Datasheet

MAX1748/MAX8726
Triple-Output TFT-LCD
DC-DC Converters
The inductance value is then given by:
Considering the typical operating circuit, the maximum
DC load current (I
MAIN(MAX)
) is 200mA with a 10V output.
A 6.8µH inductance value is then chosen, based on the
above equations and using 85% efficiency and a 1MHz
operating frequency. Smaller inductance values typically
offer a smaller physical size for a given series resistance
and current rating, allowing the smallest overall circuit
dimensions. However, due to higher peak inductor
currents, the output voltage ripple (I
PEAK
x
output filter
capacitor ESR) will be higher.
Use inductors with a ferrite core or equivalent; powder
iron cores are not recommended for use with the
MAX1748/MAX8726s’ high switching frequencies. The
inductor’s maximum current rating should exceed
I
PEAK
. Under fault conditions, inductor current may
reach up to 2.0A. The MAX1748/MAX8726s’ fast cur-
rent-limit circuitry allows the use of soft-saturation
inductors while still protecting the IC.
The inductor’s DC resistance significantly affects effi-
ciency. For best performance, select inductors with
resistance less than the internal n-channel FET resis-
tance. To minimize radiated noise in sensitive applica-
tions, use a shielded inductor.
The inductor should have as low a series resistance as
possible. For continuous inductor current, the power
loss in the inductor resistance, P
LR
, is approximated by:
P
LR
≅ (I
MAIN
x
V
MAIN
/ V
IN
)
2
x
R
L
where R
L
is the inductor series resistance.
Output Capacitor
A 10µF capacitor works well in most applications. The
equivalent series resistance (ESR) of the output-filter
capacitor affects efficiency and output ripple. Output
voltage ripple is largely the product of the peak induc-
tor current and the output capacitor ESR. Use low-ESR
ceramic capacitors for best performance. Low-ESR,
surface-mount tantalum capacitors with higher capacity
may be used for load transients with high peak cur-
rents. Voltage ratings and temperature characteristics
should be considered.
Input Capacitor
The input capacitor (C
IN
) in boost designs reduces the
current peaks drawn from the input supply and reduces
noise injection. The value of C
IN
is largely determined by
the source impedance of the input supply. High source
impedance requires high input capacitance, particularly
as the input voltage falls. Since step-up DC-DC convert-
ers act as “constant-power” loads to their input supply,
input current rises as input voltage falls. A good starting
point is to use the same capacitance value for C
IN
as for
C
OUT
. Table 1 lists suggested component suppliers.
Integrator Capacitor
The MAX1748/MAX8726 contain an internal current
integrator that improves the DC load regulation but
increases the peak-to-peak transient voltage (see the
load-transient waveforms in the Typical Operating
Characteristics). For highly accurate DC load regula-
tion, enable the current integrator by connecting a
470pF capacitor to INTG. To minimize the peak-to-peak
transient voltage at the expense of DC regulation, dis-
able the integrator by connecting INTG to REF and
adding a 100kΩ resistor to GND.
Rectifier Diode
Use a Schottky diode with an average current rating
equal to or greater than the peak inductor current, and
a voltage rating at least 1.5 times the main output volt-
age (V
MAIN
).
L
V Efficiency (V V )
VLIRI f
IN(MIN)
2
MAIN IN(MIN)
(MAIN)
2
MAIN(MAX) OSC
=
××−
×× ×
Table 1. Component Suppliers
12 ______________________________________________________________________________________
SUPPLIER PHONE FAX
INDUCTORS
Coilcraft 847-639-6400 847-639-1469
Coiltronics 561-241-7876 561-241-9339
Sumida USA 847-956-0666 847-956-0702
Toko 847-297-0070 847-699-1194
CAPACITORS
AVX 803-946-0690 803-626-3123
Kemet 408-986-0424 408-986-1442
Sanyo 619-661-6835 619-661-1055
Taiyo Yuden 408-573-4150 408-573-4159
DIODES
Central
Semiconductor
516-435-1110 516-435-1824
International
Rectifier
310-322-3331 310-322-3332
Motorola 602-303-5454 602-994-6430
Nihon 847-843-7500 847-843-2798
Zetex 516-543-7100 516-864-7630