500khz), Table 3. component suppliers – Rainbow Electronics MAX1885 User Manual
Page 28
MAX1778/MAX1880–MAX1885
Quad-Output TFT LCD DC-DC
Converters with Buffer
28
______________________________________________________________________________________
Charge-Pump Input Power and
Efficiency Considerations
The charge pumps in the MAX1778/MAX1880/
MAX1881/MAX1882 provide regulated output voltages
by controlling the voltage drop across the low-side N-
channel MOSFET, so they can be modeled as linear
regulators followed by an unregulated charge pump
when determining the input power requirements and
efficiency.
The charge pump only provides charge to the output
capacitor during half the period (50% duty cycle), so
the input current is a function of the number of stages
and the load current:
for the positive charge pump, and:
for the negative charge pump, where N is the number
of charge pump stages.
The efficiency characteristics of the MAX1778/
MAX1880/MAX1881/MAX1882 regulated charge
pumps are similar to a linear regulator. It is dominated
by quiescent current at low output currents and by the
I
I
N
SUPP
POS
(
)
=
+1
CIRCUIT #6
CIRCUIT #7
CIRCUIT #8
CIRCUIT #9
V
IN
3.3V
3.3V
3.3V
3.3V
V
MAIN
9V
9V
9V
9V
I
MAIN(MAX)
100mA
100mA
200mA
200mA
V
NEG
-5V
-5V
-5V
-5V
I
NEG
2mA
2mA
5mA
5mA
V
POS
24V
24V
24V
24V
I
POS
2mA
2mA
5mA
5mA
L
4.7
µH
10
µH
10
µH
10
µH
I
PEAK
>1A
>1A
>1A
>1A
C
OUT
4.7
µF
10
µF
10
µF
20
µF
R1
309k
Ω
309k
Ω
309k
Ω
309k
Ω
R2
49.9k
Ω
49.9k
Ω
49.9k
Ω
49.9k
Ω
R
COMP
None
None
None
20k
Ω*
C
COMP
None
None
None
200pF*
Table 2. MAX1881/MAX1882/MAX1884/MAX1885 Component Values (f
OSC
= 500kHz)
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
Table 3. Component Suppliers
*R
COMP
and C
COMP
are connected between the step-up converter’s output (V
MAIN
) and FB.
I
I
N
SUPP
POS
(
)
=
+1