TFT- LCD DC-DC Converters with Operational Amplifiers

EC9219

P 11 / 14

2009/09/29

I2R is a registered trademark of Instruments for Research and Industry, Inc.

Choose an available inductor value from an appropriate inductor family. Calculate the maximum DC input

operating point (

Calculate the ripple current at that operating point and the peak current required for the inductor:

The inductor’s saturation current rating and the EC9219s’ LX current limit (ILIM) should exceed IPEAK,

and the inductor’s DC current rating should exceed IIN(DC,MAX). For good efficiency, choose an

inductor with less than 0.1Ω series resistance. Considering the Typical Operating Circuit, the maximum

load current (IMAIN(MAX)) is 500mA with a 13V output and a typical input voltage of 5V. Choosing an ICR

of 0.5 and estimating efficiency of 85% at this operating point:

Using the circuit’s minimum input voltage (4.5V) and estimating efficiency of 80% at that operating point:

The ripple current and the peak current are:

Output-Capacitor Selection

The total output voltage ripple has two components: the capacitive ripple caused by the charging and

discharging of the output capacitance, and the ohmic ripple due to the capacitor’s equivalent series

resistance (ESR).

where IPEAK is the peak inductor current (see the Inductor Selection section). For ceramic capacitors, the

output voltage ripple is typically dominated by VRIPPLE(C). The voltage rating and temperature

characteristics of the output capacitor must also be considered.