MAX1779 Maxim, MAX1779 Datasheet - Page 10

• Manufacturer Part Number: MAX1779
• Description: Low-Power Triple-Output TFT LCD DC-DC Converter
• Manufacturer: Maxim

(V
diode (D3) turns off.
The main boost regulator does not have soft-start.
For the charge pumps, soft-start is achieved by control-
ling the rise rate of the output voltage. The output volt-
age regulates within 16ms, regardless of output
capacitance and load, limited only by the regulator’s
output impedance (see the Startup Waveforms in the
Typical Operating Characteristics).
A logic-low level on SHDN disables all three MAX1779
converters and the reference. When shut down, the
supply current drops to 0.1µA to maximize battery life
and the reference is pulled to ground. The output
Low-Power Triple-Output TFT LCD DC-DC
Converter
Figure 1. PWM Boost Converter Block Diagram
10
C3
______________________________________________________________________________________
C
+ V
INTG
SUPP
), charge flows from C3 to C4 until the
INTG
-
OSC
+
-
+
-
MAX1779
S
R
Shutdown
Soft-Start
V
IN
I
LIM
= 2.7V TO 5.5V
Σ
+
+
Q
Gm
-
+
capacitance and load current determine the rate at
which each output voltage will decay. A logic-level high
on SHDN activates the MAX1779 (see Power-Up
Sequencing). Do not leave SHDN floating. If unused,
connect SHDN to IN.
Upon power-up or exiting shutdown, the MAX1779
starts a power-up sequence. First, the reference pow-
ers up. Then the main DC-DC step-up converter pow-
ers up. Once the main boost converter reaches
regulation, the negative charge pump turns on. When
the negative output voltage reaches approximately 90%
of its nominal value (V
charge pump starts up. Finally, when the positive out-
put voltage reaches 90% of its nominal value (V
1.25V
+ -
IN
-
SLOPE
COMP
L1
PGND
GND
REF
LX
FB
C2
D1
FBN
Power-Up Sequencing
< 120mV), the positive
V
V
R2
R1
OUT
REF
R
COMP
= 1.25V
= [1 + (R1 / R2)] x V
(UP TO 13V)
C1
C
V
MAIN
COMP
REF
FBP
>