MAX1880EUG Maxim Integrated Products, MAX1880EUG Datasheet - Page 34

• Manufacturer Part Number: MAX1880EUG
• Description: DC/DC Switching Converters
• Manufacturer: Maxim Integrated Products

Specifications of MAX1880EUG

• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

Quad-Output TFT LCD DC-DC
Converters with Buffer
converter steps up the input voltage so that the linear
regulator will not drop out. Therefore, to guarantee that
the external pass transistor does not saturate, the step-
up converter’s output voltage must be set above the lin-
ear regulator’s output voltage plus the transistor’s
saturation rating (V
Power-Up Sequencing and Fault Protection
The MAX1778/MAX1880–MAX1885’s fault protection
cannot be activated until the power-up sequence is
successfully completed and the power ready output
goes low. Therefore, faults on the main output or posi-
tive charge-pump output could damage the controller
or external components. Additional fault protection may
be added as shown in Figure 10. The external MOSFET
and PNP transistor isolate the positive outputs during
startup. When the controller finishes the power-up
sequence, the power-ready output goes low, turning on
34
Figure 10. Power-Up Sequencing and Fault Protection;
______________________________________________________________________________________
V
IN
INPUT
= 3.3V
0.22μF
4.7μF
C
C
REF
MAIN
IN
0.22μF
100kΩ
≥ V
30kΩ
C1
R10
R9
LDO
+ V
REF
FLTSET
IN
SHDN
INTG
PGND
SAT
MAX1778
).
6.8μH
L1
TGND
DRVP
SUPP
GND
RDY
FBP
LX
FB
R4
49.9kΩ
0.1μF
0.1μF
C6
C4
49.9kΩ
274kΩ
R1
R2
750kΩ
the PNP transistor. Any fault on the positive charge-
pump output will pull down the charge pump’s output
voltage and trigger the fault protection; otherwise, the
MOSFET’s gate slow charges. Once the MOSFET turns
on, any faults on the main step-up converter’s output
will pull down the main output voltage and trigger the
fault protection.
The VCOM buffer does not include soft-start. Therefore,
once the VCOM buffer turns on, it draws high surge
currents while charging the output capacitance. In
some applications, the buffer’s high startup surge
current could potentially trip the fault detection circuit,
forcing the controller to shut down. In these cases,
adding a soft-start resistive divider between SUPB and
BUFOUT reduces the startup surge current and voltage
drops associated with this load (Figure 11), as shown in
R3
5.1kΩ
V
R
POS(START)
RDY
STARTUP
POSITIVE
= 20V
0.1μF
C5
1.0μF
STARTUP MAIN
V
C10
MAIN(START)
C
(2) 3.3μF
OUT
C7
1.0μF
Q3
= 8V
Q2
100kΩ
R8
VCOM Buffer Startup
R7
10kΩ
V
SYSTEM MAIN
V
POS(SYS)
MAIN(SYS)
POSITIVE
SYSTEM
= 20V
C8
3.3μF
= 8V
INPUT
V
IN
= 3.3V