Datasheet

MAX1748/MAX8726
Triple-Output TFT-LCD
DC-DC Converters
_______________________________________________________________________________________ 9
Soft-Start
For the main boost regulator, soft-start allows a gradual
increase of the internal current-limit level during startup
to reduce input surge currents. The MAX1748/MAX8726
divide the soft-start period into four phases. During
phase 1, the MAX1748/MAX8726 limit the current limit to
only 0.38A (see the Electrical Characteristics tables),
approximately a quarter of the maximum current limit
(I
LX(MAX)
). If the output does not reach regulation within
1ms, soft-start enters phase II and the current limit is
increased by another 25%. This process is repeated for
phase III. The maximum 1.5A (typ) current limit is
reached at the end of phase III or when the output
reaches regulation, whichever occurs first (see the start-
up waveforms in the Typical Operating Characteristics).
For the charge pumps, soft-start is achieved by control-
ling the rise rate of the output voltage. The output volt-
age regulates within 4ms, regardless of output
capacitance and load, limited only by the regulator’s
output impedance.
Shutdown
A logic-low level on SHDN disables all three
MAX1748/MAX8726 converters and the reference.
When shut down, 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 power activates the
MAX1748/MAX8726 (see the Power-Up Sequencing
section). Do not leave SHDN floating. If unused,
connect SHDN to IN.
Power-Up Sequencing
Upon power-up or exiting shutdown, the MAX1748 and
MAX8726 start their respective power-up sequences.
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
+
+
+
-
+
-
-
MAX1748
MAX8726
Gm
-
+
-
C
INTG
Figure 1. PWM Boost Converter Block Diagram