacitor (C5). C4 sees the same voltage as C5, and
should have the same voltage rating. A 1µF ceramic
capacitor is a practical choice for cost and performance
considerations. 2.2µF is suggested for Figure 6’s circuit.
Feedback-Compensation Capacitor
The high value of the feedback resistors (R3, R4, R5,
Figure 4) makes the feedback loop susceptible to
phase lag because of the parasitic capacitance at the
FB pin. To compensate for this, connect a capacitor
(C6, Figure 4) in parallel with R5. The value of C6
depends on the parallel combination of R3, R4, R5, and
the individual circuit layout. Typical values range from
33pF to 220pF.
Reference-Compensation Capacitor
The internal reference uses an external capacitor for
frequency compensation. Connect a ceramic capacitor
with a 0.1µF minimum value between REF and ground.
PC Board Layout and Grounding
Due to high current levels and fast switching wave-
forms, proper PC board layout is essential. In particu-
lar, keep all traces short, especially those connected to
the FB pin and those connecting N1, L1, D1, D2, C4,
and C5. Place R3, R4, and R5 as close to the feedback
pin as possible.
Use a star ground configuration: connect the grounds
of the input bypass capacitor, the output capacitor, and
the switching transistor together, close to the IC’s
PGND pin. Tie AGND and PGND together at the chip.
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P20

MAX1620EEE+

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Buy MAX1620EEE+
Manufacturer:
Maxim Integrated
Description:
Power Management Specialized - PMIC Digitally Adjustable LCD Bias Supply
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