1mhz) – Rainbow Electronics MAX1885 User Manual
Page 27
MAX1778/MAX1880–MAX1885
Quad-Output TFT LCD DC-DC
Converters with Buffer
______________________________________________________________________________________
27
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
).
Charge Pumps (MAX1778/ MAX1880/
MAX1881/MAX1882 Only)
Selecting the Number of Charge-Pump Stages
The number of charge-pump stages required to regu-
late the output voltage depends on the supply voltage,
output voltage, load current, switching frequency, the
diode’s forward voltage drop, and ceramic capacitor
values.
For positive charge-pump outputs, the number of
required stages may be determined by:
where V
SUPD
is the positive charge-pump diode supply
(Figure 4), V
DIODE
is the diode’s forward voltage drop,
and R
TX
is the charge pump’s output impedance. The
charge pump’s output impedance may be approximat-
ed using the following equation:
where the charge pump’s switching frequency (f
CHP
) is
equal to 0.5 x f
OSC
, the P-channel MOSFET’s on-resis-
tance (R
PCH(ON)
) is 10
Ω, and the N-channel MOSFET’s
on-resistance (R
NCH(ON
)) is 4
Ω (see Electrical
Characteristics).
For negative charge pump outputs, the number of
required stages may be determined by:
where N
NEG
is rounded up to the nearest integer.
N
V
V
V
R
I
NEG
NEG
SUPN
DROP
TX LOAD
. (
)
≥
+
- 1 1 2
R
R
R
C f
C
f
TX
PCH ON
NCH ON
X CHP
OUT CHP
(
)
(
)
(
)
=
+
+
+
2
1
1
N
V
V
V
V
R
I
POS
POS
SUPD
SUPP
DIODE
TX LOAD
. (
)
≥
+
-
- 1 1 2
CIRCUIT #1
CIRCUIT #2
CIRCUIT #3
CIRCUIT #4
CIRCUIT #5
V
IN
3.3V
3.3V
3.3V
5V
5V
V
MAIN
9V
9V
9V
12V
12V
I
MAIN(MAX)
100mA
200mA
200mA
220mA
220mA
V
NEG
-5V
-5V
-5V
-5V
-5V
I
NEG
2mA
5mA
5mA
5mA
5mA
V
POS
24V
24V
24V
24V
24V
I
POS
2mA
5mA
5mA
5mA
5mA
L
2.2
µH
4.7
µH
4.7
µH
6.8
µH
6.8
µH
I
PEAK
>1A
>1A
>1A
>1A
>1A
C
OUT
4.7
µF
10
µF
20
µF
10
µF
20
µF
R1
309k
Ω
309k
Ω
309k
Ω
429k
Ω
429k
Ω
R2
49.9k
Ω
49.9k
Ω
49.9k
Ω
49.9k
Ω
49.9k
Ω
R
COMP
None
None
39k
Ω*
None
20k
Ω*
C
COMP
None
None
100pF*
None
200pF*
Table 1. MAX1778/MAX1880/MAX1883 Component Values (f
OSC
= 1MHz)
*R
COMP
and C
COMP
are connected between the step-up converter’s output (V
MAIN
) and FB.