Rainbow Electronics MAX1779 - Manual (Page 10)

MAX1779
Low-Power Triple-Output TFT LCD DC-DC
Converter
10 ______________________________________________________________________________________
(V C3 + V SUPP ), charge flows from C3 to C4 until the
diode (D3) turns off.
Soft-Start
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).
Shutdown
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
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.
Power-Up Sequencing
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
FBN < 120mV), the positive
charge pump starts up. Finally, when the positive out-
put voltage reaches 90% of its nominal value (V
FBP >
LX
GND
PGND
1.25V
R2
V IN = 2.7V TO 5.5V
L1
IN
R1
Q R
S
OSC
REF
FB
V
MAIN
(UP TO 13V)
V OUT = [1 + (R1 / R2)] x V REF
V REF = 1.25V
I LIM
C1
C2 C COMP
R COMP
D1
INTG
+
+
-
+
-
-
MAX1779
Gm
- + -
C INTG
+
-
SLOPE
COMP
+
+
Σ
Figure 1. PWM Boost Converter Block Diagram