7

FN7003.2

August 16, 2005

output voltage is gradually increased to ensure proper soft-

start operation.

When the EN pin is connected to a logic low, the EL7535 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.

RSI/POR Function

When powering up, the open-collector Power-On-Reset

preset voltage. When the active-HI reset signal RSI is

issued, POR goes to low immediately and holds for the

same period of time after RSI comes back to LOW. The

output voltage is unaffected. (Please refer to the timing

diagram). When the function is not used, connect RSI to

The POR output also serves as a 100ms delayed Power

RSI pin needs to be directly (or indirectly through a resister

Output Voltage Selection

Users can set the output voltage of the converter 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 1.5µH to 2.2µH inductance 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 1.5A surge current that can occur during a

current limit condition.

Current Limit and Short-Circuit Protection

The current limit is set at about 1.5A 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 EL7535 is in a fused-lead MSOP10 package. Compared

with regular MSOP10 package, the fused-lead package

provides lower thermal resistance. The

4-layer board and 125°C/W on 2-layer board. Maximizing the

copper area around the pins will further improve the thermal

performance.

Layout Considerations

The layout is very important for the converter to function

properly. The following PC layout guidelines should be

followed:

• Separate the Power Ground ( ) and Signal Ground ( );

connect them only at one point right at the pins

as possible

• Make the following PC traces as small as possible:

• If used, connect the trace from the FB pin to R1 and R2 as

close as possible

• Maximize the copper area around the PGND pin

• 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 EL7535 Application Brief.

0.8

1

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+

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



×

=

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×

×

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∆I

IL

(

×

×

×

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EL7535