Max1729, Detailed description, Pin description – Rainbow Electronics MAX1729 User Manual
Page 6
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
I
=
K
L
PEAK
t
=
K
V
t
K
V
V
ON
IN
OFF
PS
IN
≥
−
ECB and LCD Display Bias Supply with Accurate
Output Voltage and Temperature Compensation
6
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Pin Description
Drain of the internal MOSFET Switch
LX
9
Ground
GND
10
Feedback and Mode Control Input. Connect to GND for internal feedback mode operation.
FB
5
Control Input. Drive low for more than 1.2ms to put the device into shutdown.
CTLIN
6
Bypass to GND with a 1.0µF capacitor.
OUT
7
Output of boost converter and input to LDO. Bypass to GND with a 0.068µF capacitor.
PS
8
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).
COMP
4
Reference Voltage Output. Bypass to GND with a 0.1µF capacitor.
REF
3
PIN
Temperature-Sensor Output. Bypass to GND with a 1000pF capacitor.
TC
2
Supply Input. Bypass with 0.1µF capacitor to ground. Connect to supply side of inductor (L1).
IN
1
FUNCTION
NAME