MAX749ESA+ Maxim Integrated Products, MAX749ESA+ Datasheet - Page 10

• Manufacturer Part Number: MAX749ESA+
• Description: IC SUPPLY LCD BIAS ADJ 8-SOIC
• Manufacturer: Maxim Integrated Products

Specifications of MAX749ESA+

• Applications: LCD Display
• Current - Supply: 60µA
• Voltage - Supply: 2 V ~ 6 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (3.9mm Width)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

The Sumida CD54-470N (47µH, 720mA, 370mΩ) is suit-
able for a wide range of applications, and the larger
CD105-470N (47µH, 1.17A, 170mΩ) permits higher cur-
rent levels and efficiencies.
The MAX749’s high switching frequency demands a high-
speed rectifier. Schottky diodes such as the 1N5817-
1N5822 family are recommended. Choose a diode with an
average current rating approximately equal to the peak
current, as determined by 180mV/R
down voltage greater than V + +
The MAX749 can drive a PNP transistor or a P-channel
logic-level MOSFET. The choice of a power switch is
dictated by the input voltage range, cost, and efficiency.
MOSFETs provide the highest efficiency because they
do not draw any DC gate-drive current (see Typical
Operating Characteristics graphs). However, a gate-
source voltage of several volts is needed to turn on a
MOSFET, so a 5V or greater input supply is required
(although this restriction may change as lower-thresh-
old P-channel MOSFETs become available). PNP tran-
sistors, meanwhile, may be used over the entire 2V to
6V operating voltage range of the MAX749.
When using a MOSFET, connect DHI and DLOW to its
gate (see Typical Operating Circuit ). When using a PNP
transistor, connect DHI to its base, and connect a resis-
tor between the base and DLOW (R
PNP transistor is turned off quickly by the direct pull-up
of DHI, and turned on by the base current provided
through R
drive current to (V
the input voltage, 140mV is the drop across R
is the transistor’s base-emitter voltage, and R
current-limiting resistor. For maximum efficiency, make
R
transistor is always driven into saturation.
Highest efficiency with a PNP transistor comes from
using a device with a low collector-emitter saturation
voltage and a high current gain. Use a fast-switching
type. For example the Zetex ZTX792A has switching
speeds of 40ns (t
The transistor must have a collector-to-emitter (PNP) or
drain-to-source (MOSFET) voltage rating greater than the
input-to-output voltage differential (V
case the transistor must have a current rating that exceeds
the peak current set by the current-sense resistor.
PNP transistors are generally less expensive than P-
channel MOSFETs. Table 2 lists some suppliers of
switching transistors suitable for use with the MAX749.
Digitally Adjustable LCD Bias Supply
10
BASE
_____________________________________________________________________________________
as large as possible, but small enough so that the
BASE
. This resistor limits the transistor’s base-
External Switching Transistor
ON
IN
) and 500ns (t
- 140mV - V
I
-V
BE
OUTMAX
OFF
)/R
Diode Selection
BASE
SENSE
IN
).
BASE
- V
) (Figure 1). The
I
.
OUT
, where V
and a break-
BASE
SENSE
). In either
is the
, V
IN
BE
is
Table 2. Component Suppliers
The base resistor, R
base current in the PNP transistor. A low value for R
increases base drive, which provides higher output cur-
rents and compensates for lower input voltages, but
decreases efficiency. Conversely, a high R
increases efficiency but reduces the output capability,
especially at low voltages. When using high-gain transis-
tors, e.g. the Zetex ZTX750 or ZTX792, typical values for
R
the required input voltage range and output current (see
Typical Operating Characteristics ). Lower-gain transistors
require lower values for R
R
INDUCTORS
CAPACITORS
DIODES
POWER TRANSISTORS - MOSFETS
POWER TRANSISTORS - PNP TRANSISTORS
CURRENT-SENSE RESISTORS
Coiltronics
Gowanda
Sumida USA
Sumida Japan
Kemet
Matsuo
Nichicon
Sprague
Sanyo USA
Sanyo Japan
United Chemi-Con
Motorola
Nihon USA
Nihon Japan
Harris
International
Rectifier
Siliconix
Zetex USA
Zetex UK
IRC
BASE
BASE
SUPPLIER
values are suitable if less output power is required.
are in the 150Ω to 510Ω range, but will depend on
BASE
[phone]
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[phone]
81-3-3607-511
[phone]
[phone]
[phone]
[phone]
[phone]
81-3-3837-6242
[phone]
[phone]
[phone]
81-3-3494-7411
[phone]
[phone]
[phone]
[phone]
44 (61) 727 5105
[phone]
in Figure 1, controls the amount of
BASE
PHONE
and are less efficient. Larger
Base Resistor
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[phone]
[phone]
81-3-3607-5428
[phone]
[phone]
[phone]
[phone]
[phone]
[phone]
81-3-3494-7414
[phone]
[phone]
[phone]
[phone]
44 (61) 627 5467
[phone]
FAX
BASE
value
BASE