EL7531 View Datasheet(PDF) - Renesas Electronics
Part Name
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Manufacturer
EL7531 Datasheet PDF : 12 Pages
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EL7531
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 JA is
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.
Power Good Output
The PG (pin 8) output is used to indicate when the output
voltage is properly regulating at the desired set point. It is an
open-drain output that should be tied to VIN or VCC through a
100k resistor. If no faults are detected, EN is high, and the
output voltage is within ~5% of regulation, the PG pin will be
allowed to go high. Otherwise, the open-drain NMOS will pull
PG low.
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:
VO
=
0.8
1
+
R-R----21-
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:
IINRMS = -----V----O----------V---V--I--NI--N-----–-----V----O----- IO
This is about half of the output current IO for all the VO. This
input capacitor must be able to handle this current.
The inductor peak-to-peak ripple current is given as:
IIL = ---V----LI--N------–--V--V--I--N-O---------f--S--V----O--
L is the inductance
fS the switching frequency (nominally 1.4MHz)
The inductor must be able to handle IO for the RMS load
current, and to assure that the inductor is reliable, it must
handle the 2A surge current that can occur during a current
limit condition.
In addition to decoupling capacitors and inductor value, it is
important to properly size the phase-lead capacitor C4 (Refer
to the Typical Application Diagram). The phase-lead capacitor
creates additional phase margin in the control loop by
generating a zero and a pole in the transfer function. As a
general rule of thumb, C4 should be sized to start the phase-
lead at a frequency of ~2.5kHz. The zero will always appear at
lower frequency than the pole and follow the equation below:
fZ = 2--------R--1---2---C-----4-
Over a normal range of R2 (~10-100k), C4 will range from
~470-4700pF. The pole frequency cannot be set once the zero
frequency is chosen as it is dictated by the ratio of R1 and R2,
which is solely determined by the desired output set point. The
equation below shows the pole frequency relationship:
fP = -2----------R----1----1----R----2------C-----4-
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
2. Place the input capacitor as close to VIN and PGND pins as
possible
3. Make the following PC traces as small as possible:
4. from LX pin to L
5. from CO to PGND
6. If used, connect the trace from the FB pin to R1 and R2 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.
FN7428 Rev 10.00
December 10, 2015
Page 10 of 12
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