MAX1552 Datasheet, PDF (9/12 Pages) Maxim Integrated Products – Complete Power IC for Low-Cost PDAs

Complete Power IC for
Low-Cost PDAs
0.1µF
MAX1552
IN
Li+
SWIN
LDO MAIN
CONTROL
LDO COR1
CONTROL
SDIG ON
OFF
COR2
ON
OFF
LCD
ON
OFF
TO MAIN
RESET OUT
LOW BATT OUT
0.1µF
ENSD
ENC2
ENLCD
LDO SDIG
CONTROL
LDO ^COR2
CONTROL
LCD OFF
SWITCH
SW
LX
RS
LCD
BOOST
LBO
LFB
BIAS
REF
CURRENT
REF
THSD
GND
3.3V, 300mA
1.5V, 200mA
3.3V, 200mA
1.8V, 20mA
LCD
20V
1mA
Figure 2. Block Diagram
and are commonly available in values up to 10µF. X7R
and X5R dielectrics are recommended. Note that some
ceramic dielectrics, such as Z5U and Y5V, exhibit large
capacitance and ESR variation with temperature and
require larger than the recommended values to main-
tain stability over temperature.
LCD Boost Output
Selecting an Inductor
The LCD boost is designed to operate with a wide range
of inductor values (4.7µH to 22µH). Smaller inductance
values typically offer smaller physical size for a given
series resistance or saturation current. Smaller values
make LX switch more frequently for a given load and
can reduce efficiency at low load currents. Larger val-
ues reduce switching losses due to less frequent
switching for a given load, but higher resistance may
then reduce efficiency. A 10µH inductor provides a
good balance and works well for most applications. The
inductor’s saturation current rating should be greater
than the peak switching current (250mA); however, it is
generally acceptable to bias some inductors into satura-
tion by as much as 20%, although this slightly reduces
efficiency.
Selecting a Diode
Schottky diodes rated at 250mA or more, such as the
Motorola MBRS0530 or Nihon EP05Q03L, are recom-
mended. The diode reverse-breakdown voltage rating
must be greater than the LCD output voltage.
Selecting Capacitors
For most applications, use a small 1µF LCD output
capacitor. This typically provides an output ripple of
30mV_P-P. In addition, bypass IN with 1µF, and SW with
4.7µF ceramic capacitors.
An LCD feed-forward capacitor, connected from the
output to FB, improves stability over a wide range of
battery voltages. A 33pF capacitor is sufficient for most
applications; however, this value is also affected by PC
board layout.
Setting the LCD Voltage
Adjust the output voltage by connecting a voltage-
divider from the output (V_OUT) to FB (see Figure 1).
Select R2 between 10kΩ and 200kΩ. Calculate R1 with
the following equation:
R1 = R2 [(V_OUT / V_FB) - 1]
where V_FB = 1.25V and V_OUT can range from V_IN to
28V. The input bias current of FB is typically only 5nA,
which allows large-value resistors to be used. For less
than 1% error, the current through R2 should be greater
than 100 times the feedback input bias current (I_FB).
LCD Adjustment
The LCD boost output can be digitally adjusted by
either a DAC or PWM signal.
DAC Adjustment
Adding a DAC and a resistor, R_D, to the divider circuit
(Figure 3) provides DAC adjustment of V_OUT. Ensure
that V_OUT(MAX) does not exceed the LCD panel rating.
The output voltage (V_OUT) as a function of the DAC
voltage (V_DOUT) can be calculated using the
following formula:
( ) _VOUT
=
1.25 ×

¹⁺


R1
R2




+
1.25 −
V_DOUT
R_D
× R1
Using PWM Signals
Many microprocessors have the ability to create PWM
outputs. These are digital outputs, based on either 16-
bit or 8-bit counters, with programmable duty cycle. In
many applications they are suitable for adjusting the
output of the MAX1552 (Figure 1).
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