By v, Volts. this connects c, If the voltage across c – Rainbow Electronics MAX1885 User Manual
Page 21: Plus a diode drop (v, Charge flows from c, Until the diode (d3) turns off
MAX1778/MAX1880–MAX1885
Quad-Output TFT LCD DC-DC
Converters with Buffer
______________________________________________________________________________________
21
The switching waveforms will appear noisy and asyn-
chronous when light loading causes pulse-skipping
operation; this is a normal operating condition that
improves light-load efficiency.
Dual Charge-Pump Regulator (MAX1778/
MAX1880/MAX1881/MAX1882 Only)
The MAX1778/MAX1880/MAX1881/MAX1882 con-
trollers contain two independent low-power charge
pumps (Figure 4). One charge pump inverts the input
voltage and provides a regulated negative output volt-
age. The second charge pump doubles the input volt-
age and provides a regulated positive output voltage.
The controllers contain internal P-channel and N-chan-
nel MOSFETs to control the power transfer. The
internal MOSFETs switch at a constant frequency
(fCHP = fOSC/2).
Positive Charge Pump
During the first half-cycle, the N-channel MOSFET turns
on and charges flying capacitor C
X(POS)
(Figure 4).
This initial charge is controlled by the variable N-chan-
nel on-resistance. During the second half-cycle, the N-
channel MOSFET turns off and the P-channel MOSFET
turns on, level shifting C
X(POS)
by V
SUPP
volts. This
connects C
X(POS)
in parallel with the reservoir capaci-
tor C
OUT(POS)
. If the voltage across C
OUT(POS)
plus a
diode drop (V
POS
+ V
DIODE
) is smaller than the level-
shifted flying capacitor voltage (V
CX(POS)
+ V
SUPP
),
charge flows from C
X(POS)
to C
OUT(POS)
until the diode
(D3) turns off.
INDUCTOR CURRENT
I
LOAD
t
ON
t
OFF
TIME
I
PEAK
Figure 3. Discontinuous-to-Continuous Conduction Crossover
Point
MAX1778
MAX1880
MAX1881
MAX1882
V
NEG
= -
V
REF
V
REF
= 1.25V
( )
R5
R6
R5
R6
C
OUT(NEG)
C
X(NEG)
V
SUPN
2.7V TO 13V
OSC
REF
PGND
GND
SUPN
DRVN
FBN
SUPP
DRVP
FBP
D4
D5
V
POS
= 1 +
V
REF
V
REF
= 1.25V
R3
R4
( )
V
SUPP
2.7V TO 13V
V
SUPD
D2
D3
C
REF
0.22
µF
V
REF
1.25V
R4
C
X(POS)
C
OUT(POS)
V
POS
V
NEG
R3
Figure 4. Low-Power Charge Pump Block Diagram