TFT- LCD DC-DC Converters with Operational Amplifiers

EC9219

P 10 / 14

2009/09/29

Operational Amplifiers

The EC9219 has one operational amplifier. The operational amplifiers are typically used to drive the LCD

backplane (VCOM) or the gamma-correction divider string. They feature 150mA output current, 17V/µs

slew rate, and 12MHz bandwidth. The rail-to-rail input and output capability maximizes system flexibility.

Frequency Selection

The EC9219’s frequency can be user selected to operate at either 640kHz or 1.2MHz. Tie FREQ to GND for

640kHz operation. For a 1.2MHz switching frequency, tie FREQ to VIN.

Under voltage Lockout (UVLO)

The under voltage-lockout (UVLO) circuit compares the input voltage at VIN with the UVLO threshold to

ensure the input voltage is high enough for reliable operation. The 100mV (typ) hysteresis prevents supply

transients from causing a restart. Once the input voltage exceeds the UVLO rising threshold, startup

begins. When the input voltage falls below the UVLO falling threshold, the controller turns off the main

step-up regulator, turns off the outputs, and disables the switch control block; the operational amplifier

outputs are high impedance.

Thermal-Overload Protection

Thermal-overload protection prevents excessive power dissipation from overheating the EC9219. When

the junction temperature exceeds TJ = +135℃, a thermal sensor immediately activates the fault protection,

which shuts down all outputs except the reference, allowing the device to cool down. Once the device

cools down by approximately 30℃, and reactivate the device. The thermal-overload protection protects the

controller in the event of fault conditions. For continuous operation, do not exceed the absolute maximum

junction temperature rating of TJ = +125℃.

Design Procedure Main Step-Up Regulator Inductor Selection

The minimum inductance value, peak current rating, and series resistance are factors to consider when

selecting the inductor. These factors influence the converter’s efficiency, maximum output load capability,

transient-response time, and output voltage ripple. Size and cost are also important factors to consider.

The maximum output current, input voltage, output voltage, and switching frequency determine the

inductor value. Very high inductance values minimize the current ripple and therefore reduce the peak

current, which decreases core losses in the inductor and RL losses in the entire power path. However,

large inductor values also require more energy storage and more turns of wire, which increases size and

can increase winding resistance losses in the inductor. Low inductance values decrease the size but

increase the current ripple and peak current. Finding the best inductor involves choosing the best

compromise between circuit efficiency, inductor size, and cost. The equations used here include a

constant ICR(Inductor current ripple rate), which is the ratio of the inductor peak-to-peak ripple current to

the average DC inductor current at the full load current. The best trade-off between inductor size and

circuit efficiency for step-up regulators generally has an ICR between 0.3 and 0.5. However, depending on

the AC characteristics of the inductor core material and ratio of inductor resistance to other power-path

resistances, the best ICR can shift up or down. If the inductor resistance is relatively high, more ripple can

be accepted to reduce the number of turns required and increase the wire diameter. If the inductor

resistance is relatively low, increasing inductance to lower the peak current can decrease losses

throughout the power path. If extremely thin high-resistance inductors are used, as is common for

LCD-panel applications, the best ICR can increase to between 0.5 and 1.0. Once a physical inductor is

chosen, higher and lower values of the inductor should be evaluated for efficiency improvements in typical

operating regions. Calculate the approximate inductor value using the typical input voltage (VIN), the

maximum output current (IMAIN(MAX)), the expected efficiency (

in the Typical Operating Characteristics section, and an estimate of ICR based on the above discussion: