TFT- LCD DC-DC Converters with Operational Amplifiers

EC9219

P 12 / 14

2009/09/29

Input-Capacitor Selection

The input capacitor (CIN) reduces the current peaks drawn from the input supply and reduces noise

injection into the IC. A 10μF ceramic capacitor is used in the Typical Applications Circuit (Figure 2)

because of the high source impedance seen in typical lab setups. Actual applications usually have much

lower source impedance since the step-up regulator often runs directly from the output of another

regulated supply. Typically, CIN can be reduced below the values used in the Typical Applications Circuit.

Ensure a low-noise supply at VIN by using adequate CIN. Alternately, greater voltage variation can be

tolerated on CIN if VIN is decoupled from CIN using an RC low-pass filter.

Rectifier Diode

The EC9219s’ high switching frequency demands a high-speed rectifier. Schottky diodes are recommended for

most applications because of their fast recovery time and low forward voltage. In general, a 2A Schottky diode

complements the internal MOSFET well.

Output-Voltage Selection

The output voltage of the main step-up regulator can be adjusted by connecting a resistive voltage-divider

from the output (VMAIN) to AGND with the center tap connected to FB (see Figure 2). Select R2 in the

10kΩ to 50kΩ range. Calculate R1 with the following equation:

1)

-

V

Vmain

(

R8

R3

FB

×

=

where VFB, the step-up regulator’s feedback set point, is 1.228V. Place R3 and R8 close to the IC.

Loop Compensation

The EC9219 incorporates an trans conductance amplifier in its feedback path to allow the user some

adjustment on the transient response and better regulation. The EC9219 uses current mode control

architecture which has a fast current sense loop and a slow voltage feedback loop. The fast current

feedback loop does not require any compensation. The slow voltage loop must be compensated for stable

operation. The compensation network is a series RC network from COMP pin to ground. The resistor sets

the high frequency integrator gain for fast transient response and the capacitor sets the integrator zero to

ensure loop stability. For most applications, the compensation resistor in the range of 10K to 100K and the

compensation capacitor in the range of 1nF to 0.22uF.

PC Board Layout and Grounding

Careful PC board layout is important for proper operation. Use the following guidelines for good PC board

layout:

Minimize the area of high-current loops by placing the inductor, the output diode, and the output

capacitors near the input capacitors and near the LX and PGND pins. The high-current input loop goes

from the positive terminal of the input capacitor to the inductor, to the IC’s LX pin, out of PGND, and to

the input capacitor’s negative terminal. The high-current output loop is from the positive terminal of the

input capacitor to the inductor, to the output diode (D5), and to the positive terminal of the output

capacitors, reconnecting between the output capacitor and input capacitor ground terminals. Connect

these loop components with short, wide connections. Avoid using vias in the high-current paths. If vias

are unavoidable, use many vias in parallel to reduce resistance and inductance.

Create a power-ground island (PGND) consisting of the input and output capacitor grounds, PGND pin,

and any charge-pump components. Connect all of these together with short, wide traces or a small

ground plane. Maximizing the width of the power-ground traces improves efficiency and reduces output

voltage ripple and noise spikes. Create an analog ground plane (SGND) consisting of the SGND pin, all

the feedback-divider ground connections, the COMP and SS capacitor ground connections. Connect

the SGND and PGND islands. Make no other connections between these separate ground planes.