MAX1729 Maxim, MAX1729 Datasheet - Page 7

• Manufacturer Part Number: MAX1729
• Description: ECB and LCD Display Bias Supply with Accurate Output Voltage and Temperature Compensation
• Manufacturer: Maxim

inductor current to ramp down and V
the end of t
t
verter remains idle in a low-quiescent-current state until
V
ates another cycle.
The PNP low-dropout linear regulator of the MAX1729
regulates the boost-converter output to the desired out-
put voltage. The boost converter’s regulation circuitry
holds the linear regulator’s input voltage (V
mately 0.6V above the output voltage to keep the regu-
lator out of dropout, thereby enhancing ripple rejection.
The linear regulator incorporates short-circuit protec-
tion, which limits the output current to approximately
6mA.
The MAX1729 generates a temperature sensor voltage
(V
Figure 1. Internal Block Diagram
ON
PS
CTLIN
NOTE: SWITCH STATES SHOWN FOR INTERNAL FEEDBACK MODE.
TC
ECB and LCD Display Bias Supply with Accurate
REF
TC
IN
FB
Output Voltage and Temperature Compensation
is initiated immediately. Otherwise, the boost con-
- V
) that varies at 16.5mV/°C (typ) and is nominally
122mV
OUT
TEMPERATURE
SHUTDOWN
REFERENCE
CONTROL
VOLTAGE
SENSOR
OFF
drops again and the error comparator initi-
GND
MAX1729
, V
PS
_______________________________________________________________________________________
Temperature Sensor Output
V
GND
- V
REF
OUT
CONVERTER
REGULATOR
FEEDBACK
CONTROL
LINEAR
is still too low, then another
BOOST
LDO
COMP
Linear Regulator
PS
to increase. If, at
PS
) approxi-
LX
PS
OUT
equal to the reference voltage at room temperature. TC
is capable of sinking or sourcing 50µA. This output is
used to compensate for ECB color or LCD contrast
variations caused by changes in temperature. It may
be read with an ADC and used to modify an external
PWM control signal or, in external feedback mode,
summed directly into the feedback-resistor network.
An externally generated PWM control signal on CTLIN
controls V
ences V
if CTLIN is held low for longer than 1.24ms, the
MAX1729 enters shutdown mode, decreasing the sup-
ply current below 2µA. Shutdown mode limits the mini-
mum duty cycle and frequency that may be used to
keep the device active. CTLIN frequencies between
2kHz and 12kHz are recommended.
In internal feedback mode, the signal at CTLIN is inverse-
ly buffered, level-shifted, and output at COMP through a
resistor. Internal resistance (33k typical) and C6 then fil-
ter the signal before it is used by the internal feedback
network to set V
used, the temperature sensor output voltage is read by
an ADC and used to adjust the duty cycle of the PWM
control signal. See the
Mode section for more information.
In external feedback mode, the output voltage of the
MAX1729 is controlled by the duty cycle of the PWM
control signal and an external resistor network, as
shown in Figure 3. In this mode, the signal at CTLIN is
inverted, level-shifted, and presented directly to COMP.
R3, R4, and C6 filter the signal, before it is summed into
the feedback node.
For a 3kHz PWM control signal use a 1µF low-leakage
ceramic capacitor for C6. For applications requiring a
higher-frequency PWM control signal, reduce the value
of C6 to between 1µF and 0.22µF for frequencies
between 3kHz and 12kHz. Higher C6 values reduce
output ripple. In Figure 2, V
lowing equation:
where V
13.95V/100%, as listed in the Electrical Characteristics.
Designing for Internal Feedback Mode
V
OUT
OUT
OUT(MIN)
OUT
in external feedback mode. In either mode,
= V
in internal feedback mode and influ-
OUT
OUT(MIN)
is 2.45V and Gain is nominally
. If temperature compensation is
Designing for Internal Feedback
External Feedback Mode
Internal Feedback Mode
Design Procedure
OUT
Duty Cycle
is governed by the fol-
Control Signal
Gain
7