9

FN7428.6

August 5, 2005

Forced PWM Mode/SYNC Input

Pulling the SYNC pin HI (>2.5V) forces the converter into

PWM mode in the next switching cycle regardless of output

current. The duration of the transition varies depending on

the output current. Figures 22 and 23 (under two different

loading conditions) show the device goes from PFM to PWM

mode

Start-Up and Shut-Down

approximately 2.4V, the regulator begins to switch. The

inductor current limit is gradually increased to ensure proper

soft-start operation.

When the EN pin is connected to a logic low, the EL7531 is

in the shut-down mode. All the control circuitry and both

total input current is less than 1µA.

When the EN reaches logic HI, the regulator repeats the

start-up procedure, including the soft-start function.

Current Limit and Short-Circuit Protection

The current limit is set at about 2A for the PMOS. When a

short-circuit occurs in the load, the preset current limit

restricts the amount of current available to the output, which

causes the output voltage to drop below the preset voltage.

In the meantime, the excessive current heats up the

regulator until it reaches the thermal shut-down point.

Thermal Shut-Down

Once the junction reaches about 145°C, the regulator shuts

down. Both the P channel and the N channel MOSFETs turn

off. The output voltage will drop to zero. With the output

MOSFETs turned off, the regulator will soon cool down.

Once the junction temperature drops to about 130°C, the

regulator will restart again in the same manner as EN pin

connects to logic HI.

Thermal Performance

The EL7531 is available in a fused-lead MSOP10 package.

Compared with regular MSOP10 package, the fused- lead

package provides lower thermal resistance. The

100°C/W on a 4-layer board and 125°C/W on 2-layer board.

Maximizing the copper area around the pins will further

improve the thermal performance.

Output Voltage Selection

Users can set the output voltage of the variable version with

a resister divider, which can be chosen based on the

following formula:

Component Selection

Because of the fixed internal compensation, the component

choice is relatively narrow. For a regulator with fixed output

voltage, only two capacitors and one inductor are required.

We recommend 10µf to 22µF multi-layer ceramic capacitors

with X5R or X7R rating for both the input and output

capacitors, and 1.5 to 2.2µH for the inductor.

The RMS current present at the input capacitor is decided by

the following formula:

input capacitor must be able to handle this current.

The inductor peak-to-peak ripple current is given as:

L is the inductance

current, and to assure that the inductor is reliable, it must

handle the 2A surge current that can occur during a current

limit condition.

Layout Considerations

The layout is very important for the converter to function

properly. The following PC layout guidelines should be

followed:

1. Separate the Power Ground ( ) and Signal Ground

(

); connect them only at one point right at the pins

as possible

3. Make the following PC traces as small as possible:

4. from LX pin to L

as close as possible

7. Maximize the copper area around the PGND pin

8. Place several via holes under the chip to additional

ground plane to improve heat dissipation

The demo board is a good example of layout based on this

outline. Please refer to the EL7531 Application Brief.

0.8

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EL7531