MAX1729
Detailed Description
The MAX1729 is designed to provide bias voltage for
ECB or LCD displays. It is composed of a step-up
DC-DC converter followed by a linear regulator
(Figure 1), a combination that provides step-up/step-
down voltage conversion while minimizing output rip-
ple. The device allows you to adjust a display’s color or
contrast by dynamically adjusting the MAX1729’s out-
put voltage using a PWM control signal. In internal
feedback mode, the output voltage is adjustable
between +2.5V and +16V. In external feedback mode,
the output voltage is adjustable, and its range is set by
a resistor network that is programmed to match the out-
put voltage range of LCD/ECB displays needing a max-
imum output up to +18V.
Boost Converter
The MAX1729’s DC-DC boost converter is implemented
with an on-chip N-channel MOSFET, a diode, and an
error comparator. The IC’s unique PFM control system
varies the on-time and off-time of the switch based on the
boost converter’s input and output voltage values, as fol-
lows:
where K is typically 8V-µs. This timing maintains discon-
tinuous conduction and sets the peak inductor current
(I
PEAK
) to:
where L is the inductance of L1 (Figures 2, 3, and 4).
When the error comparator detects that the drop across
the linear regulator (V
PS
- V
OUT
) is less than approximate-
ly 0.6V, the internal switch is turned on (t
ON
initiates) and
current through the inductor ramps to I
PEAK
. At the end of
t
ON
, the switch is turned off for at least t
OFF
, allowing the
ECB and LCD Display Bias Supply with Accurate
Output Voltage and Temperature Compensation
6 _______________________________________________________________________________________
Pin Description
Drain of the internal MOSFET SwitchLX9
GroundGND10
Feedback and Mode Control Input. Connect to GND for internal feedback mode operation.FB5
Control Input. Drive low for more than 1.2ms to put the device into shutdown.CTLIN6
Bypass to GND with a 1.0µF capacitor.OUT7
Output of boost converter and input to LDO. Bypass to GND with a 0.068µF capacitor.PS8
Compensation Pin. In internal feedback mode (Figure 2), bypass with a 1µF capacitor. In external feedback
mode, COMP is a buffered inverse version of CTLIN (Figure 3).
COMP4
Reference Voltage Output. Bypass to GND with a 0.1µF capacitor.REF3
PIN
Temperature-Sensor Output. Bypass to GND with a 1000pF capacitor.TC2
Supply Input. Bypass with 0.1µF capacitor to ground. Connect to supply side of inductor (L1).IN1
FUNCTIONNAME
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