Rainbow Electronics MAX749 - Manual (Page 10)

The Sumida CD54-470N (47µH, 720mA, 370m
Ω
) is suit-
able for a wide range of applications, and the larger
CD105-470N (47µH, 1.17A, 170m
Ω
) permits higher cur-
rent levels and efficiencies.
Diode Selection
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
SENSE
and a break-
down voltage greater than V+ + I -V OUTMAX
I .
External Switching Transistor
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
BASE
) (Figure 1). The
PNP transistor is turned off quickly by the direct pull-up
of DHI, and turned on by the base current provided
through R
BASE
. This resistor limits the transistor’s base-
drive current to (V
IN
- 140mV - V
BE
)/R
BASE
, where V
IN
is
the input voltage, 140mV is the drop across R
SENSE
, V
BE
is the transistor’s base-emitter voltage, and R
BASE
is the
current-limiting resistor. For maximum efficiency, make
R
BASE
as large as possible, but small enough so that the
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
ON
) and 500ns (t
OFF
).
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
IN
- V
OUT
). In either
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.
Table 2. Component Suppliers
Base Resistor
The base resistor, R
BASE
in Figure 1, controls the amount of
base current in the PNP transistor. A low value for R
BASE
increases base drive, which provides higher output cur-
rents and compensates for lower input voltages, but
decreases efficiency. Conversely, a high R
BASE
value
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
BASE
are in the 150
Ω
to 510
Ω
range, but will depend on
the required input voltage range and output current (see
Typical Operating Characteristics). Lower-gain transistors
require lower values for R
BASE
and are less efficient. Larger
R
BASE
values are suitable if less output power is required.
MAX749
Digitally Adjustable LCD Bias Supply
10 _____________________________________________________________________________________
SUPPLIER
INDUCTORS
Coiltronics (305) 781-8900 (305) 782-4163
Gowanda (716) 532-2234 (716) 532-2702
Sumida USA (708) 956-0666 (708) 956-0702
Sumida Japan 81-3-3607-511 81-3-3607-5428
CAPACITORS
Kemet (803) 963-6300 (803) 963-6322
Matsuo (714) 969-2491 (714) 960-6492
Nichicon (708) 843-7500 (708) 843-2798
Sprague (603) 224-1961 (603) 224-1430
Sanyo USA (619) 661-6322
Sanyo Japan 81-3-3837-6242
United Chemi-Con (714) 255-9500 (714) 255-9400
DIODES
Motorola (800) 521-6274
Nihon USA (805) 867-2555 (805) 867-2698
Nihon Japan 81-3-3494-7411 81-3-3494-7414
POWER TRANSISTORS - MOSFETS
Harris (407) 724-3739 (407) 724-3937
International
Rectifier
(213) 772-2000 (213) 772-9028
Siliconix (408) 988-8000 (408) 727-5414
POWER TRANSISTORS - PNP TRANSISTORS
Zetex USA (516) 543-7100 (516) 864-7630
Zetex UK 44 (61) 727 5105 44 (61) 627 5467
CURRENT-SENSE RESISTORS
IRC (512) 992-7900 (512) 992-3377
FAX PHONE