MAX1729 View Datasheet(PDF) - Maxim Integrated

Part Name

Description

Manufacturer

MAX1729

ECB and LCD Display Bias Supply with Accurate Output Voltage and Temperature Compensation

Maxim Integrated 

ECB and LCD Display Bias Supply with Accurate

Output Voltage and Temperature Compensation

IN

LX

TC

TEMPERATURE

BOOST

SENSOR

CONVERTER

PS

REF

VOLTAGE

REFERENCE

LDO

LINEAR

REGULATOR

OUT

FB

MAX1729

FEEDBACK

CONTROL

122mV

CTLIN

SHUTDOWN

CONTROL

VREF

GND

GND

COMP

NOTE: SWITCH STATES SHOWN FOR INTERNAL FEEDBACK MODE.

Figure 1. Internal Block Diagram

inductor current to ramp down and VPS to increase. If, at

the end of tOFF, VPS - VOUT is still too low, then another

tON is initiated immediately. Otherwise, the boost con-

verter remains idle in a low-quiescent-current state until

VPS - VOUT drops again and the error comparator initi-

ates another cycle.

Linear Regulator

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 (VPS) approxi-

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.

Temperature Sensor Output

The MAX1729 generates a temperature sensor voltage

(VTC) that varies at 16.5mV/°C (typ) and is nominally

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.

Control Signal

An externally generated PWM control signal on CTLIN

controls VOUT in internal feedback mode and influ-

ences VOUT in external feedback mode. In either mode,

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.

Internal Feedback Mode

In internal feedback mode, the signal at CTLIN is inverse-

ly buffered, level-shifted, and output at COMP through a

resistor. Internal resistance (33kΩ typical) and C6 then fil-

ter the signal before it is used by the internal feedback

network to set VOUT. If temperature compensation is

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 Designing for Internal Feedback

Mode section for more information.

External Feedback Mode

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.

Design Procedure

Designing for Internal Feedback Mode

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, VOUT is governed by the fol-

lowing equation:

VOUT = VOUT(MIN) + Duty Cycle ⋅ Gain

where VOUT(MIN) is 2.45V and Gain is nominally

13.95V/100%, as listed in the Electrical Characteristics.

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