iru3073 International Rectifier Corp., iru3073 Datasheet - Page 13

• Manufacturer Part Number: iru3073
• Description: Synchronous Pwm Controller With Over-current Protection / Ldo Controller -
• Manufacturer: International Rectifier Corp.

LDO Section
Output Voltage Programming
Output voltage for LDO is programmed by reference volt-
age and external voltage divider. The Fb2 pin is the in-
verting input of the error amplifier, which is internally ref-
erenced to 0.8V. The divider is ratioed to provide 0.8V at
the Fb2 pin when the output is at its desired value. The
output voltage is defined by using the following equation
LDO Power MOSFET Selection
The first step in selecting the power MOSFET for the
linear regulator is to select the maximum R
on the input to the dropout voltage and the maximum
load current.
Note that since the MOSFET R
perature, this number must be divided by ~1.5 in order
to find the R
has a maximum of 0.065V R
which meets our requirements.
Rev. 1.0
09/17/03
Figure 13 - Programming the output voltage for LDO.
V
For:
V
V
R
Results to R
R
For:
V
V
I
Results to: R
OUT2
OUT2
OUT2
REF
10
DS(ON)
IN(LDO)
OUT2
= 1KV
= 2A
= 0.8V
= V
= 1.6V
= 1.6V
DS(ON)(MAX)
=
= 2.5V
REF
V
IRU3073
IN(LDO)
3 1+
7
=1KV
DS(ON)(MAX)
I
(
OUT2
at room temperature. The IRLR2703
- V
Fb2
R
R
OUT2
10
7
= 0.45V
)
DS(ON)
DS(ON)
at room temperature,
increases with tem-
V
OUT2
R
R
7
10
DS(ON)
based
www.irf.com
Layout Consideration
The layout is very important when designing high fre-
quency switching converters. Layout will affect noise
pickup and can cause a good design to perform with
less than expected results.
Start to place the power components. Make all the con-
nections in the top layer with wide, copper filled areas.
The inductor, output capacitor and the MOSFET should
be close to each other as possible. This helps to reduce
the EMI radiated by the power traces due to the high
switching currents through them. Place input capacitor
directly to the drain of the high-side MOSFET. To reduce
the ESR, replace the single input capacitor with two par-
allel units. The feedback part of the system should be
kept away from the inductor and other noise sources
and be placed close to the IC. In multilayer PCB, use
one layer as power ground plane and have a separate
control circuit ground (analog ground), to which all sig-
nals are referenced. The goal is to localize the high cur-
rent path to a separate loop that does not interfere with
the more sensitive analog control function. These two
grounds must be connected together on the PC board
layout at a single point.
500
450
400
350
300
250
200
150
100
50
0
0
Figure 14 - Switching Frequency vs. Rt.
50
100
150
200
250
Rt (K V)
300
350
IRU3073
400
450
500
13
550