EL7535
output voltage is gradually increased to ensure proper soft-
start operation.
The inductor must be able to handle I for the RMS load
O
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.
When the EN pin is connected to a logic low, the EL7535 is
in the shut-down mode. All the control circuitry and both
MOSFETs are off, and V
falls to zero. In this mode, the
Current Limit and Short-Circuit Protection
OUT
total input current is less than 1µA.
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.
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
output holds low for about 100ms after V reaches the
O
Thermal Shut-Down
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
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.
ground and leave open the pull-up resister R at POR pin.
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The POR output also serves as a 100ms delayed Power
Good signal when the pull-up resister R is installed. The
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Thermal Performance
RSI pin needs to be directly (or indirectly through a resister
R ) connected to Ground for this to function properly.
The EL7535 is in a fused-lead MSOP10 package. Compared
with regular MSOP10 package, the fused-lead package
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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:
provides lower thermal resistance. The θ is 100°C/W on a
JA
4-layer board and 125°C/W on 2-layer board. Maximizing the
copper area around the pins will further improve the thermal
performance.
R
R
2
V
= 0.8 × 1 + ------
Layout Considerations
The layout is very important for the converter to function
properly. The following PC layout guidelines should be
followed:
O
1
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
• Separate the Power Ground ( ) and Signal Ground ( );
connect them only at one point right at the pins
• Place the input capacitor as close to V and PGND pins
IN
as possible
capacitors, and 1.5µH to 2.2µH inductance for the inductor.
• Make the following PC traces as small as possible:
The RMS current present at the input capacitor is decided by
the following formula:
- from L pin to L
X
- from C to PGND
O
V
× (V - V
)
IN
IN
O
• If used, connect the trace from the FB pin to R1 and R2 as
close as possible
-------------------------------------------------
I
=
× I
INRMS
O
V
IN
• Maximize the copper area around the PGND pin
This is about half of the output current I for all the V . This
O
O
input capacitor must be able to handle this current.
• Place several via holes under the chip to additional ground
plane to improve heat dissipation
The inductor peak-to-peak ripple current is given as:
The demo board is a good example of layout based on this
outline. Please refer to the EL7535 Application Brief.
(V - V ) × V
O
IN
O
∆I = --------------------------------------------
IL
L × V × f
IN
S
• L is the inductance
• f the switching frequency (nominally 1.4MHz)
S
FN7003.2
7
August 16, 2005
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