Rainbow Electronics MAX1885 - Manual (Page 27)

MAX1778/MAX1880–MAX1885
Quad-Output TFT LCD DC-DC
Converters with Buffer
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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.